Climate Change and...

Annotated Bibliography

Carbon Dynamics

Decomposition and Respiration

Adams, A. B., Harrison, R. B., Sletten, R. S., Strahm, B. D., Turnblom, E. C., Jensen, C. M. (2005). Nitrogen-fertilization impacts on carbon sequestration and flux in managed coastal Douglas-fir Stands of the Pacific Northwest. Forest Ecology and Management 220 (1-3): 313-325

ABSTRACT: We examined whether N-fertilization and soil origin of Douglas-fir [Psuedotsuga menziesii (Mirb.) Franco] stands in western Washington state could affect C sequestration in both the tree biomass and in soils, as well as the flux of dissolved organic carbon (DOC) through the soil profile. This study utilized four forest sites that were initially established between 1972 and 1980 as part of Regional Forest Nutrition Research Project (RFNRP). Two of the soils were derived from coarse-textured glacial outwash and two from finer-textured volcanic-source material, primarily tephra, both common soil types for forestry in the region. Between 1972 and 1996 fertilized sites received either three or four additions of 224 kg N ha-1 as urea (672-896 kg N ha-1 total). Due to enhanced tree growth, the N-fertilized sites (161 Mg C ha-1 ) had an average of 20% more C in the tree biomass compared to unfertilized sites (135 Mg C ha-1 ). Overall, N-fertilized soils (260 Mg C ha-1 ) had 48% more soil C compared to unfertilized soils (175 Mg C ha-1 ). The finer-textured volcanic-origin soils (348 Mg C ha-1 ) had 299% more C than glacial outwash soils (87.2 Mg C ha-1 ), independent of N-fertilization. Soil-solution DOC collected by lysimeters also appeared to be higher in N-fertilized, upper soil horizons compared to unfertilized controls but it was unclear what fraction of the difference was lost from decomposition or contributed to deep-profile soil C by leaching and adsorption. When soil, understory vegetation and live-tree C compartments are pooled and compared by treatment, N-fertilized plots had an average of 110 Mg C ha-1 more than unfertilized controls. These results indicate these sites generally responded to N-fertilization with increased C sequestration, but differences in stand and soil response to N-ferfilization might be partially explained by soil origin and texture.

Aires, L. M. I., Pio, C. A., Pereira, J. S. (2008). Carbon dioxide exchange above a Mediterranean C3/C4 grassland during two climatologically contrasting years. Global Change Biology 14 (3): 539-555

ABSTRACT: Eddy-covariance measurements of net ecosystem carbon exchange (NEE) were carried out above a grazed Mediterranean C3/C4 grassland in southern Portugal, during two hydrological years, 2004–2005 and 2005–2006, of contrasting rainfall. Here, we examine the seasonal and interannual variation in NEE and its major components, gross primary production (GPP) and ecosystem respiration (Reco ), in terms of the relevant biophysical controls. The first hydrological year was dry, with total precipitation 45% below the long-term mean (669 mm) and the second was normal, with total precipitation only 12% above the long-term mean. The drought conditions during the winter and early spring of the dry year limited grass production and the leaf area index (LAI) was very low. Hence, during the peak of the growth period, the maximum daily rate of NEE and the light-use and water-use efficiencies were approximately half of those observed in the normal year. In the summer of 2006, the warm-season C4 grass,Cynodon dactylon L., exerted an evident positive effect on NEE by converting the ecosystem into a carbon sink after strong rain events and extending the carbon sequestration for several days, after the end of senescence of the C3 grasses. On an annual basis, the GPP and NEE were 524 and 49 g C m−2 , respectively, for the dry year, and 1261 and −190 g C m−2 for the normal year. Therefore, the grassland was a moderate net source of carbon to the atmosphere, in the dry year, and a considerable net carbon sink, in the normal year. In these 2 years of experiment the total amount of precipitation was the main factor determining the interannual variation in NEE. In terms of relevant controls, GPP and NEE were strongly related to incident photosynthetic photon flux density on short-term time scales. Changes in LAI explained 84% and 77% of the variation found in GPP and NEE, respectively. Variations in Reco were mainly controlled by canopy photosynthesis. After each grazing event, the reduction in LAI affected negatively the NEE.

Allen, M. F., Klironomos, J. N., Treseder, K. K., Oechel, W. C. (2005). Responses of soil biota to elevated CO2 in a chaparral ecosystem. Ecological Applications 15 (5): 1701-1711

ABSTRACT: Atmospheric CO2 is rapidly increasing without an integrative understanding of the responses of soil organisms. We sampled soils in a chaparral ecosystem at 18 intervals over a 3-yr period in replicated field chambers ranging from 250 to 750 ppm CO2 at 100 ppm increments. We assessed three distinct soil energy channels: mycorrhizal fungi, saprotrophic fungi-mite/collembola, and bacteria-protozoa/nematode. C allocation below-ground increased with elevated CO2 , Standing crops of fungi and bacteria rarely changed with CO2 . Mass of bacteria-feeding nematodes increased during wet periods, but the effects on soil bacteria were not,detectable. However, grazing of fungi by mites increased with increasing CO2 up to 550 PPM CO2 . Above this threshold, allocation of C to the fungal channel declined. Direct measures of mycorrhizal fungi (percentage infection, arbuscular mycorrhizal [AM] fungal hyphal length) showed no changes with CO2 enrichment, but indirect measures (macroaggregates with newly fixed Q increased suggesting increasing allocation of C through this channel. We postulate that the lack of change in standing crop in microbes to elevated CO2 is due to increasing turnover and to increasing N deficiency. Assessing C sequestration and other impacts of elevated CO2 on ecosystems requires a comprehensive, interactive, and dynamic evaluation of soil organismal responses.

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

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

Amiro, B. D., Ian MacPherson, J., Desjardins, R. L., Chen, J. M., Liu, J. (2003). Post-fire carbon dioxide fluxes in the western Canadian boreal forest: evidence from towers, aircraft and remote sensing. Agricultural and Forest Meteorology 115 (1-2): 91-107

ABSTRACT: Recent CO2 flux measurements from towers and aircraft (net ecosystem exchange by eddy covariance) and remote sensing/modeling (net primary productivity--NPP) following fire show that the regenerating boreal forest in western Canada has a low initial flux that increases with time since fire. Daytime CO2 fluxes are downward, even after 2 years following fire, although fluxes were upward during the first year after the fire. In summer, the forest is a net carbon sink a few years following fire. A regression of all data gives a relationship where the CO2 flux relative to 10 years following fire=0.11+0.92 log10 (years since fire) (r2 =0.5). The CO2 flux reaches the same rate as that of a mature site between 10 and 30 years following fire, depending on the site and the data set. Many studies in the literature indicate that soil respiration decreases following fire, although several models assume that heterotrophic respiration increases. If fire reduces respiration and our growing season measurements showing a net sink in early years are widely applicable, it is likely that some models may have overestimated the impact of fire on the carbon balance of the boreal landscape.

Ananyeva, N. D., Susyan, E. A., Chernova, O. V., Wirth, S. (2008). Microbial respiration activities of soils from different climatic regions of European Russia. European Journal of Soil Biology 44 (2): 147-157

ABSTRACT: The aim of this study was to survey and evaluate the microbial respiration of main soil types (gleyic Cryosols, umbric Albeluvisols, albic Luvisols, luvic Chernozems, Kastanozems) across European Russia, from semiarid to polar climatic zones. Soil was sampled from 0–5 and 5–10 cm layers at natural (forest, grassland, fallow) and corresponding sites under agricultural land use. Soil microbial biomass carbon (Cmic ) determined by the substrate-induced respiration method and basal respiration (BR) were measured under standardized laboratory conditions (22 °C, 60% WHC). The ratios of BR/Cmic and Cmic /Corg were also calculated. Cmic and BR were highest in polar (gleyic Cryosols) and temperate (albic Luvisols, luvic Chernozems) climatic zones, the lowest were in boreal (umbric Luvisols) and semiarid (Kastanozems). Cmic , BR and Cmic /Corg ratios were higher in 0–5 cm layers compared to the corresponding 5–10 cm and in natural sites versus in arable. Principal component analysis yielded a clear separation of the vegetation zones with respect to the several principal components (PC). PC 1 was composed of Cmic , BR, soil chemical (Corg , Ntot ) and texture parameters. PC 2 was composed of climatic (MAT, MAP) and soil pH variables. Three-way ANOVA indicated that “soil type”, “ecosystem” and “layer” factors, and their interactions accounted for almost 98 and 99% of the total variance in Cmic and BR, respectively.

Arn Teh, Yit, Silver, Whendee L., Conrad, Mark E. (2005). Oxygen effects on methane production and oxidation in humid tropical forest soils.. Global Change Biology 11 (8): 1283-1297

ABSTRACT: We investigated the effects of oxygen (O2 ) concentration on methane (CH4 ) production and oxidation in two humid tropical forests that differ in long-term, time-averaged soil O2 concentrations. We identified sources and sinks of CH4 through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH4 oxidized in situ. Complementary laboratory experiments were conducted to determine the effects of O2 concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH4 production occurred in soils that contained between 9±1.1% and 19±0.2% O2 . For example, we observed CH4 concentrations in excess of 3% in soils with 9±1.1% O2 . CH4 emissions from the lower O2 sites were high (22–101 nmol CH4 m−2 s−1 ), and were equal in magnitude to CH4 emissions from natural wetlands. During peak periods of CH4 efflux, carbon dioxide (CO2 ) emissions became enriched in13 C because of high methanogenic activity. Gross CH4 production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48–78% of the CH4 produced was oxidized prior to atmospheric egress. O2 availability influenced CH4 oxidation more strongly than methanogenesis. Gross CH4 production was relatively insensitive to O2 concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH4 production with increasing O2 concentration, shifting theδ13 C composition of CH4 to values that were more positive. Isotopic measurements suggested that CO2 was an important source of carbon for methanogenesis in humid forests. The δ13 C value of methanogenesis was between−84‰ and −98‰, which is well within the range of CH4 produced from CO2 reduction, and considerably more depleted in13 C than CH4 formed from acetate.

Ball, T., Smith, K.A., Moncrieff, J.B. (2007). Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil. Global Change Biology 13 (10): 2128-2142

ABSTRACT: The influence of forest stand age in aPicea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO2 , CH4 and N2 O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2-year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30- and 20-year-old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30-year-old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO2 efflux was the dominant influence on the GHG budget, contributing 93–94% of the total GHG flux across the chronosequence compared with 6–7% from CH4 and N2 O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO2 were lower at the 20-year-old (20y) site than at the 30-year-old (30y) and CF sites, with soil temperature the dominant control. CH4 efflux was highest at the CF site, with peak flux 491±54.5μg m−2 h−1 and maximum annual flux 18.0±1.1 kg CH4 ha−1 yr−1 . No consistent uptake of CH4 was noted at any site. A linear relationship was found between log CH4 flux and the closeness of the water table to the soil surface across all sites. N2 O efflux was highest in the 30y site, reaching 108±38.3μgN2 O-N m−2 h−1 (171μgN2 Om−2 h−1 ) in midsummer and a maximum annual flux of 4.7±1.2 kg N2 O ha−1 yr−1 in 2001. Automatic chamber data showed a positive exponential relationship between N2 O flux and soil temperature at this site. The relationship between N2 O emission and soil volumetric moisture indicated an optimum moisture content for N2 O flux of 40–50% by volume. The relationship between C:N ratio data and integrated N2 O flux was consistent with a pattern previously noted across temperate and boreal forest soils.

Basiliko, N., Moore, T. R., Lafleur, P. M., Roulet, N. T. (2005). Seasonal and inter-annual decomposition, microbial biomass, and nitrogen dynamics in a Canadian bog. Soil Science 170 (11): 902-912

ABSTRACT: To establish the temporal and spatial variability of substrate contribution to ecosystem respiration (ER), we measured the seasonal and inter-annual microbial carbon dioxide (CO2 ) production potential, microbial biomass, and nitrogen dynamics over a period of 2 years in the upper 30 cm of a peat bog in southern Ontario. Samples collected during a warmer year with lower average summer water table position had larger inorganic and organic nitrogen (N) concentrations and microbial CO2 production potentials. Across all sampling dates, the distance of the water table beneath the surface was significantly and positively correlated with N availability, and in turn N availability was significantly and positively correlated with CO2 production, although direct correlation between water table position and CO2 production was only significant at P = 0.1. Inter-seasonal variability in CO2 production, microbial biomass, or N did not follow consistent patterns between years, and inorganic N species, particularly nitrate, concentrations varied relatively the most between sampling dates, although concentrations were always small relative to microbial biomass N and potassium sulfate- extractable organic N. Microbial CO2 production from the surface peat profile was calculated to be between 2.5 and 5.7 g CO2 m-2 day-1 . Data extrapolation showed that microbial production of CO2 can be between 41 and 67% of the CO2 emitted as ER with the larger value falling in a warmer, drier year and that inter-annual changes in production potentials may partially explain increased ER in warmer, drier years. These results suggest that changes in microbial CO2 production and microbial community and nutrient characteristics may play an important role in controlling the emission of CO2 from terrestrial ecosystems such as peatlands.

