Climate Change and...

Annotated Bibliography

Carbon Dynamics

Soil Carbon and Organic Matter

Abrahamson, D. A., Norfleet, M. L., Causarano, H. J., Williams, J. R., Shaw, J. N., Franzluebbers, A. J. (2007). Effectiveness of the soil conditioning index as a carbon management tool in the southeastern USA based on comparison with EPIC. Journal of Soil and Water Conservation 62 (2): 94-102

ABSTRACT: Models are being developed and utilized by scientists and government agencies to quantify the potential for carbon storage in soil. The Environmental Policy Integrated Climate (EPIC) v. 3060 model is a process-based model requiring detailed inputs. The soil conditioning index (SCI) is a simpler tool to predict relative change in soil organic carbon (SOC) using table values for three management components (i.e., organic matter, field operations, and erosion) within the framework of the Revised Universal Soil Loss Equation 2 model. Our objective was to determine whether SOC sequestration from no-tillage cropping systems in the southeastern USA could be simply predicted with SCI compared with detailed simulations using EPIC. Four management systems were evaluated: (1) cotton (Gossypium hirsutum L.) with conventional tillage, (2) cotton with no tillage, (3) corn (Zea mays L.)— cotton rotation with no tillage, and (4) bermudagrass (Cynodon dactylon L.)—corn—cotton rotation with no tillage. All no-tillage systems used wheat (Triticum aestivum L.) as a cover crop. Simulated SOC sequestration with EPIC was 0.46 ± 0.06 Mg ha−1 yr−1 (410 ± 51 lb ac−1 yr−1 ) under the three no-tillage management systems and -0.03 Mg ha−1 yr−1 (-30 lb ac−1 yr−1 ) under conventional tillage. The SCI also predicted a strong difference in SOC between conventional and no tillage. Differences in SOC sequestration among crop rotations were not readily apparent with EPIC but were with SCI. Predictions of SOC sequestration with SCI were comparable to those with EPIC but not necessarily in a linear manner as previously suggested. The SCI appears to be a valuable method for making reasonable, cost-effective estimates of potential changes in SOC with adoption of conservation management in the southeastern USA, although validations under actual field conditions are still needed.

Adviento-Borbe, M. A. A., Haddix, M. L., Binder, D. L., Walters, D. T., Dobermann, A. (2007). Soil greenhouse gas fluxes and global warming potential in four high-yielding maize systems. Global Change Biology 13 (9): 1972-1988

ABSTRACT: Crop intensification is often thought to increase greenhouse gas (GHG) emissions, but studies in which crop management is optimized to exploit crop yield potential are rare. We conducted a field study in eastern Nebraska, USA to quantify GHG emissions, changes in soil organic carbon (SOC) and the net global warming potential (GWP) in four irrigated systems: continuous maize with recommended best management practices (CC-rec) or intensive management (CC-int) and maize–soybean rotation with recommended (CS-rec) or intensive management (CS-int). Grain yields of maize and soybean were generally within 80–100% of the estimated site yield potential. Large soil surface carbon dioxide (CO2 ) fluxes were mostly associated with rapid crop growth, high temperature and high soil water content. Within each crop rotation, soil CO2 efflux under intensive management was not consistently higher than with recommended management. Owing to differences in residue inputs, SOC increased in the two continuous maize systems, but decreased in CS-rec or remained unchanged in CS-int. N2 O emission peaks were mainly associated with high temperature and high soil water content resulting from rainfall or irrigation events, but less clearly related to soil NO3 -N levels. N2 O fluxes in intensively managed systems were only occasionally greater than those measured in the CC-rec and CS-rec systems. Fertilizer-induced N2 O emissions ranged from 1.9% to 3.5% in 2003, from 0.8% to 1.5% in 2004 and from 0.4% to 0.5% in 2005, with no consistent differences among the four systems. All four cropping systems where net sources of GHG. However, due to increased soil C sequestration continuous maize systems had lower GWP than maize–soybean systems and intensive management did not cause a significant increase in GWP. Converting maize grain to ethanol in the two continuous maize systems resulted in a net reduction in life cycle GHG emissions of maize ethanol relative to petrol-based gasoline by 33–38%. Our study provided evidence that net GHG emissions from agricultural systems can be kept low when management is optimized toward better exploitation of the yield potential. Major components for this included (i) choosing the right combination of adopted varieties, planting date and plant population to maximize crop biomass productivity, (ii) tactical water and nitrogen (N) management decisions that contributed to high N use efficiency and avoided extreme N2 O emissions, and (iii) a deep tillage and residue management approach that favored the build-up of soil organic matter from large amounts of crop residues returned.

Akselsson, C., Berg, B., Meentemeyer, V., Westling, O. (2005). Carbon sequestration rates in organic layers of boreal and temperate forest soils — Sweden as a case study. Global Ecology and Biogeography 14 (1): 77-84

ABSTRACT:The aim of this work was to estimate C sequestration rates in the organic matter layer in Swedish forests.The region encompassed the forested area (23 × 106 ha) of Sweden ranging from about 55° N to 69° N.We used the concept of limit values to estimate recalcitrant litter remains, and combined it with amount of litter fall. Four groups of tree species were identified (pine, spruce, birch and 'other deciduous species'). Annual actual evapotranspiration (AET) was estimated for 5 × 5 km grids covering Sweden. For each grid, data of forested area and main species composition were available. The annual input of foliar litter into each grid was calculated using empirical relationships between AET and foliar litter fall in the four groups. Litter input was combined with average limit values for decomposition for the four groups of litter, based on empirical data. Finally, C sequestration rate was calculated using a constant factor of the C concentration in the litter decomposed to the limit value, thus forming soil organic matter (SOM).We obtained a value of 4.8 × 106 metric tons of C annually sequestered in SOM in soils of mature forests in Sweden, with an average of 180 kg ha−1 and a range from 40 to 410 kg ha−1 . Norway spruce forests accumulated annually an average of 200 kg C ha−1 . The pine and birch groups had an average of 150 kg ha−1 and for the group of other deciduous trees, which is limited to south Sweden, the C sequestration was around 400 kg ha−1 .There is a clear C sequestration gradient over Sweden with the highest C sequestration in the south-west, mainly corresponding to the gradient in litter fall. The limit-value method appears useful for scaling up to a regional level to describe the C sequestration in SOM. A development of the limit value approach in combination with process-orientated dynamic models may have a predictive value.

Allison, S. D. (2004). Brown ground: a soil carbon analog for the green world. The American Naturalist 167: 619-627

ABSTRACT: For many decades, ecologists have asked what prevents herbivores from consuming most of the plant biomass in terrestrial ecosystems, or “Why is the world green?” Here I ask the analogous question for detritivores: what prevents them from degrading most of the organic material in soils, or “Why is the ground brown?” For fresh plant detritus, constraints on decomposition closely parallel constraints on herbivory: both herbivore and decomposer populations may be controlled by plant tissue chemistry from the bottom up and predators from the top down. However, the majority of soil carbon is not plant litter but carbon that has been consumed by detritivores and reprocessed into humic compounds with complex and random chemical structures. This carbon persists mainly because the chemical properties of humic compounds and interactions with soil minerals constrain decomposition by extracellular enzymes in soil. Other constraints on decomposers, such as nutrient limitation of enzyme production and competition with opportunistic microbes, also contribute to brown ground. Ultimately, the oldest soil carbon persists via transformation into complex molecules that are impervious to enzymatic attack and effectively decoupled from processing by the soil food web.

Amundson, R. (2001). The carbon budget of soils. Annual Review of Earth and Planetary Science 29: 535-562

ABSTRACT: The global soil C reservoir, 1500 Gt of C (1 Gt = 1012 kg of C), is dynamic on decadal time scales and is sensitive to climate and human disturbance. At present, as a result of land use, soil C is a source of atmospheric CO2 in the tropics and possibly part of a sink in northern latitudes. Here I review the processes responsible for maintaining the global soil C reservoir and what is known about how it responds to direct and indirect human perturbations.

Ansley, R. J., Boutton, T. W., Skjemstad, J. O. (2006). Soil organic carbon and black carbon storage and dynamics under different fire regimes in temperate mixed-grass savanna. Global Biogeochemical Cycles 20 (3): B3006

ABSTRACT: We quantified the effects of repeated, seasonal fires on soil organic carbon (SOC), black carbon (BC), and total N in controls and four fire treatments differing in frequency and season of occurrence in a temperate savanna. The SOC at 0–20 cm depth increased from 2044 g C m−2 in controls to 2393–2534 g C m−2 in the three treatments that included summer fire. Similarly, soil total N (0–20 cm) increased from 224 g N m−2 in the control to 251–255 g N m−2 in the treatments that included summer fire. However, winter fires had no effect on SOC or total N. Plant species composition coupled with lowerd13 C of SOC suggested that increased soil C in summer fire treatments was related to shifts in community composition toward greater relative productivity by C3 species. Lowerd15 N of soil total N in summer fire treatments was consistent with a scenario in which N inputs > N losses. The BC storage was not altered by fire, and comprised 13–17% of SOC in all treatments. Results indicated that fire and its season of occurrence can significantly alter ecosystem processes and the storage of C and N in savanna ecosystems.

Baker, J. M., Ochsner, T. E., Venterea, R. T., Griffis, T. J. (2007). Tillage and soil carbon sequestration--What do we really know?. Agriculture, Ecosystems & Environment 118 (1-4): 1-5

ABSTRACT: It is widely believed that soil disturbance by tillage was a primary cause of the historical loss of soil organic carbon (SOC) in North America, and that substantial SOC sequestration can be accomplished by changing from conventional plowing to less intensive methods known as conservation tillage. This is based on experiments where changes in carbon storage have been estimated through soil sampling of tillage trials. However, sampling protocol may have biased the results. In essentially all cases where conservation tillage was found to sequester C, soils were only sampled to a depth of 30 cm or less, even though crop roots often extend much deeper. In the few studies where sampling extended deeper than 30 cm, conservation tillage has shown no consistent accrual of SOC, instead showing a difference in the distribution of SOC, with higher concentrations near the surface in conservation tillage and higher concentrations in deeper layers under conventional tillage. These contrasting results may be due to tillage-induced differences in thermal and physical conditions that affect root growth and distribution. Long-term, continuous gas exchange measurements have also been unable to detect C gain due to reduced tillage. Though there are other good reasons to use conservation tillage, evidence that it promotes C sequestration is not compelling.

Bauer, J., Herbst, M., Huisman, J.A., Weihermuller, L., Vereecken, H. (2008). Sensitivity of simulated soil heterotrophic respiration to temperature and moisture reduction functions. Geoderma 145 (1-2): 17-27

ABSTRACT: In this study, the influence of different soil temperature and moisture reduction functions for scaling decomposition rates of soil organic matter on the prediction of CO2 production and fluxes was analysed. For this purpose, soil temperature and moisture reduction functions of six soil carbon decomposition models (CANDY, CENTURY, DAISY, PATCIS, ROTHC, and SOILCO2) were implemented in the modified SOILCO2-ROTHC model. As a test scenario, a respiration experiment on a silt loam in Columbia (USA) was chosen, which consists of two periods both including soil respiration measurements in a wheat stand and a subsequent bare soil period. Additionally, the dataset contains measured soil temperature, soil moisture as well as CO2 concentrations within the soil profile. The cumulative CO2 fluxes simulated with different temperature reduction functions showed deviations up to 41% (1.77 t C ha−1 ) for the six-month simulation period in 1981. The influence of moisture reduction was smaller with deviations up to 2% (0.10 t C ha−1 ). A combination of corresponding temperature and moisture reduction functions resulted in the highest deviations up to 41% (1.80 t C ha−1 ). Under field conditions the sensitivity towards soil temperature reduction was 6 to 7 times higher compared to soil moisture reduction. The findings of this study show that the choice of soil temperature and soil moisture reduction functions is a crucial factor for a reliable simulation of carbon turnover.

Bedard-Haughn, A., Jongbloed, F., Akkerman, J., Uijl, A., de Jong, E., Yates, T., Pennock, D. (2006). The effects of erosional and management history on soil organic carbon stores in ephemeral wetlands of hummocky agricultural landscapes. Geoderma 135: 296-306

ABSTRACT: Carbon sequestration by agricultural soils has been widely promoted as a means of mitigating greenhouse gas emissions. In many regions agricultural fields are just one component of a complex landscape matrix and understanding the interactions between agricultural fields and other landscape components such as wetlands is crucial for comprehensive, whole-landscape accounting of soil organic carbon (SOC) change. Our objective was to assess the effects of management and erosional history on SOC storage in wetlands of a typical hummocky agricultural landscape in southern Saskatchewan. Wetlands were classed into three land management groups: native wetlands (i.e., within a native landscape), and uncultivated and cultivated wetlands within an agricultural landscape. Detailed topographic surveys were used to develop a digital elevation model of the sites and landform segmentation algorithms were used to delineate the topographic data into landform elements. SOC density to 45 cm was assessed at seven uncultivated wetlands, seven cultivated wetlands, and twelve native wetlands. Mean SOC density decreased from 175.1 mg ha−1 to 30 cm (equivalent mass depth) for the native wetlands to 168.6 mg ha−1 for the uncultivated wetlands and 87.2 mg ha−1 for the cultivated wetlands in the agricultural field. The SOC density of sediment depositional fans in the uncultivated wetlands is high but the total SOC stored in the fans is low due to their small area. The uncultivated wetlands occupy only 11% of the site but account for approximately 23% of SOC stores. Re-establishing permanent vegetation in the cultivated wetlands could provide maximum C sequestration with minimum energy inputs and a minimum loss of productive acreage but the overall consequences for the gas emissions would have to be carefully assessed.

Bellamy, P. H., Loveland, P. J., Bradley, R. I., Lark, R. M., Kirk, G. J. D. (2005). Carbon losses from all soils across England and Wales 1978-2003. Nature 437 (7056): 245-248

ABSTRACT: More than twice as much carbon is held in soils as in vegetation or the atmosphere, and changes in soil carbon content can have a large effect on the global carbon budget. The possibility that climate change is being reinforced by increased carbon dioxide emissions from soils owing to rising temperature is the subject of a continuing debate. But evidence for the suggested feedback mechanism has to date come solely from small-scale laboratory and field experiments and modelling studies. Here we use data from the National Soil Inventory of England and Wales obtained between 1978 and 2003 to show that carbon was lost from soils across England and Wales over the survey period at a mean rate of 0.6% yr-1 (relative to the existing soil carbon content). We find that the relative rate of carbon loss increased with soil carbon content and was more than 2% yr-1 in soils with carbon contents greater than 100 g kg-1 . The relationship between rate of carbon loss and carbon content is irrespective of land use, suggesting a link to climate change. Our findings indicate that losses of soil carbon in England and Wales—and by inference in other temperate regions—are likely to have been offsetting absorption of carbon by terrestrial sinks.

Bengtson, P., Bengtsson, G. (2007). Rapid turnover of DOC in temperate forests accounts for increased CO2 production at elevated temperatures. Ecology Letters 10 (9): 783-790

ABSTRACT: The evidence for the contribution of soil warming to changes in atmospheric CO2 concentrations and carbon stocks of temperate forest ecosystems is equivocal. Here, we use data from a beech/oak forest on concentrations and stable isotope ratios of dissolved organic carbon (DOC), phosphate buffer-extractable organic carbon, soil organic carbon (SOC), respiration and microbial gross assimilation of N to show that respired soil carbon originated from DOC. However, the respiration was not dependent on the DOC concentration but exceeded the daily DOC pool three to four times, suggesting that DOC was turned over several times per day. A mass flow model helped to calculate that a maximum of 40% of the daily DOC production was derived from SOC and to demonstrate that degradation of SOC is limiting respiration of DOC. The carbon flow model on SOC, DOC, microbial C mobilization/immobilization and respiration is linked by temperature-dependent microbial and enzyme activity to global warming effects of CO2 emitted to the atmosphere.

A.A. Behre, J. Harte, J.W. Harden, M.S. Torn (2007). The significance of the erosion-induced terrestrial carbon sink. BioScience 57 (4): 337-346

ABSTRACT: Estimating carbon (C) balance in erosional and depositional landscapes is complicated by the effects of soil redistribution on both net primary productivity (NPP) and decomposition. Recent studies are contradictory as to whether soil erosion does or does not constitute a C sink. Here we clarify the conceptual basis for how erosion can constitute a C sink. Specifically, the criterion for an erosional C sink is that dynamic replacement of eroded C, and reduced decomposition rates in depositional sites, must together more than compensate for erosional losses. This criterion is in fact met in many erosional settings, and thus erosion and deposition can make a net positive contribution to C sequestration. We show that, in a cultivated Mississippi watershed and a coastal California watershed, the magnitude of the erosion-induced C sink is likely to be on the order of 1% of NPP and 16% of eroded C. Although soil erosion has serious environmental impacts, the annual erosion-induced C sink offsets up to 10% of the global fossil fuel emissions of carbon dioxide for 2005.

Bernoux, Martial, Cerri, Carlos C., Neill, Christopher, De Moraes, Jener F. L. (1998). The use of stable carbon isotopes for estimating soil organic matter turnover rates. Geoderma 82 (1-3): 43-58.

ABSTRACT: In natural ecosystems, soil organic carbon (C) is derived almost exclusively from the residues of plants growing in situ. In agroecosystems, it has at least two origins: one is the remains from the previous native vegetation, and the other is the remains of the crop and the decomposition of its residues. Where vegetation has changed from plants with the C3 photosynthetic pathway to C4 pathway or vice versa, changes in the natural abundance of13 C in soil organic matter (SOM) over time can be used to identify sources of organic C in soil and to determine the turnover rate of SOM. For example, large areas of C3 tropical forest have been replaced with C4 pasture or cropland. Changes in theδ13 C values of soil organic C in these areas reflect soil organic matter turnover rate, and provide insight regarding the functional role of tropical ecosystems in the global C cycle. This paper illustrates how the stable isotope13 C can be used to estimate SOM turnover rates and the sensitivity of different models and different model parameters, using a chronosequence of forest and pastures of different ages from the Brazilian Amazon. A single-compartment exponential decay model and a two-compartment model in which SOM was divided into stable and labile components yielded similar estimates of soil C turnover time at the surface but divergent estimates at depth. The one-compartment model gave the least variable estimates of model parameters and turnover times and was also relatively insensitive to individual C stocks in single pastures of a particular age. Estimates of soil stable and labile C pools obtained using changes in forest soil δ13 C with depth differed from estimates obtained using the chronosequence. This suggests that upon burning and pasture creation, a portion of the previously stable soil C pool is rendered less stable. Model r2 was a poor criterion for selecting an appropriate soil C turnover model to apply to chronosequence data. In the absence of substantial justification for segregating SOM into different compartments based on lability, modeling should be done with the simplest models possible.

Bernoux, M., Cerri, C. C., Volkoff, B., Carvalho, M. Da Conceição S., Feller, C., Cerri, C. E. P., Eschenbrenner, V., Piccolo, M. De C., Feigl, B. (2005). Greenhouse gas fluxes and carbon storage from soil: The Brazilian inventory. Cahiers Agricultures 14 (1): 96-100

ABSTRACT: Rising levels of atmospheric CO2 have focused attention on potential CO2 emissions from terrestrial ecosystems of the world, notably from soils and biomass. The world’s mineral soils represent a large reservoir of C of about 1500 Pg C. Under the United Nations Framework Convention on Climate Change (UNFCCC) each country is required to develop, update and publish a national inventories of anthropogenic emissions (implementation of the National Communications), as well as to compile the inventories by comparable methodologies. For the last point, guidelines were developed and published as IPCC Guidelines for National Greenhouse Gas Inventories. Also, the land use, land-use changes and forestry (LULUCF) sector should be included in the national inventories. The CO2 fluxes from soils are discussed in chapter 5 for agricultural soils under the category 5D: CO2 emissions and removals from soils. These emissions are calculated from three subcategories : i) net changes in C storage in mineral soils; ii) emissions from organic soils; and iii) emissions from liming of agricultural soils. In a first step the soil organic carbon stocks up to a depth of 30 cm were estimated for Brazil based on a map of different soil-vegetation associations combined with results from a soil database. The soil-vegetation associations map was derived by intersecting soil and vegetation maps. The original soil and vegetation classification were reduced to 6 soil and 15 vegetation categories. Because this data represents sites with native vegetation in the absence of significant disturbances, it constitutes a valuable baseline for evaluating the effect of land-use change on soil C stocks for Brazil. Overall, about 36 400 million tons of carbon would be stored in the 0-30 cm soil layer under native conditions. The Brazilian Amazon region would account for 22,000 million tons. The CO2 emission from mineral soils following land-cover change in Brazil for the period 1975-1995 was estimated by Bernoux et al. who showed that the annual fluxes for Brazil indicate a net emission of CO2 to the atmosphere of 46.4 million tons of CO2 for the period 1975-1995. Intermediary calculation used to derive these annual fluxes estimated that 34 400 million tons of carbon were stored in the Brazilian soil for the year 1995. The annual CO2 emission for Brazil from liming varied from 4.9 to 9.4 million tons of CO2 per year with a mean annual CO2 emission of about 7.2 million tons. The South, Southeast and Center region accounted for a least 92% of total emission. Finally it could be calculated that the total CO2 fluxes from soils reached around 51.9 million tons of CO2 per year for the period 1975-1995.

Bird, M.I., Chivas, A.R., Head, J. (1996). A latitudinal gradient in carbon turnover in forest soils. Nature 381 (May 9)

ABSTRACT: Attempts to model the global carbon cycle, and anthropogenic modifications to carbon flow between the atmospheric, oceanic and terrestrial carbon reservoirs, commonly rely on values assumed for the13 C/12 C ratio and 'bomb-spike'14 C signature of carbon in each reservoir1,2 . A large proportion of the carbon in the terrestrial biosphere resides in the soil organic carbon (SOC) pool3 , most of which is derived from plants that assimilate carbon via the C3 photosynthetic pathway4 . Here we report measurements of the13 C and14 C signatures of particulate organic carbon from surface soils of C3 biomes from a global distribution of low-altitude, non-water-stressed locations. We find that there is currently a latitudinal gradient in the signature, with low-latitude soils being relatively depleted in13 C. The14 C signatures indicate that today's gradient is due to a latitudinal gradient in the residence time of the soil organic carbon, coupled with anthropogenic modifications to the13 C/12 C ratio of atmospheric CO2 (for example by fossil-fuel burning5 ). The long residence times (tens of years) of particulate organic carbon from high-latitude soils provide empirical evidence that if fluxes of carbon from vegetation to the soil increase, these soils have the capacity to act as a carbon sink on decadal timescales.

Black, T.A., Harden, J.W. (1995). Effects of timber harvest on soil carbon storage at Blodgett Experimental Forest, California. Canandian Journal of Forest Research 25 (8): 1385-1396

ABSTRACT: Four plots from a mixed conifer forest were similarly cleared, burned, and replanted at various times over 17 years; a plot logged 79 years before sampling was used as a control. The plots had similar slope (2 to 15%, midslope position), aspect (south to southeast), and soil type (Holland series: mesic Haploxeralf; a Gray Brown Luvisol in the Canadian classification system). Twenty sites at each plot were sampled volumetrically by horizon to 20 cm below the organic–mineral soil boundary. Samples were analyzed for bulk density, organic C, and total N. There was an initial loss (15%) of organic C from the soil within 1 to 7 years, likely the result of oxidation (burning and decomposition) and erosion. For 17 years of forest regrowth, the soil continued to lose C (another 15%), probably owing to decomposition of slash material and possibly erosion, despite the slight accumulation of new litter and roots. After 80 years of regrowth, rates of carbon accumulation exceeded rates of loss, but carbon storage had declined and was not likely to recover to preharvest levels. Timber harvest and site preparation dramatically altered soil C and N distribution, in which C/N ratios after site preparation were initially high throughout the upper 20 cm. Subsequently, C/N ratios became lower with depth and with recovery age. Although stocks of C and N varied considerably among the plots and did not change consistently as a function of recovery age, the C/N ratios did vary systematically with recovery age. We hypothesize that the amount of C ultimately stored in the soil at steady state depends largely on N reserves and potentials, which appear to vary with erosion, intensity of burning, and site treatment.

Blanchart, E., Bernoux, M., Sarda, Xavier, Neto, M. S., Cerri, C. C., Piccolo, M. De C., Douzet, J.-M., Scopel, E., Feller, C. (2007). Effect of direct seeding mulch-based systems on soil carbon storage and macrofauna in central Brazil. Agriculturae Conspectus Scientificus 72 (1): 81-87

ABSTRACT: Soils represent a large carbon pool, approximately 1500 Gt, equivalent to almost three times the quantity stored in terrestrial biomass and twice the amount stored in the atmosphere. Any modification of land-use or land management can induce variations in soil carbon stocks, even in agricultural systems that are perceived to be in a steady state. These modifications also alter soil macrofauna that is known to affect soil carbon dynamics. Direct seeding Mulch-based Cropping (DMC) systems with two crops per year without soil tillage have widely been adopted over the last 10 to 15 years in the Cerrado (central region) of Brazil. They are replacing the traditional soybean monocropping with fallow under conventional tillage (CT). The objective of this study was to examine how DMC practices affect soil organic carbon (SOC) dynamics and macrofauna (Rio Verde, Goias State). The approach was to determine soil C stocks and macrofauna in five fields under DMC aged 1, 5, 7, 11 and 13 years. In order to compare DMC systems with the native system of the region and previous land-use, a situation under native Cerrado (tree-savanna like vegetation) and a field conducted traditionally (CT) were also studied. Soil C stocks were calculated for the 0-10 and 0-40 cm soil depth and also for the first 400 kg m-2 of soil to compare the same amount of soil and to suppress the potential artefact of soil compaction when sample is based on fix layer depth. Soil macrofauna was hand-sorted from soil monoliths (30 cm depth, TSBF method). In our study, the annual rate of carbon storage was equal to ca. 1.6 Mg C ha-1 , which is in the range of values measured for DMC in different areas of Brazil, i.e., 0.4 to 1.7 Mg C ha-1 with the highest rates obtained in the Cerrado region. Compared to natural vegetation, soil macrofauna in cropped systems was strongly modified. In CT, biomass and density were very low and much lower than in DMC systems. With increasing age of DMC, total macrofauna density increased and then decreased while total macrofauna biomass continuously increased due to a strong increase in Coleoptera larvae biomass. These modifications in macrofauna density and biomass are discussed with regard to soil SOC dynamics (decomposition, mineralization and physical protection).

Blanco-Canqui, H., Lal, R. (2007). Soil and crop response to harvesting corn residues for biofuel production. Geoderma 141 (3-4): 355-362

ABSTRACT: Corn (Zea mays L.) stover is considered one of the prime lignocellulosic feedstocks for biofuel production. While producing renewable energy from biomass is necessary, impacts of harvesting corn stover on soil organic carbon (SOC) sequestration, agricultural productivity, and environmental quality must be also carefully and objectively assessed. We conducted a 2 1/2 year study of stover management in long-term (> 8 yr) no-tillage (NT) continuous corn systems under three contrasting soils in Ohio to determine changes in SOC sequestration, CO2 emissions, soil physical properties, and agronomic productivity. These measurements were made on a Rayne silt loam (RSL) (fine-loamy, mixed, active, mesic Typic Hapludult) with 6% slope, Celina silt loam (CSL) (fine, mixed, active, mesic Aquic Hapludalfs) with 2% slope, and Hoytville clay loam (HCL) (fine, illitic, mesic Mollic Epiaqualfs) with < 1% slope. Stover treatments consisted of removing 0, 25, 50, 75, and 100% of corn stover following each harvest. At the start of the experiment in May 2004, these percentages of removal corresponded to 5, 3.75, 2.5, 1.25, and 0 Mg ha−1 yr−1 of stover left on the soil surface, respectively. Annual stover removal rate of > 25% reduced SOC and soil productivity, but the magnitude of impacts depended on soil type and topographic conditions. Stover removal rate of 50% reduced grain yield by about 1.94 Mg ha−1 , stover yield by 0.97 Mg ha−1 , and SOC by 1.63 Mg ha−1 in an unglaciated, sloping, and erosion-prone soil (P < 0.05). The initial water infiltration rates were significantly reduced by > 25% of stover removal on a RSL and CSL. Plant available water reserves and earthworm population were significantly reduced by 50% of stover removal at all soils. Increases in soil compaction due to stover removal were moderate. Stover removal impacts on SOC, crop yield, and water infiltration for HCL were not significant. Results from this study following 2 1/2 yr of stover management suggest that only a small fraction (≤ 25%) of the total corn stover produced can be removed for biofuel feedstocks from sloping and erosion-prone soils.

