Climate Change and...

Annotated Bibliography

Carbon Dynamics

Agricultural Practices

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.

Adler, P. R., Del Grosso, S. J., Parton, W. J. (2007). Life-cycle assessment of net greenhouse-gas flux for bioenergy cropping systems. Ecological Applications 17 (3): 675-691

ABSTRACT: Bioenergy cropping systems could help offset greenhouse gas emissions, but quantifying that offset is complex. Bioenergy crops offset carbon dioxide emissions by converting atmospheric CO2 to organic C in crop biomass and soil, but they also emit nitrous oxide and vary in their effects on soil oxidation of methane. Growing the crops requires energy (e.g., to operate farm machinery, produce inputs such as fertilizer) and so does converting the harvested product to usable fuels (feedstock conversion efficiency). The objective of this study was to quantify all these factors to determine the net effect of several bioenergy cropping systems on greenhouse-gas (GHG) emissions. We used the DAYCENT biogeochemistry model to assess soil GHG fluxes and biomass yields for corn, soybean, alfalfa, hybrid poplar, reed canarygrass, and switchgrass as bioenergy crops in Pennsylvania, USA. DAYCENT results were combined with estimates of fossil fuels used to provide farm inputs and operate agricultural machinery and fossil-fuel offsets from biomass yields to calculate net GHG fluxes for each cropping system considered. Displaced fossil fuel was the largest GHG sink, followed by soil carbon sequestration. N2 O emissions were the largest GHG source. All cropping systems considered provided net GHG sinks, even when soil C was assumed to reach a new steady state and C sequestration in soil was not counted. Hybrid poplar and switchgrass provided the largest net GHG sinks, >200 g CO2 e-C·m-2 ·yr-1 for biomass conversion to ethanol, and >400 g CO2 e-C·m-2 ·yr-1 for biomass gasification for electricity generation. Compared with the life cycle of gasoline and diesel, ethanol and biodiesel from corn rotations reduced GHG emissions by 40%, reed canarygrass by 85%, and switchgrass and hybrid poplar by ~115%.

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.

Allamaras, R., Schomberg, H., Douglas, C., Dao, T. (2000). Soil organic carbon sequestration potential of adopting conservation tillage in U.S. croplands. Journal of Soil and Water Conservation 55 (3): 365-373

ABSTRACT: Spatial prediction and uncertainty assessment of ecological modeling and simulation systems are a difficult task because of system complexities that include multi components, their interaction and variability over space and time. Developing a general methodology and framework of uncertainty assessment for the systems' users has become very important. As the first part of a large study addressing these issues, the focus of this paper is on spatial prediction and uncertainty assessment of topographic factors involved in the Revised Universal Soil Loss Equation (RUSLE). The spatial variability of these topographic factors including slope steepness factor S, slope length factor L, and their combined LS factor were modeled with semivariogram models. Three geostatistical methods, including ordinary kriging, indicator kriging, and sequential indicator simulation, were applied and compared. The predicted value maps of these factors, their error variance or conditional variance maps, and probability maps for the predicted values larger than a given threshold value were derived. The comparison of the geostatistical methods suggests that sequential indicator simulation better than ordinary and indicator kriging.

Andren, Olof, Katterer, T. .., Karlsson, T. .. (2004). ICBM regional model for estimations of dynamics of agricultural soil carbon pools. Nutrient Cycling in Agroecosystems 70 (2): 231-239

ABSTRACT: Swedish arable land covers 3 Mha and its topsoil contains about 300 Mton C. The mineral soils seem to be close to steady-state, but the organic soils (about 10% of total arable land) have been estimated to lose ca. 1 Mton/year. We have devised a conceptual model (ICBMregion), using national agricultural crop yield/manuring statistics and allometric functions to calculate annual C input to the soil together with a five-parameter soil carbon model (ICBMr), calibrated using long-term field data. In Sweden, annual yield statistics are reported for different crops, for each of eight agricultural regions. Present topsoil carbon content and regional distribution of soil types have recently been measured. We use daily weather station data for each region together with crop type (bulked from individual crop data) and soil type to calculate an annual soil climate parameter for each crop/soil type permutation in each region. We use 14 soil types and 9 crop types, which gives 126 parameter sets for each year and region, each representing a fraction of the region's area. For each year, region, crop and soil type, ICBMregion calculates the change in young and old soil carbon per hectare, and sums up the changes to, e.g., national changes. With eight regions, we will have 1008 parameter sets per year, which easily can be handled, and what-if scenarios as well as comparisons between benchmark years are readily made. We will use the model to compare the soil C pools between the IPCC benchmark year 1990 and the present. In principle, we use inverse modelling from the sampled, recent soil C pools to estimate those in 1990. In the calculations, soil climate and yield for each year from 1990 onwards are taken into account. Then we can project soil C balances into the future under different scenarios, e.g., business as usual, land use change or changes in agricultural crops or cultivation practices. Projections of regional climate change are also available, so we can quite easily make projections of soil C dynamics under, e.g., different climate scenarios. We can follow the dynamic effects of carbon sequestration efforts – and estimate their efficiency. The approach is conceptually simple, fairly complete, and can easily be adapted to different needs and availability of data. However, perhaps the greatest advantage is that the results from this comprehensive approach used for, e.g., a 10-year period, can be condensed into a very simple spreadsheet model for calculating effects of management/land use changes on C stocks in agricultural soils.

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.

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.

Bernacchi, C. J., Hollinger, S. E., Meyers, T. (2005). The conversion of the corn/soybean ecosystem to no-till agriculture may result in a carbon sink.. Global Change Biology 11 (11): 1867-1872

ABSTRACT: Mitigating or slowing an increase in atmospheric carbon dioxide concentration ([CO2 ]) has been the focus of international efforts, most apparent with the development of the Kyoto Protocol. Sequestration of carbon (C) in agricultural soils is being advocated as a method to assist in meeting the demands of an international C credit system. The conversion of conventionally tilled agricultural lands to no till is widely accepted as having a large-scale sequestration potential. In this study, C flux measurements over a no-till corn/soybean agricultural ecosystem over 6 years were coupled with estimates of C release associated with agricultural practices to assess the net biome productivity (NBP) of this no-till ecosystem. Estimates of NBP were also calculated for the conventionally tilled corn/soybean ecosystem assuming net ecosystem exchange is C neutral. These measurements were scaled to the US as a whole to determine the sequestration potential of corn/soybean ecosystems, under current practices where 10% of agricultural land devoted to this ecosystem is no-tilled and under a hypothetical scenario where 100% of the land is not tilled. The estimates of this analysis show that current corn/soybean agriculture in the US releases ~7.2 Tg C annually, with no-till sequestering ~2.2 Tg and conventional-till releasing ~9.4 Tg. The complete conversion of land area to no till might result in 21.7 Tg C sequestered annually, representing a net C flux difference of ~29 Tg C. These results demonstrate that large-scale conversion to no-till practices, at least for the corn/soybean ecosystem, could potentially offset ca. 2% of annual US carbon emissions.

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.

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.

Bondeau, A., Smith, P.C., Zaehle, S., Schaphoff, S., Lucht, W., Cramer, W., Gerten, D., Lotze-Campen, H., Müller, C., Reichstein, M., Smith, B. (2007). Modelling the role of agriculture for the 20th century global terrestrial carbon balance. Global Change Biology 13 (3): 679-706

ABSTRACT: In order to better assess the role of agriculture within the global climate-vegetation system, we present a model of the managed planetary land surface, Lund–Potsdam–Jena managed Land (LPJmL), which simulates biophysical and biogeochemical processes as well as productivity and yield of the most important crops worldwide, using a concept of crop functional types (CFTs). Based on the LPJ-Dynamic Global Vegetation Model, LPJmL simulates the transient changes in carbon and water cycles due to land use, the specific phenology and seasonal CO2 fluxes of agricultural-dominated areas, and the production of crops and grazing land. It uses 13 CFTs (11 arable crops and two managed grass types), with specific parameterizations of phenology connected to leaf area development. Carbon is allocated daily towards four carbon pools, one being the yield-bearing storage organs. Management (irrigation, treatment of residues, intercropping) can be considered in order to capture their effect on productivity, on soil organic carbon and on carbon extracted from the ecosystem. For transient simulations for the 20th century, a global historical land use data set was developed, providing the annual cover fraction of the 13 CFTs, rain-fed and/or irrigated, within 0.5° grid cells for the period 1901–2000, using published data on land use, crop distributions and irrigated areas. Several key results are compared with observations. The simulated spatial distribution of sowing dates for temperate cereals is comparable with the reported crop calendars. The simulated seasonal canopy development agrees better with satellite observations when actual cropland distribution is taken into account. Simulated yields for temperate cereals and maize compare well with FAO statistics. Monthly carbon fluxes measured at three agricultural sites also compare well with simulations. Global simulations indicate a ~24% (respectively ~10%) reduction in global vegetation (respectively soil) carbon due to agriculture, and 6–9 Pg C of yearly harvested biomass in the 1990s. In contrast to simulations of the potential natural vegetation showing the land biosphere to be an increasing carbon sink during the 20th century, LPJmL simulates a net carbon source until the 1970s (due to land use), and a small sink (mostly due to changing climate and CO2 ) after 1970. This is comparable with earlier LPJ simulations using a more simple land use scheme, and within the uncertainty range of estimates in the 1980s and 1990s. The fluxes attributed to land use change compare well with Houghton's estimates on the land use related fluxes until the 1970s, but then they begin to diverge, probably due to the different rates of deforestation considered. The simulated impacts of agriculture on the global water cycle for the 1990s are ~5% (respectively ~20%) reduction in transpiration (respectively interception), and ~44% increase in evaporation. Global runoff, which includes a simple irrigation scheme, is practically not affected.

Bosch, D. J. (2008). Effects of rotational grazing on carbon dioxide emissions an greenhouse gas credits. Journal of Soil & Water Conservation 63 (2): 51A

EXCERPT: Intensive rotational grazing systems are being promoted as a way to increase forage production and improve soil conditions relative to conventional grazing. Converting a livestock farm operation from conventional to an intensive rotational grazing system may also reduce greenhouse gas emissions, including carbon dioxide (CO2 ), nitrous oxides, and methane. Improved forage quality and animal health may reduce animal methane emissions and increase the carbon sequestration potential of pasture soils. Conversion to intensive rotational grazing systems allows substitution of grass for harvested feed and reduces on-farm row crop production. Such substitutions have the potential to reduce nitrous oxide emissions through lower fertilizer applications and further enhance carbon sequestration in soil.

We examined the potential to reduce CO2 -equivalent emissions for a dairy farm and a cow-calf farm in Virginia. Greenhouse gas emissions were estimated under three farm boundary definitions and two emissions accounting metrics. A farm boundary analysis defines how much of the farm is considered when emissions reductions are considered. The three boundary conditions were (1) the pasture only, (2) the physical boundary of the entire farm, and (3) an extended boundary that includes the physical farm plus feed imports. The accounting metrics define the units of production activity over which …

Bricklemyer, R. S., Lawrence, R. L., Miller, P.R., Battogtokh, N. (2007). Monitoring and verifying agricultural practices related to soil carbon sequestration with satellite imagery. Agriculture, Ecosystems & Environment 118 (1-4): 201-210

ABSTRACT: The Kyoto Protocol entering into force on 16 February 2005 continues to spur interest in development of carbon trading mechanisms internationally and domestically. Critical to the development of a carbon trading effort is verification that carbon has been sequestered, and field level measurement of C change is likely cost prohibitive. Estimating C change based on agricultural management practices related to carbon sequestration seems more realistic, and analysis of satellite imagery could be used to monitor and verify these practices over large areas. We examined using Landsat imagery to verify crop rotations and quantify crop residue biomass in north central Montana. Field data were collected using a survey of farms. Standard classification tree analysis (CTA) and boosted classification and regression tree analysis (BCTA) were used to classify crop types. Linear regression (LM), regression tree analysis (RTA), and stochastic gradient boosting (SGB) were used to estimate crop residue. Six crop types were classified with 97% accuracy (BCTA) with class accuracies of 88–99%. Paired t-tests were used to compare the difference between known and predicted mean crop residue biomass. The difference between known and predicted mean residues using SGB was not different than 0 (p-value = 0.99); however root mean square error (RMSE) was large (1981 kg ha−1 ), implying that SGB accurately predicted regional crop residue biomass but not local predictions (i.e., field or farm level). The results of this study, and previous research classifying tillage practices and estimating soil disturbance, supports using satellite imagery as an effective tool for monitoring and verifying agricultural management practices related to carbon sequestration over large areas.

