Soil CO2 Efflux Rates Research Articles

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

AbstractThis study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36‐year‐old larch (Larix leptolepis) plantation in Korea. Above‐ and below‐ground C storage, litterfall, root decomposition and soil CO2 efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha−1 year−1, 75 kg P ha−1 year−1 and 37 kg K ha−1 year−1 for 2 years (May 2002, 2003). There was no significant difference in the above‐ground C storage between fertilized (41.20 Mg C ha−1) and unfertilized (42.25 Mg C ha−1) plots, and the C increment was similar between the fertilized (1.65 Mg C ha−1 year−1) and unfertilized (1.52 Mg C ha−1 year−1) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha−1 year−1 for fertilized to 1.68 Mg C ha−1 year−1 for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO2 efflux rates for the 2 years were similar between the fertilized (0.38 g CO2 m−2 h−1) and unfertilized (0.40 g CO2 m−2 h−1) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2‐year short‐term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.

Effects of Land Use on Soil Respiration

We examined constraints on soil CO2 respiration in natural oak woodlands, and adjacent vineyards that were converted approximately 30 yr ago from oak woodlands, in the Oakville Region of Napa Valley, California. All sites were located on the same soil type, a Bale (variant) gravelly loam (fine-loamy, mixed, superactive, thermic Cumulic Ultic Haploxeroll) and dominated by C3 vegetation. Seasonal soil CO2 efflux was greatest at the oak woodland sites, although during the summer drought the rates of soil CO2 efflux measured from oak sites were generally similar to those measured from the vineyards. Soil profile CO2 concentrations at the oak woodland sites were lower below 15 cm despite higher CO2 efflux rates. Soil gas diffusion coefficients for oak sites were larger than for vineyard sites, and this indicated that the apparent discrepancy in soil profile carbon dioxide concentration ([CO2]) may be caused by a diffusion limitation. Soil profile [CO2] and delta13C values showed substantial temporal changes over the course of a year. Vineyard soil CO2 was more depleted in 13CO2 below 25 cm in the soil profile during the active growing season as indicated by more negative delta13C ratios. This result indicated that different C sources were being oxidized in vineyard soils. Annual C losses were less from vineyard soils (7.02 +/- 0.58 Mg C ha(-1) yr(-1)) as compared to oak soils (15.67 +/- 1.44 Mg C ha(-1) yr(-1)), and both were comparable to losses reported in previous investigations.

Comparison of soil respiration methods in a mid-latitude deciduous forest

In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO2 efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO2 efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO2 effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates (Rsoil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO2 efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO2 efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO2 efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating Rsoil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization.

Sampling soil-derived CO2 for analysis of isotopic composition: a comparison of different techniques

A new system for soil respiration measurement [P. Rochette, L.B. Flanagan, E.G. Gregorich. Separating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13. Soil Sci. Soc. Am. J., 63, 1207–1213 (1999).] was modified in order to collect soil-derived CO2 for stable isotope analysis. The aim of this study was to assess the suitability of this modified soil respiration system to determine the isotopic composition (δ13C) of soil CO2 efflux and to measure, at the same time, the soil CO2 efflux rate, with the further advantage of collecting only one air sample. A comparison between different methods of air collection from the soil was carried out in a laboratory experiment. Our system, as well as the other dynamic chamber approach tested, appeared to sample the soil CO2, which is enriched with respect to the soil CO2 efflux, probably because of a mass dependent fractionation during diffusion and because of the atmospheric contribution in the upper soil layer. On the contrary, the static accumulation of CO2 into the chamber headspace allows sampling of δ13C-CO2 of soil CO2 efflux.

Long-term Effects of Free Air CO2 Enrichment (FACE) on Soil Respiration

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 the δ13C of soil CO2 at all but the shallowest soil depths.

