Climate Change and...

Annotated Bibliography

Carbon Dynamics

Temperate Forest Soils

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.

Aerts, R., de Caluwe, H. (1999). Nitrogen deposition effects on carbon dioxide and methane emissions from temperate peatland soils. Oikos 84 (1): 44-54

ABSTRACT: Northern peatlands are important sources of carbon dioxide and methane emissions to the atmosphere. Increased atmospheric N deposition may have a significant impact on the emission of these greenhouse gases. We studied CO2 and CH4 emissions from untreated temperate peat soils from a eutrophic and a mesotrophic fen in a high N deposition area (the Netherlands) and from a mesotrophic fen in a low N deposition area (north-east Poland). In addition, we investigated the effects of N, P and glucose amendments on the emissions of CO2 and CH4 from these soils. Nitrogen availability (extractable NH4 + in untreated peat from the high N area was 2.5-7.5 times higher than in the low N area, whereas the pH was 0.9-1.7 units lower. Using 6-week laboratory incubations of peat columns, we found that mean daily CO2 emission from untreated peat soils from the high N area was lower than that from the low N area. Both linear and multiple regression analysis showed that CO2 emission was positively related to soil pH (r2 =0.64). Additional N supply led to pH reduction and to lower CO2 emission, especially in the low N peat soils. Thus, increased atmospheric N deposition leads, probably as a result of soil acidification, to lower CO2 emission. Although glucose amendments resulted in increased CO2 and CH4 emission, we did not find evidence that this was caused by increased mineralization of native peat. Mean daily CH4 - C emission was about 1-2 orders of magnitude lower than mean daily CO2 - C emission. In the untreated peat soils from the high N eutrophic site, methane emission was higher than in the high N mesotrophic site and in the low N mesotrophic site. Linear regression analysis showed a positive relation between methane emission and soil fertility variables (r2 =0.42-0.55), whereas a multiple regression model revealed that methane emission was determined by N-related soil chemistry variables (r2 =0.93). Increased nutrient supply initially resulted in higher methane emission from soils of both mesotrophic sites, but there was no effect on the high N eutrophic soil. These results show that increased atmospheric N deposition leads to increased methane emission from low-fertility peat soils. However, the ultimate effect of atmospheric N deposition on trace gas emissions and thereby on global warming is determined by the balance between the ratios of the change in CO2 - C emission and CH4 - C emission and the ratio of their global warming potentials (1:21).

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.

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

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

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

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

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.

Carney, K. M., Hungate, B. A., Drake, B. G., Megonigal, J. P. (2007). Altered soil microbial community at elevated CO2 leads to loss of soil carbon. Proceedings of the National Academy of Sciences: PNAS 104 (12): 4990-4995

ABSTRACT: Increased carbon storage in ecosystems due to elevated CO2 may help stabilize atmospheric CO2 concentrations and slow global warming. Many field studies have found that elevated CO2 leads to higher carbon assimilation by plants, and others suggest that this can lead to higher carbon storage in soils, the largest and most stable terrestrial carbon pool. Here we show that 6 years of experimental CO2 doubling reduced soil carbon in a scrub-oak ecosystem despite higher plant growth, offsetting ≈52% of the additional carbon that had accumulated at elevated CO2 in aboveground and coarse root biomass. The decline in soil carbon was driven by changes in soil microbial composition and activity. Soils exposed to elevated CO2 had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme, which led to more rapid rates of soil organic matter degradation than soils exposed to ambient CO2 . The isotopic composition of microbial fatty acids confirmed that elevated CO2 increased microbial utilization of soil organic matter. These results show how elevated CO2 , by altering soil microbial communities, can cause a potential carbon sink to become a carbon source.

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).