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

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

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

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

Bernhardt, E. S., Barber, J. J., Pippen, J. S., Taneva, L., Andrews, J. A., Schlesinger, W. H. (2006). Long-term effects of free air CO2 enrichment (FACE) on soil Respiration. Biogeochemistry 77 (1): 91-116

ABSTRACT: Emissions of CO2 from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO2 to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO2 Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO2 concentration affects soil CO2 dynamics over 7 years of continuous enrichment. Soil respiration, soil CO2 concentrations, and the isotopic signature of soil CO2 were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO2 efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO2 enrichment effect on soil CO2 efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO2 concentrations and depletion in theδ13 C of soilCO2 at all but the shallowest soil depths.

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 .

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

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

Bonan, G.B., Van Cleve, K. (1991). Soil temperature, nitrogen mineralization, and carbon source-sink relationships in boreal forests. Canadian Journal of Forest Research 22 (5): 629-639

ABSTRACT: Boreal forests contain large quantities of soil carbon, prompting concern that climatic warming may stimulate decomposition and accentuate increasing atmospheric CO2 concentrations. While soil warming increases decomposition rates, the accompanying increase in nutrient mineralization may promote tree growth in these nutrient-poor soils and thereby compensate for the increased carbon loss during decomposition. We used a model of production and decomposition to test this hypothesis. In black spruce (Piceamariana (Mill.) B.S.P.), white spruce (Piceaglauca (Moench) Voss), and paper birch (Betulapapyrifera Marsh.) forests, decomposition increased with the soil warming caused by a 5 °C increase in air temperature. However, increased nitrogen mineralization promoted tree growth, offsetting the increased carbon loss during decomposition. In the black spruce forest, increased tree production was maintained for the 25 years of simulation. Whether this can be maintained indefinitely is unknown. In the birch forest, tree production decreased to prewarming levels after about 10 years. Our analyses examined only the consequences of belowground feedbacks that affect ecosystem carbon uptake with climatic warming. These analyses highlight the importance of interactions among net primary production, decomposition, and nitrogen mineralization in determining the response of forest ecosystems to climatic change.

Bowden, R. D., Castro, M. S., Melillo, J. M., Steudler, P. A., Aber, J. D. (1993). Fluxes of greenhouse gases between soils and the atmosphere in a temperate forest following a simulated hurricane blowdown. Biogeochemistry 21 (2): 67-71

ABSTRACT: Fluxes of nitrous oxide (N2 O), carbon dioxide (CO2 ), and methane (CH4 ) between soils and the atmosphere were measured monthly for one year in a 77-year-old temperate hardwood forest following a simulated hurricane blowdown. Emissions of CO2 and uptake of CH4 for the control plot were 4.92 MT C ha−1 y−1 and 3.87 kg C ha−1 y−1 , respectively, and were not significantly different from the blowdown plot. Annual N2 O emissions in the control plot (0.23 kg N ha−1 y−1 ) were low and were reduced 78% by the blowdown. Net N mineralization was not affected by the blowdown. Net nitrification was greater in the blowdown than in the control, however, the absolute rate of net nitrification, as well as the proportion of mineralized N that was nitrified, remained low. Fluxes of CO2 and CH4 were correlated positively to soil temperature, and CH, uptake showed a negative relationship to soil moisture. Substantial resprouting and leafing out of downed or damaged trees, and increased growth of understory vegetation following the blowdown, were probably responsible for the relatively small differences in soil temperature, moisture, N availability, and net N mineralization and net nitrification between the control and blowdown plots, thus resulting in no change in CO2 or CH4 fluxes, and no increase in N2 O emissions.

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

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

Cheng, W. (1999). Rhizosphere feedbacks in elevated CO2 . Tree Physiology 19 (4-5): 313-320.

ABSTRACT: Understanding rhizosphere processes in relation to increasing atmospheric CO2 concentrations is important for predicting the response of forest ecosystems to environmental changes, because rhizosphere processes are intimately linked with nutrient cycling and soil organic matter decomposition, both of which feedback to tree growth and soil carbon storage. Plants grown in elevated CO2 substantially increase C input to the rhizosphere. Although it is known that elevated CO2 enhances rhizosphere respiration more than it enhances root biomass, the fate and function of this extra carbon input to the rhizosphere in response to elevated CO2 are not clear. Depending on specific plant and soil conditions, the increased carbon input to the rhizosphere can result in an increase, a decrease, or no effect on soil organic matter decomposition and nutrient mineralization. Three mechanisms may account for these inconsistent results: (1) the "preferential substrate utilization" hypothesis; (2) the "priming effect" hypothesis; and (3) the "competition" hypothesis, i.e., competition for mineral nutrients between plants and soil microorganisms. A microbial growth model is developed that quantitatively links the increased rhizosphere input in response to elevated CO2 with soil organic matter decomposition. The model incorporates the three proposed mechanisms, and simulates the complexity of the rhizosphere processes. The model also illustrates mechanistically the interactions among nitrogen availability, substrate quality, and microbial dynamics when the system is exposed to elevated CO2 .

Chojnacky, D.C., Amacher, M.C., Perry, C.H., K.M. Reynolds (2006). A model for monitoring forest floor duff and litter carbon sequestration. USDA Forest Service, Pacific Northwest Research Station

ABSTRACT: The forest floor is an important part of carbon storage, biodiversity, nutrient cycling, and fire fuel hazard. This paper reports on a study of litter and duff layers of the forest floor for eastern U.S. forests. The U. S. Department of Agriculture (USDA) Forest Service, Forest Inventory Analysis (FIA) program currently measures variables related to duff and litter on a subsample of plots covering all U.S. forest lands regardless of ownership. The FIA soils protocol includes duff and litter thickness measurement and sample collection followed by lab measurement of mass and carbon content. We examined these lab data to test a model of duff and litter carbon storage based upon simple measurements of forest floor depth. Duff and litter data were compiled from 1,468 plots sampled in 2001 and 2002 from most states in the eastern U.S. These data were combined with other available FIA data for regression modeling to predict duff and litter carbon from depth of duff and litter layer and several other classification variables (R2 =0.56). Results on duff and litter model predictions show that duff and litter are an important carbon sink in eastern U.S. forests by containing about 50 percent of the forest floor carbon or 10 percent of total forest carbon (excluding mineral soil).

Cisneros-Dozal, L. M., Trumbore, S., Hanson, P. J. (2006). Partitioning sources of soil-respired CO2 and their seasonal variation using a unique radiocarbon tracer. Global Change Biology 12 (2): 194-204

ABSTRACT: Soil respiration is derived from heterotrophic (decomposition of soil organic matter) and autotrophic (root/rhizosphere respiration) sources, but there is considerable uncertainty about what factors control variations in their relative contributions in space and time. We took advantage of a unique whole-ecosystem radiocarbon label in a temperate forest to partition soil respiration into three sources: (1) recently photosynthesized carbon (C), which dominates root and rhizosphere respiration; (2) leaf litter decomposition and (3) decomposition of root litter and soil organic matter >1–2 years old.

Heterotrophic sources and specifically leaf litter decomposition were large contributors to total soil respiration during the growing season. Relative contributions from leaf litter decomposition ranged from a low of ~1±3% of total soil respiration (6± 3 mg C m−2 h−1 ) when leaf litter was extremely dry, to a high of 42±16% (96± 38 mg C m−2 h−1 ). Total soil respiration fluxes varied with the strength of the leaf litter decomposition source, indicating that moisture-dependent changes in litter decomposition drive variability in total soil respiration fluxes. In the surface mineral soil layer, decomposition of C fixed in the original labeling event (3–5 years earlier) dominated the isotopic signature of heterotrophic respiration.

Root/rhizosphere respiration accounted for 16±10% to 64±22% of total soil respiration, with highest relative contributions coinciding with low overall soil respiration fluxes. In contrast to leaf litter decomposition, root respiration fluxes did not exhibit marked temporal variation ranging from 34±14 to 40±16 mg C m−2 h−1 at different times in the growing season with a single exception (88±35 mg C m−2 h−1 ). Radiocarbon signatures of root respired CO2 changed markedly between early and late spring (March vs. May), suggesting a switch from stored nonstructural carbohydrate sources to more recent photosynthetic products.

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.

Concilio, A., Ma, S., Li, Q., LeMoine, J., Chen, J., North, M., Moorhead, D., Jensen, R. (2005). Soil respiration response to prescribed burning and thinning in mixed-conifer and hardwood forests.. Canadian Journal of Forest Research 35 (7): 1581-1591

ABSTRACT: The effects of management on soil carbon efflux in different ecosystems are still largely unknown yet crucial to both our understanding and management of global carbon flux. To compare the effects of common forest management practices on soil carbon cycling, we measured soil respiration rate (SRR) in a mixed-conifer and hardwood forest that had undergone various treatments from June to August 2003. The mixed-conifer forest, located in the Sierra Nevada Mountains of California, had been treated with thinning and burning manipulations in 2001, and the hardwood forest, located in the southeastern Missouri Ozarks, had been treated with harvesting manipulations in 1996 and 1997. Litter depth, soil temperature, and soil moisture were also measured. We found that selective thinning produced a similar effect on both forests by elevating SRR, soil moisture, and soil temperature, although the magnitude of response was greater in the mixed-conifer forest. Selective harvest increased SRR by 43% (from 3.38 to 4.82 µmol·m–2 ·s–1 ) in the mixed-conifer forest and by 14% (from 4.25 to 4.84 µmol·m–2 ·s–1 ) in the hardwood forest. Burning at the conifer site and even-aged harvesting at the mixed-hardwood site did not produce significantly different SRR from controls. Mean SRR were 3.24, 3.42, and 4.52 µmol·m–2 ·s–1 , respectively. At both sites, manipulations did significantly alter SRR by changing litter depth, soil structure, and forest microclimate. SRR response varied by vegetation patch type, the scale at which treatments altered these biotic factors. Our findings provide forest managers first-hand information on the response of soil carbon efflux to various management strategies in different forests.

Cornelissen, J. H. C., van Bodegom, P. M., Aerts, R., Callaghan, T. V., van Logtestijn, R. S. P., Alatalo, J., Stuart C. F., Gerdol, R., Gudmundsson, J., Gwynn-Jones, D., Hartley, A. E., Hik, D. S., Hofgaard, A., Jonsdottir, I. S., Karlsson, S., Klein, J. A., Laundre, J., Magnusson, B., Michelsen, A., Molau, U., Onipchenko, V. G., Quested, H. M., Sandvik, S. M., Schmidt, I. K., Shaver, G. R., Solheim, B., Soudzilovskaia, N. A., Stenstrom, A., Tolvanen, A., Totland, O., Wada, N., Welker, J. M., Zhao, X. (2007). Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes. Ecology Letters 10 (7): 619-627

ABSTRACT: Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth-form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.

Czimczik, C.I., Trumbore, S.E., Carbone, M.S., Winston, G.C. (2006). Changing sources of soil respiration with time since fire in a boreal forest. Global Change Biology 12 (6): 957-971

ABSTRACT: Radiocarbon signatures (Δ14 C) of carbon dioxide (CO2 ) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2-year period, we measured Δ14 C of soil respiration and soil CO2 in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand-replacing fire. Comparing bulk respiration Δ14 C with Δ14 C of CO2 evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ14 C of respired CO2 indicated marked variation in respiration sources in space and time.The14 C signature of respired CO2 respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ14 C greater (averaging~120‰) than autotrophic respiration. TheΔ14 C of autotrophically respired CO2 in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004).CO2 respired by black spruce roots in stands >40 years old hadΔ14 C up to 30‰ higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants.

Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ~50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soilCO2 hadΔ14 C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ14 C of soil respiration in younger successional stands dropped below those of the atmospheric CO2 .

