Changes In Soil Carbon Storage Research Articles

Forest soil carbon storage in 10‐year‐old Douglas‐fir plantations of western Oregon and Washington remains similar to pre‐harvest

AbstractForests around the world, and in the case of this study, the coastal Pacific Northwest United States, store large amounts of carbon, both above ground in the trees and below ground in soils. Understanding the effects of forest disturbance, including timber harvesting, is important in order to evaluate the role that forestry plays in the global carbon cycle. Soil carbon can be difficult to assess with enough precision to detect the kinds of changes that are expected, yet a series of small changes over time in the same direction could have important cumulative effects. In this study, eight randomly selected Douglas‐fir forest stands in Oregon and Washington were sampled at 300 points each using a fixed‐depth sampling approach to attempt to detect a 5% or higher change in soil carbon storage to 1 m, longitudinally from pre‐harvest to 10 years post‐harvest. There was moderate variability in results over time at individual sites, with some sites decreasing slightly and others increasing slightly. Only two sites achieved lower than the 5% minimum detectible difference target. The remaining six sites were able to detect 5.7%–10.7% differences. In one case, an unexpectedly large increase in mineral soil carbon 10 years post‐harvest occurred without clear explanation. On average, forest floor carbon stores were 20% larger 10 years post‐harvest than pre‐harvest. Even with the large increases excluded, both the fixed‐depth approach and equivalent soil mass correction showed there was no significant change in mineral soil carbon stores to 1 m at 10 years post‐harvest in the region.

Relationship between Greenhouse Gas Budget and Soil Carbon Storage Measured on Site in Zhalainuoer Grassland Mining Area

Grassland has great potential for carbon sequestration; however, the relationship between carbon storage (CS) and greenhouse gas (GHG) budget and their influencing factors in the natural restoration process in grassland mining areas are rarely studied. In this study, taking Zhalainuoer mining area in Inner Mongolia as an example, the subsidence soil for 1-, 2-, 5-, 10-, and 15-year and non-subsidence soil were selected as the research objects to explore the relationship between CS and the GHG budget and their influencing factors. The results show that there is a significant negative correlation between CS and the GHG budget. Soil organic carbon storage accounts for 99% of CS. CS is positively correlated with SOM and AP, and with the bacteria Entotheonellaeota. The GHG budget is mainly affected by CO2 emission, which is positively correlated with subsidence time, plant biomass, and coverage, negatively correlated with the bacteria Actinobacteriota and Deinococcota, and positively correlated with Cyanobacteria. In summary, soil plays a major role in storing carbon. Carbon sequestration is a physiological process produced by plants and organisms. Subsidence affects soil CS by changing soil properties and thus affecting its aboveground vegetation and soil microorganisms. This study investigates the changes in soil carbon storage following subsidence caused by mining activities. The findings contribute to our understanding of the impact of mining subsidence on soil CS and can inform the development of low-carbon remediation technologies.

Inundation impacts on diversified pasture biomass allocation and soil particulate organic matter stocks

AbstractNatural soil inundation caused by frequent and intense precipitation affects carbon allocation in grassland biomass, ultimately leading to changes in soil carbon storage. Increasing forage diversity could provide resiliency to inundation of grassland. The objective was to evaluate forage and root biomass and C and N stocks in the soil particulate organic matter (POM) from pastures under recurring short‐term inundation. Three forage species combinations were evaluated in an inundated (typically lasting for a few days after heavy rain events) and a non‐inundated pasture: (1) predominantly tall‐fescue (Festuca arundinacea Schreb.); (2) mixture of cool‐season perennials composed of tall‐fescue, orchardgrass (Dactylis glomerata L.), bluegrass (Poa pratensis L.) and white (Trifolium repens L.) and red clover (Trifolium pratense L.); (3) and cool‐season mixture of perennials overseeded with oats (Avena strigosa Schreb.) and rye (Secale cereale L.). Roots and forage biomass were sampled during the growing seasons of 2021 and 2022. Soil POM was evaluated 2.5 years after establishment. Inundation reduced forage and root biomass mainly during periods of higher inundation frequency, leading to lower C‐POM stocks (p < .05). Inundation caused a shift in the forage botanical composition, that is, higher occurrence of weeds and less productive grass species with shallow roots. The perennial cool‐season mixture did not increase forage yield compared with tall fescue only but did increase root mass. This occurred mainly at deeper layers and, consequently, increased C‐POM stocks (p < .05). Overseeding of winter annuals reduced overall forage production, despite increasing spring biomass when inundated in the first year, but reduced C and N‐POM stocks (p < .05). Perennial cool‐season forage mixtures can increase the resilience of pastures to inundation events and contribute to increased carbon sequestration in grasslands where inundation is prevalent.