Boerner, R EJ, Waldrop, T A, Shelburne, V B (2006). Wildfire mitigation strategies affect soil enzyme activity and soil organic carbon in loblolly pine (Pinus taeda ) forests. Canadian Journal of Forest Research 36 (12): 3148-3154

ABSTRACT: We quantified the effects of three wildfire hazard reduction treatments (prescribed fire, thinning from below, and the combination of fire and thinning), and passive management (control) on mineral soil organic C, and enzyme activity in loblolly pine (Pinus taeda L.) forests on the Piedmont of South Carolina. Soil organic C was reduced by thinning, either alone or with prescribed fire, and this effect persisted through the fourth post-treatment year. Fire also resulted in reduced soil organic C, but not until several years after treatment. Soil C/N ratio initially increased after fire, either alone or with thinning, but this difference did not persist. The activities of three soil enzymes (acid phosphatase, chitinase, and phenol oxidase) in the upper mineral soil were quantified as measures of microbial activity. During the fourth post-treatment year we observed significant stimulation of all three enzyme systems as a result of thinning or thinning and burning. Although the patterns of variation in acid phosphatase and chitinase activity among treatments were similar during the first and fourth post-treatment years, the first-year treatment effects were not statistically significant. Given the management objective of utilizing these stands for timber production, the increased potential for rapid nutrient turnover offered by thinning gives this approach advantages over prescribed fire; however, management for maximum long-term storage of soil C may be better facilitated by prescribed fire.

Bohn, H. L. (1982). Estimate of organic carbon in world soils: II. Soil Science Society Of America JournalSoil Sci So 46: 1118-1119

ABSTRACT: Soil organic carbon (C) is the largest carbon reservoir at the earth's surface, but its mass is the least certain. The completed FAO Soil Map of the World yielded these estimates: 22 x 1014 kg organic C in global soils, made up of 18 x 1014 kg C in mineral soils and 4 x 1014 kg C in the surface meter of peatlands.

M.A. Bolin, Andrén, O., Kätterer, T., de Jong, R., VandenBygaart, A.J., Angers, D.A., Parent, L.-E., Gregorich, E.G. (2007). Soil carbon dynamics in Canadian Agricultural Ecoregions: Quantifying climatic influence on soil biological activity. Agriculture, Ecosystems & Environment 122 (4): 461-470

ABSTRACT: Climate is the major determinant of soil biological activity, including the decomposition rates of soil organic carbon (SOC) components. The objective of this study was to estimate the influence of climate on SOC dynamics, using long-term standard weather station data for Canadian Agricultural Ecoregions and the Introductory Carbon Balance Model for regional applications (ICBMregion). Mean daily temperature, total precipitation and potential evapotranspiration data were used in pedotransfer, soil water balance and biological activity functions to calculate a climate factor, re_clim , used to describe both inter- (1970–1999 data, daily re_clim ) and intra-annual variation (1903–2000 data, mean annual re_clim ) of the effects of climate on SOC decomposition rates across the country. When re_clim = 1.0 the SOC decomposition rate is equal to that at a reference site in Central Sweden: when it is <1.0 the relative SOC decomposition rate is lower, and when it is >1.0 it is higher. The results show that the cool and humid eastern Canadian Agricultural Ecoregions are characterized by higher SOC decomposition rates (average re_clim = 1.20), compared to the semi-arid regions of western Canada (average re_clim = 0.95). In other words, more C input is needed to maintain a certain SOC level in eastern than in western Canada. Inter-annual variation (the difference between minimum and maximum values) in re_clim for a given agricultural ecoregion was ≈±20%. The patterns of variation in intra-annual re_clim values differed between Agricultural Ecoregions: for eastern Canada re_clim reached a peak of ≈3.5 in mid-summer, and that of western Canada peaked at ≈2.5. This concept of re_clim was tested and used in other climate zones such as northern Europe and Sub-Saharan Africa, and enabled us to integrate large data sets concerning climate and SOC dynamics and to characterize Canadian Agricultural Ecoregions by one variable, re_clim .

Bolinder, M. A., Vandenbygaart, A. J., Gregorich, E. G., Angers, D. A., Janzen, H. H. (2006). Modelling soil organic carbon stock change for estimating whole-farm greenhouse gas emissions. Canadian Journal of Soil Science 86 (3): 419-429

ABSTRACT: Modelling soil organic carbon (SOC) stock changes in agroecosystems can be performed with different approaches depending on objectives and available data. Our objective in this paper is to describe a scheme for developing a dynamic SOC algorithm for calculating net greenhouse gas emissions from Canadian farms as a function of management and local conditions. Our approach is flexible and emphasizes ease of use and the integration of available knowledge. Using this approach, we assessed the performance of several SOC models having two or more compartments for some common agroecosystems in Canada. Analysis of long-term data for conventional management practices at different sites (n = 36) in Canada, including recent model applications in the literature on some of those data, indicated that the results obtained with two-compartment models, such as the Introductory Carbon Balance Model (ICBM) and Modified Woodruff Model (MWM), yielded results comparable to those of a multi-compartment model (CENTURY). The analysis also showed that a model such as ICBM need stuning to be applied to management and conditions across Canada. Two-compartment models programmable in a simple spreadsheet format, though they may not supplant more complex models in allapplications, offer advantages of simplicity and transparency in whole-farm analyses of greenhouse gas emissions.

Bolstad, P. V., Vose, J. M. (2005). Forest and pasture carbon pools and soil respiration in the sourthern Appalachian Mountains. Forest Science 51 (4): 372-383

ABSTRACT: Our ability to estimate the changes in carbon (C) pools and fluxes due to forest conversion is hampered by a lack of comparative studies. We measured above- and belowground C pools and soil respiration flux at four forested and four pasture sites in the southern Appalachian Mountains. Above- and belowground C pools were significantly larger (P < 0.01, t-test) at forested sites relative to pasture sites. The largest differences were in aboveground live biomass, which averaged 152 Mg ha-1 C at the forested sites and 1.9 Mg ha-1 at the pasture sites. Coarse root and stump C and surface detritus were also substantially different, averaging 41.3 and 32.6 Mg ha-1 C, respectively, at the forested sites, and less than 1 Mg ha-1 at the pasture sites. Fine root C was higher and mineral soil C lower at pasture sites relative to comparable forested sites, but neither difference was statistically significant. Soil respiration at a given temperature was generally lower at pasture sites relative to forest sites. However, soil temperatures at pastures were consistently higher than at forest sites. Estimated annual soil respiration flux averaged 10.9 Mg C ha-1 at the pasture sites and 12.6 Mg C ha-1 at the forested sites.

Bonino, E. E. (2006). Changes in carbon pools associated with a land-use gradient in the Dry Chaco, Argentina. Forest Ecology and Management 223 (1-3): 183-189

ABSTRACT: Little is known about the contribution of arid and semiarid regions to the carbon balance at a global scale. The lack of information is especially noticeable for the Gran Chaco, which covers an area of about 1,200,000 km2 in South America. This study quantified carbon pools and their changes along a land-use gradient in the Dry Chaco, the driest portion of the Gran Chaco, measured in the aboveground biomass and in soils (20 cm depth). The work was conducted in the Chancani reserve, where the best preserved forests of the region are found, and in surrounding areas, including a primary forest, a secondary forest and shrubby grasslands. Previous works indicate that the entire area was originally covered by forests similar to those found at the Chancani reserve, and that the land-use changes occurred at least 30 years prior to this study. Total aboveground carbon stock, which comprises the total amount of living organic matter in trees and shrubs, was 30.31 Mg C ha-1 in the primary forest, which was reduced to 8.38 Mg C ha-1 in the secondary forest and to 1.37 Mg C ha-1 in shrubby grasslands. Carbon stock in the tree component decreased drastically between the primary and the secondary forests from 25.40 to 5.11 Mg C ha-1 . The component described as saplings of trees and shrubs also decreased significantly among the three communities from 4.91 Mg C ha-1 in the primary forest to 3.27 Mg C ha-1 in the secondary forest and to only 1.37 Mg C ha-1 in the shrubby grasslands. No significant differences were detected in the carbon content per unit area of soil, although it decreased from 34.59 Mg C ha-1 in the primary forest to 28.04 Mg C ha-1 in the secondary forest and to 22.93 Mg C ha-1 in the shrubby grassland, with a significant increase in soil bulk density in the disturbed communities. Therefore, differences in carbon stocks between communities were primarily the result of differences in vegetation biomass, whereas changes in the land-use gradient analyzed had a lower impact on soils. Nevertheless, soil constitutes the largest pool, and more severe ecological disturbances could lead to important changes in net carbon storage.

Borken, W., Xu, Y. J., Brumme, R. (1999). A climate change scenario for carbon dioxide and dissolved organic carbon fluxes from a temperate forest soil: drought and rewetting effects. Soil Science Society Of America JournalSoil Sci So 63 (6): 1848-55

ABSTRACT: Our objective was to assess the effect of changes in rainfall amount and distribution on CO2 emissions and dissolved organic C (DOC) leaching. We manipulated soil moisture, using a roof constructed below the canopy of a 65-yr-old Norway spruce plantation [Picea abies (L.) Karst.] at Solling, Germany. We simulated two scenarios: a prolonged summer drought of 172 d followed by a rewetting period of 19 d and a shorter summer drought of 108 d followed by a rewetting period of 33 d. Soil CO2 emission, DOC, soil matric potential, and soil temperature were monitored in situ for 2 yr. On an annual basis no significant influence of the droughts on DOC leaching rates below the rhizosphere was observed. Although not significantly, the droughts tended to reduce soil respiration. Rewetting increased CO2 emissions in the first 30 d by 48% (P < 0.08) in 1993 and 144% (P < 0.01) in 1994. The CO2 flush during rewetting was highest at high soil temperatures and strongly affected the annual soil respiration rate. The annual emission rate from the drought plot was not affected by the drought and rewetting treatments in 1993 (2981 kg C ha-1 yr-1 ), but increased by 51% (P < 0.05) to 4813 kg C ha-1 yr-1 in 1994. Our results suggest that reduction of rainfall or changes in rainfall distribution due to climate change will affect soil CO2 emissions and possibly C storage in temperate forest ecosystems.

Burke, R., Zepp, R.G. (1997). Effect of fire on soil-atmosphere exchange of methane and carbon dioxide in Canadian boreal forest sites. Journal of Geophysical Research 102: 29,289- 29,300

ABSTRACT: During the spring and summer of 1994 we monitored soil-atmosphere exchanges of methane and carbon dioxide at upland sites in the Canadian boreal forest near the northern study area (NSA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). The effects of fire on methane and carbon dioxide exchange in black spruce stands developed on clay soils were evaluated by measuring fluxes with dark chambers in unburned stands and stands burned in 1994, 1992, and 1987. Similar measurements were made in jack pine stands developed on sandy soils, one unburned and the other burned in 1989. All of the sites were net sinks of atmospheric methane with median fluxes ranging from −0.3 to −1.4 mg CH4 -C m−2 d−1 . Median fluxes of carbon dioxide from the forest floor to the atmosphere ranged between 1 and 2 g C m−2 d−1 . Both ecosystem characteristics (e.g., soil and vegetation type) and burning history (time since burn and fire intensity) appear to have some effect on atmospheric methane consumption and carbon dioxide emission by these forest soils. In general, the jack pine sites were stronger methane sinks and had lower carbon dioxide emissions than the black spruce sites. After a few years of recovery, the burned sites tended to be slightly stronger methane sinks than unburned controls. Our results suggest that soil CO2 effluxes from upland black spruce stands may not be immediately impacted by fire, possibly maintained at preburn levels by microbial decomposition of labile compounds released as a result of the fire. By 2 years postfire there appears to be a significant reduction in soil CO2 flux, due to the loss of tree root and moss respiration and possibly to the depletion of fire-related labile compounds. The observed recovery of soil respiration rates to preburn levels by 7 years postburn is probably due to the respiration of regrowing vegetation and the combined effects of elevated soil temperatures (about 4° to 5°C warmer than unburned sites) and improved litter quality on soil microbial activities. We estimate that soil CO2 emissions from recently burned boreal forest soils in the northern hemisphere could be of the order of 0.35 Pg C yr−1 , which is in good agreement with a previous estimate that was derived in a different manner.

Caldwell, T.G., Johnson, D.W., Miller, W.W., Qualls, R.G. (2002). Forest floor carbon and nitrogen losses due to prescription fire. Soil Science Society Of America JournalSoil Sci So 66: 262-267

ABSTRACT: Fire is the dominant factor affecting C and N losses from the semiarid forests of the eastern Sierra Nevada. As prescription fire becomes a best management practice, it is critical to develop an estimate of these fluxes. The objectives of this study were (i) to test and refine methods to estimate the volatilized C and N losses from the forest floor following fire, (ii) to investigate the interactions between O-horizon temperature and nutrient loss, and (iii) to assess measured N losses in the context of atmospheric N deposition, leaching, and N fixation. The quantities of C and N volatilized from the forest floor by prescription fire in the Sierra Nevada were measured using two different field-based methods: weight loss estimation and Ca/element ratio determination. Three sites were included in the study: Marlene, Sawtooth and Spooner. The weight method indicated C losses of 6.12, 7.39, and 17.8 Mg C ha-1 at the Sawtooth, Marlene, and Spooner sites, respectively. The ratio method indicated comparable C losses from the Sawtooth (6 Mg C ha-1 ) site, but greater losses at Marlene (16 Mg C ha-1 ) and Spooner (24 Mg C ha-1 ) sites. The weight method indicated N losses of 56.2, 60.8, and 362 kg N ha-1 , at the Sawtooth, Marlene, and Spooner sites, respectively. The ratio method indicated comparable N losses of 59.9 kg N ha-1 at the Sawtooth site, but considerably greater losses at Marlene (243 kg N ha-1 ), and Spooner (524 kg N ha-1 ) sites. The Ca-element method was preferred because of minimal needs for preburn sampling. Regardless of method, the estimated losses were significant, particularly for N, compared with deposition and leaching rates. Volatilization will represent the major mechanism for N loss from forest ecosystems of this region subjected to prescribed fire.

Callaghan, T. V., Bjorn, L. O., Chernov, Y., Chapin, T., Christensen, T. R., Huntley, B., Ims, R. A., Johansson, M., Jolly, D., Jonasson, S., Matveyeva, N., Panikov, N., Oechel, W., Shaver, G. R. (2004). Effects on the function of Arctic ecosystems in the short- and long-term perspectives. Ambio 33 (7): 448-458

ABSTRACT: Historically, the function of Arctic ecosystems in terms of cycles of nutrients and carbon has led to low levels of primary production and exchanges of energy, water and greenhouse gases have led to low local and regional cooling. Sequestration of carbon from atmospheric CO2 , in extensive, cold organic soils and the high albedo from low, snow-covered vegetation have had impacts on regional climate. However, many aspects of the functioning of Arctic ecosystems are sensitive to changes in climate and its impacts on biodiversity. The current Arctic climate results in slow rates of organic matter decomposition. Arctic ecosystems therefore tend to accumulate organic matter and elements despite low inputs. As a result, soil-available elements like nitrogen and phosphorus are key limitations to increases in carbon fixation and further biomass and organic matter accumulation. Climate warming is expected to increase carbon and element turnover, particularly in soils, which may lead to initial losses of elements but eventual, slow recovery. Individual species and species diversity have clear impacts on element inputs and retention in Arctic ecosystems. Effects of increased CO2 and UV-B on whole ecosystems, on the other hand, are likely to be small although effects on plant tissue chemisty, decomposition and nitrogen fixation may become important in the long-term. Cycling of carbon in trace gas form is mainly as CO2 and CH4 . Most carbon loss is in the form of CO2 , produced by both plants and soil biota. Carbon emissions as methane from wet and moist tundra ecosystems are about 5% of emissions as CO2 and are responsive to warming in the absence of any other changes. Winter processes and vegetation type also affect CH4 emissions as well as exchanges of energy between biosphere and atmosphere. Arctic ecosystems exhibit the largest seasonal changes in energy exchange of any terrestrial ecosystem because of the large changes in albedo from late winter, when snow reflects most incoming radiation, to summer when the ecosystem absorbs most incoming radiation. Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. Albedo during the period of snow cover declines from tundra to forest tundra to deciduous forest to evergreen forest. Shrubs and trees increase snow depth which in turn increases winter soil temperatures. Future changes in vegetation driven by climate change are therefore, very likely to profoundly alter regional climate.

Campbell,C. A. (2005). Carbon storage in soils of the North American Great Plains: effect of cropping frequency. Agronomy Journal 97 (2): 349-363

ABSTRACT: Summer fallow (fallow) is still widely used on the North American Great Plains to replenish soil moisture between crops. Our objective was to examine how fallowing affects soil organic carbon (SOC) in various agronomic and climate settings by reviewing long-term studies in the midwestern USA (five sites) and the Canadian prairies (17 sites). In most soils, SOC increased with cropping frequency though not usually in a linear fashion. In the Canadian studies, SOC response to tillage and cropping frequency varied with climate—in semiarid conditions, SOC gains under no-till were about 250 kg ha–1 yr–1 greater than for tilled systems regardless of cropping frequency; in subhumid environments, the advantage was about 50 kg ha–1 yr–1 for rotations with fallow but 250 kg ha–1 yr–1 with continuous cropping. Specific crops also influenced SOC: Replacing wheat (Triticum aestivum L.) with lentil (Lens culinaris Medikus) had little effect; replacing wheat with lower-yielding flax (Linum usitatismum L.) reduced SOC gains; and replacing wheat with erosion-preventing fall rye (Secale cereale L.) increased SOC gains. In unfertilized systems, cropping frequency did not affect SOC gains, but in fertilized systems, SOC gains often increased with cropping frequency. In a Colorado study (three sites each with three slope positions), SOC gains increased with cropping frequency, but the response tended to be highest at the lowest potential evaporation site (where residue C inputs were greatest) and least in the toeslope positions (despite their high residue C inputs). The Century and the Campbell et al. SOC models satisfactorily simulated the relative responses of SOC although they underestimated gains by about one-third.

Campbell, J. L., Law, B. E. (2005). Forest soil respiration across three climatically distinct chronosequences in Oregon. Biogeochemistry 73 (1): 109-125

ABSTRACT: To assess the relative influence of edaphoclimatic gradients and stand replacing disturbance on the soil respiration of Oregon forests, we measured annual soil respiration at 36 independent forest plots arranged as three replicates of four age classes in each of three climatically distinct forest types. Annual soil respiration for the year 2001 was computed by combining periodic chamber measurements with continuous soil temperature measurements, which were used along with site-specific temperature response curves to interpolate daily soil respiration between dates of direct measurement. Results indicate significant forest type, age, and type × age interaction effects on annual soil respiration. Average annual soil respiration was 1100–1600, 1500–2100, and 500–900 g C m−2 yr−1 for mesic spruce, montane Douglas-fir, and semi-arid pine forests respectively. Age related trends in annual soil respiration varied between forest types. The variation in annual soil respiration attributable to the climatic differences between forest types was 48%(CV). Once weighted by the age class distribution for each forest type, the variation in annual soil respiration attributable to stand replacing disturbance was 15% (CV). Sensitivity analysis suggests that the regional variation in annual soil respiration is most dependent on summer base rates (i.e. soil respiration normalized to a common temperature) and much less dependent on the site-specific temperature response curves (to which annual rates are relatively insensitive) and soil degree-days (which vary only 10% among plots).

Cardon, Z. G., Hungate, B. A., Cambardella, C. A., Chapin, F. S., Field, C. B., Holland, E. A., Mooney, H. A. (2001). Contrasting effects of elevated CO2 on old and new soil carbon pools. Soil Biology and Biochemistry 33 (3): 365-373

ABSTRACT: Soil organic carbon (SOC) is the largest reservoir of organic carbon in the terrestrial biosphere. Though the influence of increasing atmospheric CO2 on net primary productivity, on the flow of newly fixed carbon belowground, and on the quality of new plant litter in ecosystems has been examined, indirect effects of increased CO2 on breakdown of large SOC pools already in ecosystems are not well understood. We found that exposure of California grassland communities to elevated CO2 retarded decomposition of older SOC when mineral nutrients were abundant, thus increasing the turnover time of SOC already in the system. Under elevated CO2 , soil microorganisms appeared to shift from consuming older SOC to utilizing easily degraded rhizodeposits derived from increased root biomass. In contrast to this increased retention of stabilized older SOC under elevated CO2 , movement of newly fixed carbon from roots to stabilized SOC pools was retarded; though root biomass increased under elevated CO2 , new carbon in mineral-bound pools decreased. These contrasting effects of elevated CO2 on dynamics of old and new soil carbon pools contribute to a new soil carbon equilibrium that could profoundly affect long-term net carbon movement between terrestrial ecosystems and the atmosphere.

Carrasco, J., Neff, J.C., Harden, J.W. (2006). Modeling the long-term accumulation of carbon in boreal forest soils: influence of physical and chemical factors. Journal of Geophysical Research - Biogeosciences

ABSTRACT: Boreal soils are important to the global C cycle owing to large C stocks, repeated disturbance from fire, and the potential for permafrost thaw to expose previously stable, buried C. To evaluate the primary mechanisms responsible for both short- and long-term C accumulation in boreal soils, we developed a multi-isotope (12, 14 C) soil C model with dynamic soil layers that develop through time as soil organic matter burns and reaccumulates. We then evaluated the mechanisms that control organic matter turnover in boreal regions including carbon input rates, substrate recalcitrance, soil moisture and temperature, and the presence of historical permafrost to assess the importance of these factors in boreal C accumulation. Results indicate that total C accumulation is controlled by the rate of carbon input, decomposition rates, and the presence of historical permafrost. However, unlike more temperate ecosystems, one of the key mechanisms involved in C preservation in boreal soils examined here is the cooling of subsurface soil layers as soil depth increases rather than increasing recalcitrance in subsurface soils. The propagation of the14 C bomb spike into soils also illustrates the importance of historical permafrost and twentieth century warming in contemporary boreal soil respiration fluxes. Both14 C and total C simulation data also strongly suggest that boreal SOM need not be recalcitrant to accumulate; the strong role of soil temperature controls on boreal C accumulation at our modeling test site in Manitoba, Canada, indicates that carbon in the deep organic soil horizons is probably relatively labile and thus subject to perturbations that result from changing climatic conditions in the future.

Cerri, C.E.P., Easter, M., Paustian, K., Killian, K., Coleman, K., Bernoux, M., Falloon, P., Powlson, D.S., Batjes, N.H., Milne, E., Cerri, C.C. (2007). Predicted soil organic carbon stocks and changes in the Brazilian Amazon between 2000 and 2030. Agriculture, Ecosystems & Environment 122 (1): 58-72

ABSTRACT: Currently we have little understanding of the impacts of land use change on soil C stocks in the Brazilian Amazon. Such information is needed to determine impacts on the global C cycle and the sustainability of agricultural systems that are replacing native forest. The aim of this study was to predict soil carbon stocks and changes in the Brazilian Amazon during the period between 2000 and 2030, using the GEFSOC soil carbon (C) modelling system. In order to do so, we devised current and future land use scenarios for the Brazilian Amazon, taking into account: (i) deforestation rates from the past three decades, (ii) census data on land use from 1940 to 2000, including the expansion and intensification of agriculture in the region, (iii) available information on management practices, primarily related to well managed pasture versus degraded pasture and conventional systems versus no-tillage systems for soybean (Glycine max) and (iv) FAO predictions on agricultural land use and land use changes for the years 2015 and 2030. The land use scenarios were integrated with spatially explicit soils data (SOTER database), climate, potential natural vegetation and land management units using the recently developed GEFSOC soil C modelling system. Results are presented in map, table and graph form for the entire Brazilian Amazon for the current situation (1990 and 2000) and the future (2015 and 2030). Results include soil organic C (SOC) stocks and SOC stock change rates estimated by three methods: (i) the Century ecosystem model, (ii) the Rothamsted C model and (iii) the intergovernmental panel on climate change (IPCC) method for assessing soil C at regional scale. In addition, we show estimated values of above and belowground biomass for native vegetation, pasture and soybean. The results on regional SOC stocks compare reasonably well with those based on mapping approaches. The GEFSOC system provided a means of efficiently handling complex interactions among biotic-edapho-climatic conditions (>363,000 combinations) in a very large area (500 Mha) such as the Brazilian Amazon. All of the methods used showed a decline in SOC stock for the period studied; Century and RothC simulated values for 2030 being about 7% lower than those in 1990. Values from Century and RothC (30,430 and 25,000 Tg for the 0–20 cm layer for the Brazilian Amazon region were higher than those obtained from the IPCC system (23,400 Tg in the 0–30 cm layer). Finally, our results can help understand the major biogeochemical cycles that influence soil fertility and help devise management strategies that enhance the sustainability of these areas and thus slow further deforestation.

Cerri, C. E. P., Sparovek, G., Bernoux, M., Easterling, W. E., Melillo, J. M., Cerri, C. C. (2007). Tropical agriculture and global warming: impacts and mitigation options. Scientia Agricola 64 (1): 83-99

ABSTRACT: The intensive land use invariably has several negative effects on the environment and crop production if conservative practices are not adopted. Reduction in soil organic matter (SOM) quantity means gas emission (mainly CO2 , CH4 , N2 O) to the atmosphere and increased global warming. Soil sustainability is also affected, since remaining SOM quality changes. Alterations can be verified, for example, by soil desegregation and changes in structure. The consequences are erosion, reduction in nutrient availability for the plants and lower water retention capacity. These and other factors reflect negatively on crop productivity and sustainability of the soil -plant-atmosphere system. Conversely, adoption of "best management practices", such as conservation tillage, can partly reverse the process - they are aimed at increasing the input of organic matter to the soil and/or decreasing the rates at which soil organic matter decomposes.

Clifton-Brown, J.C., Breuer, J., Jones, M.B. (2007). Carbon mitigation by the energy crop, Miscanthus. Global Change Biology 13 (11): 2296-2307

ABSTRACT: Biomass crops mitigate carbon emissions by both fossil fuel substitution and sequestration of carbon in the soil. We grewMiscanthus x giganteus for 16 years at a site in southern Ireland to (i) compare methods of propagation, (ii) compare response to fertilizer application and quantify nutrient offtakes, (iii) measure long-term annual biomass yields, (iv) estimate carbon sequestration to the soil and (v) quantify the carbon mitigation by the crop. There was no significant difference in the yield between plants established from rhizome cuttings or by micro-propagation. Annual off-takes of N and P were easily met by soil reserves, but soil K reserves were low in unfertilized plots. Potassium deficiency was associated with lower harvestable yield. Yields increased for 5 years following establishment but after 10 years showed some decline which could not be accounted for by the climate driven growth model MISCANMOD. Measured yields were normalized to estimate both autumn (at first frost) and spring harvests (15 March of the subsequent year). Average autumn and spring yields over the 15 harvest years were 13.4±1.1 and 9.0±0.7 t DW ha−1 yr−1 respectively. Below ground biomass in February 2002 was 20.6±4.6 t DW ha−1 .Miscanthus derived soil organic carbon sequestration detected by a change in13 C signal was 8.9±2.4 t C ha−1 over 15 years. We estimate total carbon mitigation by this crop over 15 years ranged from 5.2 to 7.2 t C ha−1 yr−1 depending on the harvest time.

Conen, F., Leifeld, J., Seth, B., Alewell, C. (2006). Warming mineralises young and old soil carbon equally. Biogeosciences 3 (4): 515-519

ABSTRACT: The temperature sensitivity of soil organic carbon decomposition is critical for predicting future climate change because soils store 2-3 times the amount of atmospheric carbon. Of particular controversy is the question, whether temperature sensitivity differs between young or labile and old or more stable carbon pools. Ambiguities in experimental methodology have so far limited corroboration of any particular hypothesis. Here, we show in a clear-cut approach that differences in temperature sensitivity between young and old carbon are negligible. Using the change in stable isotope composition in transitional systems from C3 to C4 vegetation, we were able to directly distinguish the temperature sensitivity of carbon differing several decades in age. This method had several advantages over previously followed approaches. It allowed to identify release of much older carbon, avoided un-natural conditions of long-term incubations and did not require arguable curve-fitting. Our results demonstrate that feedbacks of the carbon cycle on climate change are driven equally by young and old soil organic carbon.

Czimczik, C. I., C. A. Masiello (2007). Controls on black carbon storage in soils. Global Biogeochemical Cycles 21 (GB3005): doi:10.1029/2006GB002798

ABSTRACT: Fire-derived black carbon (BC: charcoal and soot) has been thought to be a passive player in soils, contributing to the refractory soil organic carbon (SOC) pool, but playing no role in pedogenesis and regional short-term carbon cycling. This model, however, is at odds with recent results on the role of charcoal in soil fertility and its detection in the dissolved organic carbon (DOC) pool. For example, if BC simply accumulated passively in soils, its pattern of accumulation should match a simple model correlating fire frequency to BC storage. Instead, soil type, climate, biota, and land use practices all appear to play roles in controlling whether BC accumulates or is lost from soils. We summarize current knowledge of BC-soil interactions and construct a new paradigm describing the controls on BC storage in soils. We reconcile the refractory-labile BC paradox by proposing a model where BC storage is controlled by (1) fire frequency, (2) ecosystem presence or absence of aromatic precursor carbon and appropriate combustion conditions, (3) biological or physical mixing to remove BC from the soil surface, where it is vulnerable to combustion in future fires, (4) the presence or absence of soil mineral fractions able to sorb BC into the long-term stable carbon pool, and (5) the presence of microbial communities capable of degrading aromatic carbon. We also recognize that soil BC/SOC ratios are strongly influenced by land-use practices and add (6) human activities as a final control.