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.

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.

Chen, H., Tian, H. Q., Liu, M. L., Melillo, J., Pan, S. F., Zhang, C. (2006). Effect of land-cover change on terrestrial carbon dynamics in the southern United States. Journal of Environmental Quality 35 (4): 1533-1547

ABSTRACT: Received for publication May 17, 2005. Land-cover change has significant influence on carbon storage and fluxes in terrestrial ecosystems. The southern United States is thought to be the largest carbon sink across the conterminous United States. However, the spatial and temporary variability of carbon storage and fluxes due to land-cover change in the southern United States remains unclear. In this study, we first reconstructed the annual data set of land-cover of the southern United States from 1860 to 2003 with a spatial resolution of 8 km. Then we used a spatially explicit process-based biogeochemical model (Terrestrial Ecosystem Model [TEM] 4.3) to simulate the effects of cropland expansion and forest regrowth on the carbon dynamics in this region. The pattern of land-cover change in the southern United States was primarily driven by the change of cropland, including cropland expansion and forest regrowth on abandoned cropland. The TEM simulation estimated that total carbon storage in the southern United States in 1860 was 36.8 Pg C, which likely was overestimated, including 10.8 Pg C in the southeast and 26 Pg C in the south-central. During 1860–2003, a total of 9.4 Pg C, including 6.5 Pg C of vegetation and 2.9 Pg C of soil C pool, was released to the atmosphere in the southern United States. The net carbon flux due to cropland expansion and forest regrowth on abandoned cropland was approximately zero in the entire southern region between 1980 and 2003. The temporal and spatial variability of regional net carbon exchange was influenced by land-cover pattern, especially the distribution of cropland. The land-use analysis in this study is incomplete and preliminary. Finally, the limitations, improvements, and future research needs of this study were discussed.

Christopher, S. F., Lal, R. (2007). Nitrogen management affects carbon sequestration in North American cropland soils. Critical Reviews in Plant Sciences 26 (1): 45-64

ABSTRACT: Agricultural soils in North America can be a sink for rising atmospheric CO2 concentrations through the formation of soil organic matter (SOM) or humus. Humification is limited by the availability of nutrients such as nitrogen (N). Recommended management practices (RMPs) that optimize N availability foster humus formation. This review examines the management practices that contribute to maximizing N availability for optimizing sequestration of atmospheric CO2 into soil humus. Farming practices that enhance nutrient use, reduce or eliminate tillage, and increase crop intensity, together, affect N availability and, therefore, C sequestration. N additions, from especially, livestock manure and leguminous cover crops are necessary for increasing grain and biomass yields and returning crop residues to the soil thereby increasing soil organic carbon (SOC) concentration. Conservation tillage practices enhance also the availability of N and increase SOC concentration. Increase in cropping intensity and/or crop rotations produce higher quantity and quality of residues, increase availability of N, and therefore foster increase in C sequestration. The benefit of C sequestration from N additions may be negated by CO2 and N2 O emissions associated with production and application of N fertilizers. More studies need to be conducted to ascertain the benefits of adding N via manuring versus N fertilizer additions. Furthermore, site specific adaptive research is needed to identify RMPs that optimize soil N use efficiency while improving crop yield and C sequestration thereby curbing greenhouse gas (GHG) emissions. Due to the wide range of climate in North America, there is a large range of C sequestration potential in agricultural soils through N management. Humid croplands may have the potential to sequester 8-298 Tg C yr-1 while dry croplands may sequester 1-35 Tg C yr-1 . These estimates, however, are highly uncertain and wide-ranging. Clearly, more research is needed to quantify, more precisely, the C sequestration potential across different N management scenarios especially in Mexico and Canada.

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.

R. L. Cochran, H. P. Collins, A. Kennedy, D. F. Bezdicek (2007). Soil carbon pools and fluxes after land conversion in a semiarid shrub-steppe ecosystem. Biology and Fertility of Soils 43 (4): 479-489

ABSTRACT: Worldwide soil carbon (C) losses associated with agricultural expansion and intensification have contributed significantly to increased atmospheric CO2 . Soil disturbances resulting from land use changes were shown to modify the turnover of C and the formation of soil organic matter. A native semiarid shrub-steppe ecosystem recently converted into an irrigated agricultural development in the Columbia Basin of Washington State was evaluated for several abiotic indicators that might signal changes in an ecosystem during the initial stages of conversion and disturbance. Soil samples were collected in March of 2003 and 2004 from nine sites that included native shrub-steppe and agricultural fields converted in 2001 and 2002. Disturbance from conversion to irrigated crop production influenced total organic C and nitrogen (N) storage, C and N mineralization, and C turnover. Cultivated fields had greater concentrations of total organic C and N and higher cumulative C and N mineralization than native sites after 3 years of cultivation. Soil organic C was divided into three pools: an active pool (C a ) consisting of labile C (simple sugars, organic acids, the microbial biomass, and metabolic compounds of incorporated plant residues) with a mean residence time of days, an intermediate or slow pool (C s ) consisting of structural plant residues and physically stabilized C, and a resistant fraction (C r ) consisting of lignin and chemically stabilized C. Extended laboratory incubations of soil with measurements of CO2 were used to differentiate the size and turnover of theC a andC s functional C pools. The active pools were determined to be 4.5 and 6.5% and slow pools averaged 44 and 47% of the total C in native and cultivated fields, respectively. Cultivation, crop residue incorporation, and dairy manure compost amendments contributed to the increase in total soil C.

Cole, C. V., Flach, K., Lee, J., Sauerbeck, D., Stewart, B. (1993). Agricultural sources and sinks of carbon. Water Air and Soil Pollution 70 (1-4): 111-122

ABSTRACT: Most existing agricultural lands have been in production for sufficiently long periods that C inputs and outputs are nearly balanced and they are neither a major source nor sink of atmospheric C. As population increases, food requirements and the need for more crop land increase accordingly. An annual conversion of previously uncultivated lands up to 1.5 × 107 hectares may be expected. It is this new agricultural land which suffers the greatest losses of C during and subsequent to its conversion. The primary focus for analysis of future C fluxes in agroecosystems needs to be on current changes in land use and management as well as on direct effects of CO2 and climate change. A valid assessment of C pools and fluxes in agroecosystems requires a global soils data base and comprehensive information on land use and management practices. A comprehensive effort to assemble and analyze this information is urgently needed.

Cole, C. V., Paustian, K., Elliott, E. T., Metherell, A. K., Ojima, D. S., Parton, W. J. (1993). Analysis of agroecosystem carbon pools. Water Air and Soil Pollution 70 (1-4): 357-371

ABSTRACT: We present analyses of major driving variable controls on soil C in agroecosystems. Historical changes in soil C storage in agricultural soils are characterized by large losses during transition from natural grasslands and forests. A major driver in more recent times is the steadily increasing rate of net primary production of major land areas in agriculture. Simulation and analytical models are used to predict trajectories and potential soil C storage under possible scenarios of changed management and climate. Database and analytical requirements for extrapolation from regional to global scales are outlined.

Collins, H. P., Elliott, E. T., Paustian, K., Bundy, L. G., Dick, W. A., Huggins, D. R., Smucker, A. J. M., Paul, E. A. (2000). Soil carbon pools and fluxes in long-term corn belt agroecosystems. Soil Biology and Biochemistry. 32 (2): 157-168.

ABSTRACT: The dynamics of soil organic carbon (SOC) play an important role in long-term ecosystem productivity and the global C cycle. We used extended laboratory incubation and acid hydrolysis to analytically determine SOC pool sizes and fluxes in US Corn Belt soils derived from both forest and prairie vegetation. Measurement of the natural abundance of13 C made it possible to follow the influence of continuous corn on SOC accumulation. The active pools (Ca ) comprised 3 to 8% of the SOC with an average field mean residence time (MRT) of 100 d. The slow pools (Cs ) comprised 50% of SOC in the surface and up to 65% in subsoils. They had field MRTs from 12–28 y for C4-C and 40–80 y for C3-derived C depending on soil type and location. No-till management increased the MRT of the C3-C by 10–15 y above conventional tillage. The resistant pool (Cr) decreased from an average of 50% at the surface to 30% at depth. The isotopic composition of SOC mineralized during the early stages of incubation reflected accumulations of labile C from the incorporation of corn residues. The CO2 released later reflected13 C characteristic of the Cs pool. The13 C of the CO2 did not equal that of the whole soil until after 1000 d of incubation. The SOC dynamics determined by acid hydrolysis, incubation and13 C content were dependent on soil heritage (prairie vs. forest), texture, cultivation and parent material, depositional characteristics. Two independent methods of determining C3 pool sizes derived from C3 vegetation gave highly correlated values.

Dalal, R.C., Allen, D.E., Livesley, S.J., Richards, G. (2007). Magnitude and biophysical regulators of methane emission and consumption in the Australian agricultural, forest, and submerged landscapes: a review. Plant And SoilPlant Soil 309 (1-2): 43-76

ABSTRACT: Increases in the concentrations of atmospheric greenhouse gases, carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2 O) due to human activities are associated with global climate change. CO2 concentration in the atmosphere has increased by 33% (to 380 ppm) since 1750 ad, whilst CH4 concentration has increased by 75% (to 1,750 ppb), and as the global warming potential (GWP) of CH4 is 25 fold greater than CO2 it represents about 20% of the global warming effect. The purpose of this review is to: (a) address recent findings regarding biophysical factors governing production and consumption of CH4 , (b) identify the current level of knowledge regarding the main sources and sinks of CH4 in Australia, and (c) identify CH4 mitigation options and their potential application in Australian ecosystems. Almost one-third of CH4 emissions are from natural sources such as wetlands and lake sediments, which is poorly documented in Australia. For Australia, the major anthropogenic sources of CH4 emissions include energy production from fossil fuels (~24%), enteric fermentation in the guts of ruminant animals (~59%), landfills, animal wastes and domestic sewage (~15%), and biomass burning (~5%), with minor contributions from manure management (1.7%), land use, land-use change and forestry (1.6%), and rice cultivation (0.2%). A significant sink exists for CH4 (~6%) in aerobic soils, including agricultural and forestry soils, and potentially large areas of arid soils, however, due to limited information available in Australia, it is not accounted for in the Australian National Greenhouse Gas Inventory. CH4 emission rates from submerged soils vary greatly, but mean values ≤10 mgm−2 h−1 are common. Landfill sites may emit CH4 at one to three orders of magnitude greater than submerged soils. CH4 consumption rates in non-flooded, aerobic agricultural, pastoral and forest soils also vary greatly, but mean values are restricted to ≤100μg m−2 h−1 , and generally greatest in forest soils and least in agricultural soils, and decrease from temperate to tropical regions. Mitigation options for soil CH4 production primarily relate to enhancing soil oxygen diffusion through water management, land use change, minimised compaction and soil fertility management. Improved management of animal manure could include biogas capture for energy production or arable composting as opposed to open stockpiling or pond storage. Balanced fertiliser use may increase soil CH4 uptake, reduce soil N2 O emissions whilst improving nutrient and water use efficiency, with a positive net greenhouse gas (CO2 -e) effect. Similarly, the conversion of agricultural land to pasture, and pastoral land to forestry should increase soil CH4 sink. Conservation of native forests and afforestation of degraded agricultural land would effectively mitigate CH4 emissions by maintaining and enhancing CH4 consumption in these soils, but also by reducing N2 O emissions and increasing C sequestration. The overall impact of climate change on methanogenesis and methanotrophy is poorly understood in Australia, with a lack of data highlighting the need for long-term research and process understanding in this area. For policy addressing land-based greenhouse gas mitigation, all three major greenhouse gases (CO2 , CH4 and N2 O) should be monitored simultaneously, combined with improved understanding at process-level.