Effect of Soil Compaction and Biomass Removal on Soil CO2 Efflux in a Missouri Forest

Forest disturbances associated with harvesting activities can affect soil properties and soil respiration. A soda‐lime technique was used to measure soil carbon dioxide (CO2) efflux rates in clearcut plots of a Missouri oak‐hickory (Quercus spp. L.—Carya spp. Nutt.) forest 4 years after being treated with two levels of forest biomass removal and two levels of compaction, both separate and in combinations, and an uncut control. Respiration rates were measured twice a month from mid‐April through October. Soil CO2 efflux rates were significantly (p<0.001) higher in uncut control plots than in clearcut plots, but differences between biomass removal or soil compaction treatments were not significant. Soil CO2 efflux rates were positively correlated with soil temperature. The lack of difference between soil CO2 efflux rates in weed control and no weed control subplots suggests that several more years may be required for regenerating clearcut plots to produce soil respiration rates similar to those in uncut control plots.

Coupling of canopy gas exchange with root and rhizosphere respiration in a semi-arid forest

The strength of coupling between canopy gas exchange and root respiration was examined in ~15-yr-old ponderosa pine (Pinus ponderosa Doug. Ex Laws.) growing under seasonally drought stressed conditions. By regularly watering part of the root system to reduce tree water stress and measuring soil CO2 efflux on the dry, distant side of the tree, we were able to determine the strength of the relationship between soil autotrophic (root and rhizosphere) respiration and changes in canopy carbon uptake and water loss by comparison with control trees (no watering). After ~40 days the soil CO2 efflux rate, relative to pre-treatment conditions, was twice that of the controls. This difference, attributable to root and rhizosphere respiration, was strongly correlated with differences in transpiration rates between treatments (r2 = 0.73, p 0.99) for both treatments. At the ecosystem scale, daily soil CO2 efflux rate was linearly related to gross primary productivity (GPP) as measured by eddy-covariance technique (r2 = 0.55, p<0.01), suggesting patterns of soil CO2 release appear strongly correlated to recent carbon assimilation in this young pine stand. Collectively the observed relationships suggest some consideration should be given to the inclusion of canopy processes in future models of soil respiration.

Calibrating soil respiration measures with a dynamic flux apparatus using artificial soil media of varying porosity

SummaryMeasurement of soil respiration to quantify ecosystem carbon cycling requires absolute, not relative, estimates of soil CO2 efflux. We describe a novel, automated efflux apparatus that can be used to test the accuracy of chamber‐based soil respiration measurements by generating known CO2 fluxes. Artificial soil is supported above an air‐filled footspace wherein the CO2 concentration is manipulated by mass flow controllers. The footspace is not pressurized so that the diffusion gradient between it and the air at the soil surface drives CO2 efflux. Chamber designs or measurement techniques can be affected by soil air volume, hence properties of the soil medium are critical. We characterized and utilized three artificial soils with diffusion coefficients ranging from 2.7 × 10−7 to 11.9 × 10−7 m2 s−1 and porosities of 0.26 to 0.46. Soil CO2 efflux rates were measured using a commercial dynamic closed‐chamber system (Li‐Cor 6400 photosynthesis system equipped with a 6400‐09 soil CO2 flux chamber). On the least porous soil, small underestimates (&lt; 5%) of CO2 effluxes were observed, which increased as soil diffusivity and soil porosity increased, leading to underestimates as high as 25%. Differential measurement bias across media types illustrates the need for testing systems on several types of soil media.

Effects of fire on surface carbon, energy and water vapour fluxes over campo sujo savanna in central Brazil