Davi, H., Dufrene, E., Francois, C., Le Maire, G., Loustau, D., Bosc, A., Rambal, S., Granier, A., Moors, E. (2006). Sensitivity of water and carbon fluxes to climate changes from 1960 to 2100 in European forest ecosystems. Agricultural and Forest Meteorology 141 (1): 35-56

ABSTRACT: The effects of climate changes on carbon and water fluxes are quantified using a physiologically multi-layer, process-based model containing a carbon allocation model and coupled with a soil model (CASTANEA). The model is first evaluated on four EUROFLUX sites using eddy covariance data, which provide estimates of carbon and water fluxes at the ecosystem scale. It correctly reproduces the diurnal fluxes and the seasonal pattern. Thereafter simulations were conducted on six French forest ecosystems representative of three climatic areas (oceanic, continental and Mediterranean areas) dominated by deciduous species (Fagus sylvatica ,Quercus robur ), coniferous species (Pinus pinaster ,Pinus sylvestris ) or sclerophyllous evergreen species (Quercus ilex ). The model is driven by the results of a meteorological model (ARPEGE) following the B2 scenario of IPCC. From 1960 to 2100, the average temperature increases by 3.1 °C (30%) and the rainfall during summer decreases by 68 mm (-27%). For all the sites, between the two periods, the simulations predict on average a gross primary production (GPP) increase of 513 g(C) m-2 (+38%). This increase is relatively steep until 2020, followed by a slowing down of the GPP rise due to an increase of the effect of water stress. Contrary to GPP, the ecosystem respiration (Reco) raises at a constant rate (350 g(C) m-2 i.e. 31% from 1960 to 2100). The dynamics of the net ecosystem productivity (GPP minus Reco) is the consequence of the effect on both GPP and Reco and differs per site. The ecosystems always remain carbon sinks; however the sink strength globally decreases for coniferous (-8%), increases for sclerophyllous evergreen (+34%) and strongly increases for deciduous forest (+67%) that largely benefits by the lengthening of the foliated period. The separately quantified effects of the main variables (temperature, length of foliated season, CO2 fertilization, drought effect), show that the magnitude of these effects depends on the species and the climatic zone.

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

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

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.

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

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

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.

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%).

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

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

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.

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.

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

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

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

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

Kueppers, L. M., Harte, J. (2005). Subalpine forest carbon cycling: short- and long-term influence of climate and species. Ecological Applications 15 (6): 1984-1999

ABSTRACT: Ecosystem carbon cycle feedbacks to climate change comprise one of the largest remaining sources of uncertainty in global model predictions of future climate. Both direct climate effects on carbon cycling and indirect effects via climate-induced shifts in species composition may alter ecosystem carbon balance over the long term. In the short term, climate effects on carbon cycling may be mediated by ecosystem species composition. We used an elevational climate and tree species composition gradient in Rocky Mountain subalpine forest to quantify the sensitivity of all major ecosystem carbon stocks and fluxes to these factors. The climate sensitivities of carbon fluxes were species-specific in the cases of relative aboveground productivity and litter decomposition, whereas the climate sensitivity of dead wood decay did not differ between species, and total annual soil CO2 flux showed no strong climate trend. Lodgepole pine relative productivity increased with warmer temperatures and earlier snowmelt, while Engelmann spruce relative productivity was insensitive to climate variables. Engelmann spruce needle decomposition decreased linearly with increasing temperature (decreasing litter moisture), while lodgepole pine and subalpine fir needle decay showed a hump-shaped temperature response. We also found that total ecosystem carbon declined by 50% with a 2.8°C increase in mean annual temperature and a concurrent 63% decrease in growing season soil moisture, primarily due to large declines in mineral soil and dead wood carbon. We detected no independent effect of species composition on ecosystem C stocks. Overall, our carbon flux results suggest that, in the short term, any change in subalpine forest net carbon balance will depend on the specific climate scenario and spatial distribution of tree species. Over the long term, our carbon stock results suggest that with regional warming and drying, Rocky Mountain subalpine forest will be a net source of carbon to the atmosphere.