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.

J. D. Derner, G. E. Schuman (2007). Carbon sequestration and rangelands: A synthesis of land management and precipitation effects. Journal of Soil and Water Conservation 62 (2): 77-85

ABSTRACT: Management of rangelands can aid in the mitigation of rising atmospheric carbon dioxide concentrations via carbon storage in biomass and soil organic matter, a process termed carbon sequestration. Here we provide a review of current knowledge on the effects of land management practices (grazing, nitrogen inputs, and restoration) and precipitation on carbon sequestration in rangelands. Although there was no statistical relationship between change in soil carbon with longevity of the grazing management practice in native rangelands of the North American Great Plains, the general trend seems to suggest a decrease in carbon sequestration with longevity of the grazing management practice across stocking rates. The relationship of carbon sequestration to mean annual precipitation is negative for both the 0 to 10 cm (0 to 3.9 in) and 0 to 30 cm (0 to 11.8 in) soil depths across stocking rates. The threshold from positive to negative carbon change occurs at approximately 440 mm (17.3 in) of precipitation for the 0 to 10 cm soil depth and at 600 mm (23.6 in) for the 0 to 30 cm soil depth. We acknowledge that largely unexplored is the arena of management-environment interactions needed to increase our understanding of climate-plant-soil-microbial interactions as factors affecting nutrient cycling. Continued refinement of estimates of terrestrial carbon storage in rangelands will assist in the development of greenhouse gas emissions and carbon credit marketing policies, as well as potentially modifying government natural resource conservation programs to emphasize land management practices that increase carbon sequestration.

Fearnside, P. M., Imbrozio Barbosa, R. (1998). Soil carbon changes from conversion of forest to pasture in Brazilian Amazonia. Forest Ecology and Management 108 (1-2): 147-166

ABSTRACT: Soils in Brazilian Amazonia may contain up to 136 Gt of carbon to a depth of 8 m, of which 47 Gt are in the top meter. The current rapid conversion of Amazonian forest to cattle pasture makes disturbance of this carbon stock potentially important to the global carbon balance and net greenhouse gas emissions. Information on the response of soil carbon pools to conversion to cattle pasture is conflicting. Some of the varied results that have been reported can be explained by effects of soil compaction, clay content and seasonal changes. Most studies have compared roughly simultaneous samples taken at nearby sites with different use histories (i.e., `chronosequences'); a clear need exists for longitudinal studies in which soil carbon stocks and related parameters are monitored over time at fixed locations. Whether pasture soils are a net sink or a net source of carbon depends on their management, but an approximation of the fraction of pastures under 'typical' and 'ideal' management practices indicates that pasture soils in Brazilian Amazonia are a net carbon source, with the upper 8 m releasing an average of 12.0 t C/ha in land maintained as pasture in the equilibrium landscape that is established in the decades following deforestation. Considering the equilibrium landscape as a whole, which is dominated by pasture and secondary forest derived from pasture, the average net release of soil carbon is 8.5 t C/ha, or 11.7x106 t C for the 1.38x106 ha cleared in 1990. Only 3% of the calculated emission comes from below 1 m depth, but the ultimate contribution from deep layers may be substantially greater. The land area affected by soil C losses under pasture is not restricted to the portion of the region maintained under pasture in the equilibrium landscape, but also the portion under secondary forests derived from pasture. Pasture effects from deforestation in 1990 represent a net committed emission from soils of 9.2x106 t C, or 79% of the total release from soils from deforestation in that year. Soil emissions from Amazonian deforestation represent a quantity of carbon approximately 20% as large as Brazil's annual emission from fossil fuels.

Follett, R. F., Shafer, S. R., Jawson, M. D., Franzluebbers, A. J. (2005). Research and implementation needs to mitigate greenhouse gas emissions from agriculture in the USA. Soil and Tillage Research 83 (1): 159-166

ABSTRACT: An urgent need exists to understand which agricultural land uses and land resource types have the greatest potential to mitigate greenhouse gas (GHG) emissions contributing to global change. Global change is a natural resource issue increasingly contributed to by human activities that now joins other important issues facing agricultural scientists, such as depletion of soil organic carbon (SOC), soil degradation and contamination, and pollution of natural waters by soil sediments and nutrients. Increasing demand for food by the growing global population is resulting in increased GHG emissions, soil disturbance, fossil fuel consumption to produce agricultural products, and biomass burning. To address these issues and the threat of accelerated GHG emissions, this paper addresses: (1) current scientific facts about the attributes of soil and natural resources, (2) strategies for sustainable use of our finite, non-renewable, and fragile land resources, and (3) advances made by agricultural sciences and their potential role in forming policy.

Site-specific adaptation of appropriate conservation technologies will be needed for sequestering SOC and reducing nitrous oxide (N2 O) emission. Adoption of improved conservation technologies to mitigate GHG emission should consider: (i) the rate of C sequestration or GHG mitigation, (ii) the price offered for adopting various practices, (iii) the ease with which producers and land managers can alter land use and management activities, (iv) the potential impacts of targeting regions or practices, (v) the ancillary benefits to soil, water and air quality upon adoption of practices to sequester SOC or mitigate GHG emission, and (vi) the effectiveness and efficiency of various policies.

Development of improved conservation technologies to reduce GHG emissions could become part of more comprehensive conservation programs aimed at environmental protection, food security, and agricultural sustainability. An overarching research need is to determine the multiple benefits and trade-offs of improved conservation technologies so that land managers can systematically meet production and environmental goals and so that the most effective policies can be devised.

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.

Grace, P. R., Colunga-Garcia, M., Gage, S. H., Robertson, G. P., Safir, G. R. (2006). The potential impact of agricultural management and climate change on soil organic carbon of the North Central Region of the United States. Ecosystems 9 (5): 816-827

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. We have linked net primary production algorithms, which include the impact of enhanced atmospheric CO2 on plant growth, to the Soil Organic Carbon Resources And Transformations in EcoSystems (SOCRATES) model to develop a SOC map for the North Central Region of the United States between the years 1850 and 2100 in response to agricultural activity and climate conditions generated by the CSIRO Mk2 Global Circulation Model (GCM) and based on the Intergovernmental Panel for Climate Change (IPCC) IS92a emission scenario. We estimate that the current day (1990) stocks of SOC in the top 10 cm of the North Central Region to be 4692 Mt, and 8090 Mt in the top 20 cm of soil. This is 19% lower than the pre-settlement steady state value predicted by the SOCRATES model. By the year 2100, with temperature and precipitation increasing across the North Central Region by an average of 3.9°C and 8.1 cm, respectively, SOCRATES predicts SOC stores of the North Central Region to decline by 11.5 and 2% (in relation to 1990 values) for conventional and conservation tillage scenarios, respectively.

Grandy, A., Robertson, G. (2007). Land-use intensity effects on soil organic carbon accumulation rates and mechanisms. Ecosystems 10 (1): 59-74

ABSTRACT: Restoring soil C pools by reducing land use intensity is a potentially high impact, rapidly deployable strategy for partially offsetting atmospheric CO2 increases. However, rates of C accumulation and underlying mechanisms have rarely been determined for a range of managed and successional ecosystems on the same soil type. We determined soil organic matter (SOM) fractions with the highest potential for sequestering C in ten ecosystems on the same soil series using both density- and incubation-based fractionation methods. Ecosystems included four annual row-crop systems (conventional, low input, organic and no-till), two perennial cropping systems (alfalfa and poplar), and four native ecosystems (early successional, midsuccessional historically tilled, midsuccessional never-tilled, and late successional forest). Enhanced C storage to 5 cm relative to conventional agriculture ranged from 8.9 g C m−2 y−1 in low input row crops to 31.6 g C m−2 y−1 in the early successional ecosystem. Carbon sequestration across all ecosystems occurred in aggregate-associated pools larger than 53μm. The density-based fractionation scheme identified heavy-fraction C pools (SOM > 1.6 g cm−3 plus SOM < 53μm), particularly those in macroaggregates (>250 μm), as having the highest potential C accumulation rates, ranging from 8.79 g Cm−2 y−1 in low input row crops to 29.22 g C m−2 y−1 in the alfalfa ecosystem. Intra-aggregate light fraction pools accumulated C at slower rates, but generally faster than in inter-aggregate LF pools. Incubation-based methods that fractionated soil into active, slow and passive pools showed that C accumulated primarily in slow and resistant pools. However, crushing aggregates in a manner that simulates tillage resulted in a substantial transfer of C from slow pools with field mean residence times of decades to active pools with mean residence times of only weeks. Our results demonstrate that soil C accumulates almost entirely in soil aggregates, mostly in macroaggregates, following reductions in land use intensity. The potentially rapid destruction of macroaggregates following tillage, however, raises concerns about the long-term persistence of these C pools.

Harper, R.J., Beck, A.C., Ritson, P., Hill, M.J., Mitchell, C.D., Barrett, D.J., Smettem, K.R.J., Mann, S.S. (2007). The potential of greenhouse sinks to underwrite improved land management. Ecological Engineering 29 (4): 329-341

ABSTRACT: The current agricultural systems of broad areas of Australia are unsustainable, with large projected increases in salinization, decreases in water quality, wind erosion, and losses of biodiversity. It is well known that these problems can be partially resolved by farmland reforestation; however, a major issue is financing the scale of activity required. The international response to global warming, the United Nations Framework Convention on Climate Change and its Kyoto Protocol, includes provisions that enable greenhouse sinks (sequestration of carbon in soils and vegetation) to be used by parties to fulfil their obligations. The Kyoto Protocol also allows for trading in emission reductions, and this opens the possibility that investment in carbon sinks may help underwrite broader natural resource management objectives. This paper examines the possibilities for improved land management in Western Australia arising from the development of carbon sinks by considering: (a) the likelihood of a carbon market developing and the likely depth of that market as a result of current national and international policies, (b) the data available to provide estimates on different types of sinks, and (c) the likely benefits of wide-scale sink investment.

It was estimated that the total amount of carbon that could be sequestered by reforesting 16.8 Mha of cleared farmland is 2200 Mt CO2 -e, and between 290 and 1170 Mt CO2 -e by destocking 94.8 Mha of rangelands. There were insufficient data to produce estimates of sequestration following changes in tillage practice in cropping systems or the revegetation of already salinized land. We conclude that carbon sinks are only likely to become profitable as a broad-scale stand-alone enterprise when carbon prices reach AUD$15/t CO2 -e, with this threshold value varying with carbon yield and project costs. Below this price, their value can be significant as an adjunct to reforestation schemes that are aimed at providing other products (wood, pulp, bioenergy) and land and water conservation benefits. Irrespective of this, carbon sinks provide an opportunity to both sequester carbon in a least-cost fashion and improve soil and watershed management.