Davidson, E. A., Richardson, A. D., Savage, K. E., Hollinger, D. Y. (2006). A distinct seasonal pattern of the ratio of soil respiration to total ecosystem respiration in a spruce-dominated forest. Global Change Biology 12 (2): 230-239

ABSTRACT: Annual budgets and fitted temperature response curves for soil respiration and ecosystem respiration provide useful information for partitioning annual carbon budgets of ecosystems, but they may not adequately reveal seasonal variation in the ratios of these two fluxes. Soil respiration (Rs ) typically contributes 30–80% of annual total ecosystem respiration (Reco ) in forests, but the temporal variation of these ratios across seasons has not been investigated. The objective of this study was to investigate seasonal variation in the Rs /Reco ratio in a mature forest dominated by conifers at Howland, ME, USA. We used chamber measurements of Rs and tower-based eddy covariance measurements of Reco. The Rs /Reco ratio reached a minimum of about 0.45 in the early spring, gradually increased through the late spring and early summer, leveled off at about 0.65 for the summer, and then increased again to about 0.8 in the autumn. A spring pulse of aboveground respiration presumably causes the springtime minimum in this ratio. Soil respiration 'catches up' as the soils warm and as root growth presumably accelerates in the late spring, causing the Rs /Reco ratios to increase. The summertime plateau of Rs /Reco ratios is consistent with summer drought suppressing Rs that would otherwise be increasing, based on increasing soil temperature alone, thus causing the Rs /Reco ratios to not increase as soils continue to warm. Declining air temperatures and litter fall apparently contribute to increased Rs /Reco ratios in the autumn. Differences in phenology of growth of aboveground and belowground plant tissues, mobilization and use of stored substrates within woody plants, seasonal variation in photosynthate and litter substrates, and lags between temperature changes of air and soil contribute to a distinct seasonal pattern of Rs s/Reco ratios.

Delucia, E. H., Moore, D. J., Norby, R. J. (2005). Contrasting responses of forest ecosystems to rising atmospheric CO2: Implications for the global C cycle. Global Biogeochemical Cycles, 19 (GB3006): doi:10.1029/2004GB002346

ABSTRACT: In two parallel but independent experiments, Free Air CO2 Enrichment (FACE) technology was used to expose plots within contrasting evergreen loblolly pine (Pinus taeda L.) and deciduous sweetgum (Liquidambar styraciflua L.) forests to the level of CO2 anticipated in 2050. Net primary production (NPP) and net ecosystem production (NEP) increased in both forests. In the year 2000, after exposing pine and sweetgum to elevated CO2 for approximately 5 and 3 years, a complete budget calculation revealed increases in net ecosystem production (NEP) of 41% and 44% in the pine forest and sweetgum forest, respectively, representing the storage of an additional 174 g C m-2 and 128 g C m-2 in these forests. The stimulation of NPP without corresponding increases in leaf area index or light absorption in either forest resulted in 23 - 27% stimulation in radiation-use efficiency, defined as NPP per unit absorbed photosynthetically active radiation. Greater plant respiration contributed to lower NPP in the loblolly pine forest than in the sweetgum forest, and these forests responded differently to CO2 enrichment. Where the pine forest added C primarily to long-lived woody tissues, exposure to elevated CO2 caused a large increase in the production of labile fine roots in the sweetgum forest. Greater allocation to more labile tissues may cause more rapid cycling of C back to the atmosphere in the sweetgum forest compared to the pine forest. Imbalances in the N cycle may reduce the response of these forests to experimental exposure to elevated CO2 in the future, but even at the current stimulation observed for these forests, the effect of changes in land use on C sequestration are likely to be larger than the effect of CO2 -induced growth stimulation.

Dilustro, J. J., Collins, B., Duncan, L., Crawford, C. (2005). Moisture and soil texture effects on soil CO2 efflux components in southeastern mixed pine forests. Forest Ecology and Management 204 (1): 87-97

ABSTRACT: Monitoring soil CO2 efflux rates and identifying controlling factors, such as forest composition or soil texture, can help guide forest management and will likely gain relevance as atmospheric CO2 continues to increase. We examined soil CO2 efflux and potential controlling factors in managed mixed pine forests in southwestern Georgia. Soil CO2 efflux was monitored periodically in two stands that differed in soil texture in 2001 and 2002, and in six additional stands in 2003. We also monitored controlling factors: soil temperature, moisture, organic layer mass, and A layer depth. Soil moisture and CO2 efflux varied with soil texture differences among the stands. As expected, soil temperature had a strong influence on soil CO2 efflux. Soil moisture, organic layer mass, and A layer depth also were correlated with soil CO2 efflux during periods of water stress, but these relationships differed with soil texture. Forest management activities can alter components of soil CO2 efflux, including soil carbon pools, temperature, and moisture; understanding the underlying variation of these components and resultant CO2 efflux over soil types can help guide management toward desired forest carbon balance trends in southeastern mixed pine forests.

Fahey, T., Tierney, G., Fitzhugh, R., Wilson, G., Siccama, T. (2005). Soil respiration and soil carbon balance in a northern hardwood forest ecosystem. Canadian Journal of Forest Research 35 (2): 244-253

ABSTRACT: Soil C fluxes were measured in a northern hardwood forest ecosystem at the Hubbard Brook Experimental Forest to provide insights into the C balance of soils at this long-term study site. Soil CO2 emission (FCO2 ) was estimated using a univariate exponential model as a function of soil temperature based on 23 measurement dates over 5 years. Annual FCO2 for the undisturbed northern hardwood forest was estimated at 660 ± 54 g C·m–2 ·year–1 . Low soil moisture significantly reduced FCO2 on three of the measurement dates. The proportion of FCO2 derived from the forest floor horizons was estimated empirically to be about 58%. We estimated that respiration of root tissues contributed about 40% of FCO2 , with a higher proportion for mineral soil (46%) than for forest floor (35%). Soil C-balance calculations, based upon evidence that major soil C pools are near steady state at this site, indicated a large C flux associated with root exudation plus allocation to mycorrhizal fungi (80 g C·m–2 ·year–1 , or 17% of total root C allocation); however, uncertainty in this estimate is high owing especially to high error bounds for root respiration flux. The estimated proportion of FCO2 associated with autotrophic activity (52%) was comparable with that reported elsewhere (56%).

Fahey, T. J., Siccama, T. G., Driscoll, C. T., Likens, G. E., Campbell, J., Johnson, C. E., Battles, J. J., Aber, J. D., Cole, J. J., Fisk, M. C., Groffman, P. M., Hamburg, S. P., Holmes, R. T., Schwarz, P. A., Yanai, R. D. (2005). The biogeochemistry of carbon at Hubbard Brook. Biogeochemistry 75 (1): 109-176

ABSTRACT: The biogeochemical behavior of carbon in the forested watersheds of the Hubbard Brook Experimental Forest (HBEF) was analyzed in long-term studies. The largest pools of C in the reference watershed (W6) reside in mineral soil organic matter (43% of total ecosystem C) and living biomass (40.5%), with the remainder in surface detritus (14.5%). Repeated sampling indicated that none of these pools was changing significantly in the late-1990s, although high spatial variability precluded the detection of small changes in the soil organic matter pools, which are large; hence, net ecosystem productivity (NEP) in this 2nd growth forest was near zero (± about 20 g C/m2 -yr) and probably similar in magnitude to fluvial export of organic C. Aboveground net primary productivity (ANPP) of the forest declined by 24% between the late-1950s (462 g C/m2 -yr) and the late-1990s (354 g C/m2 -yr), illustrating age-related decline in forest NPP, effects of multiple stresses and unusual tree mortality, or both. Application of the simulation model PnET-II predicted 14% higher ANPP than was observed for 1996–1997, probably reflecting some unknown stresses. Fine litterfall flux (171 g C/m2 -yr) has not changed much since the late-1960s. Because of high annual variation, C flux in woody litterfall (including tree mortality) was not tightly constrained but averaged about 90 g C/m2 -yr. Carbon flux to soil organic matter in root turnover (128 g C/m2 -yr) was only about half as large as aboveground detritus. Balancing the soil C budget requires that large amounts of C (80 g C/m2 -yr) were transported from roots to rhizosphere carbon flux. Total soil respiration (TSR) ranged from 540 to 800 g C/m2 -yr across eight stands and decreased with increasing elevation within the northern hardwood forest near W6. The watershed-wide TSR was estimated as 660 g C/m2 -yr. Empirical measurements indicated that 58% of TSR occurred in the surface organic horizons and that root respiration comprised about 40% of TSR, most of the rest being microbial. Carbon flux directly associated with other heterotrophs in the HBEF was minor; for example, we estimated respiration of soil microarthropods, rodents, birds and moose at about 3, 5, 1 and 0.8 g C/m2 -yr, respectively, or in total less than 2% of NPP. Hence, the effects of other heterotrophs on C flux were primarily indirect, with the exception of occasional irruptions of folivorous insects. Hydrologic fluxes of C were significant in the watershed C budget, especially in comparison with NEP. Although atmospheric inputs (1.7 g C/m2 -yr) and streamflow outputs (2.7 g C/m2 -yr) were small, larger quantities of C were transported within the ecosystem and a more substantial fraction of dissolved C was transported from the soil as inorganic C and evaded from the stream as CO2 (4.0 g C/m2 -yr). Carbon pools and fluxes change rapidly in response to catastrophic disturbances such as forest harvest or major windthrow events. These changes are dominated by living vegetation and dead wood pools, including roots. If biomass removal does not accompany large-scale disturbance, the ecosystem is a large net source of C to the atmosphere (500–1200 g C/m2 -yr) for about a decade following disturbance and becomes a net sink about 15–20 years after disturbance; it remains a net sink of about 200–300 g C/m2 -yr for about 40 years before rapidly approaching steady state. Shifts in NPP and NEP associated with common small-scale or diffuse forest disturbances (e.g., forest declines, pathogen irruptions, ice storms) are brief and much less dramatic. Spatial and temporal patterns in C pools and fluxes in the mature forest at the HBEF reflect variation in environmental factors. Temperature and growing-season length undoubtedly constrain C fluxes at the HBEF; however, temperature effects on leaf respiration may largely offset the effects of growing season length on photosynthesis. Occasional severe droughts also affect C flux by reducing both photosynthesis and soil respiration. In younger stands nutrient availability strongly limits NPP, but the role of soil nutrient availability in limiting C flux in the mature forest is not known. A portion of the elevational variation of ANPP within the HBEF probably is associated with soil resource limitation; moreover, sites on more fertile soils exhibit 20–25% higher biomass and ANPP than the forest-wide average. Several prominent biotic influences on C pools and fluxes also are clear. Biomass and NPP of both the young and mature forest depend upon tree species composition as well as environment. Similarly, litter decay differs among tree species and forest types, and forest floor C accumulation is twice as great in the spruce–fir–birch forests at higher elevations than in the northern hardwood forests, partly because of inherently slow litter decay and partly because of cold temperatures. This contributes to spatial patterns in soil solution and streamwater dissolved organic carbon across the Hubbard Brook Valley. Wood decay varies markedly both among species and within species because of biochemical differences and probably differences in the decay fungi colonizing wood. Although C biogeochemistry at the HBEF is representative of mountainous terrain in the region, other sites will depart from the patterns described at the HBEF, due to differences in site history, especially agricultural use and fires during earlier logging periods. Our understanding of the C cycle in northern hardwood forests is most limited in the area of soil pool size changes, woody litter deposition and rhizosphere C flux processes.

Falk, M., Paw U, K. T., Wharton, S., Schroeder, M. (2005). Is soil respiration a major contributor to the carbon budget within a Pacific Northwest old-growth forest?. Agricultural and Forest Meteorology 135 (1-4): 269-283

ABSTRACT: Carbon dioxide (CO2 ) exchange was measured above the forest floor of a temperate Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) and Western Hemlock (Tsuga heterophylla (Raf.) Sarg.) old-growth forest in southern Washington State. Continuous flux measurements were conducted from July 1998 to December 2003 using the eddy-covariance technique. Maximum observed fluxes were +6μmol m−2 s−1 on a monthly average in summer decreasing to +1 to +2μmol m−2 s−1 in winter and early spring. Nighttime soil and understory exchange was well described by an exponential function of soil temperature at a depth of 50 mm during periods of moderate soil moisture, but moisture effects required a modification of the equation at low (0.25 m3 m−3 ) and very high (0.35 m3 m−3 ) soil moisture. Interannual variation was observed in soil respiration and understory carbon exchange and linked to interannual variability in soil moisture and temperature. Maximum CO2 exchange occurred at different times amongst years; a maximum daily CO2 flux was measured as early as May in 2000 and as late as July in 2001. Summer understory photosynthesis was shown to be up to −2μmol m−2 s−1 with some interannual variability. Understory net photosynthesis never exceeded net CO2 efflux on a half-hourly basis, implying at no time was all of the soil respiration recycled by understory photosynthesis. Maximum daily carbon exchange ranged from +5 to +7 g C m−2 day−1 in the summer months and was greatly reduced (but was still non-zero) in the wintertime due to lower soil temperatures, with daily values ranging from +0.5 to +1 g C m−2 day−1 . Annual estimates of soil and understory respiration range from 8.7 to 12.8 Mg C ha−1 year−1 for a period of 5.5 years with an average of 11.1 ± 1.5 Mg C ha−1 year−1 . The large observed annual soil efflux is consistent with the presence of large carbon stocks at the Wind River site.