Lajeunesse, S. D., Dilustro, J. J., Sharitz, R. R., Collins, B. S. (2006). Ground layer carbon and nitrogen cycling and legume nitrogen inputs following fire in mixed pine forests. American Journal of Botany 93 (1): 84-93

ABSTRACT: Many mixed pine forests in the southeastern United States undergo prescribed burning to promote open pine savannas. In these systems, soil texture can influence fire's effect on vegetation and nutrient cycling. Our objectives were to examine fire and soil texture effects on carbon (C) and nitrogen (N) pools in ground layer vegetation. We measured biomass and tissue nutrient concentrations and estimated legume N inputs via N2 fixation in frequently burned sandy and clayey sites that were in the first and second seasons following a prescribed fire in 2002 (B02) or had been unburned since 2000 (B00). Mean belowground biomass was significantly greater on sandy than on clayey sites. Total aboveground mean biomass did not differ significantly between B00 and B02 sites, but grasses had greater aboveground biomass in clayey than in sandy sites. Carbon and N pools (measured in grams per square meter) in grasses were greater in clayey than in sandy sites, yet grasses had greater tissue concentrations of C (as a percentage) in sandy sites. Legumes showed significant interaction effects between soil texture and fire frequency for tissue C and N pools, above- and belowground biomass, and acetylene reduction activities. Results suggest that soil texture can influence fire effects on ground layer vegetation in southeastern mixed pine forests.

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

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

Law, B. E., Thornton, P.E., Irvine, J., Anthoni, P.M., Van Tuyl, S. (2001). Carbon storage and fluxes in ponderosa pine forests at different developmental stages.. Global Change Biology 7 (7): 755-777

ABSTRACT: We compared carbon storage and fluxes in young and old ponderosa pine stands in Oregon, including plant and soil storage, net primary productivity, respiration fluxes, eddy flux estimates of net ecosystem exchange (NEE), and Biome-BGC simulations of fluxes. The young forest (Y site) was previously an old-growth ponderosa pine forest that had been clearcut in 1978, and the old forest (O site), which has never been logged, consists of two primary age classes (50 and 250 years old). Total ecosystem carbon content (vegetation, detritus and soil) of theO forest was about twice that of theY site (21 vs. 10 kg C m−2 ground), and significantly more of the total is stored in living vegetation at theO site (61% vs. 15%). Ecosystem respiration (Re) was higher at theO site (1014 vs. 835 g C m−2 year−1 ), and it was largely from soils at both sites (77% of Re). The biological data show that above-ground net primary productivity (ANPP), NPP and net ecosystem production (NEP) were greater at theO site than theY site. Monte Carlo estimates of NEP show that the young site is a source of CO2 to the atmosphere, and is significantly lower than NEP(O) by c. 100 g C m−2 year−1. Eddy covariance measurements also show that the O site was a stronger sink for CO2 than the Y site. Across a 15-km swath in the region, ANPP ranged from 76 g C m−2 year−1 at the Y site to 236 g C m−2 year−1 (overall mean 158 ± 14 g C m−2 year−1 ). The lowest ANPP values were for the youngest and oldest stands, but there was a large range of ANPP for mature stands. Carbon, water and nitrogen cycle simulations with the Biome-BGC model suggest that disturbance type and frequency, time since disturbance, age-dependent changes in below-ground allocation, and increasing atmospheric concentration of CO2 all exert significant control on the net ecosystem exchange of carbon at the two sites. Model estimates of major carbon flux components agree with budget-based observations to within ± 20%, with larger differences for NEP and for several storage terms. Simulations showed the period of regrowth required to replace carbon lost during and after a stand-replacing fire (O ) or a clearcut (Y ) to be between 50 and 100 years. In both cases, simulations showed a shift from net carbon source to net sink (on an annual basis) 10–20 years after disturbance. These results suggest that the net ecosystem production of young stands may be low because heterotrophic respiration, particularly from soils, is higher than the NPP of the regrowth. The amount of carbon stored in long-term pools (biomass and soils) in addition to short-term fluxes has important implications for management of forests in the Pacific North-west for carbon sequestration.

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.