Czimczik, C.I., Preston, C.M., Schmidt, M.W.I., Schulze, E. (2003). How surface fire in Siberian Scots pine forests affects soil organic carbon in the forest floor: Stocks, molecular structure, and conversion to black carbon (charcoal). Global Biogeochemical Cycles 17 (1): doi:10.1029/2002GB001956

ABSTRACT: In boreal forests, fire is a frequent disturbance and converts soil organic carbon (OC) to more degradation-resistant aromatic carbon, i.e., black carbon (BC) which might act as a long-term atmospheric-carbon sink. Little is known on the effects of fires on boreal soil OC stocks and molecular composition. We studied how a surface fire affected the composition of the forest floor of Siberian Scots pine forests by comparing the bulk elemental composition, molecular structure (13 C-MAS NMR), and the aromatic carbon fraction (BC and potentially interfering constituents like tannins) of unburned and burned forest floor. Fire reduced the mass of the forest floor by 60%, stocks of inorganic elements (Si, Al, Fe, K, Ca, Na, Mg, Mn) by 30–50%, and of OC, nitrogen, and sulfur by 40–50%. In contrast to typical findings from temperate forests, unburned OC consisted mainly of (di-)O-alkyl (polysaccharides) and few aromatic structures, probably due to dominant input of lichen biomass. Fire converted OC into alkyl and aromatic structures, the latter consisting of heterocyclic macromolecules and small clusters of condensed carbon. The small cluster size explained the small BC concentrations determined using a degradative molecular marker method. Fire increased BC stocks (16 g kg−1 OC) by 40% which translates into a net-conversion rate of 0.7% (0.35% of net primary production) unburned OC to BC. Here, however, BC was not a major fraction of soil OC pool in unburned or burned forest floor, either due to rapid in situ degradation or relocation.

Davidson, E. A., Janssens, I. A. (2006). Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature 440 (7081): 165-173

ABSTRACT: Significantly more carbon is stored in the world's soils—including peatlands, wetlands and permafrost—than is present in the atmosphere. Disagreement exists, however, regarding the effects of climate change on global soil carbon stocks. If carbon stored belowground is transferred to the atmosphere by a warming-induced acceleration of its decomposition, a positive feedback to climate change would occur. Conversely, if increases of plant-derived carbon inputs to soils exceed increases in decomposition, the feedback would be negative. Despite much research, a consensus has not yet emerged on the temperature sensitivity of soil carbon decomposition. Unravelling the feedback effect is particularly difficult, because the diverse soil organic compounds exhibit a wide range of kinetic properties, which determine the intrinsic temperature sensitivity of their decomposition. Moreover, several environmental constraints obscure the intrinsic temperature sensitivity of substrate decomposition, causing lower observed 'apparent' temperature sensitivity, and these constraints may, themselves, be sensitive to climate.

Davidson, E.A., Lefebvre, P.A. (1993). Estimating regional carbon stocks and spatially covarying edaphic factors using soil maps at three scales. Biogeochemistry 22 (2): 107-131

ABSTRACT: Most estimates of regional and global soil carbon stocks are based on extrapolations of mean soil C contents for broad categories of soil or vegetation types. Uncertainties exist in both the estimates of mean soil C contents and the area over which each mean should be extrapolated. Geographic information systems now permit spatially referenced estimates of soil C at finer scales of resolution than were previously practical. We compared estimates of total soil C stocks of the state of Maine using three methods: (1) multiplying the area of the state by published means of soil C for temperate forests and for Spodosols; (2) calculating areas of inclusions of soil taxa in the 1:5,000,000 FAO/UNESCO Soils Map of the World and multiplying those areas by selected mean carbon contents; and (3) calculating soil C for each soil series and map unit in the 1:250,000 State Soil Geographic Data Base (STATSGO) and summing these estimates for the entire state. The STATSGO estimate of total soil C was between 23% and 49% higher than the common coarse scale extrapolations, primarily because STATSGO included data on Histosols, which cover less than 5% of the area of the state, but which constitute over one-third of the soil C. Spodosols cover about 65% of the state, but contribute less than 39% of the soil C. Estimates of total soil C in Maine based on the FAO map agreed within 8% of the STATSGO estimate for one possible matching of FAO soil taxa with data on soil C, but another plausible matching overestimated soil C stocks. We also compared estimates from the 1:250,000 STATSGO database and from the 1:20,000 Soil Survey Geographic Data Base (SSURGO) for a 7.5 minute quadrangle within the state. SSURGO indicated 13% less total soil C than did STATSGO, largely because the attribute data on depths of soil horizons in SSURGO are more specific for this locality. Despite localized differences, the STATSGO database offers promise of scaling up county soil survey data to regional scales because it includes attribute data and estimates of areal coverage of C-rich inclusions within map units. The spatially referenced data also permit examination of covariation of soil C stocks with soil properties thought to affect stabilization of soil C. Clay content was a poor predictor of soil C in Maine, but drainage class covaried significantly with soil C across the state.

De Deyn, G. B., Cornelissen, J. H. C., Bardgett, R. D. (2008). Plant functional traits and soil carbon sequestration in contrasting biomes. Ecology Letters 11 (5): 516-531

ABSTRACT: Plant functional traits control a variety of terrestrial ecosystem processes, including soil carbon storage which is a key component of the global carbon cycle. Plant traits regulate net soil carbon storage by controlling carbon assimilation, its transfer and storage in belowground biomass, and its release from soil through respiration, fire and leaching. However, our mechanistic understanding of these processes is incomplete. Here, we present a mechanistic framework, based on the plant traits that drive soil carbon inputs and outputs, for understanding how alteration of vegetation composition will affect soil carbon sequestration under global changes. First, we show direct and indirect plant trait effects on soil carbon input and output through autotrophs and heterotrophs, and through modification of abiotic conditions, which need to be considered to determine the local carbon sequestration potential. Second, we explore how the composition of key plant traits and soil biota related to carbon input, release and storage prevail in different biomes across the globe, and address the biome-specific mechanisms by which plant trait composition may impact on soil carbon sequestration. We propose that a trait-based approach will help to develop strategies to preserve and promote carbon sequestration.

de Wit, H. A., Palosuo, T., Hylen, G., Liski, J. (2006). A carbon budget of forest biomass and soils in southeast Norway calculated using a widely applicable method. Forest Ecology and Management 225 (1-3): 15-26

ABSTRACT: Growing stocks of trees in Europe have increased in a magnitude that is significant in terms of carbon (C) sink strength. Estimates of the soil C sink strength that this increased stock of trees may have induced on a regional scale are scarce, uncertain and difficult to compare. This illustrates the need for a widely applicable calculation method. Here, we calculate a C budget of productive forest in southeast Norway based on forest inventory information, biomass expansion factors (BEF), biomass turnover rates and the dynamic soil model Yasso. We estimate a 29% increase (112–145 Tg) of C in biomass between 1971 and 2000, and estimate the associated increase of C in soils (including dead wood) to be 4.5% (181–189 Tg). The C sink strengths in biomass and soils (including dead wood) in 1990 are 0.38 and 0.08 Mg ha−1 yr−1 , respectively. Estimated soil C density is 58 Mg C ha−1 or ca 40% of measured soil C density in Norwegian forest soils. A sensitivity analysis – using uncertainty estimates of model inputs and parameters based on empirical data – shows that the underestimation of the soil C stock can be due to overestimation of decomposition rates of recalcitrant organic matter in the soil model and to including only trees as a source of litter. However, uncertainty in these two factors is shown to have a minimal effect on soil sink estimates. The key uncertainty in the soil sink is the initial value of the soil C stock, i.e. the assumed steady state soil C stock at the start of the time series in 1970. However, this source of uncertainty is reduced in importance for when approaching the end of the data series. This indicates that a longer time series of forest inventory data will decrease the uncertainty in the soil sink estimate due to initialisation of the soil C stock. Other, less significant, sources of uncertainty in estimates of soil stock and sink are BEF for fine roots and turnover rates of fine roots and foliage. The used method for calculation of a forest C budget can be readily applied to other regions for which similar forest resource data are available.

Dell, C. J., Sharpley, A. N. (2006). Spatial variation of soil organic carbon in a northeastern U.S. watershed. Journal of Soil and Water Conservation 61 (3): 129-136

ABSTRACT: Increasing the accumulation of organic carbon (C) in agricultural soils provides one means to reduce atmospheric carbon dioxide (CO2 ) concentrations, but detection of the relatively small changes in soil organic C is complicated by spatial variability. Soil organic C variation was assessed at various scales within a small (40 ha; 98 ac), mixed-use watershed in central Pennsylvania to determine sampling requirement for possible C credit programs. Composite soil samples (0 to 5 cm; 0 to 2 in deep) were collected on 30-m (98-ft) grid intervals across the watershed and at 10- and 0.6-m (33- and 2-ft) intervals at selected locations, and descriptive- and geo-statistical analysis utilized. Concentrations of soil organic C in pasture and forest soils were approximately two times greater than cultivated fields, where means ranged from 15 to 24 g C kg−1 (1.5 to 2.4 percent) and coefficients of variation were typically 15 to 20 percent. Soil organic C was spatially dependent over a range of approximately 200 m (660 ft) when sampled at 30-m (98-ft) intervals, and high nugget variances indicated spatially-dependent variability over lag distances shorter than 30 m (98ft). However, sampling at 10-m (33 ft) intervals appeared to adequately describe variation. Estimates of sample size requirement showed that, with the observed coefficient variances for individual fields, two- to five-fold increases in sample numbers would be required to verify statistically significant soil organic C changes ≤ 10 percent. Given the large number of samples required to provide representative measurements and the concurrent cost for labor and analysis, participation by farmers in a C credit program could be low if measured verification of soil organic C increases are required. Basing payments on modeled, rather than measured C sequestration rates, should be considered.

Deluca, T. H., Aplet, G. H. (2008). Charcoal and carbon storage in forest soils of the Rocky Mountain West. Frontiers in Ecology and the Environment 6 (1): 18-24

ABSTRACT: Charcoal represents a super-passive form of carbon (C) that is generated during fire events and is one of the few legacies of fire recorded in the soil profile; however, the importance of this material as a form of C storage has received only limited scientific attention. Here, we review the formation of charcoal in temperate and boreal forest ecosystems, discuss some of its desirable properties, and estimate the potential contribution charcoal to long-term C sequestration in forest ecosystems. Charcoal deposition over the course of several millennia probably accounts for a substantial proportion of the total soil C pool in fire-maintained forest ecosystems. Forest management processes that interfere with natural fire processes eliminate the formation of this passive form of C. We recommend that charcoal be considered in C storage budgets and modeling of forest ecosystems, especially in light of climate change and increasing occurrence of wildfire.

Dunn, A. L., Barford, C. C., Wofsy, S. C., Goulden, M. L., Daube, B. C. (2007). A long-term record of carbon exchange in a boreal black spruce forest: means, responses to interannual variability, and decadal trends. Global Change Biology 13 (3): 577-590

ABSTRACT: We present a decadal (1994–2004) record of carbon dioxide flux in a 160-year-old black spruce forest/veneer bog complex in central Manitoba, Canada. The ecosystem shifted from a source (+41 g C m−2 , 1995) to a sink (−21 g C m−2 , 2004) of CO2 over the decade, with an average net carbon balance near zero. Annual mean temperatures increased 1–2° during the period, consistent with the decadal trend across the North American boreal biome. We found that ecosystem carbon exchange responded strongly to air temperature, moisture status, potential evapotranspiration, and summertime solar radiation. The seasonal cycle of ecosystem respiration significantly lagged that of photosynthesis, limited by the rate of soil thaw and the slow drainage of the soil column. Factors acting over long time scales, especially water table depth, strongly influenced the carbon budget on annual time scales. Net uptake was enhanced and respiration inhibited by multiple years of rainfall in excess of evaporative demand. Contrary to expectations, we observed no correlation between longer growing seasons and net uptake, possibly because of offsetting increases in ecosystem respiration. The results indicate that the interactions between soil thaw and water table depth provide critical controls on carbon exchange in boreal forests underlain by peat, on seasonal to decadal time scales, and these factors must be simulated in terrestrial biosphere models to predict response of these regions to future climate.

Evans, C. D., Freeman, C., Cork, L. G., Thomas, D. N., Reynolds, B., Billett, M. F., Garnett, M. H., Norris, D. (2007). Evidence against recent climate-induced destabilisation of soil carbon from14 C analysis of riverine dissolved organic matter. Geophysical Research Letters 34 (L07407): doi:10.1029/2007GL029431

ABSTRACT; The stability of global soil carbon (C) represents a major uncertainty in forecasting future climate change. In the UK, substantial soil C losses have been reported, while at the same time dissolved organic carbon (DOC) concentrations in upland waters have increased, suggesting that soil C stocks may be destabilising in response to climate change. To investigate the link between soil carbon and DOC at a range of sites, soil organic matter, soilwater and streamwater DOC were analysed for radiocarbon (14 C). DOC exported from C-rich landscapes appears younger than the soil C itself, much of it comprising C assimilated post-1950s. DOC from more intensively managed, C-poor soils is older, in some cases >100 years. Results appear consistent with soil C destabilisation in farmed landscapes, but not in peatlands. Reported C losses may to a significant extent be explained by mechanisms other than climate change, e.g. recovery from acidification in peatlands, and agricultural intensification in managed systems.

Falloon, P., Jones, C. D., Cerri, C. E. P., Al-Adamat, R., Kamoni, P., Bhattacharyya, T., Easter, M., Paustian, K., Killian, K., Coleman, K., Milne, E. (2007). Climate change and its impact on soil and vegetation carbon storage in Kenya, Jordan, India and Brazil: Soil carbon stocks at regional scales - Assessment of Soil Organic Carbon Stocks and Change at National Scale, Final Project Presentation, The United Nations Environment Programme, Nairobi, Kenya, 23-24 May 2005. Agriculture, Ecosystems & Environment 122 (1): 114-124

ABSTRACT: The terrestrial biosphere is an important global carbon (C) sink, with the potential to drive large positive climate feedbacks. Thus a better understanding of interactions between land use change, climate change and the terrestrial biosphere is crucial in planning future land management options. Climate change has the potential to alter terrestrial C storage since changes in temperature, precipitation and carbon dioxide (CO2 ) concentrations could affect net primary production (NPP), C inputs to soil, and soil C decomposition rates. Climate change could also act as a driver for land use change, thus further altering terrestrial C fluxes. The net balance of these different effects varies considerably between regions and hence the case studies presented in this paper (the GEFSOC project countries Kenya, Jordan, Brazil, and India) provide a unique opportunity to study climate impacts on terrestrial C storage. This paper first presents predicted changes in climate for the four case study countries from a coupled climate-C cycle Global Circulation Model (HadCM3LC), followed by predicted changes in vegetation type, NPP and soil C storage. These very coarse assessments provide an initial estimate of large-scale effects. A more detailed study of climate impacts on soil C storage in the Brazilian Amazon is provided as an example application of the GEFSOC system. Interestingly in the four cases studied here precipitation seems to control the sign of the soil C changes under climate change with wetter conditions resulting in higher soil C stocks and drier conditions in lower soil C stocks, presumably because increased NPP in wetter conditions here will override any increase in respiration. In contrast, globally, it seems to be temperature that controls changes in C stocks under climate change. Even if there is a slight increase in precipitation globally, a decrease in C stocks is predicted—in other words, the regional response to precipitation differs from the global response. The reason for this may be that whilst temperature increases under climate change were predicted everywhere, the nature of precipitation changes varies greatly between regions.

Fang, C. M., Smith, P., Moncrieff, J. B., Smith, J. U. (2005). Similar response of labile and resistant soil organic matter pools to changes in temperature. Nature 433 (7021): 57-59

FIRST PARAGRAPH: Our understanding of the relationship between the decomposition of soil organic matter (SOM) and soil temperature affects our predictions of the impact of climate change on soil-stored carbon1 . One current opinion is that the decomposition of soil labile carbon is sensitive to temperature variation whereas resistant components are insensitive2, 3, 4 . The resistant carbon or organic matter in mineral soil is then assumed to be unresponsive to global warming2, 4 . But the global pattern and magnitude of the predicted future soil carbon stock will mainly rely on the temperature sensitivity of these resistant carbon pools. To investigate this sensitivity, we have incubated soils under changing temperature. Here we report that SOM decomposition or soil basal respiration rate was significantly affected by changes in SOM components associated with soil depth, sampling method and incubation time. We find, however, that the temperature sensitivity for SOM decomposition was not affected, suggesting that the temperature sensitivity for resistant organic matter pools does not differ significantly from that of labile pools, and that both types of SOM will therefore respond similarly to global warming.

Feller, C., Bernoux, M. (2008). Historical advances in the study of global terrestrial soil organic carbon sequestration. Waste Management 28 (4): 734-740

ABSTRACT: This paper serves two purposes: it provides a summarized scientific history of carbon sequestration in relation to the soil-plant system and gives a commentary on organic wastes and SOC sequestration.

The concept of soil organic carbon (SOC) sequestration has its roots in: (i) the experimental work of Lundegårdh, particularly his in situ measurements of CO2 fluxes at the soil-plant interface (1924, 1927, 1930); (ii) the first estimates of SOC stocks at the global level made by Waksman [Waksman, S.A., 1938. Humus. Origin, Chemical Composition and Importance in Nature, second ed. revised. Williams and Wilkins, Baltimore, p. 526] and Rubey [Rubey, W.W., 1951. Geologic history of sea water. Bulletin of the Geological Society of America 62, 1111–1148]; (iii) the need for models dealing with soil organic matter (SOM) or SOC dynamics beginning with a conceptual SOM model by De Saussure (1780–1796) followed by the mathematical models of Jenny [Jenny, H., 1941. Factors of Soil Formation: a System of Quantitative Pedology. Dover Publications, New York, p. 288], Hénin and Dupuis [Hénin, S., Dupuis, M., 1945. Essai de bilan de la matière organique. Annales d’Agronomie 15, 17–29] and more recently the RothC [Jenkinson, D.S., Rayner, J.H., 1977. The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science 123 (5), 298–305] and Century [Parton, W.J., Schimel, D.S., Cole, C.V., Ojima, D.S., 1987. Analysis of factors controlling soil organic matter levels in great plains grasslands. Soil Science Society of America Journal 51 (5), 1173–1179] models.

The establishment of a soil C sequestration balance is not straightforward and depends greatly on the origin and the composition of organic matter that is to be returned to the system. Wastes, which are important sources of organic carbon for soils, are taken as an example. For these organic materials the following factors have to be considered: the presence or absence of fossil C, the potential of direct and indirect emissions of non-CO2 greenhouse gases (CH4 and N2 O) following application and the agro-system which is being used as a comparative reference.

Fissore, C., C.P. Giardina, R.K. Kolka, C.C. Trettin, G.M. King, M.F. Jurgensen, C.D. Barton, S.D. McDowell (2008). Temperature and vegetation effects on soil organic carbon quality along a forested mean annual temperature gradient in North America. Global Change Biology 14 (1): 193-205

ABSTRACT: Both climate and plant species are hypothesized to influence soil organic carbon (SOC) quality, but accurate prediction of how SOC process rates respond to global change will require an improved understanding of how SOC quality varies with mean annual temperature (MAT) and forest type. We investigated SOC quality in paired hardwood and pine stands growing in coarse textured soils located along a 22 °C gradient in MAT. To do this, we conducted 80-day incubation experiments at 10 and 30 °C to quantify SOC decomposition rates, which we used to kinetically define SOC quality. We used these experiments to test the hypotheses that SOC quality decreases with MAT, and that SOC quality is higher under pine than hardwood tree species. We found that both SOC quantity and quality decreased with increasing MAT. During the 30 °C incubation, temperature sensitivity (Q10) values were strongly and positively related to SOC decomposition rates, indicating that substrate supply can influence temperature responsiveness of SOC decomposition rates. For a limited number of dates, Q10 was negatively related to MAT. Soil chemical properties could not explain observed patterns in soil quality. Soil pH and cation exchange capacity (CEC) both declined with increasing MAT, and soil C quality was positively related to pH but negatively related to CEC. Clay mineralogy of soils also could not explain patterns of SOC quality as complex (2 : 1), high CEC clay minerals occurred in cold climate soils while warm climate soils were dominated by simpler (1 : 1), low CEC clay minerals. While hardwood sites contained more SOC than pine sites, with differences declining with MAT, clay content was also higher in hardwood soils. In contrast, there was no difference in SOC quality between pine and hardwood soils. Overall, these findings indicate that SOC quantity and quality may both decrease in response to global warming, despite long-term changes in soil chemistry and mineralogy that favor decomposition.

Fontaine, S., Barot, S., Barre, P., Bdioui, N., Mary, B., Rumpel, C. (2007). Stability of organic carbon in deep soil layers controlled by fresh carbon supply. Nature 450 (7167): 277-280

ABSTRACT: The world's soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon. Supplementary material at: http://www.nature.com/nature/journal/v450/n7167/extref/nature06275-s1.pdf

Franzluebbers, A.J. (2005). Soil organic carbon sequestration and agricultural greenhouse gas emissions in the southeastern USA. Soil and Tillage Research 83 (1): 120-147

ABSTRACT: Agriculture in the southeastern USA can be highly productive (i.e., high photosynthetic fixation of atmospheric CO2 ) due to warm-moist climatic conditions. However, its impacts on greenhouse gas emissions and mitigation potential have not been thoroughly characterized. This paper is a review and synthesis of literature pertaining to soil organic C (SOC) sequestration and greenhouse gas emissions from agricultural activities in the southeastern USA. Conservation tillage is an effective strategy to regain some of the SOC lost following decades, and in some areas centuries, of intensive soil tillage and erosion. With conventional tillage (CT) as a baseline, SOC sequestration with no tillage (NT) was 0.42 ± 0.46 Mg ha−1 year−1 (10 ± 5 years). Combining cover cropping with NT enhanced SOC sequestration (0.53 ± 0.45 Mg ha−1 year−1 ) compared with NT and no cover cropping (0.28 ± 0.44 Mg ha−1 year−1 ). By increasing cropping system complexity, SOC could be increased by 0.22 Mg ha−1 year−1 , irrespective of tillage management. Taking into account an average C cost of producing and transporting N fertilizer, SOC sequestration could be optimized at 0.24 Mg ha−1 year−1 with application of 107 kg N ha−1 year−1 on N-responsive crops, irrespective of tillage management. In longer-term studies (5–21 years), poultry litter application led to SOC sequestration of 0.72 ± 0.67 Mg ha−1 year−1 (17 ± 15% of C applied). Land that was previously cropped and converted to forages sequestered SOC at a rate of 1.03 ± 0.90 Mg ha−1 year−1 (15 ± 17 years). Limited data suggest animal grazing increases SOC sequestration on upland pastures. By expanding research on SOC sequestration into more diverse pasture and manure application systems and gathering much needed data on methane and nitrous oxide fluxes under almost any agricultural operation in the region, a more complete analysis of greenhouse gas emissions and potential mitigation from agricultural management systems would be possible. This information will be necessary for developing appropriate technological and political solutions to increase agricultural sustainability and combat environmental degradation in the southeastern USA.

Ganjegunte, G. K., Vance, G. F., Preston, C. M., Schuman, G. E., Ingram, L. J., Stahl, P. D., Welker, J. M. (2005). Soil organic carbon composition in a northern mixed-grass prairie: effects of grazing. Soil Science Society of America JournalSoil Sci Soc Am J 69 (6): 1746-1756

ABSTRACT: Growing interest in the potential for soils to provide a sink for atmospheric C has prompted studies of effects of management on the amount and nature of soil organic C (SOC). In this study, we evaluated effects of different grazing management regimes (light grazing [LG], heavy grazing [HG], and non-grazed exclosures [EX]) on amount and composition of SOC at the USDA–ARS High Plains Grasslands Research Station (HPGRS), Cheyenne, WY. Soils (0–5 cm) from each treatment were analyzed for total C and N contents and lignin composition. Soil organic C and N contents were significantly greater in LG (SOC–13.8 Mg ha–1 ; total N–1.22 Mg ha–1 ) than HG (SOC–10.9 Mg ha–1 ; total N–0.94 Mg ha–1 ) or EX (SOC–10.8 Mg ha–1 ; total N–0.94 Mg ha–1 ). From CuO oxidation studies, significantly greater (P < 0.05) total lignin (Vanillyl [V] + Syringyl [S] + Cinnamyl [C] compounds) contents were noted in EX (21 g kg–1 SOC) than LG (12 g kg–1 SOC) and HG (15 g kg–1 SOC) soils. The lignin composition of humic (HA) and fulvic (FA) acids indicated that HA under LG contained significantly greater V and S than HG or EX. Fulvic acids contained S-depleted lignin compared with HAs and FAs from HG, which contained significantly greater V and C than FAs extracted from LG and EX. Nuclear magnetic resonance (NMR) spectra of HA and FA, however, did not vary significantly among the three grazing treatments. Results from CuO oxidation and NMR spectroscopy emphasized the familiar problem that determining the nature of soil organic matter (SOM) is a difficult task and sometimes different analytical techniques provide different information about the nature of SOM. Nonetheless, results of this study indicate that LG is the most sustainable grazing management system for northern mixed-grass prairies.

Gleixner,G., Kramer,C., Hahn,V., Sachse,D., M. Scherer-Lorenzen, C. Körner, E. Schulze (2005). The effect of biodiversity on carbon storage in soils. Springer-Verlag 176: 165-183

INTRODUCTION: Only limited knowledge is presently available about how carbon is stored in soils and how this process can be influenced by abiotic processes (Schimel et al. 2001). Even less is known about the effects of biodiversity on carbon storage (Catovsky et al. 2002), especially with respect to the role of tree biodiversity. Most experimental investigations on biodiversity deal with grassland experiments focusing on ecosystem functions of biomass productivity or nutrient retention (Kinzig et al. 2001; Loreau et al. 2002). In order to identify possible interactions of biodiversity with carbon storage, this chapter summarizes current knowledge on carbon storage and emphasize the importance of this process for ecosystem functioning. Several key areas will be identified where plant biodiversity might influence carbon storage.

Grace, P.R., Post, W.M., Hennessy, K. (2006). The potential impact of climate change on Australia's soil organic carbon resources. Carbon Balance and Management 1 (1): 14

ABSTRACT:Soil organic carbon (SOC) represents a significant pool of carbon within the biosphere. Climatic shifts in temperature and precipitation have a major influence on the decomposition and amount of SOC stored within an ecosystem and that released into the atmosphere. We have linked net primary production (NPP) algorithms, which include the impact of enhanced atmospheric CO2 on plant growth, to the SOCRATES terrestrial carbon model to estimate changes in SOC for the Australia continent between the years 1990 and 2100 in response to climate changes generated by the CSIRO Mark 2 Global Circulation Model (GCM).We estimate organic carbon storage in the topsoil (0–10 cm) of the Australian continent in 1990 to be 8.1 Gt. This equates to 19 and 34 Gt in the top 30 and 100 cm of soil, respectively. By the year 2100, under a low emissions scenario, topsoil organic carbon stores of the continent will have increased by 0.6% (49 Mt C). Under a high emissions scenario, the Australian continent becomes a source of CO2 with a net reduction of 6.4% (518 Mt) in topsoil carbon, when compared to no climate change. This is partially offset by the predicted increase in NPP of 20.3%Climate change impacts must be studied holistically, requiring integration of climate, plant, ecosystem and soil sciences. The SOCRATES terrestrial carbon cycling model provides realistic estimates of changes in SOC storage in response to climate change over the next century, and confirms the need for greater consideration of soils in assessing the full impact of climate change and the development of quantifiable mitigation strategies.

Gulledge, J., Schimel, J. P. (1998). Moisture control over atmospheric CH4 consumption and CO2 production in diverse Alaskan soils. Soil Biology and Biochemistry 30 (8-9): 1127-1132

ABSTRACT: Moisture is an important control on atmospheric CH4 consumption and CO2 production in soil. Wet conditions limit these microbial activities by restricting CH4 and O2 diffusion and dry conditions limit microbial activity due to physiological water stress. We examined the relationship between soil moisture and these biogeochemical activities in five Alaskan soils with varying physical properties. Three expressions of soil moisture, absolute water content (g H2O g−1 dry soil), water potential and percent of water-holding capacity (%WHC), were compared for their abilities to predict microbial activity in the different soils. We also examined the physiological responses of CH4 oxidizers and the general microbial community to changes in water potential. The quantitative relationship between absolute water content and microbial activity varied widely among soils with different textures. The relationship between microbial activity and water potential was asymmetrical and differed between upland and wetland soils. In contrast, the parabolic relationship between %WHC and CH4 consumption was symmetrical and similar among the five soils. CO2 production also related to %WHC similarly across soils. Maximum atmospheric CH4 consumption occurred between 20-40% WHC in all soils with a mean optimum of 34% WHC, whereas CO2 production was maximal above 50% WHC. For CH4 oxidation, optimum water potential was −0.3 to −0.2 MPa in upland soils, and about −0.02 MPa in a wetland soil. Our results demonstrate that %WHC is a powerful expression for quantitatively relating microbial activity responses to moisture across physically diverse soils and may be useful for modeling the response of biogeochemical processes, especially atmospheric CH4 consumption, to climate change. Our data also suggest that CH4 oxidizers in upland soils are adapted to growth on atmospheric CH4 and that CH4 consumption in upland taiga soils may be decreased by altered soil moisture, regardless of whether conditions become wetter or drier.