Hobbs, P. R. (2007). Conservation agriculture: what is it and why is it important for future sustainable food production?. Journal of Agricultural Science 145: 127-137

ABSTRACT: Conservation agriculture (CA), defined as minimal soil disturbance (no-till) and permanent soil cover (mulch) combined with rotations, is a more sustainable cultivation system for the future than those presently practised. The present paper first introduces the reasons for tillage in agriculture and discusses how this age-old agricultural practice is responsible for the degradation of natural resources and soils. The paper goes on to introduce conservation tillage (CT), a minimum tillage and surface mulch practice that was developed in response to the severe wind erosion caused by mouldboard tillage of grasslands and referred to as the American dust bowl of the 1930s. CT is then compared with CA, a suggested improvement on CT, where no-till, mulch, and rotations significantly improve soil properties (physical, biological, and chemical) and other biotic factors, enabling more efficient use of natural resources. CA can improve agriculture through improvement in water infiltration and reducing erosion, improving soil surface aggregates, reducing compaction through promotion of biological tillage, increasing surface soil organic matter and carbon content, moderating soil temperatures, and suppressing weeds. CA also helps reduce costs of production, saves time, increases yield through more timely planting, reduces diseases and pests through stimulation of biological diversity, and reduces greenhouse gas emissions. Availability of suitable equipment is a major constraint to successful CA, but advances in design and manufacture of seed drills by local manufacturers are enabling farmers to experiment and accept this technology in many parts of the world. Estimates of farmer adoption of CA are close to 100 million ha in 2005, indicating that farmers are convinced of the benefits of this technology. The paper concludes that agriculture in the next decade will have to produce more food, sustainably, from less land through more efficient use of natural resources and with minimal impact on the environment in order to meet growing population demands. This will be a significant challenge for agricultural scientists, extension personnel, and farmers. Promoting and adopting CA management systems can help meet this complex goal.

Hutchinson, J.J., Campbell, C.A., Desjardins, R.L. (2007). Some perspectives on carbon sequestration in agriculture: the contribution of agriculture to the state of climate. Agricultural and Forest Meteorology 142 (2-4): 288-302

ABSTRACT: One of the main options for greenhouse gas (GHG) mitigation identified by the IPCC is the sequestration of carbon in soils. Since the breaking of agricultural land in most regions, the carbon stocks have been depleted to such an extent, that they now represent a potential sink for CO2 removal from the atmosphere. Improved management will however, be required to increase the inputs of organic matter in the top soil and/or decrease decomposition rates. In this paper we use data from selected regions to explore the global potential for carbon sequestration in arable soils. While realising that C sequestration is not limited to the selected regions, we have, however, focussed our review on two regions: (i) Canadian Prairies and (ii) The Tropics. In temperate regions, management changes for an increase in C involve increase in cropping frequency (reducing bare fallow), increasing use of forages in crop rotations, reducing tillage intensity and frequency, better crop residue management, and adopting agroforestry. In the tropics, agroforestry remains the primary method by which sequestration rates may be significantly increased. Increases in soil C may be achieved through improved fertility of cropland/pasture; on extensive systems with shifting cultivation cropped fallows and cover crops may be beneficial, and adopting agro forestry or foresting marginal cropland is also an alternative. In addition, in the tropics it is imperative to reduce the clearing of forests for conversion to cropland. Some regional analyses of soil C sequestration and sequestration potential have been performed, mainly for temperate industrialized North America where the majority of research pertaining to C sequestration has been carried out. More research is needed, especially for the Tropics, to more accurately capture the impact of region-specific interactions between climate, soil, and management of resources on C sequestration, which are lost in global level assessments. By itself, C sequestration in agricultural soils can make only modest contributions (3–6% of fossil fuel contributions) to mitigation of overall greenhouse gas emissions. However, effective mitigation policies will not be based on any single ‘magic bullet’ solutions, but rather on many modest reductions which are economically efficient and which confer additional benefits to society. In this context, soil C sequestration is a significant mitigation option.

Jackson, R.B. (1992). On estimating agriculture's net contribution to atmospheric carbon. Water, Air and Soil Pollution 64 (1-2): 121-137

ABSTRACT: Fossil fuel combustion, chlorofluorocarbon releases, and agricultural activities (including deforestation) are the primary anthropogenic sources of greenhouse gases. Of the three sources, agriculture is the only one that also has a sink capacity. Thus, an accounting of the net carbon (C) flux is required to properly evaluate agriculture's contribution and to determine the opportunities for emissions mitigation through changes in agricultural practices. Common data sets and a standard accounting method are required to perform country-by-country net C analyses. This research used agricultural census data to determine that U.S. agriculture removed 1.3 Pg of CO2 from the atmosphere, in 1987, in the plants that it produced. The turnover times and the fate of this C were not ascertained. The research also showed that 6.4 Tg of CO4 was emitted from live U.S. agricultural animals. A net C flux was computed, but is incomplete, because rice CO4 , plant and animal waste CO4 and CO2 , and soil-atmosphere C fluxes could not be estimated from the census data. Additionally, agriculture's net contribution to atmospheric C was found to depend critically on the boundaries of the analysis.

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

Jinbo, Z., Changchun, S., Shenmin, W. (2007). Dynamics of soil organic carbon and its fractions after abandonment of cultivated wetlands in northeast China. Soil and Tillage Research 96 (1-2): 350-360

ABSTRACT: Soil organic carbon (SOC) and its different labile fractions are important in minimizing negative environmental impacts and improving soil quality. However, very little is known of the dynamics of SOC and its labile fractions after the cultivated wetlands have been abandoned in northeast China. The objectives of this study were (1) to estimate the dynamics of SOC after the abandonment of cultivated soil, (2) to investigate the most sensitive fraction for detecting changes in organic C due to the abandonment of cultivated soil, and (3) to explore the key factors affecting the dynamics of soil C after the abandonment of cultivated soil in the freshwater marsh region of northeast China. Our results showed that the abandonment of cultivated wetlands resulted in an increase in SOC and the availability of C. The SOC content increased to 31, 44, and 107 g kg−1 after these cultivated wetlands were abandoned for 1, 6, and 13 years, respectively, as compared to an SOC content of 28 g kg−1 in the soil that had been cultivated on for 9 years. In northeast China, where a cultivated wetland was abandoned, the initial regeneration of SOC pools was considerably rapid and in accordance with the Boltzmann equation. An analysis of the stepwise regression indicated that the dynamics of SOC (g kg−1 ) can be quantitatively described by a linear combination of the root density and the mean soil temperature 5 cm underground in the growing season, as expressed by the following relationship: TOC = 0.008 root density −3.264T + 96.044 (R2 = 0.67, n = 9, p < 0.05. T is the mean soil temperature 5 cm underground in the growing season), indicating that approximately 67% of the variability in SOC can be explained by these two parameters. The root biomass was the key factor affecting SOC concentration according to the observation made during the recovery of cultivated soil that was abandoned. Soil temperature indirectly influenced the SOC concentration by affecting soil microbial activity. The abandonment of cultivated wetlands resulted in an increase in the light-fraction organic C (LF-OC), microbial biomass C (MBC), and dissolved organic C (DOC) concentration. The rate of increase in LF-OC was considerably higher than that in SOC and HF-OC. Similarly, the rate of increase in MBC was also considerably higher than that in SOC in cultivated soils abandoned for 4–8 years. However, the rate of increase in DOC was far lower than that in SOC. The R2 value for the correlation between the increments of the LF-OC and SOC was significantly higher than that for the correlation between DOC and MBC (0.99 vs. 0.90), indicating that LF-OC was the most sensitive fraction for detecting changes in organic C due to the abandonment of cultivated soil.

Johnson, J.M.F., Reicosky, D.C., Allmaras, R.R., Sauer, T.J., Venterea, R.T., Dell, C.J. (2005). Greenhouse gas contributions and mitigation potential of agriculture in the central USA. Soil and Tillage Research 83 (1): 73-94

ABSTRACT: The central USA contains some of the most productive agricultural land of the world. Due to the high proportion of land area committed to crops and pasture in this region, the carbon (C) stored and greenhouse gas (GHG) emission due to agriculture represent a large percentage of the total for the USA. Our objective was to summarize potential soil organic C (SOC) sequestration and GHG emission from this region and identify how tillage and cropping system interact to modify these processes. Conservation tillage (CST), including no-tillage (NT), has become more widespread in the region abating erosion and loss of organic rich topsoil and sequestering SOC. The rate of SOC storage in NT compared to conventional tillage (CT) has been significant, but variable, averaging 0.40 ± 0.61 Mg C ha−1 year−1 (44 treatment pairs). Conversion of previous cropland to grass with the conservation reserve program increased SOC sequestration by 0.56 ± 0.60 Mg C ha−1 year−1 (five treatment pairs). The relatively few data on GHG emission from cropland and managed grazing land in the central USA suggests a need for more research to better understand the interactions of tillage, cropping system and fertilization on SOC sequestration and GHG emission.

Kern, J.S., Johnson, M.G. (1993). Conservation tillage impacts on national soil and atmospheric carbon levels. Soil Science Society Of America JournalSoil Sci So 57: 200-210

ABSTRACT: Soil organic matter is the largest global terrestrial C pool and is a source of CO2 , CH4 , and other greenhouse gases. Changes in soil organic C (SOC) content and fossil fuel C emissions inresponse to conversion of conventional tillage to conservation tillage in the contiguous USA for field crop production by the year 2020 were projected by developing a model based on published data, and geographic databases of current conservation tillage usage and agricultural SOC. Three scenarios of conservation tillage use, 27% (current usage), 57% (Scenario 2), and 76% (Scenario 3) of field cropland planted, were considered. The SOC content for major field crops to 30-cm depth was 5304 to 8654 Tg C (Tg = 1012 g), with 1710 to 2831 Tg C at 0- to 8-cm depth, and 1383 to 2240 Tg C at 8- to 15-cm depth. Maintaining current levels of conventional tillage until 2020 would result in 31 to 52 Tg SOC loss. Scenario 2 conventional tillage resulted in 18 to 30 Tg C SOC loss, and Scenario 3 yielded 9 to 16 Tg SOC loss, which were C savings of 21 to 36 Tg C over maintaining current levels of tillage. Conversion of conventional tillage to no-till resulted in 80 to 129 Tg C gain in soil for Scenario 2, and 286 to 468 Tg C for Scenario 3. No-till and conventional tillage had similar SOC contents below the 15-cm depth. Minimum tillage conserved current levels of SOC but did not consistently increase SOC above levels of conventional tillage. Fossil fuel emissions from field manipulations and herbicide production for conventional tillage are 53 kg C ha–1 yr–1 , minimum tillage is 45 kg C ha–1 yr–1 , and 29 kg C ha–1 yr–1 for no-till. Fuel emissions for maintaining current levels of tillage practices are 157 Tg C, 149 Tg C for Scenario 2, and 146 Tg C for Scenario 3 for 30 yr. Increasing the amount of conservation tillage to Scenario 3 levels will change these agricultural systems from sources of C (188–209 Tg C) to C sinks (131–306 Tg C). The SOC benefit of Scenario 3 (277–452 Tg C) is equivalent to 0.7 to 1.1% of the total projected U.S. fossil fuel C emissions for the next 30 yr.

Kroodsma, D.A., Field, C.B. (2006). Carbon sequestration in California agriculture, 1980-2000. Ecological Applications 16 (5): 1975-1985

ABSTRACT: To better understand agricultural carbon fluxes in California, USA, we estimated changes in soil carbon and woody material between 1980 and 2000 on 3.6 × 106 ha of farmland in California. Combining the CASA (Carnegie-Ames-Stanford Approach) model with data on harvest indices and yields, we calculated net primary production, woody production in orchard and vineyard crops, and soil carbon. Over the 21-yr period, two trends resulted in carbon sequestration. Yields increased an average of 20%, corresponding to greater plant biomass and more carbon returned to the soils. Also, orchards and vineyards increased in area from 0.7 × 106 ha to 1.0 × 106 ha, displacing field crops and sequestering woody carbon. Our model estimates that California's agriculture sequestered an average of 19 g C·m−2 ·yr−1 . Sequestration was lowest in non-rice annual cropland, which sequestered 9 g C·m−2 ·yr−1 of soil carbon, and highest on land that switched from annual cropland to perennial cropland. Land that switched from annual crops to vineyards sequestered 68 g C·m−2 ·yr−1 , and land that switched from annual crops to orchards sequestered 85 g C·m−2 ·yr−1 . Rice fields, because of a reduction in field burning, sequestered 55 g C·m−2 ·yr−1 in the 1990s. Over the 21 years, California's 3.6 × 106 ha of agricultural land sequestered 11.0 Tg C within soils and 3.5 Tg C in woody biomass, for a total of 14.5 Tg C statewide. This is equal to 0.7% of the state's total fossil fuel emissions over the same time period. If California's agriculture adopted conservation tillage, changed management of almond and walnut prunings, and used all of its orchard and vineyard waste wood in the biomass power plants in the state, California's agriculture could offset up to 1.6% of the fossil fuel emissions in the state.