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

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

Gough, C. M., Vogel, C. S., Kazanski, C., Nagel, L., Flower, C. E., Curtis, P. S. (2007). Coarse woody debris and the carbon balance of a north temperate forest. Forest Ecology and Management 244 (1-3): 60-67

ABSTRACT: Comprehensive estimates of forest carbon (C) mass and respiration require measurements of all C pools, including coarse woody debris (CWD). We used inventory and chamber-based methods to quantify C mass and the annual respiratory C loss from CWD and other major ecosystem components for a deciduous forest in the upper Great Lakes region. Coarse woody debris mass (MCWD , 2.2 Mg C ha−1 ) was less than that of soils (104.1 Mg C ha−1 ) and boles (71.7 Mg C ha−1 ), but similar to that of leaves (1.8 Mg C ha−1 ). Coarse woody debris respiration (RCWD ) increased with temperature and water content, with differences in RCWD among decay classes due to variation in water content rather than to variable sensitivity to environmental conditions. Sensitivity of RCWD to changing temperature, evaluated as Q10 , ranged from 2.20 to 2.57 and was variable among decay classes. Annual CWD respiration (FCWD , 0.21 Mg C ha−1 year−1 ) was 12% of bole respiration, 8% of leaf respiration, and 2% of soil respiration. The CWD decomposition rate-constant (FCWD /MCWD ) in 2004 was 0.09 year−1. When compared to the average annual ecosystem C storage of 1.53 Mg C ha−1 year−1 , FCWD represents a small, but substantial flux that is expected to increase over the next several decades in this maturing forest.

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

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

Guo, L. B., Bek, E., Gifford, R. M. (2006). Woody debris in a 16-year old Pinus radiata plantation in Australia: Mass, carbon and nitrogen stocks, and turnover. Forest Ecology and Management 228 (1-3): 145-151

ABSTRACT: Woody debris that is accumulated on the forest floor could potentially be a relatively long-term carbon (C) sink in forest ecosystems. For a 16-year oldPinus radiata D. Don. plantation in Australia, we quantified the dry mass, C and nitrogen (N) stored in woody debris (including dead logs, branches and twigs) relative to the loss of soil C that followed afforestation of the native pasture onto which the plantation had been established. This debris derived mainly from forest management (thinning and pruning) 8 years earlier. The line intersect technique was used on ten 10 m × 12 m plots to estimate the mass of woody debris on the forest floor in 10 diameter classes. There was 6.1 Mg ha−1 of oven dry woody debris, containing 3.1 Mg C ha−1 and 12.9 kg N ha−1 , on the forest floor. The largest diameter class (>50 mm) contributed most of the debris’ mass. We also estimated rates of decomposition, and C and N release from the woody debris and calculated its half-life and “life time” (95% disappearance). The overall decay rate constant (k) for all woody debris was 0.069 year−1 . The overall half-life and lifetime was 10 and 43 years, respectively. Almost half (42%) of the original C in woody debris was released in the 8 years of decay, but only 12% of the original N was released. Decay rate varied with size class with the largest diameter (>50 mm) decaying the fastest, the smallest diameter class (<5 mm) decaying the second fastest, and the intermediate size-classes being the slowest to decay. Although N was slowly released from the woody debris, this pool was an effective C sink per unit-N involved because of its high C:N ratio. The C stored in the pool offset 22% of the observed soil C-stock reduction 16 years after land use change from pasture to pine plantation.

Hashimoto, H., Nemani, R. R., White, M. A., Jolly, W. M., Piper, S. C., Keeling, C. D., Myneni, R. B., Running, S. W. (2004). El Niño–Southern Oscillation–induced variability in terrestrial carbon cycling. Journal of Geophysical Research 109 (D23110): doi:10.1029/2004JD004959

ABSTRACT: We examined the response of terrestrial carbon fluxes to climate variability induced by the El Niño–Southern Oscillation (ENSO). We estimated global net primary production (NPP) from 1982 to 1999 using a light use efficiency model driven by satellite-derived canopy parameters from the Advanced Very High Resolution Radiometer and climate data from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project. We estimated a summed heterotrophic respiration and fire carbon flux as the residual between NPP and the terrestrial net carbon flux inferred from an atmospheric inversion model, excluding the impacts of land use change. We propose that for global applications this approach may be more robust than traditional, biophysically based approaches of simulating heterotrophic respiration. NPP interannual variability was significantly related to ENSO, particularly at lower latitudes (22.5°N–22.5°S) but was weakly related to global temperature. Global heterotrophic respiration and fire carbon fluxes were strongly correlated with global temperature (7.9 pgC/°C). Our results confirm the dependence of global heterotrophic respiration and fire carbon fluxes on interannual temperature variability and strongly suggest that ENSO-mediated NPP variability influences the atmospheric CO2 growth rate.

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

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

Heinemeyer, A., Hartley, I.P., Evans, S.P., Carreira De La Fuente, J.A., Ineson, P. (2007). Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas. Global Change Biology 13 (8): 1786-1797

ABSTRACT: Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO2 efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO2 efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the 'autotrophic' root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO2 efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration.

Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO2 flux components could be divided into ~60% soil heterotrophic, ~25% EM hyphal, and ~15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO2 flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO2 flux component represents an overflow 'CO2 tap' through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO2 .

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

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

Hill, P., Marshall, C., Harmens, H., Jones, D., Farrar, J. (2005). Carbon sequestration: Do N inputs and elevated atmospheric CO2 alter soil solution chemistry and respiratory C losses?. Water Air and Soil Pollution 4 (6): 177-186

ABSTRACT: Soil respiration is a large C flux which is of primary importance in determining C sequestration. Here we ask how it is altered by atmospheric CO2 concentration and N additions. Swards ofLolium perenne L. were grown in a Eutric cambisol under controlled conditions with and without the addition of 200 kg NO3 –N ha–1 , at either 350 ppm or 700 ppm CO2 , for 3 months. Soil respiration and net canopy photosynthesis were both increased by added N and elevated CO2 , but soil respiration increased proportionately less than fixation by photosynthesis. Thus, both elevated CO2 and N appeared to increase potential C sequestration, although adding N at elevated CO2 reduced the C sequestered as a proportion of that fixed relative to elevated CO2 alone. Across all treatments below-ground respiratory C losses were predicted by root biomass, but not by soil solution C and N concentrations. Specific root-dependent respiration was increased by elevated CO2 , such that belowg-round respiration per unit biomass and per unit plant N was increased.

Humphreys, E. R., Black, T. A., Morgenstern, K., Cai, T., Drewitt, G. B., Nesic, Z., Trofymow, J.A. (2006). Carbon dioxide fluxes in coastal Douglas-fir stands at different stages of development after clearcut harvesting: The Fluxnet-Canada Research Network: Influence of Climate and Disturbance on Carbon Cycling in Forests and Peatlands. Agricultural and Forest Meteorology 140 (1-4): 6-22

ABSTRACT: Forests play a significant role in the global carbon (C) cycle. Variability in weather, species, stand age, and current and past disturbances are some of the factors that control stand-level C dynamics. This study examines the relative roles of stand age and associated structural characteristics and weather variability on the exchange of carbon dioxide between the atmosphere and three different coastal Douglas-fir stands at different stages of development after clearcut harvesting. The eddy covariance technique was used to measure carbon dioxide fluxes and a portable soil chamber system was used to measure soil respiration in the three stands located within 50 km of each other on the east coast of Vancouver Island, British Columbia, Canada. In 2002, the recently clearcut harvested stand (HDF00) was a large C source, the pole/sapling aged stand (HDF88) was a moderate C source, and the rotation-aged stand (DF49) was a moderate C sink (net ecosystem production of −606, −133, and 254 g C m−2 year−1 , respectively). Annual gross ecosystem production and ecosystem respiration also increased with increasing stand age. Differences in stand structural characteristics such as species composition and phenology were important in determining the timing and magnitude of maximum gross ecosystem production and net ecosystem production through the year. Both soil and ecosystem respiration were exponentially related to soil temperature in each stand with total ecosystem respiration differing more among stands than soil respiration. Between 1998 and 2003, annual net ecosystem production ranged from 254 to 424 g C m−2 year−1 over 6 years for DF49, from −623 to −564 g C m−2 year−1 over 3 years for HDF00, and from −154 to −133 g C m−2 year−1 over 2 years for HDF88. Interannual variations in C exchange of the oldest, most structurally stable stand (DF49) were related to variations in spring weather while the rapid growth of understory and pioneer species influenced variations in HDF00. The differences in net ecosystem production among stands (maximum of 1000 g C m−2 year−1 between the oldest and youngest stands) were an order of magnitude greater than the differences among years within a stand and emphasized the importance of age-related differences in stand structure on C exchange processes.

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

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

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

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

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

Kaye, J. P., Hart, S. C. (1998). Restoration and canopy-type effects on soil respiration in a ponderosa pine-bunchgrass ecosystem. Soil Science Society Of America JournalSoil Sci So 62 (4): 1062-1072

ABSTRACT: In ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests of the western USA, fire exclusion by Euro-American settlers facilitated pine invasion of grass openings, increased forest floor detritus, and shifted the disturbance regime toward stand-replacing fires. We evaluated the impacts of two replicated ecological restoration treatments involving tree thinning alone (thinning restoration) and a combination of tree thinning, forest floor reduction, and prescribed burning (composite restoration) on soil microbial activity, biomass, and function approximately 8 yr after initial treatments. Microbial-N levels in the two restoration treatments were not significantly different from the control during either the dry or wet periods of the growing season. Soil respiration measured in situ was significantly higher in the restoration treatments than in the control only during the dry period, while soil enzyme activities were generally higher in the composite restoration treatment than in the thinning restoration or control treatments during the wet period. Community-level physiological profiles suggested differences in the physiological capacities of bacteria and fungi in the composite restoration treatment compared with the other treatments. We also compared microbial characteristics under different canopy types to evaluate the impacts of pine invasion and establishment in grass openings on soil microorganisms. Soil respiration rates (dry period only) and enzyme activities (wet period only) were higher in grass openings than under presettlement trees, with intermediate values found under postsettlement pines that have invaded grass areas. Taken together, our results suggest that restoration treatments have long-term impacts on the soil microflora in these forests.

Kim, C. (2008). Soil CO2 efflux in clear-cut and uncut red pine (Pinus densiflora S. et Z.) stands in Korea. Forest Ecology and Management 255 (8-9): 3318-3321

ABSTRACT: This study was carried out to determine the effects of clear-cutting on soil CO2 efflux in a 42-year-old pine (Pinus densiflora S. et. Z.) stand. The variation of soil CO2 efflux rates with soil temperature, soil pH, soil moisture and soil organic carbon (C) content was measured monthly for 1 year in two pine plots; a clear-cut pine (CCP) and an uncut pine (UCP) plots. Mean soil CO2 efflux rates during the study period were significantly higher (P < 0.05) in CCP (0.52 g CO2 m−2 h−1 ) than in UCP (0.37 g CO2 m−2 h−1 ). High soil CO2 efflux rates in CCP were attributed to the change of soil temperature, soil pH, soil organic C and soil moisture content following canopy removal. In addition, soil temperature in CCP was significantly higher (1–3 °C) than in UCP except during winter (P < 0.05). Soil pH was also significantly higher (0.1–0.5 units) in CCP than in UCP (P < 0.05), suggesting a better environment for microbial or root growth activity. In contrast to soil temperature or soil pH, soil organic C and soil moisture content were significantly lower in CCP than in UCP (P < 0.05). The results indicated that the increased soil CO2 efflux rates in CCP compared with UCP could be due to the combined effect of high soil temperature, high soil pH, low soil organic C and soil moisture content.