Hagedorn, F., Maurer, S., Egli, P., Blaser, P., Bucher, J. B., Siegwolf, R. (2001). Carbon sequestration in forest soils: effects of soil type, atmospheric CO2 enrichment, and N deposition. European Journal of Soil Science 52 (4): 619-628

ABSTRACT: Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to enriching the atmosphere in CO2 and to increased N deposition is not completely understood. We studied the effects of CO2 concentrations at 370 and 570μmolCO2 mol−1 air and increased N deposition (7 against 0.7 g N m−2 year−1 ) on the dynamics of soil organic C in two types of forest soil in model ecosystems with spruce and beech established in large open-top chambers containing an acidic loam and a calcareous sand. The added CO2 was depleted in13 C and thus the net input of new C into soil organic carbon and the mineralization of native C could be quantified.

Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of new C in the acidic loam (670 ± 30 g C m−2 ) exceeded that in the calcareous sand (340 ± 40 g C m−2 ) although the soil produced less biomass. The mineralization of native organic C accounted for 700 ± 90 g C m−2 in the acidic loam and for 2800 ± 170 g C m−2 in the calcareous sand. Unfavourable conditions for mineralization and a greater physico-chemical protection of C by clay and oxides in the acidic loam are probably the main reasons for these differences. The organic C content of the acidic loam was 230 g C m−2 more under the large than under the small N treatment. As suggested by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition did not influence C concentrations. The impacts of CO2 enrichment on C concentrations were small. In the uppermost 10 cm of the acidic loam, larger CO2 concentrations increased C contents by 50–170 g C m−2 . Below 10 cm depth in the acidic loam and at all soil depths in the calcareous sand, CO2 concentrations had no significant impact on soil C concentrations. Up to 40% of the 'new' carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of the total soil volume. This suggests that a large part of the CO2 -derived 'new' C was incorporated into the labile and easily mineralizable pool in the soil.

Heath, J., Ayres, E., Possell, M., Bardgett, R. D., Black, H. I. J., Grant, H., Ineson, P., Kerstiens, G. (2005). Rising atmospheric CO2 reduces sequestration of root-derived soil carbon. Science 309 (5741): 1711-1713

ABSTRACT: Forests have a key role as carbon sinks, which could potentially mitigate the continuing increase in atmospheric carbon dioxide concentration and associated climate change. We show that carbon dioxide enrichment, although causing short-term growth stimulation in a range of European tree species, also leads to an increase in soil microbial respiration and a marked decline in sequestration of root-derived carbon in the soil. These findings indicate that, should similar processes operate in forest ecosystems, the size of the annual terrestrial carbon sink may be substantially reduced, resulting in a positive feedback on the rate of increase in atmospheric carbon dioxide concentration.

Hedde, M., Aubert, M., Decaens, T., Bureau, F. (2008). Dynamics of soil carbon in a beechwood chronosequence forest. Forest Ecology and Management 255 (1): 193-202

ABSTRACT: Accurate estimates of forest soil organic matter (OM) are now crucial to predictions of global C cycling. This work addresses soil C stocks and dynamics throughout a managed beechwood chronosequence (28–197 years old, Normandy, France). Throughout this rotation, we investigated the variation patterns of (i) C stocks in soil and humic epipedon, (ii) macro-morphological characteristics of humic epipedon, and (iii) mass, C content and C-to-N ratio in physical fractions of humic epipedon. The fractions isolated were large debris (>2000μm), coarse particular OM (cPOM, 200–2000μm), fine particular OM (fPOM, 50–200μm) and the mineral associated OM (MaOM, <50 μm).Soil C stocks remained unchanged between silvicultural phases, indicating a weak impact of this even-aged forest rotation on soil C sequestration. While humic epipedon mass and depth only slightly varied with beech development, C stocks in the holorganic layers were modified and the use of physical fractionation allowed us to discuss different aspect of quantitative and qualitative changes that occurred throughout the silvicultural rotation. Hence, changes in humic epipedon composition may be attributed to the modification of beech life-history traits with its maturation (growth vs. reproduction). Our results showed that C-POM can reached very high values (68%) in organo-mineral layers of older managed forest and that C-MaOM did not significantly change revealing the resistance of humified fractions of humic epipedon to logging and regeneration practices. C-to-N results indicated that N was probably not a limiting factor to litter degradation and explained our findings that OM did not accumulate in O horizons.This work confirms that forest harvesting and regeneration practices may have few effects on soil and humic epipedon C stocks, and that short- and long-term effects can be complex and may imply mechanisms with opposite effects.

Hibbard, K. A., Law, B. E., Reichstein, M., Sulzman, J. (2005). An analysis of soil respiration across northern hemisphere temperate ecosystems.. Biogeochemistry 73 (1): 29-70

ABSTRACT: Over two-thirds of terrestrial carbon is stored belowground and a significant amount of atmospheric CO2 is respired by roots and microbes in soils. For this analysis, soil respiration (Rs) data were assembled from 31 AmeriFlux and CarboEurope sites representing deciduous broadleaf, evergreen needleleaf, grasslands, mixed deciduous/evergreen and woodland/savanna ecosystem types. Lowest to highest rates of soil respiration averaged over the growing season were grassland and woodland/savanna < deciduous broadleaf forests < evergreen needleleaf, mixed deciduous/evergreen forests with growing season soil respiration significantly different between forested and non-forested biomes (p < 0.001). Timing of peak respiration rates during the growing season varied from March/April in grasslands to July–September for all other biomes. Biomes with overall strongest relationship between soil respiration and soil temperature were from the deciduous and mixed forests (R2 ≥ 0.65). Maximum soil respiration was weakly related to maximum fine root biomass (R2 = 0.28) and positively related to the previous years’ annual litterfall (R2 = 0.46). Published rates of annual soil respiration were linearly related to LAI and fine root carbon (R2 = 0.48, 0.47), as well as net primary production (NPP) (R2 = 0.44). At 10 sites, maximum growing season Rs was weakly correlated with annual GPP estimated from eddy covariance towersites (R2 = 0.29; p < 0.05), and annual soil respiration and total growing season Rs were not correlated with annual GPP (p > 0.1). Yet, previous studies indicate correlations on shorter time scales within site (e.g., weekly, monthly). Estimates of annual GPP from the Biome-BGC model were strongly correlated with observed annual estimates of soil respiration for six sites (R2 = 0.84; p < 0.01). Correlations from observations of Rs with NPP, LAI, fine root biomass and litterfall relate above and belowground inputs to labile pools that are available for decomposition. Our results suggest that simple empirical relationships with temperature and/or moisture that may be robust at individual sites may not be adequate to characterize soil CO2 effluxes across space and time, agreeing with other multi-site studies. Information is needed on the timing and phenological controls of substrate availability (e.g., fine roots, LAI) and inputs (e.g., root turnover, litterfall) to improve our ability to accurately quantify the relationships between soil CO2 effluxes and carbon substrate storage.

Hobbie, Sara E., Nadelhoffer, Knute J., Hogberg, Peter (2002). A synthesis: The role of nutrients as constraints on carbon balances in boreal and artic regions. Plant And SoilPlant Soil 242: 163-170

ABSTRACT: As in many ecosystems, carbon (C) cycling in arctic and boreal regions is tightly linked to the cycling of nutrients: nutrients (particularly nitrogen) are mineralized through the process of organic matter decomposition (C mineralization), and nutrient availability strongly constrains ecosystem C gain through primary production. This link between C and nutrient cycles has implications for how northern systems will respond to future climate warming and whether feedbacks to rising concentrations of atmospheric CO2 from these regions will be positive or negative. Warming is expected to cause a substantial release of C to the atmosphere because of increased decomposition of the large amounts of organic C present in high-latitude soils (a positive feedback to climate warming). However, increased nutrient mineralization associated with this decomposition is expected to stimulate primary production and ecosystem C gain, offsetting or even exceeding C lost through decomposition (a negative feedback to climate warming). Increased primary production with warming is consistent with results of numerous experiments showing increased plant growth with nutrient enrichment. Here we examine key assumptions behind this scenario: (1) temperature is a primary control of decomposition in northern regions, (2) increased decomposition and associated nutrient release are tightly coupled to plant nutrient uptake, and (3) short-term manipulations of temperature and nutrient availability accurately predict long-term responses to climate change.

Hogberg, P., Read, D. J. (2006). Towards a More Plant Physiological Perspective on Soil Ecology. Trends in Ecology & Evolution 21 (10): 548-554

ABSTRACT: Soil respiration almost balances carbon fixation by terrestrial photosynthesis and exceeds all anthropogenic carbon emissions by an order of magnitude, yet we lack precise knowledge of the sources of, and controls upon, the release of carbon dioxide from soils. Here, we discuss the increasing evidence that half of this carbon release is from living plant roots, their mycorrhizal fungi and other root-associated microbes, and that this release is driven directly by recent photosynthesis. The new studies challenge the widespread view that soil activity is dominated by decomposer organisms using older detrital material and that root litter inputs equal those of aboveground litter. The new observations emphasize the physiological continuity and dynamic interdependence of the plant-microbe-soil system and highlight the need for closer cooperation between plant and soil scientists.

Homann, P.S., Bormann, B.T., Boyle, J.R., Darbyshire, R.L., Bigley, R. (2008). Soil C and N minimum detectable changes and treatment differences in a multi-treatment forest experiment. Forest Ecology and Management 255 (5-6): 1724-1734

ABSTRACT: Detecting changes in forest soil C and N is vital to the study of global budgets and long-term ecosystem productivity. Identifying differences among land-use practices may guide future management. Our objective was to determine the relation of minimum detectable changes (MDCs) and minimum detectable differences between treatments (MDDs) to soil C and N variability at multiple spatial scales. The three study sites were 70–100-year-old coniferous forests in Washington and Oregon. Area- and volumetric-based soil measurements were made before implementation of 7 treatments on 2-ha experimental units, replicated in 3 or 4 blocks per site. In the absence of treatment effects, whole-site MDCs are 10% for mineral soil C and N masses and concentrations and 40% for O-horizon C and N masses. When treatment differences occur, MDDs are 40% for mineral soil and 150% for O-horizon. MDDs are reduced as much as two-thirds by evaluating change from pre- to post-treatment rather than only post-treatment values, and by pairing pre- and post-treatment measurements within small subplots. The magnitude of MDD reduction is quantitatively related to pre-treatment soil variability at multiple spatial scales, with the greatest reductions associated with the largest within-block:within-plot and within-plot:within-subplot variability ratios. These quantified benefits can be weighed against costs and challenges to make informed decisions when selecting the most appropriate sampling design.

Hoover, C.M., Kimble, J.M., Heath, L.S., Birdsey, R.A., Lal, R. (2003). Soil carbon sequestration and forest management: challenges and opportunities. CRC Press: 211-238

DESCRIPTION: The subject of the effects of forest management activities on soil carbon is a difficult one to address, but ongoing discussions of carbon sequestration as an emissions offset and the emergence of carbon-credit-trading systems necessitate that we broaden and deepen our understanding of the response of forest-soil carbon pools to forest management. There have been several reviews of the literature, but hard-and-fast conclusions are still difficult to draw, since many of the studies reviewed were not designed specifically to address management effects on soil carbon, were conducted on a short timescale, and differ in the methodology employed.

Huang, Z., Xu, Z., Chen, C., Boyd, S. (2008). Changes in soil carbon during the establishment of a hardwood plantation in subtropical Australia. Forest Ecology and Management 254 (1): 46-55

ABSTRACT: Soil carbon (C) pools are not only important to governing soil properties and nutrient cycling in forest ecosystems, but also play a critical role in global C cycling. Mulch and weed control treatments may alter soil C pools by changing organic matter inputs to the forest ecosystem. We studied the 12-month mulch and weed control responses on the chemical composition of soil organic C and the seasonal dynamics of water extractable organic C (WEOC), hot water extractable organic C (HWEOC), chloroform-released organic C (CHCl3-released C), and acid hydrolysed organic C (acid hydrolysable C) in a hardwood plantation of subtropical Australia. The results showed that compared with the non-mulch treatment, the mulch treatment significantly increased soil WEOC, HWEOC, and CHCl3-released C over the four sampling months. The weed control treatment significantly reduced the amount of HWEOC and CHCl3-released C compared with the no weed control treatment. Neither the mulch nor weed control treatment significantly affected soil acid hydrolysed organic C. There were no significant seasonal variations in soil WEOC, HWEOC, CHCl3-released C, and acid hydrolysed organic C in the hardwood plantation. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was used to study the structural chemistry of soil C pools in hydrofluoric acid (HF) treated soils collected 12 months after the mulch and weed control treatments were applied. Overall, O-alkyl C was the dominant C fraction, accounting for 33–43% of the total NMR signal intensity. The mulch treatment led to higher signal intensity in the alkyl C spectral region and A/O-A ratio (the ratio of alkyl C region intensity to O-alkyl C region intensity), but lower signal intensity in the aryl C and aromaticity. Compared with the no weed control treatment, the weed control treatment reduced signal intensity in the aryl C and aromaticity. Together, shifts in the amount and nature of soil C following the mulch and weed control treatments may be due to the changes in organic matter input and soil physical environment.

Ise, T., Moorcroft, P. R. (2006). The global-scale temperature and moisture dependencies of soil organic carbon decomposition: an analysis using a mechanistic decomposition model. Biogeochemistry 80 (3): 217-231

ABSTRACT: Since the decomposition rate of soil organic carbon (SOC) varies as a function of environmental conditions, global climate change is expected to alter SOC decomposition dynamics, and the resulting changes in the amount of CO2 emitted from soils will feedback onto the rate at which climate change occurs. While this soil feedback is expected to be significant because the amount of SOC is substantially more than the amount of carbon in the atmosphere, the environmental dependencies of decomposition at global scales that determine the magnitude of the soil feedback have remained poorly characterized. In this study, we address this issue by fitting a mechanistic decomposition model to a global dataset of SOC, optimizing the model’s temperature and moisture dependencies to best match the observed global distribution of SOC. The results of the analysis indicate that the temperature sensitivity of decomposition at global scales (Q10 =1.37) is significantly less than is assumed by many terrestrial ecosystem models that directly apply temperature sensitivity from small-scale studies, and that the maximal rate of decomposition occurs at higher moisture values than is assumed by many models. These findings imply that the magnitude of the soil decomposition feedback onto rate of global climate change will be less sensitive to increases in temperature, and modeling of temperature and moisture dependencies of SOC decomposition in global-scale models should consider effects of scale.

Jandl, R., Lindner, M., Vesterdal, L., Bauwens, B., Baritz, R., Hagedorn, F., Johnson, D. W., Minkkinen, K., Byrne, K. A. (2007). How strongly can forest management influence soil carbon sequestration?. Geoderma 137 (3-4): 253-268

ABSTRACT: We reviewed the experimental evidence for long-term carbon (C) sequestration in soils as consequence of specific forest management strategies. Utilization of terrestrial C sinks alleviates the burden of countries which are committed to reducing their greenhouse gas emissions. Land-use changes such as those which result from afforestation and management of fast-growing tree species, have an immediate effect on the regional rate of C sequestration by incorporating carbon dioxide (CO2 ) in plant biomass. The potential for such practices is limited in Europe by environmental and political constraints. The management of existing forests can also increase C sequestration, but earlier reviews found conflicting evidence regarding the effects of forest management on soil C pools. We analyzed the effects of harvesting, thinning, fertilization application, drainage, tree species selection, and control of natural disturbances on soil C dynamics. We focused on factors that affect the C input to the soil and the C release via decomposition of soil organic matter (SOM). The differentiation of SOM into labile and stable soil C fractions is important. There is ample evidence about the effects of management on the amount of C in the organic layers of the forest floor, but much less information about measurable effects of management on stable C pools in the mineral soil. The C storage capacity of the stable pool can be enhanced by increasing the productivity of the forest and thereby increasing the C input to the soil. Minimizing the disturbances in the stand structure and soil reduces the risk of unintended C losses. The establishment of mixed species forests increases the stability of the forest and can avoid high rates of SOM decomposition. The rate of C accumulation and its distribution within the soil profile differs between tree species. Differences in the stability of SOM as a direct species effect have not yet been reported.

Janzen, H. H. (2004). Carbon Cycling in Earth Systems - a Soil Science Perspective. 104 (3): 399-417

ABSTRACT: The carbon cycle binds together earth’s ecosystems and their inhabitants. My intent is to review the global carbon cycle, examine how humans have modified it, and contemplate (from a soil science bias) the new questions that await us on a changing earth. These thoughts are proffered, not to propose a way forward, but to invite conversation about opportunities that await us.

Terrestrial ecosystems hold a lot of carbon—about 500 Pg C in plant biomass, and 2000 Pg C in soil organic matter. Oceans contain even more. And the atmosphere, now with about 785 Pg C, connects all of these pools. The flows of carbon between the pools, and their feedbacks, have kept atmospheric CO2 reasonably constant for millennia. But humans have increasingly distorted the balance, by changing land use and by injecting fossil C back into the cycle. Consequently, atmospheric CO2 has increased recently by more than 3 Pg C per year and, by century’s end, its concentration may be twice pre-industrial levels, or more.

The changing carbon cycle poses new questions for scientists. Now we will be asked, not how things are, but how they will be. For example: How will changes in CO2 alter flows of carbon through biological carbon stocks? Can we manage ecosystems to hold more carbon? Are current carbon stores vulnerable should the earth warm, or water cycles shift, or nitrogen flows be altered? What will the C cycle look like a century from now; and will it then still provide all that we expect from it? These and other new questions may elicit from us fresh insights and approaches.We may learn to look more broadly at the C cycle, seeing all the ‘ecosystem services’ (not just C sequestration). We may insist on studies yielding deeper understanding of the C cycle, relevant beyond current issues. We may further emphasize ‘time’ in our studies, looking more at flows and changes than at describing what is—and looking long enough to see even subtle shifts.We may learn to follow C beyond the usual boundaries set by arbitrary disciplines. And we may come to see, more than before, how the carbon cycle weaves through our fields and skies and forests—and find new ways to reveal its grandeur to those who have not yet seen it. And then, it may happen that
our successors, a century from now, will look back, almost in envy, at the urgent, enticing questions we were given to solve.

Janzen, H. H. (2005). Soil carbon: A measure of ecosystem response in a changing world?. Canadian Journal of Soil Science 85 (4): 467-480

ABSTRACT: The global carbon (C) cycle is changing, as evident from abrupt increases in atmospheric CO2 . These changes have sparked interest in agricultural soils as potential repositories for excess atmospheric C. Our perspective on soil C, therefore, has shifted: once, we focused mainly on how soil C affected productivity within agroecosystems; now we see also how C dynamics in agricultural soils exert influences far beyond the farm. We have long used soil C as an indicator of soil quality; now we may want to use soil C also as a broader indicator of ecosystem response. To prompt further discussion, I offer some tentative thoughts about how we might use soil C as an indicator on a changing earth. They include: using soil C to measure changes across time, not only across space; devising more sensitive measures of soil C change; quantifying soil C across four dimensions; measuring the nature of C, as well as its amount; using soil C alongside other indicators; finding better ways of admitting our uncertainty; establishing long-term sites for our successors to measure soil C change; and following flows of C past the farm fences. Recent worries about global warming have focused our attention on “sequestering” soil C to remove atmospheric CO2 . That aim may be worthy, but perhaps too narrow; a broader goal might be to ensure the productivity, permanence, and health of our agroecosystems and adjacent environments – and use C storage as a measure of progress toward that goal. Key words: Soil organic matter, global carbon cycle, carbon sequestration, global change

Jastrow, J. D., Miller, R. M., Matamala, R., Norby, R. J., Boutton, T. W., Rice, C. W., Owensby, C. E. (2005). Elevated atmospheric carbon dioxide increases soil carbon. Global Change Biology 11 (12): 2057-2064

ABSTRACT: The general lack of significant changes in mineral soil C stocks during CO2 -enrichment experiments has cast doubt on predictions that increased soil C can partially offset rising atmospheric CO2 concentrations. Here, we show, through meta-analysis techniques, that these experiments collectively exhibited a 5.6% increase in soil C over 2–9 years, at a median rate of 19 g C m−2 yr−1 . We also measured C accrual in deciduous forest and grassland soils, at rates exceeding 40 g C m−2 yr−1 for 5–8 years, because both systems responded to CO2 enrichment with large increases in root production. Even though native C stocks were relatively large, over half of the accrued C at both sites was incorporated into microaggregates, which protect C and increase its longevity. Our data, in combination with the meta-analysis, demonstrate the potential for mineral soils in diverse temperate ecosystems to store additional C in response to CO2 enrichment.

Jiménez, J. J., Lal, R. (2006). Mechanisms of C sequestration in soils of Latin America. Critical Reviews in Plant Sciences 25 (4): 337-365

ABSTRACT: Carbon (C) sequestration, defined as the process whereby atmospheric CO2 is transferred into a long-lived C pool, is an important issue not only in the scientific community but also in the society at large because of its potential role in off-setting fossil fuel emissions. Through photosynthesis this C is stored in plants and through decomposition, trunks, branches, leaves and roots are incorporated in the soil via the action of different soil organisms, i.e., bacteria, fungi and invertebrates. This, together with the C exudates from roots that are utilized by microbial populations, constitutes the natural pathways of incorporating biomass-C into the soil. The amount of C stored in terrestrial ecosystems is the third largest among the global C pools. Soil organic carbon (SOC) up to 3 m is 2,344 Pg C (1 Petagram = 1015 g), and the SOC pool in tropical soils is approximately 30% of the global pool. Abiotic factors, which moderate C sequestration in soils are clay content, mineralogy, structural stability, landscape position, and soil moisture and temperature regimes. On the other hand, biotic factors involved in soil C sequestration are determined by the activities of soil organisms. However, models do not include the formation, stabilization and lifespan of the aggregates that have been biologically produced, including roots. This is not only due to the lack of studies on this subject, but also to overlooking the role of soil organisms in soil aggregation. Furthermore, there is a lack of comprehensive knowledge regarding the processes that control dissolved organic carbon (DOC) fluxes in soils and its role in the global budget of C sequestration. The boundaries of ecosystems are not considered in the studies of the subject, as it may be the case for terrestrial C sequestration, since the borders around the sites under study constitute pathways for the flow of C between sites and through the landscape. The concentrations of DOC in deep soil horizons and the contribution to DOC fluxes (exports) are relatively small, from 4 to 37 g DOC m−2 yr−1 retained in the mineral subsoil. In South America, although substantial research has been done under different ecosystems and land use systems in some countries, like Brazil, Colombia, Argentina, there is a need to conduct more studies with agreed standard methodologies in natural ecosystems and agricultural systems, and in other areas of Central America few studies have been undertaken to date. The principal objective of this review was to address the main mechanisms that determine SOC and SIC sequestration in soils of Latin America, and include: physical aggregate protection, SOC-clay interaction, DOC transport, bioturbation by soil organisms, and the formation of secondary carbonates. All of these mechanisms are generally explained by physical and chemical processes. In contrast, this review takes a soil ecological approach to describe the mechanisms listed above.

Jobbágy, E. G., Jackson, R.B. (2000). The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications 10 (2): 423-436

ABSTRACT: As the largest pool of terrestrial organic carbon, soils interact strongly with atmospheric composition, climate, and land cover change. Our capacity to predict and ameliorate the consequences of global change depends in part on a better understanding of the distributions and controls of soil organic carbon (SOC) and how vegetation change may affect SOC distributions with depth. The goals of this paper are (1) to examine the association of SOC content with climate and soil texture at different soil depths; (2) to test the hypothesis that vegetation type, through patterns of allocation, is a dominant control on the vertical distribution of SOC; and (3) to estimate global SOC storage to 3 m, including an analysis of the potential effects of vegetation change on soil carbon storage. We based our analysis on >2700 soil profiles in three global databases supplemented with data for climate, vegetation, and land use. The analysis focused on mineral soil layers.

Plant functional types significantly affected the vertical distribution of SOC. The percentage of SOC in the top 20 cm (relative to the first meter) averaged 33%, 42%, and 50% for shrublands, grasslands, and forests, respectively. In shrublands, the amount of SOC in the second and third meters was 77% of that in the first meter; in forests and grasslands, the totals were 56% and 43%, respectively. Globally, the relative distribution of SOC with depth had a slightly stronger association with vegetation than with climate, but the opposite was true for the absolute amount of SOC. Total SOC content increased with precipitation and clay content and decreased with temperature. The importance of these controls switched with depth, climate dominating in shallow layers and clay content dominating in deeper layers, possibly due to increasing percentages of slowly cycling SOC fractions at depth. To control for the effects of climate on vegetation, we grouped soils within climatic ranges and compared distributions for vegetation types within each range. The percentage of SOC in the top 20 cm relative to the first meter varied from 29% in cold arid shrublands to 57% in cold humid forests and, for a given climate, was always deepest in shrublands, intermediate in grasslands, and shallowest in forests (P < 0.05 in all cases). The effect of vegetation type was more important than the direct effect of precipitation in this analysis. These data suggest that shoot/root allocations combined with vertical root distributions, affect the distribution of SOC with depth.

Global SOC storage in the top 3 m of soil was 2344 Pg C, or 56% more than the 1502 Pg estimated for the first meter (which is similar to the total SOC estimates of 1500–1600 Pg made by other researchers). Global totals for the second and third meters were 491 and 351 Pg C, and the biomes with the most SOC at 1–3 m depth were tropical evergreen forests (158 Pg C) and tropical grasslands/savannas (146 Pg C).

Johnston, C. A., Groffman, P., Breshears, D. D., Cardon, Z. G., Currie, W., Emanuel, W., Gaudinski, J., Jackson, R. B., Lajtha, K., Nadelhoffer, K., Nelson, D., Post, W. M., Retallack, G., Wielopolski, L. (2004). Carbon cycling in soil. Frontiers in Ecology and the Environment 2 (10): 522-528

ABSTRACT: As yet, nobody knows what effects climate change will have on soil carbon reserves, or how those changes will affect the global carbon cycle. Soils are the primary terrestrial repository for carbon, so minor changes in the balance between belowground carbon storage and release could have major impacts on greenhouse gases. Soil fauna, roots, fungi, and microbes interact with mineral and organic matter to process soil carbon. Studies have been hampered by the difficulty of observing processes beneath the earth's surface, but advances in science and technology are improving our ability to understand belowground ecosystems.

Jones, C., McConnell, C., Coleman, K., Cox, P., Falloon, P., Jenkinson, D., Powlson, D. (2005). Global climate change and soil carbon stocks; predictions from two contrasting models for the turnover of organic carbon in soil. Global Change Biology 11 (1): 154-166

ABSTRACT: Enhanced release of CO2 to the atmosphere from soil organic carbon as a result of increased temperatures may lead to a positive feedback between climate change and the carbon cycle, resulting in much higher CO2 levels and accelerated global warming. However, the magnitude of this effect is uncertain and critically dependent on how the decomposition of soil organic C (heterotrophic respiration) responds to changes in climate. Previous studies with the Hadley Centre's coupled climate–carbon cycle general circulation model (GCM) (HadCM3LC) used a simple, single-pool soil carbon model to simulate the response. Here we present results from numerical simulations that use the more sophisticated 'RothC' multipool soil carbon model, driven with the same climate data.

The results show strong similarities in the behaviour of the two models, although RothC tends to simulate slightly smaller changes in global soil carbon stocks for the same forcing. RothC simulates global soil carbon stocks decreasing by 54 Gt C by 2100 in a climate change simulation compared with an 80 Gt C decrease in HadCM3LC. The multipool carbon dynamics of RothC cause it to exhibit a slower magnitude of transient response to both increased organic carbon inputs and changes in climate. We conclude that the projection of a positive feedback between climate and carbon cycle is robust, but the magnitude of the feedback is dependent on the structure of the soil carbon model.

Jones, R. J. A., Hiederer, R., Rusco, E., Montanarella, L. (2005). Estimating organic carbon in the soils of Europe for policy support. European Journal of Soil Science 56: 655-671

ABSTRACT: The estimation of soil carbon content is of pressing concern for soil protection and in mitigation strategies for global warming. This paper describes the methodology developed and the results obtained in a study aimed at estimating organic carbon contents (%) in topsoils across Europe. The information presented in map form provides policy-makers with estimates of current topsoil organic carbon contents for developing strategies for soil protection at regional level. Such baseline data are also of importance in global change modelling and may be used to estimate regional differences in soil organic carbon (SOC) stocks and projected changes therein, as required for example under the Kyoto Protocol to the United Nations Framework Convention on Climate Change, after having taken into account regional differences in bulk density.

The study uses a novel approach combining a rule-based system with detailed thematic spatial data layers to arrive at a much-improved result over either method, using advanced methods for spatial data processing. The rule-based system is provided by the pedo-transfer rules, which were developed for use with the European Soil Database. The strong effects of vegetation and land use on SOC have been taken into account in the calculations, and the influence of temperature on organic carbon contents has been considered in the form of a heuristic function. Processing of all thematic data was performed on harmonized spatial data layers in raster format with a 1 km × 1 km grid spacing. This resolution is regarded as appropriate for planning effective soil protection measures at the European level. The approach is thought to be transferable to other regions of the world that are facing similar questions, provided adequate data are available for these regions. However, there will always be an element of uncertainty in estimating or determining the spatial distribution of organic carbon contents of soils.