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. (2008). Soils and sustainable agriculture. A review. Agronomy for Sustainable Development 28 (1): 57-64

ABSTRACT: Enhancing food production and supporting civil/engineering structures have been the principal foci of soil science research during most of the 19th and the first seven or eight decades of the 20th century. Demands on soil resources during the 21st century and beyond include: (i) increasing agronomic production to meet the food needs of additional 3.5 billion people that will reside in developing countries along with likely shift in food habits from plant-based to animal-based diet, (ii) producing ligno-cellulosic biomass through establishment of energy plantations on agriculturally surplus/marginal soils or other specifically identified lands, (iii) converting degraded/desertified soils to restorative land use for enhancing biodiversity and improving the environment, (iv) sequestering carbon in terrestrial (soil and trees) and aquatic ecosystems to off-set industrial emissions and stabilize the atmospheric abundance of CO2 and other greenhouse gases, (v) developing farming/cropping systems which improve water use efficiency and minimize risks of water pollution, contamination and eutrophication, and (vi) creating reserves for species preservation, recreation and enhancing aesthetic value of soil resources. Realization of these multifarious soil functions necessitate establishment of inter-disciplinary approach with close linkages between soil scientists and chemists, physicists, geologists, hydrologists, climatologists, biologists, system engineers (nano technologists), computer scientists and information technologists, economists, social scientists and molecular geneticists dealing with human, animal and microbial processes. While advancing the study of basic principles and processes, soil scientists must also reach out to other disciplines to address the global issues of the 21st century and beyond.

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.

Lehuger, S., Gabrielle, B., Larmanou, E., Laville, P., Cellier, P., Loubet, B. (2007). Predicting the global warming potential of agro-ecosystems. Biogeosciences Discussions 4 (2): 1059-1092

ABSTRACT: Nitrous oxide, carbon dioxide and methane are the main biogenic greenhouse gases (GHG) contributing to the global warming potential (GWP) of agro-ecosystems. Evaluating the impact of agriculture on climate thus requires a capacity to predict the net exchanges of these gases in an integrated manner, as related to environmental conditions and crop management. Here, we used two year-round data sets from two intensively monitored cropping systems in northern France to test the ability of the biophysical crop model CERES-EGC to simulate GHG exchanges at the plot-scale. The experiments involved maize and rapeseed crops on a loam and rendzina soils, respectively. The model was subsequently extrapolated to predict CO2 and N2 O fluxes over an entire crop rotation. Indirect emissions (IE) arising from the production of agricultural inputs and from cropping operations were also added to the final GWP. One experimental site (involving a wheat-maize-barley rotation on a loamy soil) was a net source of GHG with a GWP of 350 kg CO2 -C eq ha-1 yr-1 , of which 75% were due to IE and 25% to direct N2 O emissions. The other site (involving an oilseed rape-wheat-barley rotation on a rendzina) was a net sink of GHG for -250kg CO2 -C eq ha-1 yr-1 , mainly due to a higher predicted C sequestration potential and C return from crops. Such modelling approach makes it possible to test various agronomic management scenarios, in order to design productive agro-ecosystems with low global warming impact.

Liebig, M.A., Morgan, J.A., Reeder, J.D., Ellert, B.H., Gollany, H.T., Schuman, G.E. (2005). Greenhouse gas contributions and mitigation potential of agricultural practices in northwestern USA and western Canada. Soil and Tillage Research 83 (1): 25-52

ABSTRACT: Concern over human impact on the global environment has generated increased interest in quantifying agricultural contributions to greenhouse gas fluxes. As part of a research effort called GRACEnet (Greenhouse Gas Reduction through Agricultural Carbon Enhancement Network), this paper summarizes available information concerning management effects on soil organic carbon (SOC) and carbon dioxide (CO2 ), nitrous oxide (N2 O), and methane (CH4 ) fluxes in cropland and rangeland in northwestern USA and western Canada, a region characterized by its inherently productive soils and highly variable climate. Continuous cropping under no-tillage in the region increased SOC by 0.27 ± 0.19 Mg C ha-1 yr-1 , which is similar to the Intergovernmental Panel on Climate Change (IPCC) estimate for net annual change in C stocks from improved cropland management. Soil organic C sequestration potential for rangelands was highly variable due to the diversity of plant communities, soils, and landscapes, underscoring the need for additional long-term C cycling research on rangeland. Despite high variability, grazing increased SOC by 0.16 ± 0.12 Mg C ha-1 yr-1 and converting cropland or reclaimed mineland to grass increased SOC by 0.94 ± 0.86 Mg C ha-1 yr-1 . Although there was generally poor geographical coverage throughout the region with respect to estimates of N2 O and CH4 flux, emission of N2 O was greatest in irrigated cropland, followed by non-irrigated cropland, and rangeland. Rangeland and non-irrigated cropland appeared to be a sink for atmospheric CH4 , but the size of this sink was difficult to determine given the few studies conducted. Researchers in the region are challenged to fill the large voids of knowledge regarding CO2 , N2 O, and CH4 flux from cropland and rangeland in the northwestern USA and western Canada, as well as integrate such data to determine the net effect of agricultural management on radiative forcing of the atmosphere.

Lobell, D.B., Bala, G., Duffy, P.B. (2006). Biogeophysical impacts of cropland management changes on climate. Geophysical Research Letters 33 (L06708): doi:10.1029/2005GL025492

ABSTRACT: It is well known that expansion of agriculture into natural ecosystems can have important climatic consequences, but changes occurring within existing croplands also have the potential to effect local and global climate. To better understand the impacts of cropland management practices, we used the NCAR CAM3 general circulation model coupled to a slab-ocean model to simulate climate change under extreme scenarios of irrigation, tillage, and crop productivity. Compared to a control scenario, increases in irrigation and leaf area index and reductions in tillage all have a physical cooling effect by causing increases in planetary albedo. The cooling is most pronounced for irrigation, with simulated local cooling up to -8°C and global land surface cooling of 1.3°C. Increases in soil albedo through reduced tillage are found to have a global cooling effect (-0.2°C) comparable to the biogeochemical cooling from reported carbon sequestration potentials. By identifying the impacts of extreme scenarios at local and global scales, this study effectively shows the importance of considering different aspects of crop management in the development of climate models, analysis of observed climate trends, and design of policy intended to mitigate climate change.

Lokupitiya, E., Paustian, K. (2006). Agricultural soil greenhouse gas emissions: a review of national inventory methods. Journal of Environmental Quality 35 (4): 1413-1427

ABSTRACT: Received for publication May 1, 2005. Parties to the United Nations Framework Convention on Climate Change (UNFCCC) are required to submit national greenhouse gas (GHG) inventories, together with information on methods used in estimating their emissions. Currently agricultural activities contribute a significant portion (approximately 20%) of global anthropogenic GHG emissions, and agricultural soils have been identified as one of the main GHG source categories within the agricultural sector. However, compared to many other GHG sources, inventory methods for soils are relatively more complex and have been implemented only to varying degrees among member countries. This review summarizes and evaluates the methods used by Annex 1 countries in estimating CO2 and N2 O emissions in agricultural soils. While most countries utilize the Intergovernmental Panel on Climate Change (IPCC) default methodology, several Annex 1 countries are developing more advanced methods that are tailored for specific country circumstances. Based on the latest national inventory reporting, about 56% of the Annex 1 countries use IPCC Tier 1 methods, about 26% use Tier 2 methods, and about 18% do not estimate or report N2 O emissions from agricultural soils. More than 65% of the countries do not report CO2 emissions from the cultivation of mineral soils, organic soils, or liming, and only a handful of countries have used country-specific, Tier 3 methods. Tier 3 methods usually involve process-based models and detailed, geographically specific activity data. Such methods can provide more robust, accurate estimates of emissions and removals but require greater diligence in documentation, transparency, and uncertainty assessment to ensure comparability between countries. Availability of detailed, spatially explicit activity data is a major constraint to implementing higher tiered methods in many countries.

Martens, D. A., Emmerich, W., McLain, J. E.T., Johnsen, T. N. (2005). Atmospheric carbon mitigation potential of agricultural management in the southwestern USA. Soil and Tillage Research 83 (1): 95-119

ABSTRACT: Agriculture in the southwestern USA is limited by water supply due to high evaporation and limited seasonal precipitation. Where water is available, irrigation allows for production of a variety of agricultural and horticultural crops. This review assesses the impacts of agriculture on greenhouse gas emission and sequestration of atmospheric C in soils of the hot, dry region of the southwestern USA. In Texas, conservation tillage increased soil organic C by 0.28 Mg C ha-1 year-1 compared with more intensive tillage. Conversion of tilled row crops to the conservation reserve program or permanent pastures increased soil organic C by 0.32 ± 0.50 Mg C ha-1 year-1 . Soil organic C sequestration was dependent on rotation, previous cropping, and type of conservation tillage employed. Relatively few studies have interfaced management and C cycling to investigate the impacts of grazing management on soil organic C, and therefore, no estimate of C balance was available. Irrigated crop and pasture land in Idaho had soil organic C content 10-40 Mg C ha-1 greater than in dryland, native grassland. Soil salinity must be controlled in cropland as soil organic C content was lower with increasing salinity. Despite 75% of the region's soils being classified as calcic, the potential for sequestration of C as soil carbonate has been only scantly investigated. The region may be a significant sink for atmospheric methane, although in general, trace gas flux from semiarid soils lacks adequate characterization. Agricultural impacts on C cycling will have to be better understood in order for effective C sequestration strategies to emerge.

McLauchlan, K. (2006). The nature and longevity of agricultural impacts on soil carbon and nutrients: a review. Ecosystems 9 (8): 1364-1382

ABSTRACT: Since the domestication of plant and animal species around 10,000 years ago, cultivation and animal husbandry have been major components of global change. Agricultural activities such as tillage, fertilization, and biomass alteration lead to fundamental changes in the pools and fluxes of carbon (C), nitrogen (N), and phosphorus (P) that originally existed in native ecosystems. Land is often taken out of agricultural production for economic, social, or biological reasons, and the ability to predict the biogeochemical trajectory of this land is important to our understanding of ecosystem development and our projections of food security for the future. Tillage generally decreases soil organic matter (SOM) due to erosion and disruption of the physical, biochemical, and chemical mechanisms of SOM stabilization, but SOM can generally reaccumulate after the cessation of cultivation. The use of organic amendments causes increases in SOM on agricultural fields that can last for centuries to millennia after the termination of applications, although the locations that provide the organic amendments are concurrently depleted. The legacy of agriculture is therefore highly variable on decadal to millennial time scales and depends on the specific management practices that are followed during the agricultural period. State factors such as climate and parent material (particularly clay content and mineralogy) modify ecosystem processes such that they may be useful predictors of rates of postagricultural biogeochemical change. In addition to accurate biogeochemical budgets of postagricultural systems, ecosystem models that more explicitly incorporate mechanisms of SOM loss and formation with agricultural practices will be helpful. Developing this predictive capacity will aid in ecological restoration efforts and improve the management of modern agroecosystems as demands on agriculture become more pressing.