Vegetation succession accelerated the accumulation of soil organic carbon on road-cut slopes by changing the structure of the bacterial community

More emphasis on the soil carbon sequestration of restored rock-cut slopes will be beneficial for reducing carbon emissions from the transportation sector. However, the changes in soil carbon storage with vegetation succession are unclear. We selected two different highways where the cut slopes had been restored for 3 and 10–20 years in Guangdong Province, China, as the study plots. By examining the two restored slopes at different recovery times, we analyzed the differences in soil organic carbon, soil physicochemical properties and the diversity of the soil microbial communities of road-cut slopes with vegetation succession. The soil organic carbon of restored slopes increased significantly with vegetation succession. Moreover, vegetation diversity increased soil organic carbon storage by reducing the microbial activity of Proteobacteria. Additionally, the results indicated that pH remained a key factor in controlling microbial activity and that the effect of soil moisture was site specific. Additionally, our study highlights the importance of investigating the microbial community, pH, and vegetation diversity for a more complete understanding of the mechanisms that regulate soil organic carbon dynamics in restored slope ecosystems. Understanding the multivariate processes controlling soil organic carbon dynamics should be an important goal of future soil organic carbon research for restoring ecosystems on bare slopes.

Fungal community and functional responses to soil warming are greater than for soil nitrogen enrichment

Soil fungi are key regulators of forest carbon cycling and their responses to global change have effects that ripple throughout ecosystems. Global changes are expected to push many fungi beyond their environmental niches, but there are relatively few studies involving multiple, simultaneous global change factors. Here, we studied soil fungal diversity, community composition, co-occurrence patterns, and decomposition gene responses to 10 years of soil warming and nitrogen addition, alone and in combination. We specifically examined whether there were fungal community characteristics that could explain changes in soil carbon storage and organic matter chemistry in chronically warmed and fertilized soil. We found that fungal communities in warmed soils are less diverse and shift in composition. Warming also favored hyperdominance by a few mycorrhizal fungal species and lowered manganese peroxidase but increased hydrolytic enzyme encoding gene potentials. Nitrogen addition did not significantly affect fungal community composition but, like warming, did reduce fungal diversity and favored overdominance by a unique set of mycorrhizal taxa. Warming alone and in combination with nitrogen addition also reduced negative but increased positive fungal co-occurrence probabilities, promoting species coexistence. Negative fungal co-occurrence was positively correlated to soil carbon content, while the proportion of fungal hydrolytic enzyme encoding genes was negatively correlated with soil carbon content. This may reflect fungal life history trade-offs between competition (e.g., reduced negative co-occurrence) and resource acquisition (e.g., higher abundance of hydrolytic enzyme encoding genes) with implications for carbon storage.

Soil burial reduces decomposition and offsets erosion-induced soil carbon losses in the Indian Himalaya.

The extent to which soil erosion is a net source or sink of carbon globally remains unresolved but has the potential to play a key role in determining the magnitude of CO2 emissions from land-use change in rapidly eroding landscapes. The effects of soil erosion on carbon storage in low-input agricultural systems, in acknowledged global soil erosion hotspots in developing countries, are especially poorly understood. Working in one such hotspot, the Indian Himalaya, we measured and modelled field-scale soil budgets, to quantify erosion-induced changes in soil carbon storage. In addition, we used long-term (1-year) incubations of separate and mixed soil horizons to better understand the mechanisms controlling erosion-induced changes in soil carbon cycling. We demonstrate that high rates of soil erosion did not promote a net carbon loss to the atmosphere at the field scale. Furthermore, our experiments showed that rates of decomposition in the organic matter-rich subsoil layers in depositional areas were lower per unit of soil carbon than from other landscape positions; however, these rates could be increased by mixing with topsoils. The results indicate that, the burial of soil carbon, and separation from fresh carbon inputs, led to reduced rates of decomposition offsetting potential carbon losses during soil erosion and transport within the cultivated fields. We conclude that the high rates of erosion experienced in these Himalayan soils do not, in isolation, drive substantial emissions of organic carbon, and there is the potential to promote carbon storage through sustainable agricultural practice.

Taxonomic and structural diversity indices predict soil carbon storage better than functional diversity indices along a dieback intensity gradient in semi-arid oak forests