Karberg, N. J., Pregitzer, K. S., King, J. S., Friend, A. L., Wood, J. R. (2005). Soil carbon dioxide partial pressure and dissolved inorganic carbonate chemistry under elevated carbon dioxide and ozone. Oecologia 142 (2): 296-306

ABSTRACT: Global emissions of atmospheric CO2 and tropospheric O3 are rising and expected to impact large areas of the Earths forests. While CO2 stimulates net primary production, O3 reduces photosynthesis, altering plant C allocation and reducing ecosystem C storage. The effects of multiple air pollutants can alter belowground C allocation, leading to changes in the partial pressure of CO2 (p CO2 ) in the soil , chemistry of dissolved inorganic carbonate (DIC) and the rate of mineral weathering. As this system represents a linkage between the long- and short-term C cycles and sequestration of atmospheric CO2 , changes in atmospheric chemistry that affect net primary production may alter the fate of C in these ecosystems. To date, little is known about the combined effects of elevated CO2 and O3 on the inorganic C cycle in forest systems. Free air CO2 and O3 enrichment (FACE) technology was used at the Aspen FACE project in Rhinelander, Wisconsin to understand how elevated atmospheric CO2 and O3 interact to alter pCO2 and DIC concentrations in the soil. Ambient and elevated CO2 levels were 360±16 and 542±81 l l–1 , respectively; ambient and elevated O3 levels were 33±14 and 49±24 nl l–1 , respectively. Measured concentrations of soil CO2 and calculated concentrations of DIC increased over the growing season by 14 and 22%, respectively, under elevated atmospheric CO2 and were unaffected by elevated tropospheric O3 . The increased concentration of DIC altered inorganic carbonate chemistry by increasing system total alkalinity by 210%, likely due to enhanced chemical weathering. The study also demonstrated the close coupling between the seasonal13 C of soilp CO2 and DIC, as a mixing model showed that new atmospheric CO2 accounted for approximately 90% of the C leaving the system as DIC. This study illustrates the potential of using stable isotopic techniques and FACE technology to examine long- and short-term ecosystem C sequestration.

Kirschbaum, Miko U. F. (2006). The temperature dependence of soil organic matter decomposition, and the effect of global warming on soil organic C storage. Soil Biology and Biochemistry 27 (6): 753-760

ABSTRACT: One of the key questions in climate change research relates to the future dynamics of the large amount of C that is currently stored in soil organic matter. Will the amount of C in this pool increase or decrease with global warming? The future trend in amounts of soil organic C will depend on the relative temperature sensitivities of net primary productivity and soil organic matter decomposition rate. Equations for the temperature dependence of net primary productivity have been widely used, but the temperature dependence of decomposition rate is less clear. The literature was surveyed to obtain the temperature dependencies of soil respiration and N dynamics reported in different studies. Only laboratory-based measurements were used to avoid confounding effects with differences in litter input rates, litter quality, soil moisture or other environmental factors. A considerable range of values has been reported, with the greatest relative sensitivity of decomposition processes to temperature having been observed at low temperatures. A relationship fitted to the literature data indicated that the rate of decomposition increases with temperature at 0°C with a Q10 of almost 8. The temperature sensitivity of organic matter decomposition decreases with increasing temperature, indicated by the Q10 decreasing with temperature to be about 4.5 at 10°C and 2.5 at 20°C. At low temperatures, the temperature sensitivity of decomposition was consequently much greater than the temperature sensitivity of net primary productivity, whereas the temperature sensitivities became more similar at higher temperatures. The much higher temperature sensitivity of decomposition than for net primary productivity has important implications for the store of soil organic C in the soil. The data suggest that a 1°C increase in temperature could ultimately lead to a loss of over 10% of soil organic C in regions of the world with an annual mean temperature of 5°C, whereas the same temperature increase would lead to a loss of only 3% of soil organic C for a soil at 30°C. These differences are even greater in absolute amounts as cooler soils contain greater amounts of soil organic C. This analysis supports the conclusion of previous studies which indicated that soil organic C contents may decrease greatly with global warming and thereby provide a positive feed-back in the global C cycle.

Dan Berggren Kleja, Magnus Svensson, Hooshang Majdi, Per-Erik Jansson, Ola Langvall, Bo Bergkvist, Maj-Britt Johansson, Per Weslien, Laimi Truusb, Anders Lindroth, Göran I. Ågren (2007). Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden. Biogeochemistry 89 (1): 7-25

ABSTRACT: This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m−2 year−1 ). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m−2 year−1 in the north to +9 g C m−2 year−1 in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m−2 year−1 .

Knorr,W., Prentice,I. C., House,J. I., Holland,E. A. (2005). Long-term sensitivity of soil carbon turnover to warming. Nature 433 (20 January): 298-301

ABSTRACT: The sensitivity of soil carbon to warming is a major uncertainty in projections of carbon dioxide concentration and climate1 . Experimental studies overwhelmingly indicate increased soil organic carbon (SOC) decomposition2, 3, 4, 5, 6, 7, 8 at higher temperatures, resulting in increased carbon dioxide emissions from soils. However, recent findings have been cited as evidence against increased soil carbon emissions in a warmer world9, 10 . In soil warming experiments, the initially increased carbon dioxide efflux returns to pre-warming rates within one to three years10, 11, 12, 13, 14 , and apparent carbon pool turnover times are insensitive to temperature15 . It has already been suggested that the apparent lack of temperature dependence could be an artefact due to neglecting the extreme heterogeneity of soil carbon16 , but no explicit model has yet been presented that can reconcile all the above findings. Here we present a simple three-pool model that partitions SOC into components with different intrinsic turnover rates. Using this model, we show that the results of all the soil-warming experiments are compatible with long-term temperature sensitivity of SOC turnover: they can be explained by rapid depletion of labile SOC combined with the negligible response of non-labile SOC on experimental timescales. Furthermore, we present evidence that non-labile SOC is more sensitive to temperature than labile SOC, implying that the long-term positive feedback of soil decomposition in a warming world may be even stronger than predicted by global models1, 17, 18, 19, 20 .

Kögel-Knabner , I., Ekschmitt, K., Flessa, H., Guggenberger, G., Matzner, E., Marschner, B., von Lützow , M. (2008). An integrative approach of organic matter stabilization in temperate soils: Linking chemistry, physics, and biology. Journal of Plant Nutrition and Soil Science 171 (1): 5-13

ABSTRACT: With this topical issue, we present the work of the Priority Program 1090 of the German Research Foundation (Deutsche Forschungsgemeinschaft DFG): Soils as a source and sink for CO2 - mechanisms and regulation of organic matter stabilisation in soils. This introduction gives an overview on the sites investigated and the major research approaches, including a glossary of major terms used in the field of soil organic matter research. We point out the advantages of integration of data from a broad field of different soil-science disciplines and the progress achieved by application and combination of new analytical methods describing the quality and turnover of soil organic matter.

Komarov, A. S., Kubasova, T. S. (2007). Modeling organic matter dynamics in conifer-broadleaf forests in different site types upon fires: A computational experiment. Biology Bulletin 34 (4): 408-416

ABSTRACT: The effect of forest fires differing in intensity on organic matter dynamics in forest soils has been assessed in different types of forest sites using the EFIMOD system of models. Differences between the patterns of organic matter dynamics according to scenarios of forest ecosystem development under normal conditions and upon forest fires have been analyzed. Recovery rates of soil organic matter pools after fires depend on their intensity and frequency. The most profound changes take place upon high-intensity crown fires, which may even result in ecosystem destruction.

Lal, R. (2005). Forest soils and carbon sequestration. Forest Ecology and Management 220 (1-3): 242-258

ABSTRACT: Soils in equilibrium with a natural forest ecosystem have high carbon (C) density. The ratio of soil:vegetation C density increases with latitude. Land use change, particularly conversion to agricultural ecosystems, depletes the soil C stock. Thus, degraded agricultural soils have lower soil organic carbon (SOC) stock than their potential capacity. Consequently, afforestation of agricultural soils and management of forest plantations can enhance SOC stock through C sequestration. The rate of SOC sequestration, and the magnitude and quality of soil C stock depend on the complex interaction between climate, soils, tree species and management, and chemical composition of the litter as determined by the dominant tree species. Increasing production of forest biomass per se may not necessarily increase the SOC stocks. Fire, natural or managed, is an important perturbation that can affect soil C stock for a long period after the event. The soil C stock can be greatly enhanced by a careful site preparation, adequate soil drainage, growing species with a high NPP, applying N and micronutrients (Fe) as fertilizers or biosolids, and conserving soil and water resources. Climate change may also stimulate forest growth by enhancing availability of mineral N and through the CO2 fertilization effect, which may partly compensate release of soil C in response to warming. There are significant advances in measurement of soil C stock and fluxes, and scaling of C stock from pedon/plot scale to regional and national scales. Soil C sequestration in boreal and temperate forests may be an important strategy to ameliorate changes in atmospheric chemistry.

R. Lal (2007). Carbon management in agricultural soils. Mitigation and Adaptation Strategies for Global Change 12 (2): 303-322

ABSTRACT: World soils have been a major source of enrichment of atmospheric concentration of CO2 ever since the dawn of settled agriculture, about 10,000 years ago. Historic emission of soil C is estimated at 78 ± 12 Pg out of the total terrestrial emission of 136 ± 55 Pg, and post-industrial fossil fuel emission of 270 ± 30 Pg. Most soils in agricultural ecosystems have lost 50 to 75% of their antecedent soil C pool, with the magnitude of loss ranging from 30 to 60 Mg C/ha. The depletion of soil organic carbon (SOC) pool is exacerbated by soil drainage, plowing, removal of crop residue, biomass burning, subsistence or low-input agriculture, and soil degradation by erosion and other processes. The magnitude of soil C depletion is high in coarse-textured soils (e.g., sandy texture, excessive internal drainage, low activity clays and poor aggregation), prone to soil erosion and other degradative processes. Thus, most agricultural soils contain soil C pool below their ecological potential. Adoption of recommend management practices (e.g., no-till farming with crop residue mulch, incorporation of forages in the rotation cycle, maintaining a positive nutrient balance, use of manure and other biosolids), conversion of agriculturally marginal soils to a perennial land use, and restoration of degraded soils and wetlands can enhance the SOC pool. Cultivation of peatlands and harvesting of peatland moss must be strongly discouraged, and restoration of degraded soils and ecosystems encouraged especially in developing countries. The rate of SOC sequestration is 300 to 500 Kg C/ha/yr under intensive agricultural practices, and 0.8 to 1.0 Mg/ha/yr through restoration of wetlands. In soils with severe depletion of SOC pool, the rate of SOC sequestration with adoption of restorative measures which add a considerable amount of biomass to the soil, and irrigated farming may be 1.0 to 1.5 Mg/ha/yr. Principal mechanisms of soil C sequestration include aggregation, high humification rate of biosolids applied to soil, deep transfer into the sub-soil horizons, formation of secondary carbonates and leaching of bicarbonates into the ground water. The rate of formation of secondary carbonates may be 10 to 15 Kg/ha/yr, and the rate of leaching of bicarbonates with good quality irrigation water may be 0.25 to 1.0 Mg C/ha/yr. The global potential of soil C sequestration is 0.6 to 1.2 Pg C/yr which can off-set about 15% of the fossil fuel emissions.

Lal, R. (2007). Soil carbon stocks under present and future climate with specific reference to European ecoregions. Nutrient Cycling in Agroecosystems 81 (2): 113-127

ABSTRACT; World soils and terrestrial ecosystems have been a source of atmospheric abundance of CO2 ever since settled agriculture began about 10–13 millennia ago. The amount of CO2 -C emitted into the atmosphere is estimated at 136 ± 55 Pg from terrestrial ecosystems, of which emission from world soils is estimated at 78 ± 12 Pg. Conversion of natural to agricultural ecosystems decreases soil organic carbon (SOC) pool by 30–50% over 50–100 years in temperate regions, and 50–75% over 20–50 years in tropical climates. The projected global warming, with estimated increase in mean annual temperature of 4–6°C by 2100, may have a profound impact on the total soil C pool and its dynamics. The SOC pool may increase due to increase in biomass production and accretion into the soil due to the so-called “CO2 fertilization effect”, which may also enhance production of the root biomass. Increase in weathering of silicates due to increase in temperature, and that of the formation of secondary carbonates due to increase in partial pressure of CO2 in soil air may also increase the total C pool. In contrast, however, SOC pool may decrease because of: (i) increase in rate of respiration and mineralization, (ii) increase in losses by soil erosion, and (iii) decrease in protective effects of stable aggregates which encapsulate organic matter. Furthermore, the relative increase in temperature projected to be more in arctic and boreal regions, will render Cryosols under permafrost from a net sink to a net source of CO2 if and when permafrost thaws. Thus, SOC pool of world soils may decrease with increase in mean global temperature. In contrast, the biotic pool may increase primarily because of the CO2 fertilization effect. The magnitude of CO2 fertilization effect may be constrained by lack of essential nutrients (e.g., N, P) and water. The potential of SOC sequestration in agricultural soils of Europe is 70–190 Tg C yr−1 . This potential is realizable through adoption of recommended land use and management, and restoration of degraded soils and ecosystems including wetlands.

Lal, R., Kimble, J. M. (1997). Conservation tillage for carbon sequestration. Nutrient Cycling in Agroecosystems 49 (1-3): 243-253.

ABSTRACT: World soils represent the largest terrestrial pool of organic carbon (C), about 1550 Pg compared with about 700 Pg in the atmosphere and 600 Pg in land biota. Agricultural activities (e.g., deforestation, burning, plowing, intensive grazing) contribute considerably to the atmospheric pool. Expansion of agriculture may have contributed substantially to the atmospheric carbon pool. However, the exact magnitude of carbon fluxes from soil to the atmosphere and from land biota to the soil are not known. An important objective of the sustainable management of soil resources is to increase soil organic carbon (SOC) pool by increasing passive or non-labile fraction. Soil surface management, soil water conservation and management, and soil fertility regulation are all important aspects of carbon sequestration in soil. Conservation tillage, a generic term implying all tillage methods that reduce runoff and soil erosion in comparison with plow-based tillage, is known to increase SOC content of the surface layer. Principal mechanisms of carbon sequestration with conservation tillage are increase in micro-aggregation and deep placement of SOC in the sub-soil horizons. Other useful agricultural practices associated with conservation tillage are those that increase biomass production (e.g., soil fertility enhancement, improved crops and species, cover crops and fallowing, improved pastures and deep-rooted crops). It is also relevant to adopt soil and crop management systems that accentuate humification and increase the passive fraction of SOC. Because of the importance of C sequestration, soil quality should be evaluated in terms of its SOC content.

Lawrence, D., Slater, A. (2008). Incorporating organic soil into a global climate model. Climate Dynamics 30 (2): 145-160

ABSTRACT: Organic matter significantly alters a soil’s thermal and hydraulic properties but is not typically included in land-surface schemes used in global climate models. This omission has consequences for ground thermal and moisture regimes, particularly in the high-latitudes where soil carbon content is generally high. Global soil carbon data is used to build a geographically distributed, profiled soil carbon density dataset for the Community Land Model (CLM). CLM parameterizations for soil thermal and hydraulic properties are modified to accommodate both mineral and organic soil matter. Offline simulations including organic soil are characterized by cooler annual mean soil temperatures (up to ~2.5°C cooler for regions of high soil carbon content). Cooling is strong in summer due to modulation of early and mid-summer soil heat flux. Winter temperatures are slightly warmer as organic soils do not cool as efficiently during fall and winter. High porosity and hydraulic conductivity of organic soil leads to a wetter soil column but with comparatively low surface layer saturation levels and correspondingly low soil evaporation. When CLM is coupled to the Community Atmosphere Model, the reduced latent heat flux drives deeper boundary layers, associated reductions in low cloud fraction, and warmer summer air temperatures in the Arctic. Lastly, the insulative properties of organic soil reduce interannual soil temperature variability, but only marginally. This result suggests that, although the mean soil temperature cooling will delay the simulated date at which frozen soil begins to thaw, organic matter may provide only limited insulation from surface warming.

Leduc, S. D., Rothstein, D. E. (2007). Initial recovery of soil carbon and nitrogen pools and dynamics following disturbance in jack pine forests: A comparison of wildfire and clearcut harvesting. Soil Biology and Biochemistry 39 (11): 2865-2876

ABSTRACT: Forests naturally maintained by stand-replacing wildfires are often managed with clearcut harvesting, yet we know little about how replacing wildfire with clearcutting affects soil processes and properties. We compared the initial recovery of carbon (C) and nitrogen (N) pools and dynamics following disturbance in jack pine (Pinus banksiana ) stands in northern Lower Michigan, USA, by sampling soils (Oa+A horizons) from three “treatments”: 3–6-year-old harvest-regenerated stands, 3–6-year-old wildfire-regenerated stands and 40–55-year-old intact, mature stands (n=4 stands per treatment). We measured total C and N; microbial biomass and potentially mineralizable C and N; net nitrification; and gross rates of N mineralization and nitrification. Burned stands exhibited reduced soil N but not C, whereas clearcut and mature stands had similar quantities of soil organic matter. Both disturbance types reduced microbial biomass C compared to mature stands; however, microbial biomass N was reduced in burned stands but not in clearcut stands. The experimental C and N mineralization values were fit to a first-order rate equation to estimate potentially mineralizable pool size (C0 and N0 ) and rate parameters. Values for C0 in burned and clearcut stands were approximately half that of the mature treatment, with no difference between disturbance types. In contrast, N0 was lowest in the wildfire stands (170.2μg N g−1 ), intermediate in the clearcuts (215.4μg N g−1 ) and highest in the mature stands (244.6μg N g−1 ). The most pronounced difference between disturbance types was for net nitrification. These data were fit to a sigmoidal growth equation to estimate potential NO3 accumulation (Nitmax) and kinetic parameters. Values of Nitmax in clearcut soils exceeded that of wildfire and mature soils (149.2 vs. 83.5 vs. 96.5μg NO3 –N g−1 , respectively). Moreover, the clearcut treatment exhibited no lag period for net NO3 production, whereas the burned and mature treatments exhibited an approximate 8-week lag period before producing appreciable quantities of NO3 . There were no differences between disturbances in gross rates of mineralization or nitrification; rather, lower NO3 immobilization rates in the clearcut soils, 0.20μg NO3 g−1 d−1 compared to 0.65 in the burned soils, explained the difference in net nitrification. Because the mobility of NO3 and NH4 + differs markedly in soil, our results suggest that differences in nitrification between wildfire and clearcutting could have important consequences for plant nutrition and leaching losses following disturbance.

Lehmann, J., Gaunt, J., Rondon, M. (2006). Bio-char sequestration in terrestrial ecosystems - A review. Mitigation and Adaptation Strategies for Global Change 11 (2): 395-419

ABSTRACT: The application of bio-char (charcoal or biomass-derived black carbon (C)) to soil is proposed as a novel approach to establish a significant, long-term, sink for atmospheric carbon dioxide in terrestrial ecosystems. Apart from positive effects in both reducing emissions and increasing the sequestration of greenhouse gases, the production of bio-char and its application to soil will deliver immediate benefits through improved soil fertility and increased crop production. Conversion of biomass C to bio-char C leads to sequestration of about 50% of the initial C compared to the low amounts retained after burning (3%) and biological decomposition (< 10–20% after 5–10 years), therefore yielding more stable soil C than burning or direct land application of biomass. This efficiency of C conversion of biomass to bio-char is highly dependent on the type of feedstock, but is not significantly affected by the pyrolysis temperature (within 350–500 ∘C common for pyrolysis). Existing slash-and-burn systems cause significant degradation of soil and release of greenhouse gases and opportunies may exist to enhance this system by conversion to slash-and-char systems. Our global analysis revealed that up to 12% of the total anthropogenic C emissions by land use change (0.21 Pg C) can be off-set annually in soil, if slash-and-burn is replaced by slash-and-char. Agricultural and forestry wastes such as forest residues, mill residues, field crop residues, or urban wastes add a conservatively estimated 0.16 Pg C yr−1 . Biofuel production using modern biomass can produce a bio-char by-product through pyrolysis which results in 30.6 kg C sequestration for each GJ of energy produced. Using published projections of the use of renewable fuels in the year 2100, bio-char sequestration could amount to 5.5–9.5 Pg C yr−1 if this demand for energy was met through pyrolysis, which would exceed current emissions from fossil fuels (5.4 Pg C yr−1 ). Bio-char soil management systems can deliver tradable C emissions reduction, and C sequestered is easily accountable, and verifiable.

Leifeld, J. (2006). Soils as sources and sinks of greenhouse gases. : 23-44

ABSTRACT: Soils annually emit between 6.8 and 7.9 Gt CO2 equivalents, mainly as CH4 from intact peatlands and from rice agriculture; as N2 O from unmanaged and managed soils; and as CO2 from land-use change. Methane emissions attributable to other wetlands add another 1.6–3.8 Gt CO2 equivalents. From a global standpoint, N2 O from unmanaged soils and CH4 from peatlands and other wetlands make soils naturally net greenhouse gas emitters. In addition, the storage of carbon in soils and the fluxes of CH4 and N2 O have been changed by anthropogenic effects towards emission rates 52 to 72% above those under natural conditions before the dawn of intensive agriculture and land-use change. Land-use changes on mineral soils induced most of the recorded losses of soil organic matter (SOM), but there is evidence that proper agricultural management of soil resources is able to recover some of these losses and to maintain soil functions. However, the discrepancy between so-called ‘sequestration potentials’ and the measures already adopted is amazingly large. Globally, only about 5% of the cropped areas is managed according to practices such as no tillage or organic farming. The contribution of soil loss by erosion, desertification and sealing to global oxidative SOM losses is uncertain; however, in the case of soil erosion, it is considered to be a major factor in global SOM decline. Mitigation options calculated for SOM restoration, reduced CH4 and N2O emissions are able to alleviate mean annual emissions by 1.2 to 2.9 Gt CO2 equivalents, mainly as a result of carbon sequestration, which is the most efficient measure for the next few decades. In the longer term, however, the large potential for reducing CH4 and N2 O emissions outweigh the finite capacity of soils to recover C. Integrated assessment of net greenhouse-gas fluxes is key for evaluating management practices aimed at reducing overall emissions. From the viewpoint of climate change and taking into consideration the mean fluxes of CO2 , CH4 and N2 O, peatland protection is more favourable than peatland cultivation in the long term. The most important gaps in our understanding appear to be with regard to estimating fluxes along with soil erosion and desertification processes, in the extent of peatland cultivation; the role of black carbon formation, natural ‘background’ sequestration rates of undisturbed soils; and the net response of soils, particularly in cold regions, to global warming. With regard to the societal perception of soil contributing to the global cycling of greenhouse gases, it is important to emphasize that significant proportions of the emissions are inevitably linked to intensive agriculture.

Leifeld, Jens, Fuhrer, Juerg (2005). The temperature response of CO2 production from bulk soils and soil fractions is related to soil organic matter quality. Biogeochemistry 75 (3): 433-453

ABSTRACT: The projected increase in global mean temperature could accelerate the turnover of soil organic matter (SOM). Enhanced soil CO2 emissions could feedback on the climate system, depending on the balance between the sensitivity to temperature of net carbon fixation by vegetation and SOM decomposition. Most of the SOM is stabilised by several physico-chemical mechanisms within the soil architecture, but the response of this quantitatively important fraction to increasing temperature is largely unknown. The aim of this study was to relate the temperature sensitivity of decomposition of physical and chemical soil fractions (size fractions, hydrolysis residues), and of bulk soil, to their quality and turnover time. Soil samples were taken from arable and grassland soils from the Swiss Central Plateau, and CO2 production was measured under strictly controlled conditions at 5, 15, 25, and 35 °C by using sequential incubation. Physico-chemical properties of the samples were characterised by measuring elemental composition, surface area,14 C age, and by using DRIFT spectroscopy. CO2 production rates per unit (g) organic carbon (OC) strongly varied between samples, in relation to the difference in the biochemical quality of the substrates. The temperature response of all samples was exponential up to 25 °C, with the largest variability at lower temperatures. Q10 values were negatively related to CO2 production over the whole temperature range, indicating higher temperature sensitivity of SOM of lower quality. In particular, hydrolysis residues, representing a more stabilised SOM pool containing older C, produced less CO2 g−1 OC than non-hydrolysed fractions or bulk samples at lower temperatures, but similar rates at ≥25 °C, leading to higher Q10 values than in other samples. Based on these results and provided that they apply also to other soils it is suggested that because of the higher sensitivity of passive SOM the overall response of SOM to increasing temperatures might be higher than previously expected from SOM models. Finally, surface area measurements revealed that micro-aggregation rather than organo-mineral association mainly contributes to the longer turnover time of SOM isolated by acid hydrolysis.

Lemma, B., Kleja, D. B.n, Olsson, M., Nilsson, I. (2007). Factors controlling soil organic carbon sequestration under exotic tree plantations: A case study using the CO2Fix model in southwestern Ethiopia. Forest Ecology and Management 252 (1-3): 124-131

ABSTRACT: Models are important research tools for predicting the build-up of soil organic carbon (SOC), because they provide an increased insight into factors that are involved in the build-up process. The CO2Fix (v. 3.1) model was used to examine the influence of litter production, litter quality and microclimate on differences in SOC accretion under exotic tree species established on farmland in southwestern Ethiopia. The SOC storage underCupressus lusitanica was larger than that under the other two investigated species (Pinus patula andEucalyptus grandis ). This was mainly because of the higher total litter input and higher proportion of fine woody litter (branches and coarse roots) in theCupressus stand. SOC accretion was greater underPinus than underEucalyptus . However, the total litter input in thePinus andEucalyptus stands was nearly the same. The difference between thePinus andEucalyptus stands was best explained by the fact thatPinus produced more fine woody litter than didEucalyptus . Litter quality and microclimate only accounted for a minor part of the differences in SOC storage in theCupressus ,Pinus andEucalyptus stands. Therefore, the results suggested that total litter input and the proportion of fine woody litter were the main factors that accounted for the inter-specific differences in SOC accretion.

Liski, J., Ivelsniemi, H., Makela, A., Starr, M. (1998). Model analysis of the efects of soil age, fires and harvesting on the carbon storage of boreal forest soils. European Journal of the Soil Science 49: 407-416

ABSTRACT: Potential causes for changes in the amounts of carbon (C) stored in the soils of boreal forests were studied by measuring the C in the soil along a 5000-year chronosequence in coastal western Finland and using a simple dynamic model of decomposition. The amount of soil C stabilized at an age of about 2000 years. This suggests that the youth of many boreal soils does not make them sinks for atmospheric C. Simulated repeated fires kept the amount of soil C reduced by about 25%, but if fires were prevented then the C in the soil increased. Stored C may thus be less than the potential storage where fires are frequent, and it could be increased by preventing fires. Simulated clear-cutting caused a temporary 5–10% decrease in the amount of soil C over a 20-year period after the harvesting. It also caused a long-term decrease in the amount of soil C such that, after two 100-year rotations, the amount had been decreased by 14%. Stored C is almost certainly less than the potential storage and decreasing where forests are harvested.

Liski, J., Westman, C. J. (1997). Carbon storage in forest soil of Finland: 2. Size and regional patterns. Biogeochemistry 36 (3): 261-274

ABSTRACT: For confidently estimating the amount of carbon stored in boreal forest soil, better knowledge of smaller regions is needed. In order to estimate the amount of soil C in forests on mineral soil in Finland, i.e. excluding peatland forests, and illustrate the regional patterns of the storage, statistical models were first made for the C densities of the organic and 0–1 m mineral soil layers. A forest type, which indicated site productivity, and the effective temperature sum were used as explanatory variables of the models. In addition, a constant C density was applied for the soil layer below the depth of 1 m on sorted sediments. Using these models the C densities were calculated for a total of 46673 sites of the National Forest Inventory (NFI). The amount of the soil C was then calculated in two ways: 1) weighting the C densities of the NFI sites by the land area represented by these sites and 2) interpolating the C densities of the NFI sites for 4 ha blocks to cover the whole land area of Finland and summing up the blocks on forested mineral soil. The soil C storage totalled 1109 Tg and 1315 Tg, when calculated by the areal weighting and the interpolated blocks, respectively. Of that storage, 28% was in the organic layer, 68% in the 0–1 m mineral soil layer and 4% in the layer below 1 m. The total soil C equals more than two times the amount of C in tree biomass and 20% of the amount of C in peat in Finland. Soil C maps made using the interpolated blocks indicated that the largest soil C reserves are located in central parts of southern Finland. The C storage of the organic layer was assessed to be overestimated at largest by 13% and that of the 0–1 m mineral soil layer by 29%. The largest error in the organic layer estimate is associated with the effects of forest harvesting and in the mineral soil estimate with the stone content of the soil.

Liski, J., Westman, C. J. (1997). Carbon storage in forest soil of Finland: 1. Effect of thermoclimate. Biogeochemistry 36 (3): 239-260

ABSTRACT: A total of 30 coniferous forest sites representing two productivity classes, forest types, were investigated on a temperature gradient (effective temperature sum using +5°C threshold 800–1300 degree-days and annual mean temperature –0.6–+3.9°C) in Finland for studying the effect of thermoclimate on the soil C storage. Other soil forming factors were standardized within the forest types so that the variation in the soil C density could be related to temperature. According to the applied regression model, the C density of the 0–1 m mineral soil layer increased 0.266 kg m–2 for every 100 degree-day increase in the temperature sum, and the layer contained 57% and 28% more C under the warmest conditions of the gradient compared to the coolest in the less and more productive forest type, respectively. Accordingly, this soil layer was estimated to contain 23 more C in a new equilibrium with a 4°C higher annual mean temperature in Finland. The C density of the organic layer was not associated with temperature. Both soil layers contained more C at the sites of the more productive forest type, and the forest type explained 36% and 70% of the variation in the C density of the organic and 0–1 m layers, respectively. Within the forest types, the temperature sum accounted for 33–41% of the variation in the 0–1 m layer. These results suggest that site productivity is a cause for the large variation inthe soil C density within the boreal zone, and relating the soil C density to site productivity and temperature would help to estimate the soil C reserves more accurately in the boreal zone.