Mosier,A. R., Halvorson,A. D., Peterson,G. A., Robertson,G. P., Sherrod,L. (2005). Measurement of net global warming potential in three agroecosystems. Nutrient Cycling in Agroecosystems 72 (1): 67-76

ABSTRACT: When appraising the impact of food and fiber production systems on the composition of the Earth's atmosphere and the ‘greenhouse’ effect, the entire suite of biogenic greenhouse gases – carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2 O) – needs to be considered. Storage of atmospheric CO2 into stable organic carbon pools in the soil can sequester CO2 while common crop production practices can produce CO2 , generate N2 O, and decrease the soil sink for atmospheric CH4 . The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40–44% of the GWP from both rain-fed sites and contributed 16–33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether a system is a sink or source of CO2 , i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO2 equivalents ha−1 y−1 . Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N2 O emission rates decline or increase with time after conversion to NT.

Mosier, A. R., Halvorson, A. D., Reule, C. A., Liu, X. J. J. (2006). Net global warming potential and greenhouse gas intensity in irrigated cropping systems in northeastern Colorado. Journal of Environmental Quality 35 (4): 1584-1598

ABSTRACT: The impact of management on global warming potential (GWP), crop production, and greenhouse gas intensity (GHGI) in irrigated agriculture is not well documented. A no-till (NT) cropping systems study initiated in 1999 to evaluate soil organic carbon (SOC) sequestration potential in irrigated agriculture was used in this study to make trace gas flux measurements for 3 yr to facilitate a complete greenhouse gas accounting of GWP and GHGI. Fluxes of CO2 , CH4 , and N2 O were measured using static, vented chambers, one to three times per week, year round, from April 2002 through October 2004 within conventional-till continuous corn (CT-CC) and NT continuous corn (NT-CC) plots and in NT corn–soybean rotation (NT-CB) plots. Nitrogen fertilizer rates ranged from 0 to 224 kg N ha–1 . Methane fluxes were small and did not differ between tillage systems. Nitrous oxide fluxes increased linearly with increasing N fertilizer rate each year, but emission rates varied with years. Carbon dioxide efflux was higher in CT compared to NT in 2002 but was not different by tillage in 2003 or 2004. Based on soil respiration and residue C inputs, NT soils were net sinks of GWP when adequate fertilizer was added to maintain crop production. The CT soils were smaller net sinks for GWP than NT soils. The determinant for the net GWP relationship was a balance between soil respiration and N2 O emissions. Based on soil C sequestration, only NT soils were net sinks for GWP. Both estimates of GWP and GHGI indicate that when appropriate crop production levels are achieved, net CO2 emissions are reduced. The results suggest that economic viability and environmental conservation can be achieved by minimizing tillage and utilizing appropriate levels of fertilizer.

Omonode, R. A., Vyn, T. J., Smith, D. R., Hegymegi, P., Gal, A. (2007). Soil carbon dioxide and methane fluxes from long-term tillage systems in continuous corn and corn-soybean rotations. Soil and Tillage Research 95 (1-2): 182-195

ABSTRACT: Although the Midwestern United States is one of the world's major agricultural production areas, few studies have assessed the effects of the region's predominant tillage and rotation practices on greenhouse gas emissions from the soil surface. Our objectives were to (a) assess short-term chisel (CP) and moldboard plow (MP) effects on soil CO2 and CH4 fluxes relative to no-till (NT) and, (b) determine how tillage and rotation interactions affect seasonal gas emissions in continuous corn and corn–soybean rotations on a poorly drained Chalmers silty clay loam (Typic Endoaquoll) in Indiana. The field experiment itself began in 1975. Short-term gas emissions were measured immediately before, and at increasing hourly intervals following primary tillage in the fall of 2004, and after secondary tillage in the spring of 2005, for up to 168 h. To quantify treatment effects on seasonal emissions, gas fluxes were measured at weekly or biweekly intervals for up to 14 sampling dates in the growing season for corn. Both CO2 and CH4 emissions were significantly affected by tillage but not by rotation in the short-term following tillage, and by rotation during the growing season. Soil temperature and moisture conditions in the surface 10 cm were significantly related to CO2 emissions, although the proportion of variation explained by temperature and moisture was generally very low (never exceeded 27%) and varied with the tillage system being measured. In the short-term, CO2 emissions were significantly higher for CP than MP and NT. Similarly, mean seasonal CO2 emissions during the 2-year period were higher for CP (6.2 Mg CO2 -C ha−1 year−1 ) than for MP (5.9 Mg CO2 -C ha−1 year−1 ) and NT (5.7 Mg CO2 -C ha−1 year−1 ). Both CP and MP resulted in low net CH4 uptake (7.6 and 2.4 kg CH4-C ha−1 year−1 , respectively) while NT resulted in net emissions of 7.7 kg CH4 -C ha−1 year−1 . Mean emissions of CO2 were 16% higher from continuous corn than from rotation corn during the two growing seasons. After 3 decades of consistent tillage and crop rotation management for corn and soybean producing grain yields well above average in the Midwest, continuous NT production in the corn–soybean rotation was identified as the system with the least soil-derived C emissions to the atmosphere from among those evaluated prior to and during corn production.

Pathak, H., Wassmann, R. (2007). Introducing greenhouse gas mitigation as a development objective in rice-based agriculture: I. Generation of technical coefficients. Agricultural Systems 94 (3): 807-825

ABSTRACT: This study presents a modeling tool to assess emission of greenhouse gases (GHG) from the agricultural sector as affected by land-use and residue utilization options. The overall purpose of this tool is twofold: (i) a spreadsheet model for comprehensive compilation of the direct and indirect emissions from land management, residue-burning and fossil fuel consumption through on-farm and off-farm operations and (ii) a decision support tool to explore economically viable mitigation options through detailed cost–benefit analysis of different technological options. We developed TechnoGAS (technical coefficient generator for mitigation technologies of greenhouse gas emissions from agricultural sectors), which integrates analytical and expert knowledge with regional databases on bio-physical, agronomic and socio-economic features to establish input–output relationships (‘Technical Coefficients’) related to GHG emissions in agriculture. The approach includes emissions of methane (CH4 ) from rice fields, rice straw burning and cattle; carbon dioxide (CO2 ) from fossil fuel and soil organic carbon decline as well as nitrous oxide (N2 O) from soil, rice straw burning and fertilizer use. To illustrate the approach of the spreadsheet model for comprehensive compilation of emissions, we applied TechnoGAS for an entire rice–wheat cropping cycle in the state of Haryana in northern India as a case study. Twenty technologies of rice production, which can be adopted by farmers, are analysed for their operation-specific emissions including their global warming potential (GWP). The technologies differ in terms of water regime, residue management/utilization, soil management and additives, which represent different mitigation options for GHG emissions. With the current farmers’ practice in various districts in Haryana, soil-borne emissions are the major source of GHG contributing 53% of the average GWP (3288 kg CO2 equivalent ha−1 ) in rice followed by burning of rice straw (13% of the GWP). Cattle, farm operations, off-farm and inorganic fertilizer contributes 12%, 10%, 10% and 2% of the GWP, respectively. Emissions from wheat are relatively low (1204 kg CO2 equivalent ha−1 ) as there is no CH4 emission and wheat straw is not burnt. Different mitigation technologies show pronounced effects on the GWP of the rice crop and varied between 1715 kg CO2 equivalent ha−1 with continuous flooding, urea and rice straw used for building materials and 10,020 kg CO2 equivalent ha−1 with continuous flooding, and application of nutrients through organic manure. Compared to current farmers’ practice, 13 technologies are found to have the potential to reduce the GWP by 8–51%, but they also reduce the net income of farmers. Upscaling of the estimates to the entire state of Haryana shows that the GWP with the current farmers’ practice in rice is 2617 Gg CO2 equivalent. Modification of water management from continuous flooding to alternate flooding or application of urea alone instead of urea plus FYM will reduce the GWP by 15% and 29%, respectively, while feeding of rice straw to cattle and supplying N through urea will reduce it by 41% compared to the current practice of burning rice straw and use of FYM. The study shows that the TechnoGAS tool can be used for estimating GHG emission from various land-use types and for identifying promising mitigation options. A detailed cost/benefit analysis is supplied by Wassmann and Pathak [Wassmann, R., Pathak, H., this volume. Introducing greenhouse gas mitigation as a development objective in rice-based agriculture: II. Cost–benefit assessment for different technologies, regions and scales.].

Paustian, K., Cole, C. V., Sauerbeck, D., Sampson, N. (1998). CO2 mitigation by agriculture: An overview. Climatic Change 40 (1): 135-162.

ABSTRACT: Agriculture currently contributes significantly to the increase of CO2 in the atmosphere, primarily through the conversion of native ecosystems to agricultural uses in the tropics. Yet there are major opportunities for mitigation of CO2 and other greenhouse gas emissions through changes in the use and management of agricultural lands. Agricultural mitigation options can be broadly divided into two categories: (I) strategies to maintain and increase stocks of organic C in soils (and biomass), and (ii) reductions in fossil C consumption, including reduced emissions by the agricultural sector itself and through agricultural production of biofuels to substitute for fossil fuels. Reducing the conversion of new land to agriculture in the tropics could substantially reduce CO2 emissions, but this option faces several difficult issues including population increase, land tenure and other socio-political factors in developing countries. The most significant opportunities for reducing tropical land conversions are in the humid tropics and in tropical wetlands. An important linkage is to improve the productivity and sustainability of existing agricultural lands in these regions. Globally, we estimate potential agricultural CO2 mitigation through soil C sequestration to be 0.4-0.9 Pg C y-1 , through better management of existing agricultural soils, restoration of degraded lands, permanent "set-asides" of surplus agricultural lands in temperate developed countries and restoration of 10-20% of former wetlands now being used for agriculture. However, soils have a finite capacity to store additional C and therefore any increases in C stocks following changes in management would be largely realized within 50-100 years. Mitigation potential through reducing direct agricultural emissions is modest, 0.01-0.05 Pg C y-1 . However, the potential to offset fossil C consumption through the use of biofuels produced by agriculture is substantial, 0.5-1.6 Pg C y-1 , mainly through the production of dedicated biofuel crops with a smaller contribution (0.2-0.3 Pg C y-1 ) from crop residues. Many agricultural mitigation options represent "win-win" situations, in that there are important side benefits, in addition to CO2 mitigation, that could be achieved, e.g. improved soil fertility with higher soil organic matter, protection of lands poorly suited for permanent agriculture, cost saving for fossil fuel inputs and diversification of agricultural production (e.g. biofuels). However, the needs for global food production and farmer/societal acceptability suggest that mitigation technologies should conform to: (I) the enhancement of agricultural production levels in parts of the world where food production and population demand are in delicate balance and (ii) the accrual of additional benefits to the farmer (e.g., reduced labor, reduced or more efficient use of inputs) and society at large.

Pennock, D. J. (2005). Precision conservation for co-management of carbon and nitrogen on the Canadian Prairies. Journal of Soil and Water Conservation 60 (6): 396-401

ABSTRACT: Agriculture is targeted to make a substantial contribution to Canada's greenhouse gas reduction targets under the Kyoto Protocol. To achieve a net reduction in emissions any gains in soil organic carbon storage cannot come at the expense of enhanced nitrous oxide emissions from the soil. In nonlevel agricultural landscapes of the Canadian Prairies the potential for significant soil organic carbon gain due to adoption of soil conserving practices is greatest on convex upper slope positions, which have experienced major losses of soil organic carbon due to cultivation. The potential for soil organic carbon gain in lower slope positions is limited due to their high soil organic carbon contents, but targeted wetland and riparian vegetation restoration programs could lead to significant above ground carbon gains. Several studies have shown that emissions of nitrous oxide from lower slope positions are significantly higher than the convex slope positions, and that improvements in nitrogen fertilizer use efficiency through site-specific management has the potential to significantly reduce nitrous oxide (N2 O) emissions from these positions. Because of the close relationship between landform position and the key carbon and nitrogen processes, quantitative landform segmentation procedures can be used to delineate precision conservation management zones in these landscapes. Site-specific management practices such as reduced or no-till, seeding to grass, wetland restoration, and site-specific nitrogen (N) management can then be implemented to simultaneously increase soil organic carbon stores on eroded upper slope segments while preserving existing stores of soil organic carbon and reducing N2 O emissions from lower slope segments. Close cooperation between precision conservation professionals and agronomists is required to ensure that information required by producers is available to guide them in their decision making and implementation of precision conservation for co-management of carbon and nitrogen.