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

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

Mosier, A. R., Milchunas, D. G., Morgan, J. A., King, J. Y., LeCain, D. (2002). Soil-atmosphere exchange of CH4 , CO2, NOx , and N2 O in the Colorado shortgrass steppe under elevated CO2 . Plant And SoilPlant Soil 240 (2): 201-211

ABSTRACT: In late March 1997, an open-top-chamber (OTC) CO2 enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO2 (~720m mol mol–1 ) on plant production, photosynthesis, and water use of this mixed C3/C4 plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO2 on trace gas exchange. From this study, we report here our weekly measurements of CO2 , CH4 , NOx and N2 O fluxes within control (unchambered), ambient CO2 and elevated CO2 OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C3 and C4 plant biomass increased under elevated CO2 and soil moisture content was typically higher than under ambient CO2 conditions, none of the trace gas fluxes were significantly altered by CO2 enrichment. Over the 43 month period of observation NOx and N2 O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7m g N m–2 hr–1 in elevated CO2 OTCs, respectively. NOx flux was negatively correlated to plant biomass production. Methane oxidation rates averaged –31 and –34m g C m–2 hr–1 and ecosystem respiration averaged 43 and 44 mg C m–2 hr–1 under ambient and elevated CO2 , respectively, over the same time period.

Neary, D.G., Overby, S.T., Hart, S.C., Kimble, J.M., Heath, L.S., Birdsey, R.A., Lal, R. (2003). Soil carbon in arid and semiarid forest ecosystems. CRC Press: 293-310

INTRODUCTION: Forests of the semiarid and arid zones of the interior western United States (US) are some of the most unique in North America. They occupy 11 to 34% of the landscape at mostly higher elevations (USDA Forest Service, 1981). These forests are characterized by a high diversity of flora, fauna, climates, elevations, soils, geology, hydrology, and productivity. Within the space of a few dozen kilometers, forests can change from desert shrublands to spruce and fir, the equivalent of going from northern Mexico to the Arctic. Because of the hydrologic cycle in the Interior West of the US, these forests are generally not known for their high productivity or contribution to the nation’s wood supply. Some exceptions do exist in the ponderosa pine ecosystem. However, these forests have a high value due to other resources and amenities that they supply. In much of the western US, the primary source of municipal drinking water is runoff that emanates from the high elevation portions of these forests (Dissmeyer, 2000). These forests also provide habitat for wildlife, including many threatened and endangered species. Arid zone forests are now being utilized for recreation to such a degree by a rapidly expanding western US population that the value of the recreation amenities they supply exceed those of extractive resources (i.e., wood and minerals; Lyons, 1998).

Forests in the dry regions of the interior western US are characterized by an overriding system driver, water availability, which controls the type, amount, and productivity of vegetation. This driver controls key ecosystem processes and ultimately carbon (C) reserves. The arid region forests that are examined in this chapter are characteristically ones in which annual water losses through evapotranspiration exceed or are slightly less than annual precipitation. The presence of water, interacting with soil processes, soil properties, soil biota, and vegetation ultimately determines the amount, quality, and state of C.

The main objective of this paper is to describe the types of dry forests that occur in the Intermountain West of the US, the soils that they occupy, the interactions with current and future land management activities, and the potentials for additional C sequestration. High elevation forests that are part of the larger intermountain forest ecoregions are discussed in Chapter 17.

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.

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

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

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

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

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

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.

K. van Groenigen, J. Six, B. A. Hungate, M. de Graaff, N. van Breemen, C. van Kessel (2006). Element interactions limit soil carbon storage. Proceedings Of The National Academy Of Sciences Of The United States Of America 103 (17): 6571-6574

ABSTRACT: Rising levels of atmospheric CO2 are thought to increase C sinks in terrestrial ecosystems. The potential of these sinks to mitigate CO2 emissions, however, may be constrained by nutrients. By using metaanalysis, we found that elevated CO2 only causes accumulation of soil C when N is added at rates well above typical atmospheric N inputs. Similarly, elevated CO2 only enhances N2 fixation, the major natural process providing soil N input, when other nutrients (e.g., phosphorus, molybdenum, and potassium) are added. Hence, soil C sequestration under elevated CO2 is constrained both directly by N availability and indirectly by nutrients needed to support N2 fixation.

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.

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

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

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

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

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

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

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