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

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

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

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

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

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

Liu, L., King, J. Y., Giardina, C.P. (2007). Effects of elevated atmospheric CO2 and tropospheric O3 on nutrient dynamics: decomposition of leaf litter in trembling aspen and paper birch communities. Plant And SoilPlant Soil 299 (1): 65-82

ABSTRACT: Atmospheric changes could strongly influence how terrestrial ecosystems function by altering nutrient cycling. We examined how the dynamics of nutrient release from leaf litter responded to two important atmospheric changes: rising atmospheric CO2 and tropospheric O3 . We evaluated the independent and combined effects of these gases on foliar litter nutrient dynamics in aspen (Populus tremuloides Michx) and birch (Betula papyrifera Marsh)/aspen communities at the Aspen FACE Project in Rhinelander, WI. Naturally senesced leaf litter was incubated in litter bags in the field for 735 days. Decomposing litter was sampled six times during incubation and was analyzed for carbon, and both macro (N, P, K, S, Ca, and Mg) and micro (Mn, B, Zn and Cu) nutrient concentrations. Elevated CO2 significantly decreased the initial litter concentrations of N (−10.7%) and B (−14.4%), and increased the concentrations of K (+23.7%) and P (+19.7%), with no change in the other elements. Elevated O3 significantly decreased the initial litter concentrations of P (−11.2%), S (−8.1%), Ca (−12.1%), and Zn (−19.5%), with no change in the other elements. Pairing concentration data with litterfall data, we estimated that elevated CO2 significantly increased the fluxes to soil of all nutrients: N (+12.5%), P (+61.0%), K (+67.1%), S (+28.0%), and Mg (+40.7%), Ca (+44.0%), Cu (+38.9%), Mn (+62.8%), and Zn (+33.1%). Elevated O3 had the opposite effect: N (−22.4%), P (−25.4%), K (−27.2%), S (−23.6%), Ca (−27.6%), Mg (−21.7%), B (−16.2%), Cu (−20.8%), and Zn (−31.6%). The relative release rates of the nine elements during the incubation was: K ≥ P ≥ mass ≥ Mg ≥B ≥ Ca ≥ S ≥ N ≥ Mn ≥ Cu ≥ Zn. Atmospheric changes had little effect on nutrient release rates, except for decreasing Ca and B release under elevated CO2 and decreasing N and Ca release under elevated O3 . We conclude that elevated CO2 and elevated O3 will alter nutrient cycling more through effects on litter production, rather than litter nutrient concentrations or release rates.

Liu, X., Monger, H. C., Whitford, W. G. (2007). Calcium carbonate in termite galleries - biomineralization or upward transport?. Biogeochemistry 82 (3): 241-250

ABSTRACT: Termites and soil calcium carbonate are major factors in the global carbon cycle: termites by their role in decomposition of organic matter and methane production, and soil calcium carbonate by its storage of atmospheric carbon dioxide. In arid and semiarid soils, these two factors potentially come together by means of biomineralization of calcium carbonate by termites. In this study, we evaluated this possibility by testing two hypotheses. Hypothesis 1 states that termites biomineralize calcium carbonate internally and use it as a cementing agent for building aboveground galleries. Hypothesis 2 states that termites transport calcium carbonate particles from subsoil horizons to aboveground termite galleries where the carbonate detritus becomes part of the gallery construction. These hypotheses were tested by using (1) field documentation that determined if carbonate-containing galleries only occurred on soils containing calcic horizons, (2)13 C/12 C ratios, (3) X-ray diffraction, (4) petrographic thin sections, (5) scanning electron microscopy, and (6) X-ray mapping. Four study sites were evaluated: a C4-grassland site with no calcic horizons in the underlying soil, a C4 -grassland site with calcic horizons, a C3 -shrubland site with no calcic horizons, and a C3 -shrubland site with calcic horizons. The results revealed that carbonate is not ubiquitously present in termite galleries. It only occurs in galleries if subsoil carbonate exists within a depth of 100 cm.13 C/12 C ratios of carbonate in termite galleries typically matched13 C/12 C ratios of subsoil carbonate. X-ray diffraction revealed that the carbonate mineralogy is calcite in all galleries, in all soils, and in the termites themselves. Thin sections, scanning electron microscopy, and X-ray mapping revealed that carbonate exists in the termite gut along with other soil particles and plant opal. Each test argued against the biomineralization hypothesis and for the upward-transport hypothesis. We conclude, therefore, that the gallery carbonate originated from upward transport and that this CaCO3 plays a less active role in short-term carbon sequestration than it would have otherwise played if it had been biomineralized directly by the termites.

Lloyd, J., Kolle, O., Fritsch, H., De Freitas, S. R., Dias, Mafs, Artaxo, P., Nobre, A. D., De Araujo, A. C., Kruijt, B., Sogacheva, L., Fisch, G., Thielmann, A., Kuhn, U., Andreae, M. O. (2007). An airborne regional carbon balance for Central Amazonia. Biogeosciences Discussions 4 (1): 99-123

ABSTRACT: We obtained regional estimates of surface CO2 exchange rates using atmospheric boundary layer budgeting techniques above tropical forest near Manaus, Brazil. Comparisons were made with simultaneous measurements from two eddy covariance towers below. Although there was good agreement for daytime measurements, large differences emerged for integrating periods dominated by the night-time fluxes. These results suggest that a systematic underestimation of night time respiratory effluxes may be responsible for the high Amazonian carbon sink suggested by several previous eddy covariance studies. Large CO2 fluxes from riverine sources or high respiratory losses from recently disturbed forests do not need to be invoked in order to balance the carbon budget of the Amazon. Our results do not, however, discount some contribution of these processes to the overall Amazon carbon budget.

Mariko, S., Urano, T., Asanuma, J. (2007). Effects of irrigation on CO2 and CH4 fluxes from Mongolian steppe soil. Journal of Hydrology 333 (1): 118-123

SUMMARY: Semi-arid temperate steppes comprise approximately 30% of the world’s temperate grassland, and consequently, are a significant component of the global carbon cycle. To better understand how precipitation affects soil carbon fluxes in semi-arid steppes, we examined the effects of irrigation (simulated rainfall) on CO2 and CH4 fluxes from Mongolian semi-arid steppe soil on 10–12 August 2002 and 19–22 August 2003. Meteorological data revealed that the soil was dry in 2002 and wet in 2003. Summer flux measurements in both years showed that the soil emitted CO2 at 75–250 mg m−2 h−1 and consumed atmospheric CH4 at 30–90μg m−2 h−1 . In 2002, the CO2 flux of the irrigated soil showed an increase of 50% over one day following irrigation compared to the non-irrigated soil, and thereafter, no increase. This enhancing effect of irrigation was found only immediately following irrigation in 2003. Soil CH4 fluxes showed little difference between the irrigated and non-irrigated soils in 2002 and 2003. There was also little difference in soil temperatures (at the surface and 5 cm depth) between the soils in 2002 and 2003. The water content of the irrigated soil increased following irrigation then rapidly decreased with time. These results demonstrate that rainfall events enhance carbon loss from semi-arid steppe soil at least within the day following irrigation. However, long-term meteorological observations of precipitation and soil water content in 2003 and 2004 suggest that usual rainfall pulses throughout the growing season (June–September) do not markedly enhance CO2 emission from such soils.

Martin, D., Beringer, J., Hutley, L. B., McHugh, I. (2007). Carbon cycling in a mountain ash forest: Analysis of below ground respiration. Agricultural and Forest Meteorology 147 (1-2): 58-70

ABSTRACT: Soils are responsible for storing up to 75% of forest carbon uptake making them extremely large carbon pools. However, soil carbon is eventually released to the atmosphere by below ground respiration, consisting of soil respiration (microbial activity) and root respiration, which is influenced by environmental climate variables (soil temperature and moisture), soil characteristics (chemical and physical properties) and stand characteristics (stand age). We investigated the impact of stand age of cool temperate mountain ash forests (E. regnans ) in Wallaby Creek, Victoria on carbon cycling between the soil and atmosphere using a chronosequence of three sites of different ages (regrowth from bushfires in 1730, 1926 and 1983). Below ground respiration was measured between January (Summer) and May (Autumn) in 2005 across all three sites, with the highest rates found in the old growth forest (5.3μmol CO2 m−2 s−1 ) and with lowest rates in the youngest site (2.9μmol CO2 m−2 s−1 ). Within sites, below ground respiration rates increased with temperature, with Q10 values ranging between 1.42 and 1.55. Rates were further influenced by soil moisture, and soil physical and chemical properties, including root biomass and levels of soil carbon. Litterfall was also measured and was highest at the youngest site (140 g biomass m−2 month−1 ) and lowest (92 g biomass m−2 month−1 ) at the old growth site. Greater understanding of forest carbon cycling will result in an improved understanding of forests and their influence on global warming.

McCarthy, D. R., Brown, K. J. (2006). Soil respiration responses to topography, canopy cover, and prescribed burning in an oak-hickory forest in southeastern Ohio. Forest Ecology and Management 237 (1-3): 94-102

ABSTRACT: Soil respiration (Rs ) is an important component of carbon loss from forest ecosystems. As forest management (e.g. prescribed burning) is becoming increasingly more common, it is important to understand the relationship between Rs and prescribed fire. Unfortunately, this relationship is still misunderstood due to the heterogeneity of physical and biological factors over the landscape and between ecosystems. To examine the effects of landscape position, canopy cover (CC), and prescribed burning on soil moisture, soil temperature, and Rs , while controlling for variation in soil properties, we utilized a randomized complete block (RCB) design with five treatments within each block. Each block consisted of five 2 m × 2 m treatment subplots: control, cool burn, hot burn, lime fertilization, and leaf litter removal. A total of 20 blocks were nested within a 2 × 2 factorial design with two effects, landscape position (upland or lowland) and canopy cover (100 or 60%). Rs , soil temperature, and soil moisture were measured monthly from June to November 2004. Repeated measures analysis of variance revealed significant effects of treatment and time on Rs . However, Rs was not significantly affected by prescribed fire, landscape position, or canopy cover. Soil temperature and moisture were significantly affected by landscape position, canopy cover, and time. By eliminating within-site variability between control and prescribed burning treatments, Rs rates were found to be unchanged in burn plots during the growing season following the fire. These results highlight the importance of environmental variability in determining the effects of prescribed fire on Rs rates.

Melillo, J.M., Aber, J.D., Linkins, A.E., Ricca, A., Fry, B., Nadelhoffer, K.J. (1989). Carbon and nitrogen dynamics along the decay continuum: Plant litter to soil organic matter. Plant And SoilPlant Soil 115 (2): 189-198

ABSTRACT: Decay processes in an ecosystem can be thought of as a continuum beginning with the input of plant litter and leading to the formation of soil organic matter. As an example of this continuum, we review a 77-month study of the decay of red pine (Pinus resinosa Ait.) needle litter. We tracked the changes in C chemistry and the N pool in red pine (Pinus resinosa Ait.) needle litter during the 77-month period using standard chemical techniques and stable isotope, analyses of C and N.

Mass loss is best described by a two-phase model: an initial phase of constant mass loss and a phase of very slow loss dominated by degradation of lignocellulose (acid soluble sugars plus acid insoluble C compounds). As the decaying litter enters the second phase, the ratio of lignin to lignin and cellulose (the lignocellulose index, LCI) approaches 0.7. Thereafter, the LCI increases only slightly throughout the decay continuum indicating that acid insoluble materials (lignin) dominate decay in the latter part of the continuum.

Nitrogen dynamics are also best described by a two-phase model: a phase of N net immobilization followed by a phase of N net mineralization. Small changes in C and N isotopic composition were observed during litter decay. Larger changes were observed with depth in the soil profile.

An understanding of factors that control lignin degradation is key to predicting the patterns of mass loss and N dynamics late in decay. The hypothesis that labile C is needed for lignin degradation must be evaluated and the sources of this C must be identified. Also, the hypothesis that the availability of inorganic N slows lignin decay must be evaluated in soil systems.