Loya, W. M., Johnson, L. C., Kling, G. W., King, J. Y., Reeburgh, W. S., Nadelhoffer, K. J. (2002). Pulse-labeling studies of carbon cycling in arctic tundra ecosystems: Contribution of photosynthates to soil organic matter. Global Biogeochemical Cycles 16 (4): 1101

ABSTRACT: To increase our understanding of carbon (C) cycling and storage in soils, we used14 C to trace C from roots into four soil organic matter (SOM) fractions and the movement of soil microbes in arctic wet sedge and tussock tundra. For both tundra types, the proportion of14 C activity in the soil was 6% of the total14 C-CO2 taken up by plants at each of the four harvests conducted 1, 7, 21, and 68 days after labeling. In tussock tundra, we observed rapid microbial transformation of labile C from root exudates into more stable SOM. In wet sedge tundra, there appears to be delayed or indirect microbial use of root exudates. The net amount of14 C label transferred to SOM by the end of the season in both tundra types was approximately equal to the amount transferred to soils 1 day after labeling, suggesting that transfer of14 C tracer from roots to soils continued through the growing season. Overall, C inputs from living roots contributes 24 g C m−2 yr−1 in tussock tundra and 8.8 g C m−2 yr−1 in wet sedge tundra. These results suggest rapid belowground allocation of C by plants and subsequent incorporation of much of this C into storage in the SOM.

Luca, C., Si, B.C., Farrell, R.E. (2007). Upslope length improves spatial estimation of soil organic carbon content. Canadian Journal of Soil Science 87 (3): 291-300

ABSTRACT: Quantifying soil organic carbon (SOC) is important to aide in assessing carbon (C) sequestration potential, and as an indicator of soil quality. However, intensive s ampling of SOC for quantification can be expensive and time consuming. The objectives of this study were to identify which topographic index correlated best with SOC and determine if incorporating the index improved interpolation of limited SOC data. A transect with 93 sample points spaced 6 m apart was set up, and four topographical indices (curvature, wetness index, upslope length, and elevation) were evaluated for their potential as secondary variables. Three Kriging-based interpolation methods, ordinary kriging, cokriging, and simple kriging with varying local means were compared to determine if incorporating topographical indices improved interpolation of SOC. The upslope length, which takes into consideration the quantity of water that will be redistributed to a point, was found to have the strongest relationship with SOC (R2 = 0.48, P < 0.01) and was used as a secondary variable for kriging. Thirty points from the SOC data were randomly selected and used in the kriging algorithms to estimate the remain ing 63 points. The sum of squared differences (SSD) showed a significant reduction (from 1677 to 1455 for SKlm and from 1677 to 1464 for cokriging) in estimates when upslope length was used as a secondary variable. These results indicate that fewer samples may be taken to estimate SOC accurately and precisely if upslope length is incorporated. On a landscape scale this could facilitate quantification of carbon credits and management decisions in precision farming systems.

Luo, Y. Q., Wan, S. Q., Hui, D. F., Wallace, L. L. (2001). Acclimatization of soil respiration to warming in a tall grass prairie. Nature 413 (6856): 622-625

ABSTRACT: The latest report by the Intergovernmental Panel on Climate Change (IPCC) predicts a 1.4–5.8 °C average increase in the global surface temperature over the period 1990 to 2100 (ref. 1). These estimates of future warming are greater than earlier projections, which is partly due to incorporation of a positive feedback. This feedback results from further release of greenhouse gases from terrestrial ecosystems in response to climatic warming2, 3, 4 . The feedback mechanism is usually based on the assumption that observed sensitivity of soil respiration to temperature under current climate conditions would hold in a warmer climate5 . However, this assumption has not been carefully examined. We have therefore conducted an experiment in a tall grass prairie ecosystem in the US Great Plains to study the response of soil respiration (the sum of root and heterotrophic respiration) to artificial warming of about 2 °C. Our observations indicate that the temperature sensitivity of soil respiration decreases—or acclimatizes—under warming and that the acclimatization is greater at high temperatures. This acclimatization of soil respiration to warming may therefore weaken the positive feedback between the terrestrial carbon cycle and climate.

Manson, A.D., Jewitt, D., Short, A.D. (2007). Effects of season and frequency of burning on soils and landscape functioning in a moist montane grassland. African Journal of Range and Forage Science 24 (1): 9-18

ABSTRACT: The effects of burning on soil properties and landscape function were investigated in a long-term experiment comparing different burning strategies in a moist montane grassland. Total C, total N, total S, bulk density, plant-available nutrients, and soil acidity were determined in the top 200mm of soil, together with vegetation basal cover at the soil surface. The no-burn treatment had the lowest basal cover (14.8%). Basal cover for the burnt treatments ranged from 19.0% (five-year spring burn) to 25.4% (alternate autumn/spring, burnt every 18 months). The organic matter content of these soils was very high with total carbon ranging from 114g kg−1 in the 0-50mm layer to 77g kg−1 in the 150-200mm layer. Bulk density was very low, being 0.57g ml−1 in the 0-50mm layer. There were no significant effects of burning on the quantity of soil organic matter. The C:N ratio was significantly affected throughout the top 200mm by burning treatments; in the 0-50mm layer it ranged from 14.43 in the no-burn treatment to 16.14 in the treatment burnt every 18 months. Higher C:N ratios in frequently burnt treatments suggests that grassland productivity is N-limited in these treatments. In the top 50mm, soil pH is lower in treatments burnt infrequently (5 year and no burn) than in those burnt frequently, whereas concentrations of basic exchangeable cations (K, Ca and Mg) were lower in treatments burnt infrequently (five-year and no burn) than in those burnt frequently. The higher pH and concentrations of basic cations in frequently burnt treatments was probably due to greater cycling of nutrients to the soil surface as a result of higher productivity and deposition of nutrients in ash, together with reduced leaching of cations with nitrate. Landscape Function Analysis was used to measure the functioning of the landscape in terms of scarce resources and the processes that maintain these resources. All sites were highly functional, irrespective of the burning treatment applied. The infrequently burned sites had significantly higher nutrient cycling and infiltration indices than frequently burnt sites and these indices were correlated well with soil chemical properties (acidity, acid saturation, Ca, Cu, K, Mg, P and pH). No significant differences were found between treatments for the stability index.

McGuire, A.D., Melillo, J.M., Kicklighter, D.W., Joyce, L. A. (1995). Equilibrium responses of soil carbon to climate change: empirical and process-based estimates. Journal of Biogeography 22 (4/5): 785-796

ABSTRACT: We use a new version of the Terrestrial Ecosystem Model (TEM), which has been parameterized to control for reactive soil organic carbon (SOC) across climatic gradients, to evaluate the sensitivity of SOC to a 1°C warming in both empirical and process-based analyses. In the empirical analyses we use the steady state SOC estimates of TEM to derive SOC-response equations that depend on temperature and volumetric soil moisture, and extrapolate them across the terrestrial biosphere at 0.5° spatial resolution. For contemporary climate and atmospheric CO2 , mean annual temperature explains 34.8% of the variance in the natural logarithm of TEM-estimated SOC. Because the inclusion of mean annual volumetric soil moisture in the regression explains an additional 19.6%, a soil mosture term in an equation of SOC response should improve estimates. For a 1°C warming, the globally derived empirical model estimates a terrestrial SOC loss of 22.6 x 1015 g (Pg), with 77.9% of the loss in extra-tropical ecosystems. To explore whether loss estimates SOC are affected by the spatial scale at which the response equations are derived equations for each of the eighteen ecosystems considered in this study. The sensitivity of terrestrial SOC estimated by summing the losses predicted by each of the ecosystem empirical models is greater (27.9 Pg per °C) than that estimated by the global empirical model; the 12.2 Pg loss (43.7%) in tropical ecosystems suggests that they may be more sensitive to warming. The global process-based loss of SOC estimated by TEM in response to a 1°C warming (26.3 Pg) is similar to the sum of the ecosystem empirical losses, but the 13.6 Pg loss (51.7%) in extra-tropical ecosystems suggests that they may be slightly less sensitive to warming. For the modelling of SOC responses, these results suggest that soil moisture is useful to incorporate in empirical models of SOC response and that globally derived empirical models may conceal regional sensitivity of SOC to warming. The analyses in this study suggest that the maximum loss of SOC to the atmosphere per °C warming is less than 2% of the terrestrial soil carbon inventory. Because the NPP response to elevated CO2 has the potential to compensate for this loss, the scenario of warming enhancing soil carbon loss to further enhance warming is unlikely in the absence of land use or changes in vegetation distribution.

Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T., Morrisseau, S. (2002). Soil warming and carbon-cycle feedbacks to the climate system. Science 298 (5601): 2173-2176

ABSTRACT: In a decade-long soil warming experiment in a mid-latitude hardwood forest, we documented changes in soil carbon and nitrogen cycling in order to investigate the consequences of these changes for the climate system. Here we show that whereas soil warming accelerates soil organic matter decay and carbon dioxide fluxes to the atmosphere, this response is small and short-lived for a mid-latitude forest, because of the limited size of the labile soil carbon pool. We also show that warming increases the availability of mineral nitrogen to plants. Because plant growth in many mid-latitude forests is nitrogen-limited, warming has the potential to indirectly stimulate enough carbon storage in plants to at least compensate for the carbon losses from soils. Our results challenge assumptions made in some climate models that lead to projections of large long-term releases of soil carbon in response to warming of forest ecosystems.

Michel, K., Matzner, E. (2002). Nitrogen content of forest floor Oa layers affects carbon pathways and nitrogen mineralization. Soil Biology and Biochemistry 34 (11): 1807-1813

ABSTRACT: As a consequence of chronically high N depositions in forest ecosystems, the C-to-N ratio of forest floors has narrowed in many forest ecosystems. This might affect the sequestration of soil C and the partitioning of C during decomposition. We investigated samples from Oa layers of 15 different forest floors under Norway spruce (Picea abies Karst. L.) with varying C-to-N ratios in respect to soil respiration, N mineralization and dissolved organic carbon (DOC) release under standardized laboratory conditions. Samples were incubated aerobically at 15 °C and water holding capacity over a period of 10 months. Soil respiration decreased significantly with decreasing C-to-N and increasing N content. The release of DOC increased with increasing C-to-N ratio, while N-mineralization was not affected by C-to-N ratio and N content. Our results support the hypothesis that low C-to-N ratios in later stages of decomposition stabilize soil organic matter and that chronically high N deposition will lead to increased accumulation of C in forest floors.

Nakane, K., Lee, N.-J. (1995). Simulation of soil carbon cycling and carbon balance following clear-cutting in a mid-temperate forest and contribution to the sink of atmospheric CO2 . Vegetatio 121 (1-2): 147-156

ABSTRACT: A simulation model of soil carbon cycling was developed based on the data observed in a mid-temperate forest in Yoshiwa, Hiroshima Prefecture, Japan, and soil carbon cycling and carbon budget in a mature forest stand and following clear-cutting were calculated on a daily basis using daily air temperature and precipitation data. The seasonal change in the amount of the A0 layer was characterized by a decrease from spring to autumn due to rapid decomposition of litter, and recovery in late autumn due to a large litterfall input. There was little change in the amount of humus in mineral soil. These estimates coincides closely with those observed in the field. Most flow rates and the accumulation of soil carbon decreased very markedly just after clear-cutting. The A0 layer reached its minimum in 10 years, and recovered its loss within 50–60 years after cutting. A large loss of carbon was observed just after cutting, but the balance changed from negative to positive in 15 years after cutting. The total loss of soil carbon following cutting recovered within 30 years, and nearly the same amount of carbon as that stocked in the timber before harvesting accumulated 70–80 years after cutting. The calculation by the simulation model was made using the assumption that the increase in atmospheric CO2 promoted the primary production rate by 10% over the last three decades. The result suggests that about 8 t C ha-1 was sunk into soils of the mid-temperate forest over the same period. It indicates that forest soils may be one of the main sinks for atmospheric CO2 .

Niklaus, P.A., Falloon, P. (2006). Estimating soil carbon sequestration under elevated CO2 by combining carbon isotope labelling with soil carbon cycle modelling. Global Change Biology 12 (10): 1909-1921

ABSTRACT: Elevated CO2 concentrations generally stimulate grassland productivity, but herbaceous plants have only a limited capacity to sequester extra carbon (C) in biomass. However, increased primary productivity under elevated CO2 could result in increased transfer of C into soils where it could be stored for prolonged periods and exercise a negative feedback on the rise in atmospheric CO2 .

Measuring soil C sequestration directly is notoriously difficult for a number of methodological reasons. Here, we present a method that combines C isotope labelling with soil C cycle modelling to partition net soil sequestration into changes in new C fixed over the experimental duration (Cnew ) and pre-experimental C (Cold ). This partitioning is advantageous because the Cnew accumulates whereas Cold is lost in the course of time (ΔCnew >0 whereas ΔCold <0).We applied this method to calcareous grassland exposed to 600 μLCO2 L−1 for 6 years. The CO2 used for atmospheric enrichment was depleted in13 C relative to the background atmosphere, and this distinct isotopic signature was used to quantify net soil Cnew fluxes under elevated CO2 . Using13 C/12 C mass balance and inverse modelling, the Rothamsted model 'RothC' predicted gross soil Cnew inputs under elevated CO2 and the decomposition of Cold . The modelled soil C pools and fluxes were in good agreement with experimental data. C isotope data indicated a net sequestration of ≈90 g Cnew m−2 yr−1 in elevated CO2 . Accounting for Cold -losses, this figure was reduced to ≈30 g C m−2 yr−1 at elevated CO2 ; the elevated CO2 -effect on net C sequestration was in the range of ≈10 g C m−2 yr−1 . A sensitivity and error analysis suggests that the modelled data are relatively robust. However, elevated CO2 -specific mechanisms may necessitate a separate parameterization at ambient and elevated CO2 ; these include increased soil moisture due to reduced leaf conductance, soil disaggregation as a consequence of increased soil moisture, and priming effects. These effects could accelerate decomposition of Cold in elevated CO2 so that the CO2 enrichment effect may be zero or even negative. Overall, our findings suggest that the C sequestration potential of this grassland under elevated CO2 is rather limited.

Oelbermann, M., English, M., Schiff, S. L. (2008). Evaluating carbon dynamics and microbial activity in arctic soils under warmer temperatures. Canadian Journal of Soil Science 88 (1): 31-44

ABSTRACT: A large portion of carbon (C) is stored in the world’s soils, including those of peatlands, wetlands and permafrost. However, there is disagreement regarding the effects of climate change on the rate of organic matter decomposition in permafrost soils of the arctic. In this study it was hypothesized that soil exposed to a higher ambient temperature would have a greater flux of CO2 as well as a change in the metabolic diversity of culturable soil microorganisms. To evaluate this hypothesis we determined soil C dynamics, soil microbial respiration and activity, and13 C and15 N fractionation in laboratory incubations (at 14 and 21°C) for an organic-rich soil (Mesic Organic Cryosol) and a mineral soil (Turbic Cryosol) collected at the Daring Lake Research Station in Canada’s Northwest Territories. Soil organic C (SOC) and nitrogen (N) stocks (g m-2) and concentration (%) were significantly different (P < 0.05) between soil horizons for both soil types. Stable isotope analysis showed a significant enrichment ind13 C andd15 N with depth and a depletion ind13 C andd15 N with increasing SOC and N concentration. In laboratory incubations, microbial respiration showed three distinct phases of decomposition: a phase with a rapidly increasing rate of respiration (phase 1), a phase in which respiration reached a peak midway through the incubation (phase 2), and a phase in the latter part of the incubation in which respiration stabilized at a lower flux than that of the first phase (phase 3). Fluxes of CO2 were significantly greater at 21°C than at 14°C. Thed13C of the evolved CO2 became significantly enriched with time with the greatest enrichment occurring in phase 2 of the incubation. Soil microbial activity, as measured using Biolog EcoplatesTM, showed a significantly greater average well color development, richness, and Shannon index at 21°C; again the greatest change occurred in phase 2 of the incubation. Principal component analysis (PCA) of the Biolog data also showed a change in the distinct clustering of the soil microbial activity, showing that C sources from the soil were metabolized differently with time at 21 than at 14°C, and between soil horizons. Our results show that Canadian arctic soils contain large stores of C, which readily decompose, and that substantial increases in CO2 emissions and changes in the metabolic diversity of culturable soil microorganisms may occur when ambient temperatures increase from 14 to 21°C.

Ojima, D. S., Schimel, D. S., Parton, W. J., Owensby, C. E. (1994). Long- and short-term effects of fire on nitrogen cycling in tallgrass prairie. Biogeochemistry 24 (2): 67-84

ABSTRACT: Fires in the tallgrass prairie are frequent and significantly alter nutrient cycling processes. We evaluated the short-term changes in plant production and microbial activity due to fire and the long-term consequences of annual burning on soil organic matter (SOM), plant production, and nutrient cycling using a combination of field, laboratory, and modeling studies. In the short-term, fire in the tallgrass prairie enhances microbial activity, increases both above-and belowground plant production, and increases nitrogen use efficiency (NUE). However, repeated annual burning results in greater inputs of lower quality plant residues causing a significant reduction in soil organic N, lower microbial biomass, lower N availability, and higher C:N ratios in SOM. Changes in amount and quality of below-ground inputs increased N immobilization and resulted in no net increases in N availability with burning. This response occurred rapidly (e.g., within two years) and persisted during 50 years of annual burning. Plant production at a long-term burned site was not adversely affected due to shifts in plant NUE and carbon allocation. Modeling results indicate that the tallgrass ecosystem responds to the combined changes in plant resource allocation and NUE. No single factor dominates the impact of fire on tallgrass plant production.

Overby, S.T., Hart, S.C., Neary, D.G., Kimble, J.M., Birdsey, R.A., Lal, R., Heath, L.S. (2003). Impacts of natural disturbance on soil carbon dynamics in forest ecosystems. CRC Press: 159-172

DESCRIPTION: Forest soils are entities within themselves, self-organized and highly resilient over time. The transfer of energy bound in carbon (C) molecules drives the organization and functions of this biological system (Fisher and Binkley, 2000; Paul and Clark, 1996). Photosynthetic organisms reduce atmospheric C and store energy from solar radiation in the formation of complex C molecules. This bound energy is transferred to mineral soil in the form of litterfall, root turnover, and root exudates supporting an intricate detrital trophic structure (Fisher, 1995). Much of the C moving through this detrital food web is released annually back to the atmosphere as CO2 from respiration (see Chapter 7), but resident in the mineral soil is a large pool of C that is recalcitrant to decomposition.

Interest in the ability of forest soils to store atmospheric C derived from anthropogenic sources has grown in recent years (Johnson, 1992; Heath and Smith, 2000; Cardon et al., 2001; Johnson and Curtis, 2001). Prior to the 1920s, deforestation was the primary source of increasing atmospheric C, but has since been surpassed by fossil fuel combustion (Vitousek, 1991). Reduced harvests on National Forest lands and reforestation on abandoned agricultural lands since the 1950s have increased some terrestrial C pools in the United States (Houghton et. al. 1999), yet this increase may be at risk due to altered temporal and spatial scales of disturbances (Murray et al., 2000). The extent to which these altered disturbance events have already affected many of the forests within the United States is considerable (see Chapter 2). This paper examines the importance of natural disturbance in shaping forest landscapes and the relationship between aboveground impacts and mineral soil carbon dynamics.

Page-Dumroese, D. S, Jurgensen, M. F (2006). Soil carbon and nitrogen pools in mid- to late-successional forest stands of the northwestern United States: potential impact of fire. Canadian Journal of Forest Research 36 (9): 2270-2284

ABSTRACT: When sampling woody residue (WR) and organic matter (OM) present in forest floor, soil wood, and surface mineral soil (0–30 cm) in 14 mid- to late-successional stands across a wide variety of soil types and climatic regimes in the northwestern USA, we found that 44%–84% of carbon (C) was in WR and surface OM, whereas >80% of nitrogen (N) was in the mineral soil. In many northwestern forests fire suppression and natural changes in stand composition have increased the amounts of WR and soil OM susceptible to wildfire losses. Stands with high OM concentrations on the soil surface are at greater risk of losing large amounts of C and N after high-severity surface fires. Using the USDA Forest Service Regional Soil Quality Standards and Guidelines, we estimate that 6%–80% of the pooled C to a mineral-soil depth of 30 cm could be lost during a fire considered detrimental to soil productivity. These estimates will vary with local climatic regimes, fire severity across the burned area, the size and decay class of WR, and the distribution of OM in the surface organic and mineral soil. Estimated N losses due to fire were much lower (<1%–19%). Further studies on the amounts and distribution of OM in these stands are needed to assess wildfire risk, determine the impacts of different fire severities on WR and soil OM pools, and develop a link between C and N losses and stand productivity.

Pastor, J., Post, W.M. (1986). Influence of climate, soil moisture, and succession on forest carbon and nitrogen cycles. Biogeochemistry 2 (1): 3-27

ABSTRACT: The interactions between the biotic processes of reproduction, growth, and death and the abiotic processes which regulate temperature and water availability, and the interplay between the biotic and abiotic processes regulating N and light availabilities are important in the dynamics of forest ecosystems. We have developed a computer simulation that assembles a model ecosystem which links these biotic and abiotic interactions through equations that predict decomposition processes, actual evapo-transpiration, soil water balance, nutrient uptake, growth of trees, and light penetration through the canopy. The equations and parameters are derived directly from field studies and observations of forests in eastern North America, resulting in a model that can make accurate quantitative predictions of biomass accumulation, N availability, soil humus development and net primary production.

Post, J., Hattermann, F. F., Krysanova, V., Suckow, F. (2008). Parameter and input data uncertainty estimation for the assessment of long-term soil organic carbon dynamics. Environmental Modelling & Software 23 (2): 125-138

ABSTRACT: The use of integrated soil organic matter (SOM) models to assess SOM dynamics under climate change, land use change and different land management practices require a quantification of uncertainties and key sensitive factors related to the respective modelling framework. Most uncertainty studies hereby focus on model parameter uncertainty, neglecting other sources like input data derived uncertainties, and spatial and temporal properties of uncertainty. Sources of uncertainties assessed in this study stem from uncertainties in model parameterisation and from uncertainties in model input data (climate, soil data, and land management assumptions). Thereby, Monte Carlo based global sensitivity and uncertainty analysis using a latin hypercube stratified sampling technique was applied to derive plot scale (focusing on temporal propagation) and river basin scale propagation of uncertainty for long-term soil organic carbon (SOC) dynamics. The model used is the eco-hydrological river basin model SWIM (Soil and Water Integrated Model), which has been extended by a process-based multi-compartment model for SOM turnover. Results obtained by this study can be transferred and used in other simulation models of this kind. Uncertainties resulting from all input factors used (model parameters + model input data) show a coefficient of variation between 5.1 and 6.7% and accounted for ± 0.065 to ± 0.3% soil carbon content (0.06–0.15 t C ha−1 yr−1 ). Parameter derived uncertainty contributed most to overall uncertainty. Concerning input data contributions, uncertainties stemming from soil and climate input data variations are striking. At the river basin scale, cropland and forest ecosystems, loess and gleyic soils possess the highest degree of uncertainty. Quantified magnitudes of uncertainty stemming from the examined sources vary temporally and spatially due to specific natural settings (e.g. climate, land use and soil properties) and deliver useful information for interpreting simulation results on long-term soil organic carbon dynamics under environmental change. Derived from this analysis, key sensitive model parameters and interactions between them were identified: the mineralization rate coefficient, the carbon use efficiency parameter (synthesis coefficient) along with parameters determining the soil temperature influence on SOM turnover (mainly Q10 value) and the soil input related data (soil bulk density and initial soil C content) introduced the highest degree of model uncertainty. The here gained information can be transferred to other process-based SOM turnover models to consider stronger most crucial parameters introducing highest uncertainty contribution to soil C storage assessment under changing environmental conditions.

Post, W.M., Emanuel, William R., Zinke, Paul J., Stangenberger, Alan G. (1982). Soil carbon pools and world life zones. Nature 298 (July 8): 156-159

ABSTRACT: Soil organic carbon in active exchange with the atmosphere constitutes approximately two-thirds of the carbon in terrestrial ecosystems1,2 . The relatively large size and long residence time of this pool (of the order of 1,200 yr) make it a potentially important sink for carbon released to the atmosphere by fossil fuel combustion; however, in many cases, human disturbance has caused a decrease in soil carbon storage3,4. Various recent estimates place the global total of soil carbon between 700 (ref. 2) and 2,946 x 1015 g (ref. 5) with several intermediate estimates: 1,080 (ref. 1), 1,392 (ref. 6), 1,456 (ref. 3), and 2,070 x 1015 g (ref. 7). Schlesinger's3 estimate seems to be based on the most extensive data base (200 observations, some of which are mean values derived from large studies in particular areas) and is widely cited in carbon cycle studies. In addition to estimating the world soil carbon pool, it is important to establish the relationships between the geographical distribution of soil carbon and climate, vegetation, human development and other factors as a basis for assessing the influence of changes in any of these factors on the global carbon cycle. Our analysis of 2,700 soil profiles, organized on a climate basis using the Holdridge life-zone classification system8 , indicates relationships between soil carbon density and climate, a major soil forming factor. Soil carbon density generally increases with increasing precipitation, and there is an increase in soil carbon with decreasing temperature for any particular level of precipitation. When the potential evapotranspiration equals annual precipitation, soil carbon density9 is ~10 kg m-2 , exceptions to this being warm temperate and subtropical soils. Based on recent estimates of the areal extent of major ecosystem complexes9,10 which correspond well with climatic life zones, the global soil organic carbon pool is estimated to be 1,395 x 1015 g.

Potter, K. N., Derner, J. D. (2006). Soil carbon pools in central Texas: Prairies, restored grasslands, and croplands. Journal of Soil and Water Conservation 61 (3): 124-128

ABSTRACT: Establishment of perennial grasses on degraded soils has been suggested as a means to improve soil quality and sequester carbon in the soil. Particulate organic carbon may be an important component in the increased soil carbon content. We measured particulate organic carbon [defined as organic carbon in the 53 to 2000μm (0.002 to 0.08 in) size fraction] and mineral associated organic carbon (defined as the less than 53 μm (0.002 in) size fraction) at three locations in central Texas. Each location had a never-tilled native grassland site, a long-term agricultural site and a restored grassland on a previously tilled site. Organic carbon pool sizes varied in the surface 40 cm (16 in) of native grassland, restored grasslands and agricultural soils. The native grasslands contained the largest amounts of total organic carbon, while the restored grasslands and agricultural soils contained similar amounts of total organic carbon. Both particulate organic carbon and mineral associated carbon pools were reduced beyond the depth of tillage in the restored grass and agricultural soils compared to the native grassland soils. The restored grassland soils had a larger particulate organic carbon content than the agricultural soils, but the increase in particulate organic carbon was limited to the surface 5 cm (2 in) of soil. Trends in particulate organic carbon accumulation over time from nine to 30 years were not significant in this study.

K. N. Potter, J. Velazquez-Garcia, E. Scopel, H. A. Torbert (2007). Residue removal and climatic effects on soil carbon content of no-till soils. Journal of Soil and Water Conservation 62 (2): 110-114

ABSTRACT: While no-till management practices usually result in increased soil organic carbon (SOC) contents, the effect of residue removal with no-till is not well understood, especially in warmer climates. A multi-year study was conducted at six locations having a wide range of climatic conditions in central Mexico to determine the effect of varying rates of residue removal with no-till on SOC. Mean annual temperatures ranged from 16°C to 27°C (61°F to 81°F). Mean annual rainfall ranged from 618 to 1099 mm yr−1 (24 to 43 in yr−1 ). Treatments consisted of annual moldboard plowing under residue and no-till with 100%, 66%, 33%, and no corn (Zea mays L.) residue retained on the no-till surface. At five of the six locations, no-till with all surface residues removed maintained SOC levels above that of moldboard plowing which incorporated all residues. Retaining 100% of the crop residues with no-till always increased or maintained the SOC content. SOC increased in cooler climates, but asmean annual temperature increased, more retained crop residues were needed to increase the SOC. In tropical (mean annual temperature > 20°C) conditions, 100% corn residue retention with no-till only maintained SOC levels. Mean annual temperature had a greater impact on SOC than did annual rainfall. It appears that, in warmer climates, residue in excess of that needed for erosion control may be used for animal fodder or energy production. At the higher temperatures, most of the residue will decompose if left on the soil surface without improving soil carbon contents.