Summary Tower‐based eddy covariance measurements were used to quantify the effect of fire on subsequent carbon dioxide fluxes and water and surface energy balance characteristics for campo sujo savanna located near Brasília in Central Brazil (15°56′ S, 47°51′ W). Campo sujo is a xeromorphic, open shrub savanna with very scattered but definitely visible shrubs and tree‐like shrub elements. We studied two areas, one exposed to a prescribed fire late in the dry season, and a second that had not been burned for the previous 4 years. The fire on 22 September 1998 consumed an estimated 26 mol C m−2. Immediately after the fire, evapotranspiration rates decreased and the savanna became a stronger net source of CO2 to the atmosphere. This was attributed to the removal of the still slightly physiologically active grass layer and higher soil CO2 efflux rates as a consequence of elevated surface soil temperatures post‐burning. On the commencement of the first rains in early October 1998, this situation was reversed, with the burned area rapidly becoming a stronger sink for CO2 and with higher evapotranspiration rates than a nearby unburned (control) area. This difference persisted throughout the wet season (until at least June 1999) and was attributable to greater physiological activity of the regrowing vegetation in the burned area. Early in the growing season, higher soil evaporation rates may also have contributed to faster water use by the previously burned area. Overall, we estimate an annual gross primary productivity for the burned area of 135 mol C m−2 year−1, with that for the unburned area being 106 mol C m−2 year−1. Estimated ecosystem respiration rates were more similar on an annual basis (96 and 82 mol C m−2 year−1 for the burned and unburned areas, respectively), giving rise to a substantially higher net ecosystem productivity for the previously burned area (38 vs 24 mol C m−2 year−1). Stimulation of photosynthetic activity in the rapid post‐fire growth phase means that the negative effects of fire on the ecosystem carbon balance were more or less neutralized after only 12 months.

Automated Monitoring of Soil Respiration

We designed, constructed, and tested an automated chamber system for continuously monitoring soil respiration. Our objective was to design a chamber that would permit monitoring of CO2 efflux rates over long time periods without altering the natural microclimate within the chamber. Furthermore, the design would permit accurate measurements even in a highly fluctuating CO2 environment. We built a chamber that operates by closing over the soil in response to a control signal and remains closed for a 14-min period before opening again. Thus, the chamber allows normal drying and wetting of the soil between measurements. An automated switching system was programmed to sequentially open and close chambers in concert with an infrared gas analysis system (IRGA). The IRGA was operated in a differential mode, and equivalent flow rates of reference gas (ambient air) and sample gas (air exiting chamber) were maintained with mass flow controllers. A flexible neoprene lid, stretched tightly over each chamber when closed, provided an airtight seal. This feature and the use of a large mixing bottle (for buffering frequent changes in ambient CO2 concentration) permitted us to measure soil CO2 efflux rates even in an environment with highly variable atmospheric CO2 concentration. Soil respiration rates, measured over a period of several weeks with the automated chambers, were in agreement with traditional point-in-time measurements, and the soil microclimate was not affected by the chambers. However, when extrapolated over a period of several weeks the point-in-time measurements overestimated CO2 efflux rates based on continuous measurements with the automated system.

Seasonal and spatial variability in soil CO2 efflux rates for a central Siberian Pinus sylvestris forest

Rates of CO2 efflux from the floor of a central Siberian Scots pine (Pinus sylvestris) forest were measured using a dynamic closed chamber system and by a eddy covariance system placed 2.5 m above the forest floor. Measurements were undertaken for a full growing season: from early May to early October 1999. Spatial variability as determined by the chamber measurements showed the rate of CO2 efflux to depend on location, with rates from relatively open areas (“glades”) only being about 50% those observed below or around trees. This was despite generally higher temperatures in the glade during the day. A strong relationship between CO2 efflux rate and root density was observed in early spring, suggesting that lower rates in open areas may have been attributable to fewer roots there. Continuous measurements with the eddy covariance system provided good temporal coverage. This method, however, provided estimates of ground CO2 efflux rate rates that were about 50% lower than chamber measurements that were undertaken in areas considered to be representative of the forest as a whole. An examination of the seasonal pattern of soil CO2 efflux rates suggests that much of the variability in CO2 efflux rate could be accounted for by variations in soil temperature. Nevertheless, there were also some indications that the soil water deficits served to reduce soil CO2 efflux rates during mid-summer. Overall the sensitivity of CO2 efflux rate to temperature seems to be greater for this boreal ecosystem than has been the case for most other studies.