Meyer, H., Kaiser, C., Biasi, C., Haemmerle, R., Rusalimova, O., Lashchinsky, N., Baranyi, C., Daims, H., Barsukov, P., Richter, A. (2006). Soil carbon and nitrogen dynamics along a latitudinal transect in western Siberia, Russia. Biogeochemistry 81 (2): 239-252

ABSTRACT: An 1800-km South to North transect (N 53°43′ to 69°43′) through Western Siberia was established to study the interaction of nitrogen and carbon cycles. The transect comprised all major vegetation zones from steppe, through taiga to tundra and corresponded to a natural temperature gradient of 9.5°C mean annual temperature (MAT). In order to elucidate changes in the control of C and N cycling along this transect, we analyzed physical and chemical properties of soils and microbial structure and activity in the organic and in the mineral horizons, respectively. The impact of vegetation and climate exerted major controls on soil C and N pools (e.g., soil organic matter, total C and dissolved inorganic nitrogen) and process rates (gross N mineralization and heterotrophic respiration) in the organic horizons. In the mineral horizons, however, the impact of climate and vegetation was less pronounced. Gross N mineralization rates decreased in the organic horizons from south to north, while remaining nearly constant in the mineral horizons. Especially, in the northern taiga and southern tundra gross nitrogen mineralization rates were higher in the mineral compared to organic horizons, pointing to strong N limitation in these biomes. Heterotrophic respiration rates did not exhibit a clear trend along the transect, but were generally higher in the organic horizon compared to mineral horizons. Therefore, C and N mineralization were spatially decoupled at the northern taiga and tundra. The climate change implications of these findings (specifically for the Arctic) are discussed.

ONeill,Katherine, Richter,Daniel, Kasischke,Eric (2006). Succession-driven changes in soil respiration following fire in black spruce stands of interior Alaska. Biogeochemistry 80 (1): 1-20

ABSTRACT: This study examined the nitrogen (N) dynamics of a black spruce (Picea mariana (Mill.) BSP)-dominated chronosequence in Manitoba, Canada. The seven sites studied each contained separate well- and poorly drained stands, originated from stand-killing wildfires, and were between 3 and 151 years old. Our goals were to (i) measure total N concentration ([N]) of all biomass components and major soil horizons; (ii) compare N content and select vegetation N cycle processes among the stands; and (iii) examine relationships between ecosystem C and N cycling for these stands. Vegetation [N] varied significantly by tissue type, species, soil drainage, and stand age; woody debris [N] increased with decay state and decreased with debris size. Soil [N] declined with horizon depth but did not vary with stand age. Total (live + dead) biomass N content ranged from 18.4 to 99.7 g N m−2 in the well-drained stands and 37.8–154.6 g N m−2 in the poorly drained stands. Mean soil N content (380.6 g N m−2 ) was unaffected by stand age. Annual vegetation N requirement (5.9 and 8.4 g N m−2 yr−1 in the middle-aged well- and poorly drained stands, respectively) was dominated by trees and fine roots in the well-drained stands, and bryophytes in the poorly drained stands. Fraction N retranslocated was significantly higher in deciduous than evergreen tree species, and in older than younger stands. Nitrogen use efficiency (NUE) was significantly lower in bryophytes than in trees, and in deciduous than in evergreen trees. Tree NUE increased with stand age, but overall stand NUE was roughly constant (~150 g g−1 N) across the entire chronosequence.

Park, J. H., Kalbitz, K., Matzner, E. (2002). Resource control on the production of dissolved organic carbon and nitrogen in a deciduous forest floor. Soil Biology and Biochemistry 34 (6): 813-822

ABSTRACT: The forest floor in temperate forests has become recognized for its importance in the retention of elevated inputs of dissolved inorganic nitrogen (DIN) and as a source of dissolved organic matter (DOM). A laboratory leaching experiment was conducted over the period of 98 d to examine the origin of dissolved organic carbon (DOC) and nitrogen (DON) in a deciduous forest floor, and the effect of resource availability and microbial activity on the production mechanisms involved. The experiment was composed of different types of treatments: exclusion of specific forest floor layers (no Oi, no Oe) and addition of carbon sources (glucose, cellulose, leaf, wood) and NH4 NO3 (nitrogen). The cumulative amount of CO2 evolution was positively related to the availability of C sources at each treatment: glucose>leaf=wood=cellulose>control=no Oe=nitrogen>no Oi. DOC release was related to the amount of C sources but showed no clear correlation with CO2 evolution. An increase in C availability generally led to a reduction in the release of DON as well as DIN. In contrast, the amendment of NH4 NO3 reduced the cumulative DOC release but enhanced the release of both DON and DIN. Fresh leaf litter was a more important DOC source than labile substrates (glucose and cellulose) as well as more stable substrates (forest floor materials and wood). Among forest floor layers, more humified horizons (Oe and Oa) were the primary source of DIN and made a similar contribution to DOM release as the Oi layer. The changes in DOM composition detected by a humification index of the leachates, in combination with a shift in the final microbial biomass C, suggested that DOM released from the soluble pools of added litter or the Oi layer contained a substantial amount of microbially processed organic matter. Our study demonstrated the importance of C availability in regulating microbial activity and immobilization of dissolved N in an N-enriched forest floor. However, the discrepancy between substrate lability and DOC production, in combination with a rapid microbial processing of DOC released from labile C pools, illustrated the complicated nature of microbial production and consumption of DOC in the forest floor.

Pregitzer, Kurt, Loya, Wendy, Kubiske, Mark, Zak, Donald (2006). Soil respiration in northern forests exposed to elevated atmospheric carbon dioxide and ozone. Oecologia 148 (3): 503-516

ABSTRACT: The aspen free-air CO2 and O3 enrichment (FACTS II–FACE) study in Rhinelander, Wisconsin, USA, is designed to understand the mechanisms by which young northern deciduous forest ecosystems respond to elevated atmospheric carbon dioxide (CO2 ) and elevated tropospheric ozone (O3 ) in a replicated, factorial, field experiment. Soil respiration is the second largest flux of carbon (C) in these ecosystems, and the objective of this study was to understand how soil respiration responded to the experimental treatments as these fast-growing stands of pure aspen and birch + aspen approached maximum leaf area. Rates of soil respiration were typically lowest in the elevated O3 treatment. Elevated CO2 significantly stimulated soil respiration (8–26%) compared to the control treatment in both community types over all three growing seasons. In years 6–7 of the experiment, the greatest rates of soil respiration occurred in the interaction treatment (CO2 + O3 ), and rates of soil respiration were 15–25% greater in this treatment than in the elevated CO2 treatment, depending on year and community type. Two of the treatments, elevated CO2 and elevated CO2 + O3 , were fumigated with13 C-depleted CO2 , and in these two treatments we used standard isotope mixing models to understand the proportions of new and old C in soil respiration. During the peak of the growing season, C fixed since the initiation of the experiment in 1998 (new C) accounted for 60–80% of total soil respiration. The isotope measurements independently confirmed that more new C was respired from the interaction treatment compared to the elevated CO2 treatment. A period of low soil moisture late in the 2003 growing season resulted in soil respiration with an isotopic signature 4–6‰ enriched in13 C compared to sample dates when the percentage soil moisture was higher. In 2004, an extended period of low soil moisture during August and early September, punctuated by a significant rainfall event, resulted in soil respiration that was temporarily 4–6‰ more depleted in13 C. Up to 50% of the Earth’s forests will see elevated concentrations of both CO2 and O3 in the coming decades and these interacting atmospheric trace gases stimulated soil respiration in this study.

Raich, J.W., Schleisinger, W.H. (1992). The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate. Tellus Series B Chemical and Physical Meteorology 44 (2): 81-99

ABSTRACT: We review measured rates of soil respiration from terrestrial and wetland ecosystems to define the annual global CO2 flux from soils, to identify uncertainties in the global flux estimate, and to investigate the influences of temperature, precipitation, and vegetation on soil respiration rates. The annual global CO2 flux from soils is estimated to average (± S.D.) 68 ± 4 PgC/ yr, based on extrapolations from biome land areas. Relatively few measurements of soil respiration exist from arid, semi-arid, and tropical regions; these regions should be priorities for additional research. On a global scale, soil respiration rates are positively correlated with mean annual air temperatures and mean annual precipitation. There is a close correlation between mean annual net primary productivity (NPP) of different vegetation biomes and their mean annual soil respiration rates, with soil respiration averaging 24% higher than mean annual NPP. This difference represents a minimum estimate of the contribution of root respiration to the total soil CO2 efflux. Estimates of soil C turnover rates range from 500 years in tundra and peaty wetlands to 10 years in tropical savannas. We also evaluate the potential impacts of human activities on soil respiration rates, with particular focus on land use changes, soil fertilization, irrigation and drainage, and climate changes. The impacts of human activities on soil respiration rates are poorly documented, and vary among sites. Of particular importance are potential changes in temperatures and precipitation. Based on a review of in situ measurements, the Q10 value for total soil respiration has a median value of 2.4. Increased soil respiration with global warming is likely to provide a positive feedback to the greenhouse effect.

Schedlbauer, J. L., Kavanagh, K. L. (2008). Soil carbon dynamics in a chronosequence of secondary forests in northeastern Costa Rica. Forest Ecology and Management 255 (3-4): 1326-1335

ABSTRACT: Secondary forests are becoming an increasingly important tropical landscape component with the potential to provide environmental services such as soil carbon storage. Substantial losses of soil carbon can occur with tropical forest conversion to pasture, but stocks can sometimes be restored with the development of secondary forest. Few studies have taken advantage of shifts in vegetation from C4 to C3 communities to determine soil carbon turnover following secondary forest development on pasture. Because trees quickly colonize abandoned pastures in northeastern Costa Rica, we expected to find evidence of increased soil carbon storage and gradual soil carbon turnover following pasture abandonment. Three early successional and nine late successional secondary sites ranging in age from 2.6 to 33 years, as well as four pastures were used in this study. At each site, mineral soil samples up to 30 cm depth were collected from three plots to determine bulk density, percent soil carbon, and stable carbon isotope values (d13 C). Thed13 C of soil respired CO2 was also determined at each site. Contrary to expectations, soil carbon storage did not increase with secondary forest age and was unrelated to increases in aboveground carbon storage. However, pastures stored 19% more carbon than early and late successional sites in the top 10 cm of mineral soil, and successional sites stored 14-18% more carbon than pastures between 10 and 30 cm.d13 C data indicated that most pasture-derived soil carbon in the top 30 cm of soil turned over within 10 years of pasture abandonment and subsequent colonization by trees. Overall, these data indicate that total soil carbon storage remains relatively unchanged following land use transitions from pasture to secondary forest. This is likely due to the presence of large passive pools of mineral-stabilized soil carbon in this region of Costa Rica. The contribution of these forests to increased carbon storage on the landscape is primarily confined to aboveground carbon stocks, though other environmental services may be derived from these forests. In the context of global carbon accounting, it appears that future carbon credits may be best applied to aboveground carbon storage in secondary forests regrowing on soils with large mineral-stabilized soil carbon pools.

Schimel, J. P., Gulledge, J. M., Clein-Curley, J. S., Lindstrom, J. E., Braddock, J. F. (1999). Moisture effects on microbial activity and community structure in decomposing birch litter in the Alaskan taiga. Soil Biology and Biochemistry 31 (6): 831-838

ABSTRACT: We carried out a field experiment to evaluate the effect of moisture regime on microbial biomass and activity in birch litter in the Alaskan taiga. Litter bags were placed in one of three treatments: continuously moist (0.5 cm water d−1 ), cycling (0.5 cm water weekly), and ‘natural', which experienced two natural dry–wet cycles of 2 weeks dry followed by rain. The experiment lasted for 1 month. Each week we collected litter bags and analyzed microbial respiration and biomass C and N. In the last two cycles we analyzed bacterial substrate use on Biolog GN plates. There were strong overall correlations between biomass, respiration and litter moisture content. However, the different treatments had significantly different rates of respiration, biomass and respiratory quotient (qCO2 ) that could not be explained by moisture content directly. The natural treatment had lower respiration rates and biomass than the wet or cycling samples, indicating that the 2-week droughts in the natural treatment reduced microbial populations and activity to a greater degree than did shorter droughts. Episodic drying and rewetting considerably decreased the number of Biolog substrates used. This experiment showed that the size and functioning of the litter microbial community was strongly affected by its stress history.

Schlesinger, W. H., Andrews, J. A. (2000). Soil respiration and the global carbon cycle. Biogeochemistry 48 (1): 7-20.