Powlson, D. S., Hirsch, P. R., Brookes, P. C. (2001). The role of soil microorganisms in soil organic matter conservation in the tropics. Nutrient Cycling in Agroecosystems 61 (1-2): 41-51

ABSTRACT: Soil is a large sink for organic carbon within the terrestrial biosphere. Practices which cause a decline in soil organic matter cause CO2 release, in addition to damaging soil resilience and, often, agricultural productivity. The soil micro-organisms (collectively the soil microbial biomass) are the agents of transformation of soil organic matter, nutrients and of most key soil processes. Their activities are much influenced by soil physico-chemical and ecological interactions. This paper addresses two key issues. Firstly, ways of managing, and the extent to which it is possible to manage, soil biological functions. Secondly, the methodologies currently available for studying soil micro-organisms, and the functions they mediate, are discussed. It is concluded that, as the world population develops in this new millennium, there will be an increased dependence upon biological processes in soil to provide adequate crop nutrition for the majority of the world's farmers. Although a major increase in the use of artificial fertilisers will be necessary on a global scale, this will not be an option for large numbers of farmers due to their poverty. Instead they will rely on recycling of nutrients from animal and vegetable composts and urban wastes, and biological cycling from nitrogen fixation and mycorrhizae. The challenge is to select the most appropriate topics for further research. Not all aspects are likely to lead to significantly improved agricultural productivity, or sustainability within the foreseeable future.

Ritchie, J. C., McCarty, G. W., Venteris, E. R., Kaspar, T.C. (2007). Soil and soil organic carbon redistribution on the landscape. Geomorphology 89 (1-2): 163-171

ABSTRACT: Patterns of soil organic carbon (SOC) vary widely across the landscape leading to large uncertainties in the SOC budget especially for agricultural landscapes where water, tillage and wind erosion redistributes soil and SOC across the landscape. It is often assumed that soil erosion results in a loss of SOC from the agricultural ecosystem but recent studies indicate that soil erosion and its subsequent redistribution within fields can stimulate carbon sequestration in agricultural ecosystems. This study investigates the relationship between SOC and soil redistribution patterns on agricultural landscapes. Soil redistribution (erosion and deposition) patterns were estimated in three tilled agricultural fields using the fallout137 Cesium technique.137 Cs and SOC concentrations of upland soils are significantly correlated in our study areas. Upland areas (eroding) have significantly less SOC than soils in deposition areas. SOC decreased as gradient slope increases and soils on concave slopes had higher SOC than soils on convex slopes. These data suggest that soil redistribution patterns and topographic patterns may be used to help understand SOC dynamics on the landscape. Different productivity and oxidation rates of SOC of eroded versus deposited soils also contribute to SOC spatial patterns. However, the strong significant relationships between soil redistribution and SOC concentrations in the upland soil suggest that they are moving along similar physical pathways in these systems. Our study also indicates that geomorphic position is important for understanding soil movement and redistribution patterns within a field or watershed. Such information can help develop or implement management systems to increase SOC in agricultural ecosystems.

Segers, R., Kengen, S. W. M. (1998). Methane production as a function of anaerobic carbon mineralization: A process model. Soil Biology and Biochemistry 30 (8-9): 1107-1117

ABSTRACT: Anaerobic carbon mineralization is a major regulator of soil methane production, but the relationship between these processes is variable. To explain the dynamics of this relationship a model was developed, which comprises the dynamics of alternative electron-acceptors, of acetate and of methanogenic biomass. Major assumptions are: (i) alternative electron-acceptors suppress methanogenesis and (ii) the rate of electron-acceptor reduction is controlled by anaerobic carbon mineralization. The model was applied to anaerobic incubation experiments with slurried soil samples from a drained and an undrained peat soil in the Netherlands to test the model and to further interpret the data. Three parameters were fitted with a Monte Carlo method, using experimentally determined time series of methane, carbon dioxide and acetate. The fitted parameters were the initial concentration of electron-acceptors, the initial concentration of methanogenic biomass and the maximum relative growth rate of methanogenic biomass. Simulated and measured time courses of methane corresponded reasonably well. The model as such stresses the importance of alternative electron-acceptors. At the drained site initial alternative electron-acceptor concentrations were between 0.3 and 0.8 mol electron equivalents (el. eqv.) kg−1 dw soil, whereas at the undrained site they were between 0.0 and 0.3 mol el. eqv. kg−1 dw soil, depending on the experimental treatments. The sum of measured NO−3 and SO2−4 concentrations and estimated maximum Fe3+ and Mn4+ concentrations was much lower than the fitted concentrations of alternative electron-acceptors. Apparently, reduction of unknown electron-acceptors consumed a large part of anaerobically-mineralized carbon which, therefore, was not available for methanogenesis.

Six, J., Elliott, E. T., Paustian, K. (2000). Soil macroaggregate turnover and microaggregate formation: A mechanism for C sequestration under no-tillage agriculture. Soil Biology and Biochemistry 32 (14): 2099-2103

ABSTRACT: Soil disturbance from tillage is a major cause of organic matter depletion and reduction in the number and stability of soil aggregates when native ecosystems are converted to agriculture. No-till (NT) cropping systems usually exhibit increased aggregation and soil organic matter relative to conventional tillage (CT). However, the extent of soil organic matter changes in response to NT management varies between soils and the mechanisms of organic matter stabilization in NT systems are unclear. We evaluated a conceptual model which links the turnover of aggregates to soil organic matter dynamics in NT and CT systems; we argue that the rate of macroaggregate formation and degradation (i.e. aggregate turnover) is reduced under NT compared to CT and leads to a formation of stable microaggregates in which carbon is stabilized and sequestered in the long term. Therefore, the link between macroaggregate turnover, microaggregate formation, and C stabilization within microaggregates partly determines the observed soil organic matter increases under NT.

Smith, P. (2005). An overview of the permanence of soil organic carbon stocks: influence of direct human-induced, indirect and natural effects.. European Journal of Soil Science 56 (5): 673-680

ABSTRACT: If biospheric sinks, such as soil organic carbon, are to be used to meet obligations for greenhouse gas emission reduction under the Kyoto Protocol, the permanence of these sinks needs to be considered. Further, since only direct human-induced carbon sinks can be included, and sinks resulting from indirect and natural effects cannot be used, there is a pressing need to separate direct human-induced effects from indirect and natural effects. Since these effects also influence the permanence of soil organic stocks, this paper attempts to synthesize existing knowledge in soil science, and use models to examine the likely influence of direct, indirect and natural effects on the permanence of soil organic carbon stocks.

Sperow, M. (2007). The marginal costs of carbon sequestration: Implications of one greenhouse gas mitigation activity. Journal of Soil & Water Conservation 62 (6): 367-375

ABSTRACT: This paper provides a method for estimating the marginal cost of soil carbon (C) derived from setting aside highly erodible cropland in the United States. Increases in soil carbon are estimated using a modified Intergovernmental Panel on Climate Change soil organic carbon inventory method and National Resources Inventory data. Marginal costs of soil carbon sequestration activities are based on the opportunity cost of removing highly erodible land from crop production using land rental rates adjusted to account for the productivity of cropland and Conservation Reserve Program rental rates. Total soil carbon sequestration from setting aside highly erodible land is over 10 Tg C yr−1 (11.0 Mtn C yr−1 ) on the 21.9 Mha (54.1 Mac) where corn, cotton, sorghum, soybean, wheat, or fallow were grown in 1997. The marginal cost of stored carbon based on these estimates range from $11 to $4,492 Mg−1 C ($10 to $4,075 tn−1 C) with a US weighted average of $288 Mg−1 C ($261tn−1 C). Changes in US crop production levels from removing land from crop production are also estimated.

Steinbeiss, S., Temperton, V.M., Gleixner, G. (2007). Mechanisms of soil carbon storage in experimental grasslands. Biogeosciences Discussions 4 (5): 3829-3862

ABSTRACT: We investigated the fate of root and litter derived carbon into soil organic matter and dissolved organic matter in soil profiles, in order to explain unexpected positive effects of plant diversity on carbon storage. A time series of soil and soil solution samples was investigated at the field site of The Jena Experiment. In addition to the main biodiversity experiment with C3 plants, a C4 species (Amaranthus retroflexus L.) naturally labeled with13 C was grown on an extra plot. Changes in organic carbon concentration in soil and soil solution were combined with stable isotope measurements to follow the fate of plant carbon into the soil and soil solution. A split plot design with plant litter removal versus double litter input simulated differences in biomass input. After 2 years, the no litter and double litter treatment, respectively, showed an increase of 381 g Cm-2 and 263g C m-2 to 20 cm depth, while 71 g C m-2 and 393 g C m-2 were lost between 20 and 30 cm depth. The isotopic label in the top 5 cm indicated that 11 and 15% of soil organic carbon were derived from plant material on the no litter and the double litter treatment, respectively. Without litter, this equals the total amount of carbon newly stored in soil, whereas with double litter this corresponds to twice the amount of stored carbon. Our results indicate that litter input resulted in lower carbon storage and larger carbon losses and consequently accelerated turnover of soil organic carbon. Isotopic evidence showed that inherited soil organic carbon was replaced by fresh plant carbon near the soil surface. Our results suggest that primarily carbon released from soil organic matter, not newly introduced plant organic matter, was transported in the soil solution and contributed to the observed carbon storage in deeper horizons.

Tan, Z. X., Liu, S. G., Johnston, C. A., Loveland, T. R., Tieszen, L. L., Liu, J. X., Kurtz, R. (2005). Soil organic carbon dynamics as related to land use history in the northwestern Great Plains. Global Biogeochemical Cycles 19 (GB3011): doi:10.1029/2005GB002536

ABSTRACT: Strategies for mitigating the global greenhouse effect must account for soil organic carbon (SOC) dynamics at both spatial and temporal scales, which is usually challenging owing to limitations in data and approach. This study was conducted to characterize the SOC dynamics associated with land use change history in the northwestern Great Plains ecoregion. A sampling framework (40 sample blocks of 10 × 10 km2 randomly located in the ecoregion) and the General Ensemble Biogeochemical Modeling System (GEMS) were used to quantify the spatial and temporal variability in the SOC stock from 1972 to 2001. Results indicate that C source and sink areas coexisted within the ecoregion, and the SOC stock in the upper 20-cm depth increased by 3.93 Mg ha−1 over the 29 years. About 17.5% of the area was evaluated as a C source at 122 kg C ha−1 yr−1 . The spatial variability of SOC stock was attributed to the dynamics of both slow and passive fractions, while the temporal variation depended on the slow fraction only. The SOC change at the block scale was positively related to either grassland proportion or negatively related to cropland proportion. We concluded that the slow C pool determined whether soils behaved as sources or sinks of atmospheric CO2 , but the strength depended on antecedent SOC contents, land cover type, and land use change history in the ecoregion.

Terra, J. A., Reeves, D. W., Shaw, J. N., Raper, R. L. (2005). Impacts of landscape attributes on carbon sequestration during the transition from conventional to conservation management practices on a Coastal Plain field. Journal of Soil and Water Conservation 60 (6): 438-446

ABSTRACT: Field-scale experiments on degraded soils comparing management systems would facilitate a better understanding of the soil organic carbon (C) landscape dynamics associated with transition to conservation systems. We assessed the effects of soil management practices and terrain attributes on soil organic C in a 9 ha (22.2 ac) Alabama field (Typic and Aquic Paleudults). Treatments were established in strips across the landscape in a corn (Zea mays L.)-cotton (Gossypium hirsutum L.) rotation. Treatments included a conventional system (chisel plowing/disking without cover crops) with or without dairy manure, and a conservation system (no-till and cover crops) with and without manure. A soil survey, topography, soil electrical conductivity, initial soil organic C and soil texture were used to delineate management zones or clusters. After one rotation cycle (30 months), averaged across 240 positions distributed over the entire field, no-till or conventional tillage + manure increased soil organic C (0 to 5 cm; 0 to 2 in depth) by -50 percent compared to conventional tillage (7.34 and 7.62 vs. 5.02 Mg ha−1 ; 3.28 and 3.40 vs. 2.24 t ac−1 , respectively); but no-till+manure increased soil organic C by 157 percent. Initial soil organic C content was the most common correlated variable with soil organic C changes (SOC) across the landscape for all treatments and conservation systems had greater soil organic C increases relative to conventional systems at low soil quality landscape positions. Our results show the potential to sequester C using high-residue producing conservation systems and manure is scale dependent, and may be higher than previously expected for degraded soils in the southeastern United States.

Thompson, J. A., Kolka, R. K. (2005). Soil carbon storage estimation in a forested watershed using Quantitative soil-landscape modeling.. Soil Science Society Of America JournalSoil Sci So 69 (4): 1086-1093

ABSTRACT: Carbon storage in soils is important to forest ecosystems. Moreover, forest soils may serve as important C sinks for ameliorating excess atmospheric CO2 . Spatial estimates of soil organic C (SOC) storage have traditionally relied upon soil survey maps and laboratory characterization data. This approach does not account for inherent variability within map units, and often relies on incomplete, unrepresentative, or biased data. Our objective was to develop soil-landscape models that quantify relationships between SOC and topographic variables derived from digital elevation models. Within a 1500-ha watershed in eastern Kentucky, the amount of SOC stored in the soil to a depth of 0.3 m was estimated using triplicate cores at each node of a 380-m grid. We stratified the data into four aspect classes and used robust linear regression to generate empirical models. Despite low coefficients of correlation between measured SOC and individual terrain attributes, we developed and validated models that explain up to 71% of SOC variability using three to five terrain attributes. Mean SOC content in the upper 30 cm, as predicted from our models, is 5.3 kg m–2 , compared with an estimate of 2.9 kg m–2 from soil survey data. Total SOC storage in the upper 30 cm within the entire watershed is 82.0 Gg, compared with an estimate of 44.8 Gg from soil survey data. A soil-landscape modeling approach may prove useful for future SOC spatial modeling because it incorporates the continuous variability of SOC across landscapes and may be transportable to similar landscapes.

Thuille, A., Schulze, E. (2006). Carbon dynamics in successional and afforested spruce stands in Thuringia and the Alps. Global Change Biology 12 (2): 325-342

ABSTRACT: Changes in the carbon stocks of stem biomass, organic layers and the upper 50 cm of the mineral soil during succession and afforestation of spruce (Picea abies ) on former grassland were examined along six chronosequences in Thuringia and the Alps. Three chronosequences were established on calcareous and three on acidic bedrocks. Stand elevation and mean annual precipitation of the chronosequences were different. Maximum stand age was 93 years on acid and 112 years on calcareous bedrocks. Stem biomass increased with stand age and reached values of 250–400 t C ha−1 in the oldest successional stands. On acidic bedrocks, the organic layers accumulated linearly during forest succession at a rate of 0.34 t C ha−1 yr−1 . On calcareous bedrocks, a maximum carbon stock in the humus layers was reached at an age of 60 years.

Total carbon stocks in stem biomass, organic layers and the mineral soil increased during forest development from 75 t C ha−1 in the meadows to 350 t C ha−1 in the oldest successional forest stands (2.75 t C ha−1 yr−1 ). Carbon sequestration occurred in stem biomass and in the organic layers (0.34 t C ha−1 yr−1 on acid bedrock), while mineral soil carbon stocks declined.

Mineral soil carbon stocks were larger in areas with higher precipitation. During forest succession, mineral soil carbon stocks of the upper 50 cm decreased until they reached approximately 80% of the meadow level and increased slightly thereafter. Carbon dynamics in soil layers were examined by a process model. Results showed that sustained input of meadow fine roots is the factor, which most likely reduces carbon losses in the upper 10 cm. Carbon losses in 10–20 cm depth were lower on acidic than on calcareous bedrocks. In this depth, continuous dissolved organic carbon inputs and low soil respiration rates could promote carbon sequestration following initial carbon loss.

At least 80 years are necessary to regain former stock levels in the mineral soil. Despite the comparatively larger amount of carbon stored in the regrowing vegetation, afforestation projects under the Kyoto protocol should also aim at the preservation or increase of carbon in the mineral soil regarding its greater stability of compared with stocks in biomass and humus layers. If grassland afforestation is planned, suitable management options and a sufficient rotation length should be chosen to achieve these objectives. Maintenance of grass cover reduces the initial loss.

Trumbore, Susan, Da Costa, Enir Salazar, Nepstad, Daniel C., Barbosa De Camargo, Pl+¡nio, Martinelli, Luiz A., Ray, David, Restom, Teresa, Silver, Whendee (2006). Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration.. Global Change Biology 12 (2): 217-229

ABSTRACT: Radiocarbon (14 C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of14 C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2-year-old) photosynthetic products. HighΔ14 C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil.Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using14 C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the14 C values in soil pore space CO2 , in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time.

Trumbore, Susan E. (1997). Potential responses of soil organic carbon to global environmental change. Proceedings Of The National Academy Of Sciences Of The United States Of America 94 (16): 8284-8291.

ABSTRACT: Recent improvements in our understanding of the dynamics of soil carbon have shown that 20–40% of the approximately 1,500 Pg of C stored as organic matter in the upper meter of soils has turnover times of centuries or less. This fast-cycling organic matter is largely comprised of undecomposed plant material and hydrolyzable components associated with mineral surfaces. Turnover times of fast-cycling carbon vary with climate and vegetation, and range from <20 years at low latitudes to >60 years at high latitudes. The amount and turnover time of C in passive soil carbon pools (organic matter strongly stabilized on mineral surfaces with turnover times of millennia and longer) depend on factors like soil maturity and mineralogy, which, in turn, reflect long-term climate conditions. Transient sources or sinks in terrestrial carbon pools result from the time lag between photosynthetic uptake of CO2 by plants and the subsequent return of C to the atmosphere through plant, heterotrophic, and microbial respiration. Differential responses of primary production and respiration to climate change or ecosystem fertilization have the potential to cause significant interrannual to decadal imbalances in terrestrial C storage and release. Rates of carbon storage and release in recently disturbed ecosystems can be much larger than rates in more mature ecosystems. Changes in disturbance frequency and regime resulting from future climate change may be more important than equilibrium responses in determining the carbon balance of terrestrial ecosystems.

Soil carbon inventories and turnover rates are influenced by climate, vegetation, parent material, topography, and time, the fundamental state factors outlined by Jenny (1, 2). Studies attempting to understand the influence of a specific factor (e.g., temperature or moisture) on soil properties have found it useful to identify a suite of soils for which the factor in question varies whereas the others are held constant (1–4). This approach has been used successfully to look at the role of temperature (3, 5) and time (6–10) on the turnover of soil C. Ecosystem models such as century (11, 12), casa (13), or the Rothamsted model (14, 15) predict the sensitivity of soil C inventory and turnover to climate, vegetation, and parent material, but as yet few data exist to test these predictions. Parameterizations of decomposition used in these models are based on empirical fits to specific calibration sites and may not include enough basic understanding of the interaction between plant substrates and the soil environment to make successful predictions in different environments (16).

The reservoir of soil carbon has been proposed as both a significant source and sink of atmospheric CO2 . A soil source results when net decomposition exceeds C inputs to the soil, either as a result of human activities such as clearing forests for agriculture (17, 18) or because of increased decomposition rates due to global warming (12, 14, 19, 20). Net sinks of C in soils are postulated from the difference between net ecosystem C uptake and tree growth rates (21) or from presumed increases in net C inputs from CO2 or N-fertilization of plants (19, 20, 22–24). In both cases, the magnitude and timing of the response depends on the amount of carbon in pools that respond quickly to changes in climate and vegetation, and to the time lag between fixation of C by plants and its subsequent release to the atmosphere during decomposition.

This paper will describe recent approaches used to study soil C dynamics, and preliminary applications of these tools to the problems of soil C response to global environmental changes. The results indicate the importance of the global soil C pool to the global C cycle on interrannual to century time scales and suggest profitable areas for future research.

Ullah, S., Frasier, R., King, L., Picotte-Anderson, N., Moore, T.R. (2008). Potential fluxes of N2 O and CH4 from soils of three forest types in Eastern Canada. Soil Biology and Biochemistry 40 (4): 986-994

ABSTRACT: We conducted laboratory incubation experiments to elucidate the influence of forest type and topographic position on emission and/or consumption potentials of nitrous oxide (N2 O) and methane (CH4 ) from soils of three forest types in Eastern Canada. Soil samples collected from deciduous, black spruce and white pine forests were incubated under a control, an NH4 NO3 amendment and an elevated headspace CH4 concentration at 70% water-filled pore space (WFPS), except the poorly drained wetland soils which were incubated at 100% WFPS. Deciduous and boreal forest soils exhibited greater potential of N2 O and CH4 fluxes than did white pine forest soils. Mineral N addition resulted in significant increases in N2 O emissions from wetland forest soils compared to the unamended soils, whereas well-drained soils exhibited no significant increase in N2 O emissions in-response to mineral N additions. Soils in deciduous, boreal and white pine forests consumed CH4 when incubated under an elevated headspace CH4 concentration, except the poorly drained soils in the deciduous forest, which emitted CH4 . CH4 consumption rates in deciduous and boreal forest soils were twice the amount consumed by the white pine forest soils. The results suggest that an episodic increase in reactive N input in these forests is not likely to increase N2 O emissions, except from the poorly drained wetland soils; however, long-term in situ N fertilization studies are required to validate the observed results. Moreover, wetland soils in the deciduous forest are net sources of CH4 unlike the well-drained soils, which are net sinks of atmospheric CH4 . Because wetland soils can produce a substantial amount of CH4 and N2 O, the contribution of these wetlands to the total trace gas fluxes need to be accounted for when modeling fluxes from forest soils in Eastern Canada.

Van Kessel, C., Horwath, W. R., Hartwig, U., Harris, D., Luscher, A. (2000). Net soil carbon input under ambient and elevated CO2 concentrations: isotopic evidence after 4 years. Global Change Biology 6 (4): 435-444

SUMMARY: Elevation of atmospheric CO2 concentration is predicted to increase net primary production, which could lead to additional C sequestration in terrestrial ecosystems. Soil C input was determined under ambient and Free Atmospheric Carbon dioxide Enrichment (FACE) conditions forLolium perenne L. andTrifolium repens L. grown for four years in a sandy-loam soil. The13 C content of the soil organic matter C had been increased by 5‰ compared to the native soil by prior cropping to corn (Zea mays) for > 20 years. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above-ground biomass produced by L. perenne during the 4-year period was strongly dependent on the amount of N fertilizer applied but did not respond to increased CO2 . In contrast, the total accumulated above-ground biomass of T. repens doubled under elevated CO2 but remained independent of N fertilizer rate. The C:N ratio of above-ground biomass for both species increased under elevated CO2 whereas only the C:N ratio ofL. perenne roots increased under elevated CO2 . Root biomass ofL. perenne doubled under elevated CO2 and again under high N fertilization. Total soil C was unaffected by CO2 treatment but dependent on species. After 4 years and for both crops, the fraction of new C (F-value) under ambient conditions was higher (P= 0.076) than under FACE conditions: 0.43 vs. 0.38. Soil underL. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO2 had no effect on total soil organic matter content for both systems. The net amount of C sequestered in 4 years was unaffected by the CO2 concentration (overall average of 8.5 g C kg−1 soil). There was a significant species effect and more new C was sequestered under highly fertilizedL. perenne . The amount of new C sequestered in the soil was primarily dependent on plant species and the response of root biomass to CO2 and N fertilization. Therefore, in this FACE study net soil C sequestration was largely depended on how the species responded to N rather than to elevated CO2 .

Van Miegroet, H., Boettinger, J. L., Baker, M. A., Nielsen, J., Evans, D., Stum, A. (2005). Soil carbon distribution and quality in a montane rangeland-forest mosaic in northern Utah. Forest Ecology and Management 220 (1-3): 284-299

ABSTRACT: Relatively little is known about soil organic carbon (SOC) dynamics in montane ecosystems of the semi-arid western U.S. or the stability of current SOC pools under future climate change scenarios. We measured the distribution and quality of SOC in a mosaic of rangeland-forest vegetation types that occurs under similar climatic conditions on non-calcareous soils at Utah State University's T.W. Daniel Experimental Forest in northern Utah: the forest types were aspen [Populus tremuloides ] and conifer (mixture of fir [Abies lasiocarpa ] and spruce [Picea engelmannii ]); the rangeland types were sagebrush steppe [Artemisia tridentata], grass-forb meadow, and a meadow-conifer ecotone. Total SOC was calculated from OC concentrations, estimates of bulk density by texture and rock-free soil volume in five pedons. The SOC quality was expressed in terms of leaching potential and decomposability. Amount and aromaticity of water-soluble organic carbon (DOC) was determined by water extraction and specific ultra violet absorbance at 254 nm (SUVA) of leached DOC. Decomposability of SOC and DOC was derived from laboratory incubation of soil samples and water extracts, respectively.

Although there was little difference in total SOC between soils sampled under different vegetation types, vertical distribution, and quality of SOC appeared to be influenced by vegetation. Forest soils had a distinct O horizon and higher SOC concentration in near-surface mineral horizons that declined sharply with depth. Rangeland soils lacked O horizons and SOC concentration declined more gradually. Quality of SOC under rangelands was more uniform with depth and SOC was less soluble and less decomposable (i.e., more stable) than under forests. However, DOC in grass-forb meadow soils was less aromatic and more bioavailable, likely promoting C retention through cycling. The SOC in forest soils was notably more leachable and decomposable, especially near the soil surface, with stability increasing with soil depth. Across the entire dataset, there was a weak inverse relationship between the decomposability and the aromaticity of DOC. Our data indicate that despite similar SOC pools, vegetation type may affect SOC retention capacity under future climate projections by influencing potential SOC losses via leaching and decomposition.

Van Noordwijk, M., Cerri, C. C., Woomer, P. L., Nugroho, K., Bernoux, M. (1997). Soil carbon dynamics in the humid tropical forest zone. Geoderma 79 (1-4): 187-225.

ABSTRACT: Conversion of natural forests to agriculture in the humid tropics leads to a reduction in ecosystem carbon storage due to the immediate removal of aboveground biomass and a gradual subsequent reduction in soil organic carbon. A considerable part of soil carbon is protected from microbial attack by a range of physical and chemical mechanisms and is not sensitive to landuse change. We analyzed the soils data base for Sumatra (Indonesia) developed by the Center for Soil and Agroclimate Research (CSAR) to estimate effects of landuse on soil C content. Sumatra has a considerable diversity of soils ranging from those of recent origin in the highlands, to older sedimentary and heavily leached soils in the pedimont peneplain and large areas of wetland soils along the coast. Peat soils (Histosols) and other wetland soils (Aquic and Fluvic suborders) contain the greatest soil C reserves, followed by young volcanic soils (Andisols). Agricultural use of these soils can have a disproportionately large effect on C release to the atmosphere. On the major part of the upland soils the difference in (top) soil C content between natural forest and agricultural land is in the range 0.5–1.0% C, equivalent to a change in total C stock of 10–20 Mg ha−1 . These results agree with data collected in S. Sumatra in the 1930s. Corg of forest soils is related to soil pH, and is lowest in the pH range 5.0–6.0. Wetland conditions, lower pH, higher altititude (lower temperature) and higher clay and silt content all contributed to higher soil C contents in a multiple regression analysis of the whole data set. Existing models and data sets are insufficient to predict changes in soil C contents under various landuse practices. Carbon isotope studies, and especially thed13 C method may be used to study the effects of landuse change, especially when the vegetation was changed from one dominated by C3 plants (most forest species) to one dominated by grasses and crops with a C4 photosynthetic pathway. Results from Brazil documented a gradual decline of organic matter originating from the forest system and its partial replacement by organic matter derived from inputs of sugarcane during the first fifty years of cultivation. Forest conversion to well managed grasslands may lead to an increased soil C storage, after an initial decline. The consequences of erosion on losses of soil C depend on the scale at which these losses are considered, because of sedimentation processes. When net erosion losses are not expressed per unit area, but per length scale to the power l.6, erosion losses appear to be largely independent of scale. The'fractal dimension of erosion’ (on average around 1.6) probably is a landscape characteristic and estimates of its value are needed for extrapolation. Better understanding of soil C deposition sites is needed to evaluate overall erosion effects and test whether or not erosion can contribute to net C sequestration.

Miko U. F. Kirschbaum (2006). The temperature dependence of organic-matter decomposition—still a topic of debate. Soil Biology and Biochemistry 38 (9): 2510-2518

ABSTRACT: The temperature dependence of organic matter decomposition is of considerable ecosphysiological importance, especially in the context of possible climate-change feedback effects. It effectively controls whether, or how much, carbon will be released with global warming, and to what extent that release of carbon constitutes a dangerous positive feedback effect that leads to further warming.

The present paper is an invited contribution in a series of Citation Classics based on a review paper of the temperature dependence of organic matter decomposition that was published in 1995. It discusses the context and main findings of the 1995 study, the progress has been made since then and what issues still remain unresolved.

Despite the continuation of much further experimental work and repeated publication of summary articles, there is still no scientific consensus on the temperature dependence of organic matter decomposition. It is likely that this lack of consensus is largely due to different studies referring to different experimental conditions where confounding factors play a greater or lesser role.

Substrate availability is particularly important. If it changes during the course of measurements, it can greatly confound the derived apparent temperature dependence. This confounding effect is illustrated through simulations and examples of experimental work drawn from the literature. The paper speculates that much of the current disagreement between studies might disappear if different studies would ensure that they are all studying the same system attributes, and if confounding factors were always considered and, if possible, eliminated.