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.

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.

Schnitzer, M., Mcarthur, D. F. E., Schulten, H. R., Kozak, L. M., Huang, P. M. (2006). Long-term cultivation effects on the quantity and quality of organic matter in selected Canadian prairie soils. Geoderma 130 (1-2): 141-156

ABSTRACT: Sixteen Orthic Chernozemic surface soil samples, one half from virgin prairie and one half from adjacent cultivated prairie (cultivated for 31 to 94 years), were collected from eight sites throughout Southern Saskatchewan, Canada. Samples were analyzed for total organic C and a number of other chemical and physical properties. The virgin and cultivated soils at site No. 4 were selected for more detailed analysis by CP-MAS13 C NMR, Curie-point-pyrolysis-gas chromatography/mass spectrometry (Cp-Py-GC/MS), and by pyrolysis-field ionization mass spectrometry (Py-FIMS). Long-term cultivation resulted in large significant decreases in total SOM (soil organic matter), as represented by total soil organic C. There were significant increases in aromaticity of the SOM as a result of long-term cultivation as indicated by CP-MAS13 C NMR spectroscopy. This was mainly attributable to the result of cultivation-enhanced degradation of aliphatic C relative to aromatic C. Organic compounds identified in the Cp-Py-GC/MS spectra of the virgin and cultivated soils at site No. 4 consisted of n-alkanes (ranging from C11 to C22) and alkenes (ranging from C7:1 to C21:1), with the virgin soil being richer in alkenes than the cultivated soil. Other components identified were cyclic aromatics, carbocyclics, N-containing aromatics, N-heterocyclics, benzene and substituted benzenes, phenols and substituted phenols and substituted furans. The compounds identified appeared to originate from long-chain aliphatics, lignins, polyphenols, aromatics, polysaccharides, and N-containing compounds in the two soils. While qualitatively similar compounds were identified by Py-FIMS in the two soils, the total ion intensity (TII) of the virgin soil was almost 2.5 times as high as that of the cultivated soil. This suggests that cultivation made the organic matter less volatile, either by favouring the formation of higher molecular weight organic matter or by promoting the formation of non-volatile metal-organic matter complexes. The Py-FIMS spectra showed that the virgin soil contained relatively more lignin dimers, lipids, sterols, and n-C16 to n-C34 fatty acids than the cultivated soil. Thus, conversely, the cultivated soil was richer in carbohydrates, phenols and lignin monomers, alkyl aromatics and N-containing compounds, including peptides, than the virgin soil.

Singh, B. R., Lal, R. (2005). The potential of soil carbon sequestration through improved management practices in Norway. Environment, Development and Sustainability 7 (1): 161-184

ABSTRACT: The study was conducted to assess the potential of Norwegian agricultural ecosystems to sequester carbon (C) based on the data from some long-term agronomic and land use experiments. The total emission of CO2 in Norway in 1998 was 41.4 million metric ton (MMT), of which agriculture contributed only 0.157 MMT, or <0.4% of the total emissions. With regards to methane (CH4) and nitrous oxide (N2 O) gases, however, agricultural activities contributed 32.5% and 51.3% of their respective emissions in Norway. The soil organic carbon (SOC) losses associated with accelerated soil erosion in Norway are estimated at 0.475 MMTC yr–1 . Land use changes and soil/crop management practices with potential for SOC sequestration include conservation tillage methods, judicious use of fertilizers and manures, use of crop residues, diverse crop rotations, and erosion control measures. The potential for SOC sequestration is 0.146 MMTC yr–1 for adopting conservation tillage, 0.011–0.035 MMTC yr–1 for crop residue management, 0.026 MMTC yr–1 for judicious use of mineral fertilizer, 0.016–0.135 MMTC yr–1 for manure application, and 0.036 MMTC yr–1 for adopting crop rotations. The overall potential of these practices for SOC sequestration ranges from 0.591 to 1.022 MMTC yr–1 with an average value of 0.806 MMTC yr–1 . Of the total potential, 59% is due to adoption of erosion control measures, 5.8% to restoration of peat lands, 21% to conversion to conservation tillage and residue management, and 14% to adoption of improved cropping systems. Enhancing SOC sequestration and improving soil quality, through adoption of judicious land use and improved system of soil and crop management, are prudent strategies for sustainable management of soil, water and environment resources.

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. (2008). Land use change and soil organic carbon dynamics. Nutrient Cycling in Agroecosystems 81 (2): 169-178

ABSTRACT: Historically, soils have lost 40–90 Pg carbon (C) globally through cultivation and disturbance with current rates of C loss due to land use change of about 1.6 ± 0.8 Pg C y−1 , mainly in the tropics. Since soils contain more than twice the C found in the atmosphere, loss of C from soils can have a significant effect of atmospheric CO2 concentration, and thereby on climate. Halting land-use conversion would be an effective mechanism to reduce soil C losses, but with a growing population and changing dietary preferences in the developing world, more land is likely to be required for agriculture. Maximizing the productivity of existing agricultural land and applying best management practices to that land would slow the loss of, or is some cases restore, soil C. There are, however, many barriers to implementing best management practices, the most significant of which in developing countries are driven by poverty. Management practices that also improve food security and profitability are most likely to be adopted. Soil C management needs to considered within a broader framework of sustainable development. Policies to encourage fair trade, reduced subsidies for agriculture in developed countries and less onerous interest on loans and foreign debt would encourage sustainable development, which in turn would encourage the adoption of successful soil C management in developing countries. If soil management is to be used to help address the problem of global warming, priority needs to be given to implementing such policies.

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.

Tan, Z. X., Lal, R. (2005). Carbon sequestration potential estimates with changes in land use and tillage practice in Ohio, USA. Agriculture, Ecosystems & Environment 111 (1-4): 140-152

ABSTRACT: Soil C sequestration through changes in land use and management is one of the important strategies to mitigate the global greenhouse effect. This study was conducted to estimate C sequestration potential of the top 20 cm depth of soil for two scenarios in Ohio, USA: (1) with reforestation of both current cropland and grassland where SOC pools are less than the baseline SOC pool under current forest; (2) with the adoption of NT on all current cropland. Based on Ohio Soil Survey Characterization Database and long-term experimental data of paired conservation tillage (CT) versus no-till (NT), we specified spatial variations of current SOC pools and C sequestration potentials associated with soil taxa within each major land resource area (MLRA). For scenario I, there would be 4.56 Mha of cropland having an average SOC sequestration capacity of 1.55 kg C m−2 and 0.80 Mha of grassland with that of 1.35 kg C m−2 . Of all potential area, 73% are associated with Alfisols and 15% with Mollisols, but the achievable potential could vary significantly with individual MLRAs. Alternately, an average SOC sequestration rate of 62 g C m−2 year−2 was estimated with conversion from CT to NT for cultivated Alfisols, by which a cumulative increase of 71 Tg C resulted from reforestation of cropland could be realized in 25 years. Soils with lower antecedent C contents have higher C sequestration rates. In comparison with the results obtained at the state scale, the estimates of SOC sequestration potentials taxonomically associated with each specific MLRA may be more useful to the formulation of C credit trading programs.

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.

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 .

J. A. Entry, R. E. Sojka, G. E. Shewmaker (2002). Management of irrigated agriculture to increase organic carbon storage in soils. Soil Science Society Of America JournalSoil Sci So 66 (6): 1957-1964

ABSTRACT: Increasing the amount of C in soils may be one method to reduce the concentration of CO2 in the atmosphere. We measured organic C stored in southern Idaho soils having long term cropping histories that supported native sagebrush vegetation (NSB), irrigated moldboard plowed crops (IMP), irrigated conservation-chisel-tilled crops (ICT), and irrigated pasture systems (IP). The CO2 emitted as a result of fertilizer production, farm operations, and CO2 lost via dissolved carbonate in irrigation water, over a 30-yr period, was included. Net organic C in ecosystems decreased in the order IP > ICT > NSB > IMP. In this study, if NSB were converted to IMP, 0.15 g C m-2 would be emitted to the atmosphere, but if converted to IP 3.56 g C m-2 could be sequestered. If IMP land were converted to ICT, 0.95 g C m-2 could be sequestered in soil and if converted to IP 3.71 g C m-2 could be sequestered. There are 2.6 x 108 ha of land worldwide presently irrigated. If irrigated agriculture were expanded 10% and the same amount of rainfed land were converted back to native grassland, an increase of 3.4 x 109 Mg C (5.9% of the total C emitted in the next 30 yr) could potentially be sequestered. The total projected release of CO2 is 5.7 x 1010 Mg C worldwide during the next 30 yr. Converting rainfed agriculture back to native vegetation while modestly increasing areas in irrigated agriculture could have a significant impact on CO2 atmospheric concentrations while maintaining or increasing food production.

P. Smith, D. S. Powlson, J. U. Smith, P. Falloon, K. Coleman (2000). Meeting Europe's climate change commitments: quantitative estimates of the potential for carbon mitigation by agriculture. Global Change Biology 6 (5): 525-539

ABSTRACT: Under the Kyoto Protocol, the European Union is committed to a reduction in CO2 emissions to 92% of baseline (1990) levels during the first commitment period (2008–2012). The Kyoto Protocol allows carbon emissions to be offset by demonstrable removal of carbon from the atmosphere. Thus, land-use/land-management change and forestry activities that are shown to reduce atmospheric CO2 levels can be included in the Kyoto targets. These activities include afforestation, reforestation and deforestation (article 3.3 of the Kyoto Protocol) and the improved management of agricultural soils (article 3.4). In this paper, we estimate the carbon mitigation potential of various agricultural land-management strategies and examine the consequences of European policy options on carbon mitigation potential, by examining combinations of changes in agricultural land-use/land-management.

We show that no single land-management change in isolation can mitigate all of the carbon needed to meet Europe's climate change commitments, but integrated combinations of land-management strategies show considerable potential for carbon mitigation. Three of the combined scenarios, one of which is an optimal realistic scenario, are by themselves able to meet Europe's emission limitation or reduction commitments.

Through combined land-management scenarios, we show that the most important resource for carbon mitigation in agriculture is the surplus arable land. We conclude that in order to fully exploit the potential of arable land for carbon mitigation, policies will need to be implemented to allow surplus arable land to be put into alternative long-term land-use.

Of all options examined, bioenergy crops show the greatest potential for carbon mitigation. Bioenergy crop production also shows an indefinite mitigation potential compared to other options where the mitigation potential is finite. We suggest that in order to exploit fully the bioenergy option, the infrastructure for bioenergy production needs to be significantly enhanced before the beginning of the first Kyoto commitment period in 2008.

It is not expected that Europe will attempt to meet its climate change commitments solely through changes in agricultural land-use. A reduction in CO2 -carbon emissions will be key to meeting Europe's Kyoto targets, and forestry activities (Kyoto Article 3.3) will play a major role. In this study, however, we demonstrate the considerable potential of changes in agricultural land-use and -management (Kyoto Article 3.4) for carbon mitigation and highlight the policies needed to promote these agricultural activities. As all sources of carbon mitigation will be important in meeting Europe's climate change commitments, agricultural carbon mitigation options should be taken very seriously.