ABSTRACT: Soil respiration is the primary path by which CO2 fixed by land plants returns to the atmosphere. Estimated at approximately 75 × 1015 g C/yr, this large natural flux is likely to increase due changes in the Earth's condition. The objective of this paper is to provide a brief scientific review for policymakers who are concerned that changes in soil respiration may contribute to the rise in CO2 in Earth's atmosphere. Rising concentrations of CO2 in the atmosphere will increase the flux of CO2 from soils, while simultaneously leaving a greater store of carbon in the soil. Traditional tillage cultivation and rising temperature increase the flux of CO2 from soils without increasing the stock of soil organic matter. Increasing deposition of nitrogen from the atmosphere may lead to the sequestration of carbon in vegetation and soils. The response of the land biosphere to simultaneous changes in all of these factors is unknown, but a large increase in the soil carbon pool seems unlikely to moderate the rise in atmospheric CO2 during the next century.

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

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

Sotta, E.D., Veldkamp, E., Guimaraes, B.R., Paixao, R.K., Ruivo, M.L.P., Almeida, S.S. (2006). Landscape and climatic controls on spatial and temporal variation in soil CO2 efflux in an Eastern Amazonian Rainforest, Caxiuanã, Brazil. Forest Ecology and Management 237 (1-3): 57-64

ABSTRACT: Quantification of temporal and spatial variation of soil CO2 emissions is essential for an accurate interpretation of tower-based measurements of net ecosystem exchange. Here, we measured in the old-growth forest of Caxiuana, Eastern Amazonia soil CO2 efflux and its environmental controls from two Oxisol sites with contrasting soil texture and at different landscape positions. Average CO2 efflux was 21% higher for sand (3.93 +/- 0.06 [mu]mol CO2 m-2 s-1 ) than for the clay (3.08 +/- 0.07 [mu]mol CO2 m-2 s-1 ). No difference was detected for soil temperature between sites, while soil water content in sandy soil (23.2 +/- 0.33%) was much lower than the clay soil (34.5 +/- 0.98%), for the 2-year period. Soil CO2 efflux did not differ between dry and wet season, but we detected a significant interaction between season and topographic position. The variation caused by topography was in the same order of magnitude as temporal variation. Mean contribution of the litter layer to the CO2 efflux rates was 20% and varied from 25% during the wet season to close to 0% during the dry season. The relation between soil water content and soil CO2 efflux showed an optimum for both soil textures but the shape and optimum of the curves were different. The results of our study illustrate that soil moisture is an important driver of temporal variations in soil CO2 efflux in this old-growth forest. When extrapolating soil CO2 efflux to larger areas, the significant influences of soil texture, litter, and the interaction of topographical position and time illustrate that it is necessary to include some of the complexity of landscapes.

Sponseller, R. A. (2007). Precipitation pulses and soil CO2 flux in a Sonoran Desert ecosystem. Global Change Biology 13 (2): 426-436

ABSTRACT: Precipitation is a major driver of biological processes in arid and semiarid ecosystems. Soil biogeochemical processes in these water-limited systems are closely linked to episodic rainfall events, and the relationship between microbial activity and the amount and timing of rainfall has implications for the whole-system carbon (C) balance. Here, the influences of storm size and time between events on pulses of soil respiration were explored in an upper Sonoran Desert ecosystem. Immediately following experimental rewetting in the field, CO2 efflux increased up to 30-fold, but generally returned to background levels within 48 h. CO2 production integrated over 48 h ranged from 2.5 to 19.3 g C m−2 and was greater beneath shrubs than in interplant spaces. When water was applied on sequential days, postwetting losses of CO2 were only half a large as initial fluxes, and the size of the second pulse increased with time between consecutive events. Soil respiration was more closely linked to the organic matter content of surface soils than to rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size, reaching an asymptote at approximately 0.5 cm simulated storms. This nonlinear relationship stems from (1) resource limitation of microbial activity that is manifest at small time scales, and (2) greatly reduced process rates in deeper soil strata. Thus, beyond some threshold in storm size, increasing the duration or depth of soil moisture has little consequence for short-term losses of CO2 . In addition, laboratory rewetting across a broad range in soil water content suggest that microbial activity and CO2 efflux following rainfall may be further modified by the routing and redistribution of water along hillslopes. Finally, analysis of long-term precipitation data suggests that half the monsoon storms in this system are sufficient to induce soil heterotrophic activity and C losses, but are not large enough to elicit autotrophic activity and C accrual by desert shrubs.

Taneve, L., J. S. Pippen, W. H. Schlesinger, M. A. Gonzalez-Meler (2006). The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration. Global Change Biology 12 (6): 983-994

ABSTRACT: Soil carbon is returned to the atmosphere through the process of soil respiration, which represents one of the largest fluxes in the terrestrial C cycle. The effects of climate change on the components of soil respiration can affect the sink or source capacity of ecosystems for atmospheric carbon, but no current techniques can unambiguously separate soil respiration into its components. Long-term free air CO2 enrichment (FACE) experiments provide a unique opportunity to study soil C dynamics because the CO2 used for fumigation has a distinct isotopic signature and serves as a continuous label at the ecosystem level. We used the13 C tracer at the Duke Forest FACE site to follow the disappearance of C fixed before fumigation began in 1996 (pretreatment C) from soil CO2 and soil-respired CO2 , as an index of belowground C dynamics during the first 8 years of the experiment. The decay of pretreatment C as detected in the isotopic composition of soil-respired CO2 and soil CO2 at 15, 30, 70, and 200 cm soil depth was best described by a model having one to three exponential pools within the soil system. The majority of soil-respired CO2 (71%) originated in soil C pools with a turnover time of about 35 days. About 55%, 50%, and 68% of soil CO2 at 15, 30, and 70 cm, respectively, originated in soil pools with turnover times of less than 1 year. The rest of soil CO2 and soil-respired CO2 originated in soil pools that turn over at decadal time scales. Our results suggest that a large fraction of the C returned to the atmosphere through soil respiration results from dynamic soil C pools that cannot be easily detected in traditionally defined soil organic matter standing stocks. Fast oxidation of labile C substrates may prevent increases in soil C accumulation in forests exposed to elevated [CO2 ] and may consequently result in shorter ecosystem C residence times.

Tang, J., Baldocchi, D. D., Xu, L. (2005). Tree photosynthesis modulates soil respiration on a diurnal time scale.. Global Change Biology 11 (8): 1298-1304

ABSTRACT: To estimate how tree photosynthesis modulates soil respiration, we simultaneously and continuously measured soil respiration and canopy photosynthesis over an oak-grass savanna during the summer, when the annual grass between trees was dead. Soil respiration measured under a tree crown reflected the sum of rhizosphere respiration and heterotrophic respiration; soil respiration measured in an open area represented heterotrophic respiration. Soil respiration was measured using solid-state CO2 sensors buried in soils and the flux-gradient method. Canopy photosynthesis was obtained from overstory and understory flux measurements using the eddy covariance method. We found that the diurnal pattern of soil respiration in the open was driven by soil temperature, while soil respiration under the tree was decoupled with soil temperature. Although soil moisture controlled the seasonal pattern of soil respiration, it did not influence the diurnal pattern of soil respiration. Soil respiration under the tree controlled by the root component was strongly correlated with tree photosynthesis, but with a time lag of 7–12 h. These results indicate that photosynthesis drives soil respiration in addition to soil temperature and moisture.

Thormann, M. N. (2006). Diversity and function of fungi in peatlands: A carbon cycling perspective. Canadian Journal of Soil Science 86 (2): 281-293

ABSTRACT: Peatlands are a dominant landform in the northern hemisphere, accumulating carbon in the form of peat due to an imbalance between decomposition and plant production rates. Decomposer (saprobes) and mycorrhizal fungi significantly influence carbon dynamics by degrading organic matter via the synthesis of extracellular enzymes. As organic matter decomposes, litter quality variables figure most prominently in the succession of fungi. Hence, litters composed primarily of complex polymers decompose very slowly. Surprisingly, recalcitrant polymer degraders (mostly basidiomycetes) are rarely isolated from peat, which may explain the accumulation of complex polymers in peat profiles. While enzymatic profiles of mycorrhizal fungi and other root endophytes may be more limited compared with saprobes, many of these fungi can degrade polymers of varying complexity as well and hence may also be significant decomposers of organic matter. To date, anamorphic ascomycetes and zygomycetes are the most frequently isolated fungi from peatlands (63 and 10% of all taxa, respectively), and chytridiomycetes, teleomorphic ascomycetes, and basidiomycetes appear to be less common (11% of all taxa). The remaining 16% of taxa remain unidentified or are sterile taxa. How disturbances affect peatland microbial communities and their roles is virtually unknown. This aspect of peatland microbial ecology requires immediate attention. The objective of this paper is to review the current state of knowledge of the diversity of fungi and their roles in carbon cycling dynamics in peatlands.

Topp, E., Pattey, E. (1997). Soils as sources and sinks for atmospheric methane. Canadian Journal of Soil Science 77 (2): 167-178

ABSTRACT: Methane is considered to be a significant greenhouse gas. Methane is produced in soils as the end product of the anaerobic decomposition of organic matter. In the absence of oxygen, methane is very stable, but under aerobic conditions it is mineralized to carbon dioxide by methanotrophic bacteria. Soil methane emissions, primarily from natural wetlands, landfills and rice paddies, are estimated to represent about half of the annual global methane production. Oxidation of atmospheric methane by well-drained soils accounts for about 10% of the global methane sink. Whether a soil is a net source or sink for methane depends on the relative rates of methanogenic and methanotrophic activity. A number of factors including pH, Eh, temperature and moisture content influence methane transforming bacterial populations and soil fluxes. Several techniques are available for measuring methane fluxes. Flux estimation is complicated by spatial and temporal variability. Soil management can impact methane transformations. For example, landfilling of organic matter can result in significant methane emissions, whereas some cultural practices such as nitrogen fertilization inhibit methane oxidation by agricultural soils.

Trumbore, S. (2006). Carbon respired by terrestrial ecosystems - recent progress and challenges. Global Change Biology 12 (2): 141-153

ABSTRACT: Net ecosystem production is the residual of two much larger fluxes: photosynthesis and respiration. While photosynthesis is a single process with a well-established theoretical underpinning, respiration integrates the variety of plant and microbial processes by which CO2 returns from ecosystems to the atmosphere. Limits to current capacity for predicting ecosystem respiration fluxes across biomes or years result from the mismatch between what is usually measured – bulk CO2 fluxes – and what process-based models can predict – fluxes of CO2 from plant (autotrophic) or microbial (heterotrophic) respiration. Papers in this Thematic Issue and in the recent literature, document advances in methods for separating respiration into autotrophic and heterotrophic components using three approaches: (1) continuous measurements of CO2 fluxes and assimilation of these data into process-based models; (2) application of isotope measurements, particularly radiocarbon; and (3) manipulation experiments. They highlight the role of allocation of C fixed by plants to respiration, storage, growth or transfer to other organisms as a control of seasonal and interannual variability in soil respiration and the oxidation state of C in the terrestrial biosphere. A second theme is the potential for comparing C isotope signatures in organic matter, CO2 evolved in incubations and microbial biomarkers to elucidate the pathways (respiration, recycling, or transformation) of C during decomposition. Together, these factors determine the continuum of timescales over which C is returned to the atmosphere by respiration and enable testing of theories of plant and microbial respiration that go beyond empirical models and allow predictions of future respiration responses to future change in climate, pollution and land use.

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

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

Trumbore, S.E., Chadwick, O.A., Amundson, R. (1996). Rapid exchange between soil carbon and atmospheric carbon dioxide driven by temperature changes. Science 272 (5260): 393-396

ABSTRACT: Comparison of14 C (carbon-14) in archived (pre-1963) and contemporary soils taken along an elevation gradient in the Sierra Nevada, California, demonstrates rapid (7 to 65 years) turnover for 50 to 90 percent of carbon in the upper 20 centimeters of soil (A horizon soil carbon). Carbon turnover times increased with elevation (decreasing temperature) along the Sierra transect. This trend was consistent with results from other locations, which indicates that temperature is a dominant control of soil carbon dynamics. When extrapolated to large regions, the observed relation between carbon turnover and temperature suggests that soils should act as significant sources or sinks of atmospheric carbon dioxide in response to global temperature changes.