Contrasting soil respiration in young and old‐growth ponderosa pine forests

AbstractThree years of fully automated and manual measurements of soil CO2 efflux, soil moisture and temperature were used to explore the diel, seasonal and inter‐annual patterns of soil efflux in an old‐growth (250‐year‐old, O site) and recently regenerating (14‐year‐old, Y site) ponderosa pine forest in central Oregon. The data were used in conjunction with empirical models to determine which variables could be used to predict soil efflux in forests of contrasting ages and disturbance histories. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. Soil CO2 efflux at both sites showed large inter‐annual variability that could be attributed to soil moisture availability in the deeper soil horizons (O site) and the quantity of summer rainfall (Y site). Seasonal patterns of soil CO2 efflux at the O site showed a strong positive correlation between diel mean soil CO2 efflux and soil temperature at 64 cm depth whereas diel mean soil efflux at the Y site declined before maximum soil temperature occurred during summer drought. The use of diel mean soil temperature and soil water potential inferred from predawn foliage water potential measurements could account for 80% of the variance of diel mean soil efflux across 3 years at both sites, however, the functional shape of the soil water potential constraint was site‐specific. Based on the similarity of the decomposition rates of litter and fine roots between sites, but greater productivity and amount of fine litter detritus available for decomposition at the O site, we would expect higher rates of soil CO2 efflux at the O site. However, annual rates were only higher at the O site in one of the 3 years (597 ± 45 vs. 427 ± 80 g C m−2). Seasonal patterns of soil efflux at both sites showed influences of soil water limitations that were also reflected in patterns of canopy stomatal conductance, suggesting strong linkages between above and below ground processes.

Belowground carbon pools and processes in different age stands of Douglas-fir

Forest floor material and soil organic matter may act as both a source and a sink in global CO2 cycles. Thus, the ecosystem processes controlling these pools are central to understanding the transfers of carbon (C) between the atmosphere and terrestrial systems. To examine these ecosystem processes, the effect of stand age on temporal carbon source-sink relationships was examined in 20-year-old, 40-year-old and old-growth stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Cascade Mountains of south-central Washington State. Belowground C and nitrogen (N) storage and soil respiration were measured. In addition, nylon mesh bags containing homogenized soils from each site were buried at the respective sites to quantify root ingrowth and potential C sequestration and loss. The sites supporting the 20- and 40-year-old stands had soil C stores reflecting the C contributions from logging residue, coarse woody debris and stumps left after harvest. Because the N-fixer red alder (Alnus rubra Bong.) comprised 33% of the 40-year-old stand, this site had significantly greater concentrations and pools of N in the forest floor than sites without red alder. This N-rich site had consistently lower soil CO2 efflux rates during the growing season than the sites supporting the 20-year-old and old-growth stands. Estimated annual soil C efflux was 1367, 883 and 1194 g m-2 for the sites supporting the 20-, 40- and old-growth stands, respectively. These values are higher than previously reported values. Root ingrowth was significantly less in the 40-year-old stand than in the 20-year-old stand, and both young stands showed markedly less fine root growth than the old-growth stand. At the sites supporting the young stands, C and N were lost from the soil bags, whereas there was an increase in C and N in the soil bags at the site supporting the old-growth stand. The fine root growth and soil respiration data support the hypothesis that belowground C allocation decreases with increasing fertility. Quantification of the source-sink relationship of soil C at the three stands based on litterfall, relative root ingrowth and soil respiration measurements was compromised because of significant CO2 flux from decaying organic matter in the young stands.

Seasonal and spatial variability in soil CO2 efflux rates for a central SiberianPinus sylvestris forest