B. A. Hungate, K. van Groenigen, J. Six, J.D. Jastrow, Y. Luo, M. de Graaff, C. van Kessel, C.W. Osenberg (2009). Assessing the effect of elevated carbon dioxide on soil carbon: a comparison of four meta-analyses. Global Change Biology 15 (8): 2020-2034

ABSTRACT: Soil is the largest reservoir of organic carbon (C) in the terrestrial biosphere and soil C has a relatively long mean residence time. Rising atmospheric carbon dioxide (CO2) concentrations generally increase plant growth and C input to soil, suggesting that soil might help mitigate atmospheric CO2 rise and global warming. But to what extent mitigation will occur is unclear. The large size of the soil C pool not only makes it a potential buffer against rising atmospheric CO2 , but also makes it difficult to measure changes amid the existing background. Meta-analysis is one tool that can overcome the limited power of single studies. Four recent meta-analyses addressed this issue but reached somewhat different conclusions about the effect of elevated CO2 on soil C accumulation, especially regarding the role of nitrogen (N) inputs. Here, we assess the extent of differences between these conclusions and propose a new analysis of the data. The four meta-analyses included different studies, derived different effect size estimates from common studies, used different weighting functions and metrics of effect size, and used different approaches to address nonindependence of effect sizes. Although all factors influenced the mean effect size estimates and subsequent inferences, the approach to independence had the largest influence. We recommend that meta-analysts critically assess and report choices about effect size metrics and weighting functions, and criteria for study selection and independence. Such decisions need to be justified carefully because they affect the basis for inference. Our new analysis, with a combined data set, confirms that the effect of elevated CO2 on net soil C accumulation increases with the addition of N fertilizers. Although the effect at low N inputs was not significant, statistical power to detect biogeochemically important effect sizes at low N is limited, even with meta-analysis, suggesting the continued need for long-term experiments.

M. Easter, K. Paustian, K. Killian, S. Williams, T. Feng, R. Al-Adamat, N.H. Batjes, M. Bernoux, T. Bhattacharyya, C.C. Cerri, C.E.P. Cerri, K. Coleman, P. Falloon, C. Feller, P. Gicheru, P. Kamoni, E. Milne, D.K. Pal, D.S. Powlson, Z. Rawajfih, M. Sessay, S. Wokabi (2007). The GEFSOC soil carbon modelling system: A tool for conducting regional-scale soil carbon inventories and assessing the impacts of land use change on soil carbon. 122 (1): 13-25

ABSTRACT: The GEFSOC soil carbon modelling system was built to provide interdisciplinary teams of scientists, natural resource managers and policy analysts (who have the appropriate computing skills) with the necessary tools to conduct regional-scale soil carbon (C) inventories. It allows users to assess the effects of land use change on soil organic C (SOC) stocks, soil fertility and the potential for soil C sequestration. The tool was developed in conjunction with case-studies of land use and management impacts on SOC in Brazil, Jordan, Kenya and India, which represent a diversity of land use and land management patterns and are countries where sustaining soil organic matter and fertility for food security is an on-going problem. The tool was designed to run using two common desktop computers, connected via a local area network. It utilizes open-source software that is freely available. All new software and user interfaces developed for the tool are available in an open source environment allowing users to examine system details, suggest improvements or write additional modules to interface with the system. The tool incorporates three widely used models for estimating soil C dynamics: (1) the Century ecosystem model; (2) the RothC soil C decomposition model; and (3) the Intergovernmental Panel on Climate Change (IPCC) method for assessing soil C at regional scales. The tool interacts with a Soil and Terrain Digital Database (SOTER) built for the specific country or region the user intends to model. A demonstration of the tool and results from an assessment of land use change in a sample region of North America are presented.

Torn, M. S., Trumbore, S. E., Chadwick, O. A., Vitousek, P. M., Hendricks, D. M. (1997). Mineral control of soil organic carbon storage and turnover. Nature 389 (6647): 170-173

ABSTRACT: A large source of uncertainty in present understanding of the global carbon cycle is the distribution and dynamics of the soil organic carbon reservoir. Most of the organic carbon in soils is degraded to inorganic forms slowly, on timescales from centuries to millennia. Soil minerals are known to play a stabilizing role, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long-residence-time organic carbon is not well known. Here we use radiocarbon analyses to explore interactions between soil mineralogy and soil organic carbon along two natural gradients - of soil-age and of climate - in volcanic soil environments. During the first approximates 150,000 years of soil development, the volcanic parent material weathered to metastable, non-crystalline minerals. Thereafter, the amount of non-crystalline minerals declined, and more stable crystalline minerals accumulated. Soil organic carbon content followed a similar trend, accumulating to a maximum after 150,000 years, and then decreasing by 50% over the next four million years. A positive relationship between noncrystalline minerals and organic carbon was also observed in soils through the climate gradient, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere-ecosystem carbon fluxes during long-term soil development; this conclusion should be generalizable at least to other humid environments

Van Oost, K., Quine, T. A., Govers, G., De Gryze, S., Six, J., Harden, J. W., Ritchie, J. C., McCarty, G. W., Heckrath, G., Kosmas, C., Giraldez, J. V., da Silva, J. R. Marques, Merckx, R. (2007). The impact of agricultural soil erosion on the global carbon cycle. Science 318 (5850): 626-629

ABSTRACT: Agricultural soil erosion is thought to perturb the global carbon cycle, but estimates of its effect range from a source of 1 petagram per year1 to a sink of the same magnitude. By using Caesium-137 and carbon inventory measurements from a large-scale survey, we found consistent evidence for an erosion-induced sink of atmospheric carbon equivalent to approximately 26% of the carbon transported by erosion. Based on this relationship, we estimated a global carbon sink of 0.12 (range 0.06 to 0.27) petagrams of carbon per year resulting from erosion in the world's agricultural landscapes. Our analysis directly challenges the view that agricultural erosion represents an important source or sink for atmospheric CO2 .

R. Lal (2004). Soil carbon sequestration to mitigate climate change. Geoderma 123 (1-2): 1-22

ABSTRACT: The increase in atmospheric concentration of CO2 by 31% since 1750 from fossil fuel combustion and land use change necessitates identification of strategies for mitigating the threat of the attendant global warming. Since the industrial revolution, global emissions of carbon (C) are estimated at 270±30 Pg (Pg=petagram=1015 G=1 billion ton) due to fossil fuel combustion and 136±55 Pg due to land use change and soil cultivation. Emissions due to land use change include those by deforestation, biomass burning, conversion of natural to agricultural ecosystems, drainage of wetlands and soil cultivation. Depletion of soil organic C (SOC) pool have contributed 78±12 Pg of C to the atmosphere. Some cultivated soils have lost one-half to two-thirds of the original SOC pool with a cumulative loss of 30–40 Mg C/ha (Mg=megagram=106 G=1 ton). The depletion of soil C is accentuated by soil degradation and exacerbated by land misuse and soil mismanagement. Thus, adoption of a restorative land use and recommended management practices (RMPs) on agricultural soils can reduce the rate of enrichment of atmospheric CO2 while having positive impacts on food security, agro-industries, water quality and the environment. A considerable part of the depleted SOC pool can be restored through conversion of marginal lands into restorative land uses, adoption of conservation tillage with cover crops and crop residue mulch, nutrient cycling including the use of compost and manure, and other systems of sustainable management of soil and water resources. Measured rates of soil C sequestration through adoption of RMPs range from 50 to 1000 kg/ha/year. The global potential of SOC sequestration through these practices is 0.9±0.3 Pg C/year, which may offset one-fourth to one-third of the annual increase in atmospheric CO2 estimated at 3.3 Pg C/year. The cumulative potential of soil C sequestration over 25–50 years is 30–60 Pg. The soil C sequestration is a truly win–win strategy. It restores degraded soils, enhances biomass production, purifies surface and ground waters, and reduces the rate of enrichment of atmospheric CO2 by offsetting emissions due to fossil fuel.

Luo, Y. (2007). Terrestrial carbon-cycle feedback to climate warming. Annual Review of Ecology, Evolution, and Systematics 38: 683-712

ABSTRACT: The coupled carbon-climate models reported in the literature all demonstrate a positive feedback between terrestrial carbon cycles and climate warming. A primary mechanism underlying the modeled positive feedback is the kinetic sensitivity of photosynthesis and respiration to temperature. Field experiments, however, suggest much richer mechanisms driving ecosystem responses to climate warming, including extended growing seasons, enhanced nutrient availability, shifted species composition, and altered ecosystem-water dynamics. The diverse mechanisms likely define more possibilities of carbon-climate feedbacks than projected by the kinetics-based models. Nonetheless, experimental results are so variable that we have not generated the necessary insights on ecosystem responses to effectively improve global models. To constrain model projections of carbon-climate feedbacks, we need more empirical data from whole ecosystem warming experiments across a wide range of biomes, particularly in tropic regions, and closer interactions between models and experiments.

Garten, C.T., Jr., Post. W.M., III, P. J. Hanson, L. W. Cooper (1999). Forest soil carbon inventories and dynamics along an elevation gradient in the southern Appalachian Mountains. Biogeochemistry 45 (2): 115-145

ABSTRACT: Soil organic carbon (SOC) was partitioned between unprotected and protected pools in six forests along an elevation gradient in the southern Appalachian Mountains using two physical methods: flotation in aqueous CaCl2 (1.4 g/mL) and wet sieving through a 0.053 mm sieve. Both methods produced results that were qualitatively and quantitatively similar. Along the elevation gradient, 28 to 53% of the SOC was associated with an unprotected pool that included forest floor O-layers and other labile soil organic matter (SOM) in various stages of decomposition. Most (71 to 83%) of the C in the mineral soil at the six forest sites was identified as protected because of its association with a heavy soil fraction (> 1.4 g/mL) or a silt-clay soil fraction. Total inventories of SOC in the forests (to a depth of 30 cm) ranged from 384 to 1244 mg C/cm2 .The turnover time of the unprotected SOC was negatively correlated (r = –0.95, p < 0.05) with mean annual air temperature (MAT) across the elevation gradient. Measured SOC inventories, annual C returns to the forest floor, and estimates of C turnover associated with the protected soil pool were used to parameterize a simple model of SOC dynamics. Steady-state predictions with the model indicated that, with no change in C inputs, the low- (235–335 m), mid- (940–1000 m), and high- (1650–1670 m) elevation forests under study might surrender 40 to 45% of their current SOC inventory following a 4°C increase in MAT. Substantial losses of unprotected SOM as a result of a warmer climate could have long-term impacts on hydrology, soil quality, and plant nutrition in forest ecosystems throughout the southern Appalachian Mountains.

Fissore, C., C.P. Giardina, C.W. Swanston, G.M. King, R.K. Kolka (2009). Variable temperature sensitivity of soil organic carbon in North American forests. Global Change Biology 15 (9): 2295-2310

ABSTRACT: We investigated mean residence time (MRT) for soil organic carbon (SOC) sampled from paired hardwood and pine forests located along a 22 °C mean annual temperature (MAT) gradient in North America. We used acid hydrolysis fractionation, radiocarbon analyses, long-term laboratory incubations (525-d), and a three-pool model to describe the size and kinetics of the acid insoluble C (AIC), active and slow SOC fractions in soil. We found that active SOC was 2 ± 0.2% (mean ± SE) of total SOC, with an MRT of 33 ± 6 days that decreased strongly with increasing MAT. In contrast, MRT for slow SOC and AIC (70 ± 6% and 27 ± 6% of total SOC, respectively) ranged from decades to thousands of years, and neither was significantly related to MAT. The accumulation of AIC (as a percent of total SOC) was greater in hardwood than pine stands (36% and 21%, respectively) although the MRT for AIC was longer in pine stands. Based on these results, we suggest that the responsiveness of most SOC decomposition in upland forests to global warming will be less than currently modeled, but any shifts in vegetation from hardwood to pine may alter the size and MRT of SOC fractions

S. Hu, D. C. Coleman, C. R. Carroll, P. F. Hendrix, M. H. Beare (1997). Labile soil carbon pools in subtropical forest and agricultural ecosystems as influenced by management practices and vegetation types. Agriculture Ecosystems & Environment 65 (1): 69-78

ABSTRACT: Carbon storage in agricultural and forest soils has attracted attention recently due to its potential as a substantial carbon sink. Labile soil C pools are especially important because they are more vulnerable to climatic change and disturbance and play vital roles in nutrient cycling. Southern Appalachian forest soils and those from conventional tillage (CT), no-tillage (NT) and fescue sods at three sites in the Georgia piedmont were analyzed for total C, total N, carbohydrates, and microbial biomass C. The sizes of soil labile C pools (carbohydrates and microbial biomass) and their contributions to the total soil C pool differed significantly among ecosystems. The highest carbohydrate contents and microbial biomass C were found in forest soils, but agricultural soils had a significantly higher proportion of the soil organic matter present as carbohydrates and as microbial biomass. This difference probably reflects the quality of soil organic matter. Soil microbial biomass C was more sensitive to changes in management regimes than soil carbohydrates. Management practices signfiicantly affected organic C, carbohydrate contents, microbial biomass C and organic C turnover rates in agricultural soils, whereas differences in the quality of organic input due to different vegetation types substantially influenced soil labile C pools in forest soils. High mannose-to-xylose ratios in highly sandy agricultural soils indicate that plant-derived materials are rapidly metabolized by microorganisms and that organic C protection in sandy soils is largely dependent on reducing microbial access through effective residue management such as surface placement.

P. D. Falloon, P. Smith, J. U. Smith, J. Szabó, K. Coleman, S. Marshall (1998). Regional estimates of carbon sequestration potential: linking the Rothamsted Carbon Model to GIS databases. Biology and Fertility of Soils 27 (3): 236-241

ABSTRACT: Soil organic matter (SOM) represents a major pool of carbon within the biosphere. It is estimated at about 1400 Pg globally, which is roughly twice that in atmospheric CO2 . The soil can act as both a source and a sink for carbon and nutrients. Changes in agricultural land use and climate can lead to changes in the amount of carbon held in soils, thus, affecting the fluxes of CO2 to and from the atmosphere. Some agricultural management practices will lead to a net sequestration of carbon in the soil. Regional estimates of the carbon sequestration potential of these practices are crucial if policy makers are to plan future land uses to reduce national CO2 emissions. In Europe, carbon sequestration potential has previously been estimated using data from the Global Change and Terrestrial Ecosystems Soil Organic Matter Network (GCTE SOMNET). Linear relationships between management practices and yearly changes in soil organic carbon were developed and used to estimate changes in the total carbon stock of European soils. To refine these semi-quantitative estimates, the local soil type, meteorological conditions and land use must also be taken into account. To this end, we have modified the Rothamsted Carbon Model, so that it can be used in a predictive manner, with SOMNET data. The data is then adjusted for local conditions using Geographical Information Systems databases. In this paper, we describe how these developments can be used to estimate carbon sequestration at the regional level using a dynamic simulation model linked to spatially explicit data. Some calculations of the potential effects of afforestation on soil carbon stocks in Central Hungary provide a simple example of the system in use.

Batjes, N.H. (1996). Total carbon and nitrogen in the soils of the world. European Journal of Soil Science 47 (2): 151-163

ABSTRACT: The soil is important in sequestering atmospheric CO2 and in emitting trace gases (e.g. CO2 , CH4 and N2 O) that are radiatively active and enhance the 'greenhouse' effect. Land use changes and predicted global warming, through their effects on net primary productivity, the plant community and soil conditions, may have important effects on the size of the organic matter pool in the soil and directly affect the atmospheric concentration of these trace gases.

A discrepancy of approximately 350 × 1015 g (or Pg) of C in two recent estimates of soil carbon reserves worldwide is evaluated using the geo-referenced database developed for the World Inventory of Soil Emission Potentials (WISE) project. This database holds 4353 soil profiles distributed globally which are considered to represent the soil units shown on a 1/2° latitude by 1/2° longitude version of the corrected and digitized 1:5 M FAO–UNESCO Soil Map of the World.

Total soil carbon pools for the entire land area of the world, excluding carbon held in the litter layer and charcoal, amounts to 2157–2293 Pg of C in the upper 100 cm. Soil organic carbon is estimated to be 684–724 Pg of C in the upper 30 cm, 1462–1548 Pg of C in the upper 100 cm, and 2376–2456 Pg of C in the upper 200 cm. Although deforestation, changes in land use and predicted climate change can alter the amount of organic carbon held in the superficial soil layers rapidly, this is less so for the soil carbonate carbon. An estimated 695–748 Pg of carbonate-C is held in the upper 100 cm of the world's soils. Mean C: N ratios of soil organic matter range from 9.9 for arid Yermosols to 25.8 for Histosols. Global amounts of soil nitrogen are estimated to be 133–140 Pg of N for the upper 100 cm. Possible changes in soil organic carbon and nitrogen dynamics caused by increased concentrations of atmospheric CO2 and the predicted associated rise in temperature are discussed.

G. E. Schuman, H. H. Janzen, J. E. Herrick (2002). Soil carbon dynamics and potential carbon sequestration by rangelands. Environmental Pollution 116 (3): 391-396

ABSTRACT: The USA has about 336 Mha of grazing lands of which rangelands account for 48%. Changes in rangeland soil C can occur in response to a wide range of management and environmental factors. Grazing, fire, and fertilization have been shown to affect soil C storage in rangelands, as has converting marginal croplands into grasslands. Carbon losses due to soil erosion can influence soil C storage on rangelands both by reducing soil productivity in source areas and potentially increasing it in depositional areas, and by redistributing the C to areas where soil organic matter mineralization rates are different. Proper grazing management has been estimated to increase soil C storage on US rangelands from 0.1 to 0.3 Mg C ha−1 year−1 and new grasslands have been shown to store as much as 0.6 Mg C ha−1 year−1 . Grazing lands are estimated to contain 10–30% of the world’s soil organic carbon. Given the size of the C pool in grazing lands we need to better understand the current and potential effects of management on soil C storage.

M. W. Williams, P. D. Brooks, T. Seastedt (1998). Nitrogen and carbon soil dynamics in response to climate change in a high-elevation ecosystem in the Rocky Mountains, U.S.A.. Arctic and Alpine Research 30 (1): 26-30

ABSTRACT: We have implemented a long-term snow-fence experiment at the Niwot Ridge Long-Term Ecological Research (NWT) site in the Colorado Front Range of the Rocky Mountains, U.S.A., to assess the effects of climate change on alpine ecology and biogeochemical cycles. The responses of carbon (C) and nitrogen (N) dynamics in high-elevation mountains to changes in climate are investigated by manipulating the length and duration of snow cover with the 2.6 X 60 m snow fence, providing a proxy for climate change. Results from the first year of operation in 1994 showed that the period of continuous snow cover was increased by 90 d. The deeper and earlier snowpack behind the fence insulated soils from winter air temperatures, resulting in a 9 degrees C increase in annual minimum temperature at the soil surface. The extended period of snow cover resulted in subnivial microbial activity playing a major role in annual C and N cycling. The amount of C mineralized under the snow as measured by CO2 production was 22 g m-2 in 1993 and 35 g m-2 in 1994, accounting for 20 net primary above-ground production before construction of the snow fence in 1993 and 31fashion, maximum subnivial N2 O flux increased 3-fold behind the snow fence, from 75 mg N m-2 d-1 in 1993 to 250 mg N m-2 d-1 in 1994. The amount of N lost from denitrification was greater than the annual atmospheric input of N in snowfall. Surface litter decomposition studies show that there was a significant increase in the litter mass loss under deep and early snow, with no significant change under medium and little snow conditions. Changes in climate that result in differences in snow duration, depth, and extent may therefore produce large changes in the C and N soil dynamics of alpine ecosystems.

Post, W.M., K.C. Kwon (2000). Soil carbon sequestration and land use change: processes and potential. Global Change Biology 6 (3): 317-328

ABSTRACT: When agricultural land is no longer used for cultivation and allowed to revert to natural vegetation or replanted to perennial vegetation, soil organic carbon can accumulate by processes that essentially reverse some of the effects responsible for soil organic carbon losses from when the land was converted from perennial vegetation. We discuss the essential elements of what is known about soil organic matter dynamics that may result in enhanced soil carbon sequestration with changes in land-use and soil management. We review literature that reports changes in soil organic carbon after changes in land-use that favor carbon accumulation. This data summary provides a guide to approximate rates of SOC sequestration that are possible with management, and indicates the relative importance of some factors that influence the rates of organic carbon sequestration in soil. There is a large amount of variation in rates and the length of time that carbon may accumulate in soil that are related to the productivity of the recovering vegetation, physical and biological conditions in the soil, and the past history of soil organic carbon inputs and physical disturbance. Maximum rates of C accumulation during the early aggrading stage of perennial vegetation growth, while substantial, are usually much less than 100 g C m-2 y-1 . Average rates of accumulation are similar for forest or grassland establishment: 33.8 g C m-2 y-1 and 33.2 g C m-2 y-1 respectively. These observed rates of soil organic C accumulation, when combined with the small amount of land area involved, are insufficient to account for a significant fraction of the missing C in the global carbon cycle as accumulating in the soils of formerly agricultural land.

Neff, J.C., G.P. Asner (2001). Dissolved organic carbon in terrestrial ecosystems: synthesis and a model. Ecosystems 4 (1): 29-48

ABSTRACT: The movement of dissolved organic carbon (DOC) through soils is an important process for the transport of carbon within ecosystems and the formation of soil organic matter. In some cases, DOC fluxes may also contribute to the carbon balance of terrestrial ecosystems; in most ecosystems, they are an important source of energy, carbon, and nutrient transfers from terrestrial to aquatic ecosystems. Despite their importance for terrestrial and aquatic biogeochemistry, these fluxes are rarely represented in conceptual or numerical models of terrestrial biogeochemistry. In part, this is due to the lack of a comprehensive understanding of the suite of processes that control DOC dynamics in soils. In this article, we synthesize information on the geochemical and biological factors that control DOC fluxes through soils. We focus on conceptual issues and quantitative evaluations of key process rates to present a general numerical model of DOC dynamics. We then test the sensitivity of the model to variation in the controlling parameters to highlight both the significance of DOC fluxes to terrestrial carbon processes and the key uncertainties that require additional experiments and data. Simulation model results indicate the importance of representing both root carbon inputs and soluble carbon fluxes to predict the quantity and distribution of soil carbon in soil layers. For a test case in a temperate forest, DOC contributed 25% of the total soil profile carbon, whereas roots provided the remainder. The analysis also shows that physical factors-most notably, sorption dynamics and hydrology-play the dominant role in regulating DOC losses from terrestrial ecosystems but that interactions between hydrology and microbial-DOC relationships are important in regulating the fluxes of DOC in the litter and surface soil horizons. The model also indicates that DOC fluxes to deeper soil layers can support a large fraction (up to 30%) of microbial activity below 40 cm.

P. J. Zinke, A. G. Stangenberger (2000). Elemental storage of forest soil from local to global scales. Forest Ecology and Management 138 (1-3): 159-165

ABSTRACT; The storage of available soil fertility elements is a cornerstone of the sustainability of forest elemental cycles at the local scale, as well as to those on the global scale. Total soil elemental storage per m2 for relatively undisturbed sites utilizing cumulative depth functions of amounts of storage for local soil pit profiles was eventually expanded to a world wide database from more than 3000 sites. Examples are presented that apply these data to carbon and nitrogen storage problems ranging in scale from local sites in California’s Lake Tahoe Basin to soils at global levels.

P. S. Homann, M. Harmon, S. Remillard, E. A.H. Smithwick (2005). What the soil reveals: Potential total ecosystem C stores of the Pacific Northwest region, USA. Forest Ecology and Management 220 (1-3): 270-283

ABSTRACT: How much organic C can a region naturally store in its ecosystems? How can this be determined, when land management has altered the vegetation of the landscape substantially? The answers may lie in the soil: this study synthesized the spatial distribution of soil properties derived from the state soils geographic database with empirical measurements of old-growth forest ecosystem C to yield a regional distribution of potential maximum total-ecosystem organic C stores. The region under consideration is 179,000 square kilometers extending from the southern Oregon border to the northern Washington border, and from the Pacific Ocean to the east side of the Cascade Mountains. Total ecosystem organic C (TEC) was measured in 16 diverse old-growth forests encompassing 35 stands and 79 pedons to a depth of 100 cm. The TEC ranged between 185 and 1200 Mg C ha-1 . On an average, 63% of TEC was in the vegetation, 13% in woody detritus, 3% in the forest floor, 7% in the 0–20 cm mineral soil, and 13% in 20–100 cm mineral soil. The TEC was strongly related to soil organic C (SOC) in the 0–20 cm mineral soil, yielding a monotonically increasing, curvilinear relation. To apply this relation to estimate the TEC distribution throughout the region, 211 map units of the state soils geographic database (STATSGO) were used. The SOC in the 0–20 cm mineral soil of the map units was consistent with values from previously measured pedons distributed throughout the region. Resampling of 13 second-growth forests 25 years after initial sampling indicated no regional change in mineral SOC, and supported the use of a static state soils map. The SOC spatial distribution combined with the quantitative old-growth TEC–SOC relation yielded an estimate of potential TEC storage throughout the region under the hypothetical condition of old-growth forest coverage. The area-weighted TEC was 760 Mg C ha-1 . This is ~100 Mg C ha-1 more than a previous estimate based on a coarser resolution of six physiographic provinces, and ~400 Mg C ha-1 more than current regional stores. The map of potential TEC may be useful in forecasting regional C dynamics and in land-management decisions related to C sequestration.

Litton, C. M., Giardina, C. P. (2008). Below-ground carbon flux and partitioning: global patterns and response to temperature. Functional Ecology 22 (6): 941-954

ABSTRACT: 1. The fraction of gross primary production (GPP) that is total below-ground carbon flux (TBCF) and the fraction of TBCF that is below-ground net primary production (BNPP) represent globally significant C fluxes that are fundamental in regulating ecosystem C balance. However, global estimates of the partitioning of GPP to TBCF and of TBCF to BNPP, as well as the absolute size of these fluxes, remain highly uncertain.
2. Efforts to model below-ground processes are hindered by methodological difficulties for estimating below-ground C cycling, the complexity of below-ground interactions, and an incomplete understanding of the response of GPP, TBCF and BNPP to climate change. Due to a paucity of available data, many terrestrial ecosystem models and ecosystem-level studies of whole stand C use efficiency rely on assumptions that: (i) C allocation patterns across large geographic, climatic and taxonomic scales are fixed; and (ii) c. 50% of TBCF is BNPP.
3. Here, we examine available information on GPP, TBCF, BNPP, TBCF : GPP and BNPP : TBCF from a diverse global data base of forest ecosystems to understand patterns in below-ground C flux and partitioning, and their response to mean annual temperature (MAT).
4. MAT and mean annual precipitation (MAP) covaried strongly across the global forest data base (37 mm increase in MAP for every 1 °C increase in MAT). In all analyses, however, MAT was the most important variable explaining observed patterns in below-ground C processes.
5. GPP, TBCF and BNPP all increased linearly across the global scale range of MAT. TBCF : GPP increased significantly with MAT for temperate and tropical ecosystems (> 5 °C), but variability was high across the data set. BNPP : TBCF varied from 0·26 to 0·53 across the entire MAT gradient (−5 to 30 °C), with a much narrower range of 0·42 to 0·53 for temperate and tropical ecosystems (5 to 30 °C).
6. Variability in the data sets was moderate and clear exceptions to the general patterns exist that likely relate to other factors important for determining below-ground C flux and partitioning, in particular water availability and nutrient supply. Still, our results highlight global patterns in below-ground C flux and partitioning in forests in response to MAT that in part confirm previously held assumptions.

Crow, S. E., Lajtha, K., Filley, T. R., Swanston, C. W., Bowden, R. D., Caldwell, B. A. (2009). Sources of plant-derived carbon and stability of organic matter in soil: implications for global change. Global Change Biology 15 (8): 2003-2019

ABSTRACT: Alterations in forest productivity and changes in the relative proportion of above- and belowground biomass may have nonlinear effects on soil organic matter (SOM) storage. To study the influence of plant litter inputs on SOM accumulation, the Detritus Input Removal and Transfer (DIRT) Experiment continuously alters above- and belowground plant inputs to soil by a combination of trenching, screening, and litter addition. Here, we used biogeochemical indicators [i.e., cupric oxide extractable lignin-derived phenols and suberin/cutin-derived substituted fatty acids (SFA)] to identify the dominant sources of plant biopolymers in SOM and various measures [i.e., soil density fractionation, laboratory incubation, and radiocarbon-based mean residence time (MRT)] to assess the stability of SOM in two contrasting forests within the DIRT Experiment: an aggrading deciduous forest and an old-growth coniferous forest. In the deciduous forest, removal of both above- and belowground inputs increased the total amount of SFA over threefold compared with the control, and shifted the SFA signature towards a root-dominated source. Concurrently, light fraction MRT increased by 101 years and C mineralization during incubation decreased compared with the control. Together, these data suggest that root-derived aliphatic compounds are a source of SOM with greater relative stability than leaf inputs at this site. In the coniferous forest, roots were an important source of soil lignin-derived phenols but needle-derived, rather than root-derived, aliphatic compounds were preferentially preserved in soil. Fresh wood additions elevated the amount of soil C recovered as light fraction material but also elevated mineralization during incubation compared with other DIRT treatments, suggesting that not all of the added soil C is directly stabilized. Aboveground needle litter additions, which are more N-rich than wood debris, resulted in accelerated mineralization of previously stored soil carbon. In summary, our work demonstrates that the dominant plant sources of SOM differed substantially between forest types. Furthermore, inputs to and losses from soil C pools likely will not be altered uniformly by changes in litter input rates.

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