Lal, R. (2009). Soils and food sufficiency. A review. Agronomy for Sustainable Development 29 (1): 113-133

ABSTRACT: Soil degradation, caused by land misuse and soil mismanagement, has plagued humanity since the dawn of settled agriculture. Many once thriving civilizations collapsed due to erosion, salinization, nutrient depletion and other soil degradation processes. The Green Revolution of the 1960s and 1970s, that saved hundreds of millions from starvation in Asia and elsewhere, by-passed Sub-Saharan Africa. This remains the only region in the world where the number of hungry and food-insecure populations will still be on the increase even by 2020. The serious technological and political challenges are being exacerbated by the rising energy costs. Resource-poor and small-size land-holders can neither afford the expensive input nor are they sure of their effectiveness because of degraded soils and the harsh, changing climate. Consequently, crop yields are adversely impacted by accelerated erosion, and depletion of soil organic matter (SOM) and nutrients because of the extractive farming practices. Low crop yields, despite growing improved varieties, are due to the severe soil degradation, especially the low SOM reserves and poor soil structure that aggravate drought stress. Components of recommended technology include: no-till; residue mulch and cover crops; integrated nutrient management; and biochar used in conjunction with improved crops (genetically modified, biotechnology) and cropping systems, and energy plantation for biofuel production. However, its low acceptance, e.g. for no-till farming, is due to a range of biophysical, social and economic factors. Competing uses of crop residues for other needs is among numerous factors limiting the adoption of no-till farming. Creating another income stream for resource-poor farmers, through payments for ecosystem services, e.g., C sequestration in terrestrial ecosystems, is an important strategy for promoting the adoption of recommended technologies. Adoption of improved soil management practices is essential to adapt to the changing climate, and meeting the needs of growing populations for food, fodder, fuel and fabrics. Soil restoration and sustainable management are essential to achieving food security, and global peace and stability.

J. M. Antle, B. A. McCarl, T. Tietenberg, H. Folmer (2001). The economics of carbon sequestration in agricultural soils. Edward Elgar Publishing: 278-310

FIRST PARAGRAPH: Science has established conclusively that concentrations of greenhouse gases (GHG) in the Earth’s atmosphere have been rising rapidly since the Industrial Revolution (e.g., see IPCC, 1996). While these increasing concentrations are associated primarily with fossil fuel consumption, a significant share (estimated in the range of 12 to 42 per cent) is believed to be caused by changes in land use, including deforestation and the expansion of agriculture (Watson et al., 2000, p. 5). While the consequences of increasing atmospheric GHG concentrations remains the subject of intensive scientific study and debate (see the US National Assessment [US Global Change Research Program, 2000] and [IPPC, 1996, 2001] for current literature reviews), there is growing national and international momentum to implement policies to reduce GHG emissions. The most obvious -- but not necessarily least costly -- way to do that is to reduce fossil fuel consumption. However GHGs can also be removed from the atmosphere by reversing some of the processes associated with land use changes.

Lal, R., Follett, R. F., Stewart, B. A., Kimble, J. M. (2007). Soil carbon sequestration to mitigate climate change and advance food security. Soil Science 172 (12): 943-956

ABSTRACT: World soils have been a source of atmospheric carbon dioxide since the dawn of settled agriculture, which began about 10 millennia ago. Most agricultural soils have lost 30% to 75% of their antecedent soil organic carbon (SOC) pool or 30 to 40 t C ha-1 . The magnitude of loss is often more in soils prone to accelerated erosion and other degradative processes. On a global scale, CO2-C emissions since 1850 are estimated at 270 ± 30 giga ton (billion ton or Gt) from fossil fuel combustion compared with 78 ± 12 Gt from soils. Consequently, the SOC pool in agricultural soils is much lower than their potential capacity. Furthermore, depletion of the SOC pool also leads to degradation in soil quality and declining agronomic/biomass productivity. Therefore, conversion to restorative land uses (e.g., afforestation, improved pastures) and adoption of recommended management practices (RMP) can enhance SOC and improve soil quality. Important RMP for enhancing SOC include conservation tillage, mulch farming, cover crops, integrated nutrient management including use of manure and compost, and agroforestry. Restoration of degraded/desertified soils and ecosystems is an important strategy. The rate of SOC sequestration, ranging from 100 to 1000 kg ha-1 year-1 , depends on climate, soil type, and site-specific management. Total potential of SOC sequestration in the United States of 144 to 432 Mt year-1 (288 Mt year-1 ) comprises 45 to 98 Mt in cropland, 13 to 70 Mt in grazing land, and 25 to 102 Mt in forestland. The global potential of SOC sequestration is estimated at 0.6 to 1.2 Gt C year-1 , comprising 0.4 to 0.8 Gt C year-1 through adoption of RMP on cropland (1350 Mha), and 0.01 to 0.03 Gt C year-1 on irrigated soils (275 Mha), and 0.01 to 0.3 Gt C year-1 through improvements of rangelands and grasslands (3700 Mha). In addition, there is a large potential of C sequestration in biomass in forest plantations, short rotation woody perennials, and so on. The attendant improvement in soil quality with increase in SOC pool size has a strong positive impact on agronomic productivity and world food security. An increase in the SOC pool within the root zone by 1 t C ha-1 year-1 can enhance food production in developing countries by 30 to 50 Mt year-1 including 24 to 40 Mt year-1 of cereal and legumes, and 6 to 10 Mt year-1 of roots and tubers. Despite the enormous challenge of SOC sequestration, especially in regions of warm and arid climates and predominantly resource-poor farmers, it is a truly a win-win strategy. While improving ecosystem services and ensuring sustainable use of soil resources, SOC sequestration also mitigates global warming by offsetting fossil fuel emissions and improving water quality by reducing nonpoint source pollution.

W. M. Post, K. C. Kwon (2008). Soil carbon sequestration and land-use change: processes and potential. Global Change Biology 6 (3): 317-327

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. This accumulation process essentially reverses 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 favour 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 variation in the length of time for and the rate at which carbon may accumulate in soil, 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.

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.

D.R. Sauerbeck (2001). CO2 emissions and C sequestration by agriculture – perspectives and limitations. Nutrient Cycling in Agroecosystems 60 (1-3): 253-266

ABSTRACT: This overview paper concentrates on carbon dioxide, discussing its agricultural sources and the possibilities to minimize their respective emissions. Besides such source-related emissions reductions, agriculture is also expected to help slowing down the CO2 increase in the atmosphere by sequestering part of it in soil organic matter, and by producing suitable biomass as a substitute for fossil fuel.

The share of agriculture in the consumption of fossil fuel is comparatively low. Even the high-intensity farming of industrialized countries does not consume more than about 3–4.5% of their total energy budget, at least as far as the fuel inputs into primary farm production are concerned. Possible savings are e.g. reduced soil tillage, optimized fertilizer efficiency, improved irrigation techniques and enhanced solar drying. The pertinent literature comes to the optimistic conclusion that by exploiting all these possibilities a 10–40% reduction of the present agricultural energy requirements might be achieved. Accordingly, theoretical fuel savings might be in the order of 0.01–0.05 Gt C yr−1 . Although this should be aimed at for many reasons, it unfortunately corresponds to less than 1% of the present overall CO2 release from fossil fuel.

Between 0.25 and 1 Gt fossil fuel carbon could theoretically be substituted per year by agricultural biofuels, and 0.06–0.25 Gt yr−1 by shelterbelts and agroforestry. Together with 25% of the crop residues, this comes to a potential fossil fuel offset of somewhere between 0.5 and 1.5 Gt C yr−1 . This would be an impressive figure, suggesting a potential saving of 10–25%, while providing the same amount of energy without enriching the atmosphere with much additional CO2 . In reality, however, there remain lots of questions as yet unsolved, in addition to many environmental problems which one would have to expect. As an example, a concomitant increase of non-CO2 greenhouse gases can not be ruled out. Furthermore, the use of crop residues as biofuels, even though it is customary in many developing countries, could have deleterious effects on soil fertility.

Land use changes from forest or grassland to arable agriculture have been and still are a significant source for the release of former plant and soil carbon into the atmosphere. The reasons for decreasing soil carbon contents are a reduced input of plant biomass into cropland on the one hand, and an accelerated decomposition of the existing organic matter in agricultural soils on the other. The combined losses from the earth's native biomass and from soils due to cultivation between the year 1700 and today amount to about 170 Gt carbon, which is now largely in the atmosphere. A further CO2 emission in the range of 1.2 Gt C per year is still going on due to additional land clearing for agriculture in the tropics. The only way to escape from this forest conversion is a more sustainable use and improved productivity of the already existing farmland.

Soil organic matter of cropland increases only if either the additions can be enhanced or the decomposition rates be reduced. There are opportunities by which such improvements can be achieved. Taking the global historical loss of about 42 Gt former soil C from mineral soils as a reference, and assuming a practically feasible restoration by one half to two thirds, this would correspond to somewhere between 20 and 30 Gt C altogether, or to an average CO2 offset of 0.4–0.6 Gt C yr−1 . The drawback, however, is that this carbon-sink option is of limited duration only. The humus enrichment in crop soil always follows a saturation curve, approaching a new equilibrium level after not more than 50–100 years. Furthermore, this new soil carbon level drops rapidly again, as soon as the required most careful management can no longer be sustained.

W. E. Easterling (1996). Adapting North American agriculture to climate change in review. Agricultural and Forest Meteorology 80 (1): 1-53

ABSTRACT: The adaptability of North American agriculture to climate change is assessed through a review of current literature. A baseline of North American agriculture without climate change suggests that farming faces serious challenges in the future (e.g. declining domestic demand, loss of comparative advantage, rising environmental costs). Climate change adjustments at the farm-level and in government policy, including international trade policy, are inventoried from the literature. The adaptive potential of agriculture is demonstrated historically with situations that are analogous to climate change, including the translocation of crops across natural climate gradients, the rapid introduction of new crops such as soybeans in the US and canola in Canada, and resource substitutions prompted by changes in prices of production inputs. A wide selection of modeling studies is reviewed which, in net, suggests several agronomic and economic adaptation strategies that are available to agriculture. Agronomic strategies include changes in crop varieties and species, timing of operations, and land management including irrigation. Economic strategies include investment in new technologies, infrastructure and labor, and shifts in international trade. Overall, such agronomic strategies were found to offset either partially or completely the loss of productivity caused by climate change. Economic adaptations were found to render the agricultural costs of climate change small by comparison with the overall expansion of agricultural production. New avenues of adaptive research are recommended including the formalization of the incorporation of adaptation strategies into modeling, linkage of adaptation to the terrestrial carbon cycle, anticipation of future technologies, attention to scaling from in situ modeling to the landscape scale, expansion of data sets and the measurement and modeling of unpriced costs. The final assessment is that climate change should not pose an insurmountable obstacle to North American agriculture. The portfolio of assets needed to adapt is large in terms of land, water, energy, genetic diversity, physical infrastructure and human resources, research capacity and information systems, and political institutions and world trade—the research reviewed here gives ample evidence of the ability of agriculture to utilize such assets. In conclusion, the apparent efficiency with which North American agriculture may adapt to climate changes provides little inducement for diverting agricultural adaptation resources to efforts to slow or halt the climate changes.

Stavins, R.N., K.R. Richards (2005). The cost of U.S. forest-based carbon sequestration. Pew Center on Global Climate Change: 52 p.

FIRST PARAGRAPH: When and if the United States decides on mandatory policies to address global climate change, it will be necessary to decide whether carbon sequestration should be part of the domestic portfolio of compliance activities. The potential costs of carbon sequestration policies will presumably be a major criterion, so it is important to assess the cost of supplying forest-based carbon sequestration in the United States. In this report we survey major studies, examine the factors that have affected their carbon sequestration cost estimates, and synthesize the results.

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