Rates of CO2 efflux from the floor of a central Siberian Scots pine (Pinus sylvestris) forest were measured using a dynamic closed chamber system and by a eddy covariance system placed 2.5 m above the forest floor. Measurements were undertaken for a full growing season: from early May to early October 1999. Spatial variability as determined by the chamber measurements showed the rate of CO2 efflux to depend on location, with rates from relatively open areas (“glades”) only being about 50% those observed below or around trees. This was despite generally higher temperatures in the glade during the day. A strong relationship between CO2 efflux rate and root density was observed in early spring, suggesting that lower rates in open areas may have been attributable to fewer roots there. Continuous measurements with the eddy covariance system provided good temporal coverage. This method, however, provided estimates of ground CO2 efflux rate rates that were about 50% lower than chamber measurements that were undertaken in areas considered to be representative of the forest as a whole. An examination of the seasonal pattern of soil CO2 efflux rates suggests that much of the variability in CO2 efflux rate could be accounted for by variations in soil temperature. Nevertheless, there were also some indications that the soil water deficits served to reduce soil CO2 efflux rates during mid-summer. Overall the sensitivity of CO2 efflux rate to temperature seems to be greater for this boreal ecosystem than has been the case for most other studies.

Seasonal patterns of soil carbon dioxide efflux from a wet-dry tropical savanna of northern Australia

Soil CO2 efflux rates were measured in a eucalypt open forest in a tropical savanna of northern Australia, with a portable closed chamber and CO2 gas analyser. Both abiotic (soil temperature and water content) and biotic (litterfall and fine-root growth) factors that may influence soil CO2 efflux were examined. Daytime rates of soil CO2 efflux rate were consistently higher than nocturnal values. Maximal rates occurred during late afternoons when soil temperatures were also maximal and minimum values were recorded during the early morning (0400–0800 hours). Average soil CO2 efflux was 5.37 mol m–2 s–1 (range 3.5–6.7 mol m–2 s–1 during the wet season and declined to 2.20 mol m–2 s–1 (range 1.2–3.6 mol m–2 s–1) during the dry season. The amount of carbon released from soil was 14.3 t ha–1 year–1, with approximately 70&amp;percnt; released during the wet season and 30&amp;percnt; during the dry season. The rate of efflux was correlated with soil moisture content and soil temperature only during the wet season, when root growth and respiration were high. During the dry season there was no correlation with soil temperature. These results are discussed in relation to the carbon balance of tropical savannas.

Corrigendum to: Seasonal patterns of soil carbon dioxide efflux from a wet¿dry tropical savanna of northern Australia

Soil CO2 efflux rates were measured in a eucalypt open forest in a tropical savanna of northern Australia, with a portable closed chamber and CO2 gas analyser. Both abiotic (soil temperature and water content) and biotic (litterfall and fine-root growth) factors that may influence soil CO2 efflux were examined. Daytime rates of soil CO2 efflux rate were consistently higher than nocturnal values. Maximal rates occurred during late afternoons when soil temperatures were also maximal and minimum values were recorded during the early morning (0400–0800 hours). Average soil CO2 efflux was 5.37 mol m–2 s–1 (range 3.5–6.7 mol m–2 s–1 during the wet season and declined to 2.20 mol m–2 s–1 (range 1.2–3.6 mol m–2 s–1) during the dry season. The amount of carbon released from soil was 14.3 t ha–1 year–1, with approximately 70&amp;percnt; released during the wet season and 30&amp;percnt; during the dry season. The rate of efflux was correlated with soil moisture content and soil temperature only during the wet season, when root growth and respiration were high. During the dry season there was no correlation with soil temperature. These results are discussed in relation to the carbon balance of tropical savannas.

Response of soil CO2 efflux to water manipulation in a tallgrass prairie ecosystem