Turetsky, M. R., Ripley, S. (2005). Decomposition in extreme-rich fens of boreal Alberta, Canada. Soil Science Society Of America JournalSoil Sci So 69 (6): 1856-1860

ABSTRACT: Rich fens (minerotrophic peatlands with surface water pH > 5.5) have greater alkalinity and species richness than other boreal peatlands. We used short-term laboratory incubations to quantify CO2 and CH4 production in peat from five extreme-rich fens in Alberta. Carbon dioxide production rates averaged 48.29 ± 1.36 µmol CO2 g organic matter–1 d–1 across sites and sampling events. Peat from all sites produced CH4 during anaerobic incubations, leading to average anaerobic CH4 production rates of 359.53 ± 138.7 nmol CH4 g organic matter–1 d–1 . However, methane frequently was consumed (oxidized) during aerobic incubations, leading to aerobic CH4 consumption rates averaging 75.2 ± 63.7 nmol CH4 g organic matter–1 d–1 across sites. Calculated rates of dissolved H2 CO3 + HCO3 – production averaged 59.7 ± 13.4mmol g organic matter–1 d–1 , suggesting that dissolved inorganic C is important to the overall C fluxes in these rich fens. Our results suggest that changing hydrologic conditions will influence the balance between methanogenesis and methanotrophy in rich fens, but that surface water chemistry, likely influenced by marl precipitation, also is important to decomposition. Rich fens are estimated to represent the most common wetland type in Alberta, and these peatland ecosystems could play an important role in trace gas emissions across boreal regions.

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

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

Van Dijk, A., Dolman, A. J. (2004). Estimates of CO2 uptake and release among European forests based on eddy covariance data. Global Change Biology 10 (9): 1445-1459

ABSTRACT: We used a spatially nested hierarchy of field and remote-sensing observations and a process model, Biome-BGC, to produce a carbon budget for the forested region of Oregon, and to determine the relative influence of differences in climate and disturbance among the ecoregions on carbon stocks and fluxes. The simulations suggest that annual net uptake (net ecosystem production (NEP)) for the whole forested region (8.2 million hectares) was 13.8 Tg C (168 g C m−2 yr−1 ), with the highest mean uptake in the Coast Range ecoregion (226 g C m−2 yr−1 ), and the lowest mean NEP in the East Cascades (EC) ecoregion (88 g C m−2 yr−1 ). Carbon stocks totaled 2765 Tg C (33 700 g C m−2 ), with wide variability among ecoregions in the mean stock and in the partitioning above- and belowground. The flux of carbon from the land to the atmosphere that is driven by wildfire was relatively low during the late 1990s (~0.1 Tg C yr−1 ), however, wildfires in 2002 generated a much larger C source (~4.1 Tg C). Annual harvest removals from the study area over the period 1995–2000 were ~5.5 Tg C yr−1 . The removals were disproportionately from the Coast Range, which is heavily managed for timber production (approximately 50% of all of Oregon's forest land has been managed for timber in the past 5 years). The estimate for the annual increase in C stored in long-lived forest products and land fills was 1.4 Tg C yr−1 . Net biome production (NBP) on the land, the net effect of NEP, harvest removals, and wildfire emissions indicates that the study area was a sink (8.2 Tg C yr−1 ). NBP of the study area, which is the more heavily forested half of the state, compensated for ~52% of Oregon's fossil carbon dioxide emissions of 15.6 Tg C yr−1 in 2000. The Biscuit Fire in 2002 reduced NBP dramatically, exacerbating net emissions that year. The regional total reflects the strong east–west gradient in potential productivity associated with the climatic gradient, and a disturbance regime that has been dominated in recent decades by commercial forestry.

Van Miegroet, H., Moore, P.T., Tewksbury, C.E., Nicholas, N.S. (2007). Carbon sources and sinks in high-elevation spruce-fir forests of the Southeastern US. Forest Ecology and Management 238 (1-3): 249-260

ABSTRACT: This paper examines carbon (C) pools, fluxes, and net ecosystem balance for a high-elevation red spruce–Fraser fir forest [Picea rubens Sarg./Abies fraseri (Pursh.) Poir.] in the Great Smoky Mountains National Park (GSMNP), based on measurements in fifty-four 20 m × 20 m permanent plots located between 1525 and 1970 m elevation. Forest floor and mineral soil C was determined from destructive sampling of the O horizon and incremental soil cores (to a depth of 50 cm) in each plot. Overstory C pools and net C sequestration in live trees was estimated from periodic inventories between 1993 and 2003. The CO2 release from standing and downed wood was based on biomass and C concentration estimates and published decomposition constants by decay class and species. Soil respiration was measured in situ between 2002 and 2004 in a subset of eight plots along an elevation gradient. Litterfall was collected from a total of 16 plots over a 2–5-year period.

The forest contained on average 403 Mg C ha−1 , almost half of which stored belowground. Live trees, predominantly spruce, represented a large but highly variable C pool (mean: 126 Mg C ha−1 , CV = 39%); while dead wood (61 Mg C ha−1 ), mostly fir, accounted for as much as 15% of total ecosystem C. The 10-year mean C sequestration in living trees was 2700 kg C ha−1 year−1 , but increased from 2180 kg C ha−1 year−1 in 1993–1998 to 3110 kg C ha−1 year−1 in 1998–2003, especially at higher elevations. Dead wood also increased during that period, releasing on average 1600 kg C ha−1 year−1 . Estimated net soil C efflux ranged between 1000 and 1450 kg C ha−1 year−1 , depending on the calculation of total belowground C allocation. Based on current flux estimates, this old-growth system was close to C neutral.

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

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

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

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

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

Litton, C.M., J.W. Raich, M.G. Ryan (2007). Carbon allocation in forest ecosystems. Global Change Biology 13 (10): 2089-2109

ABSTRACT: Carbon allocation plays a critical role in forest ecosystem carbon cycling. We reviewed existing literature and compiled annual carbon budgets for forest ecosystems to test a series of hypotheses addressing the patterns, plasticity, and limits of three components of allocation: biomass, the amount of material present; flux, the flow of carbon to a component per unit time; and partitioning, the fraction of gross primary productivity (GPP) used by a component.Can annual carbon flux and partitioning be inferred from biomass? Our survey revealed that biomass was poorly related to carbon flux and to partitioning of photosynthetically derived carbon, and should not be used to infer either.Are component fluxes correlated? Carbon fluxes to foliage, wood, and belowground production and respiration all increased linearly with increasing GPP (a rising tide lifts all boats). Autotrophic respiration was strongly linked to production for foliage, wood and roots, and aboveground net primary productivity and total belowground carbon flux (TBCF) were positively correlated across a broad productivity gradient.How does carbon partitioning respond to variability in resources and environment? Within sites, partitioning to aboveground wood production and TBCF responded to changes in stand age and resource availability, but not to competition (tree density). Increasing resource supply and stand age, with one exception, resulted in increased partitioning to aboveground wood production and decreased partitioning to TBCF. Partitioning to foliage production was much less sensitive to changes in resources and environment. Overall, changes in partitioning within a site in response to resource supply and age were small (<15% of GPP), but much greater than those inferred from global relationships. Across all sites, foliage production plus respiration, and total autotrophic respiration appear to use relatively constant fractions of GPP – partitioning to both was conservative across a broad range of GPP – but values did vary across sites. Partitioning to aboveground wood production and to TBCF were the most variable – conditions that favored high GPP increased partitioning to aboveground wood production and decreased partitioning to TBCF.Do priorities exist for the products of photosynthesis? The available data do not support the concept of priorities for the products of photosynthesis, because increasing GPP increased all fluxes. All facets of carbon allocation are important to understanding carbon cycling in forest ecosystems. Terrestrial ecosystem models require information on partitioning, yet we found few studies that measured all components of the carbon budget to allow estimation of partitioning coefficients. Future studies that measure complete annual carbon budgets contribute the most to understanding carbon allocation.

Chen, S., G. Lin, J. Huang, G.D. Jenerette (2009). Dependence of carbon sequestration on the differential responses of ecosystem photosynthesis and respiration to rain pulses in a semiarid steppe. Global Change Biology 15 (10): 2450-2461

ABSTRACT: Precipitation pulses play an important role in regulating ecosystem carbon exchange and balance of semiarid steppe ecosystems. It has been predicted that the frequency of extreme rain events will increase in the future, especially in the arid and semiarid regions. We hypothesize that large rain pulses favor carbon sequestration, while small ones cause more carbon release in the semiarid steppes. To understand the potential response in carbon sequestration capacity of semiarid steppes to the changes in rain pulse size, we conducted a manipulative experiment with five simulated rain pulse sizes (0, 5, 10, 25, and 75 mm) in Inner Mongolia steppe. Our results showed that both gross ecosystem productivity (GEP) and ecosystem respiration (Re ) responded rapidly (within 24 h) to rain pulses and the initial response time was independent of pulse size. However, the time of peak GEP was 1–3 days later than that of Re , which depended on pulse size. Larger pulses caused greater magnitude and longer duration of variations in GEP and Re . Differences in the response time of microbes and plants to wetting events constrained the response pattern of heterotrophic (Rh ) and autotrophic (Ra ) components of Re following a rain event. Rh contributed more to the increase of Re in the early stage of rain pulse response, while Ra played an more important role later, and determined the duration of pulse response, especially for large rain events of >10 mm. The distinct responses of ecosystem photosynthesis and respiration to increasing pulse sizes led to a threshold in rain pulse size between 10 and 25 mm, above which post wetting responses favored carbon sequestration. The disproportionate increase of the primary productivity of higher plants, compared with those in the activities of microbial decomposers to larger pulse events suggests that the carbon sequestration capacity of Inner Mongolia steppes will be sensitive to changes in precipitation size distribution rather than just precipitation amount.

J. B. Gaudinski, S. E. Trumbore, E. A. Davidson, S. Zheng (2000). Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes. Biogeochemistry 51 (1): 33-69

ABSTRACT: Temperate forests of North America are thought to be significant sinks of atmospheric CO2 . We developed a below-ground carbon (C) budget for well-drained soils in Harvard Forest Massachusetts, an ecosystem that is storing C. Measurements of carbon and radio carbon (14 C) inventory were used to determine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM):recognizable litter fragments (L), humified low density material (H), and high density or mineral-associated organic matter (M). Turnover times in all fractions increased with soil depth and were 2–5 years for recognizable leaf litter, 5–10 years for root litter, 40–100+ years for low density humified material and >100 years for carbon associated with minerals. These turnover times represent the time carbon resides in the plant + soil system, and may underestimate actual decomposition rates if carbon resides for several years in living root, plant or woody material.

Soil respiration was partitioned into two components using14 C: recent photosynthate which is metabolized by roots and microorganisms within a year of initial fixation (Recent-C), and C that is respired during microbial decomposition of SOM that resides in the soil for several years or longer (Reservoir-C).For the whole soil, we calculate that decomposition of Reservoir-C contributes approximately 41% of the total annual soil respiration. Of this 41%,recognizable leaf or root detritus accounts for 80% of the flux, and 20% is from the more humified fractions that dominate the soil carbon stocks. Measurements of CO2 and14 CO2 in the soil atmosphere and in total soil respiration were combined with surface CO2 fluxes and a soil gas diffusion model to determine the flux and isotopic signature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cm of the soil (O + A + Ap horizons). The average residence time of Reservoir-C in the plant + soil system is 8±1 years and the average age of carbon in total soil respiration (Recent-C + Reservoir-C) is 4±1 years.

The O and A horizons have accumulated 4.4 kg C m–2 above the plow layer since abandonment by settlers in the late-1800's. C pools contributing the most to soil respiration have short enough turnover times that they are likely in steady state. However, most C is stored as humified organic matter within both the O and A horizons and has turnover times from 40 to 100+ years respectively. These reservoirs continue to accumulate carbon at a combined rate of 10–30 g C mminus 2 yr–1 . This rate of accumulation is only 5–15% of the total ecosystem C sink measured in this stand using eddy covariance methods.

R. Valentini, G. Matteucci, A. J. Dolman, E.-D. Schulze, C. Rebmann, E. J. Moors, A. Granier, P. Gross, N. O. Jensen, K. Pilegaard, A. Lindroth, A. Grelle, C. Bernhofer, T. Grünwald, M. Aubinet, R. Ceulemans, A. S. Kowalski, T. Vesala, Ü. Rannik, P. Berbigier, D. Loustau, J. Gudmundsson, H. Thorgeirsson, A. Ibrom, K. Morgenstern, R. Clement, J. Moncrieff, L. Montagnani, S. Minerbi, P. G. Jarvis (2000). Respiration as the main determinant of carbon balance in European forests. Nature 404 (20 April): 861-865

ABSTRACT: Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol1 . Several studies suggest that the terrestrial biosphere is gaining carbon2, 3, 4, 5, 6, 7, 8 , but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha-1 yr-1 , with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

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