Although CO2 efflux plays a critical role in carbon exchange between the biosphere and atmosphere, our understanding of its regulation by soil moisture is rather limited. This study was designed to examine the relationship between soil CO2 efflux and soil moisture in a natural ecosystem by taking advantage of the historically long drought period from 29 July to 21 September 2000 in the southern Central Great Plain, USA. At the end of August when soil moisture content at the top 50 mm was reduced to less than 50 g kg −1 gravimetrically, we applied 8 levels of water treatments (simulated to rainfall of 0, 10, 25, 50, 100, 150, 200, and 300 mm) with three replicates to 24 plots in a Tallgrass Prairie ecosystem in Central Oklahoma, USA. In order to quantify root-free soil CO2 efflux, we applied the same 8 levels of water treatments to 24 500-mm soil columns using soil from field adjacent to the experimental plots. We characterized dynamic patterns of soil moisture and soil CO2 efflux over the experimental period of 21 days. Both soil moisture content and CO2 efflux showed dramatic increases immediately after the water addition, followed by a gradual decline. The time courses in response to water treatments are well described by Y = Y0 + ate −bt ,w hereY is either soil moisture or CO2 efflux, t is time, Y0, a ,a ndb are coefficients. Among the 8 water treatments, the maximal soil CO2 efflux rate occurred at the 50 mm water level in the field and 100 mm in the root-free soil 1 day after the treatment. The maximal soil CO2 efflux gradually shifted to higher water levels as the experiment continued. We found the relationship between soil CO2 efflux and soil moisture using the data from the 21-day experiment was highly scattered, suggesting complex mechanisms determining soil CO2 efflux by soil moisture.

CARBON ISOTOPE RATIOS IN BELOWGROUND CARBON CYCLE PROCESSES

Analyses of carbon isotope ratios (δ13C) in soil organic matter (SOM) and soil respired CO2 provide insights into dynamics of the carbon cycle. δ13C analyses do not provide direct measures of soil CO2 efflux rates but are useful as a constraint in carbon cycle models. In many cases, δ13C analyses allow the identification of components of soil CO2 efflux as well as the relative contribution of soil to overall ecosystem CO2 fluxes. δ13C values provide a unique tool for quantifying historical shifts between C3 and C4 ecosystems over decadal to millennial time scales, which are relevant to climate change and land-use change issues. We identify the need to distinguish between δ13C analyses of SOM and those of soil CO2 efflux in carbon cycle studies, because time lags in the turnover rates of different soil carbon components can result in fluxes and stocks that differ in isotopic composition (disequilibrium effect). We suggest that the frequently observed progressive δ13C enrichment of SOM may be related to a g...

Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas‐fir terracosms

AbstractAlthough numerous studies indicate that increasing atmospheric CO2 or temperature stimulate soil CO2 efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO2 and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO2 and elevated temperature on these components of soil CO2 efflux in Douglas‐fir terracosms. We measured both soil CO2 efflux rates and the 13C and 18O isotopic compositions of soil CO2 efflux in 12 sun‐lit and environmentally controlled terracosms with 4‐year‐old Douglas fir seedlings and reconstructed forest soils under two CO2 concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO2 efflux. In most cases, litter decomposition was the dominant component of soil CO2 efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO2 concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO2, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual‐isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.

Global patterns of carbon dioxide emissions from soils

We use semi‐mechanistic, empirically based statistical models to predict the spatial and temporal patterns of global carbon dioxide emissions from terrestrial soils. Emissions include the respiration of both soil organisms and plant roots. At the global scale, rates of soil CO2 efflux correlate significantly with temperature and precipitation; they do not correlate well with soil carbon pools, soil nitrogen pools, or soil C:N. Wetlands cover about 3% of the land area but diminish predicted CO2 emissions by only about 1%. The estimated annual flux of CO2 from soils to the atmosphere is estimated to be 76.5 Pg C yr−1, 1–9 Pg greater than previous global estimates, and 30–60% greater than terrestrial net primary productivity. Historic land cover changes are estimated to have reduced current annual soil CO2 emissions by 0.2–2.0 Pg C yr−1 in comparison with an undisturbed vegetation cover. Soil CO2 fluxes have a pronounced seasonal pattern in most locations, with maximum emissions coinciding with periods of active plant growth. Our models suggest that soils produce CO2 throughout the year and thereby contribute to the observed wintertime increases in atmospheric CO2 concentrations. Our derivation of statistically based estimates of soil CO2 emissions at a 0.5° latitude by longitude spatial and monthly temporal resolution represents the best‐resolved estimate to date of global CO2 fluxes from soils and should facilitate investigations of net carbon exchanges between the atmosphere and terrestrial biosphere.