Key messageThe use of structural diversity indices can be efficient tools to increase understanding of changes in soil carbon storage as a key ecosystem function along the oak dieback intensity.Investigating the relationships between biodiversity and ecosystem functions within the mass‐ratio and niche complementary hypotheses is still a challenging issue in terrestrial ecology. These relationships have not been studied along a dieback gradient of disturbance in semi-arid forest ecosystems. To fill this gap, we investigated the relationships between a main ecosystem function -soil carbon storage- and various diversity indices along a dieback intensity gradient (no, low, moderate or high dieback intensity) in protected or intensively managed mixed oak forests in western Iran. We used different diversity approaches and calculated the functional divergence index (FDvar), community weighted mean trait values; CWM, taxonomic diversity indices (richness, Shannon–Wiener diversity and evenness) and structural diversity indices (MI: mingling index, HD: height differentiation, DD: diameter differentiation). Soil carbon storage was significantly influenced by the type of management, the intensity of dieback and their interactions and was higher in the protected areas (95.90 ± 4.62 ton ha−1) than in the intensively managed areas (76.52 ± 2.04 ton ha−1). It showed a humped-shaped pattern along the dieback intensity gradient in the protected areas, as it peaked at the low dieback intensity (122.47 ± 10.12 ton ha-1), indicating that soil function was maximized at a low disturbance level. Soil carbon storage was positively and significantly correlated with all structural and taxonomic diversity indices, except for evenness, but negatively with most functional composition indices (CWM of LNC: leaf nitrogen concentration, H: mean of woody species height and LDMC: leaf dry-matter content). It was best predicted by a structural index (Mingling index: R2 = 0.214) followed by a taxonomic index (species richness, R2 = 0.173) and a CWM index (CWM LDMC, R2 = 0.158). Our results emphasize the role played by the diversity indices to predict ecosystem functions in contrasted management conditions and along a dieback gradient. They also provide evidence to support both the mass‐ratio and niche complementary hypotheses.

Effect of Paddy-Upland Rotation System on the Net Greenhouse Gas Balance as the Sum of Methane and Nitrous Oxide Emissions and Soil Carbon Storage: A Case in Western Japan

To investigate the effect of paddy-upland (PU) rotation system on greenhouse gas emissions, methane (CH4) and nitrous oxide (N2O) emissions were monitored for three years for a PU rotation field (four cultivations (wheat-soybean-rice-rice) over three years) and continuous paddy (CP) field on alluvial soil in western Japan. Soil carbon storage was also calculated using an improved Rothamsted Carbon (RothC) model. The net greenhouse gas balance was finally evaluated as the sum of CO2eq of the CH4, N2O and changes in soil carbon storage. The average CH4 emissions were significantly lower and the average N2O emissions were significantly higher in the PU field than those in the CP field (p < 0.01). On CO2 equivalent basis, CH4 emissions were much higher than N2O emission. In total, the average CO2eq emissions of CH4 plus N2O in the PU field (1.81 Mg CO2 ha−1 year−1) were significantly lower than those in the CP field (7.42 Mg CO2 ha−1 year−1) (p < 0.01). The RothC model revealed that the changes in soil carbon storage corresponded to CO2eq emissions of 0.57 and 0.09 Mg CO2 ha−1 year−1 in the both fields, respectively. Consequently, the net greenhouse gas balance in the PU and CP fields were estimated to be 2.38 and 7.51 Mg CO2 ha−1 year−1, respectively, suggesting a 68% reduction in the PU system. In conclusion, PU rotation system can be regarded as one type of the climate-smart soil management.

Refining benchmarks for soil organic carbon in Australia’s temperate forests

Soil organic carbon (SOC) stocks in Australia’s temperate forests have been overlooked in national soil databases and in global SOC analyses of natural ecosystems despite the importance of temperate forests to the global terrestrial carbon balance. This limits the potential to both predict change in SOC stocks in temperate Australia and to identify where and how SOC stocks can be managed to mitigate climate change. Based on data from 707 sites, we examine variations in SOC concentrations and stocks across a range of natural temperate broadleaf forests in south-eastern Australia. Comparisons with current Australia-wide databases highlight previous under-estimation of forest SOC concentrations, leading to substantial underprediction of SOC stocks in the most productive forests (e.g. this study’s mean of 207 Mg C ha−1 compared with a database mean estimate of 141 Mg C ha−1 for Tall open-forests to 30-cm soil depth). Random Forest models involving 27 environmental variables (representing climate, terrain, parent material, soil attributes, vegetation, and fire history) explained up to 79% of the variation in SOC concentrations and 77% of the variation in SOC stocks to 30-cm depth. Climate variables (precipitation, temperature) were of greatest importance to the prediction of both SOC concentrations and SOC stocks, tending to override the importance of terrain and fire-history variables at this study’s regional scale. While patterns in SOC concentrations and stocks were correlated, SOC concentrations were not a reliable proxy for SOC stocks to 10-cm depth, reiterating the importance of mass equivalent measures (i.e. soil bulk density) to assessing changes in soil carbon storage. Our study provides a timely check of the model-based estimates of SOC concentrations and stocks in Australia’s temperate forests that are currently available in nation-wide databases and improves the available information for defining benchmarks, and for identifying potential areas of SOC loss and gain, in programs that aim to mitigate climate change.

Community-based seasonal movement grazing maintains lower greenhouse gas emission intensity on Qinghai-Tibet Plateau of China

Global livestock husbandry provides ecosystem goods and services but also emits 7.1 Gt CO2-eq. of greenhouse gases (GHGs) per year. To lower GHG emissions intensity, appropriate production management systems should be identified. Since the 1980s, grassland livestock husbandry in China has transformed gradually from pastoralism into individual household management under the Grassland Household Contract System Policy. However, little is known about how this transition influences GHG emissions. We selected two case study sites representing two different forms of rangeland management systems in Ruoergai county of the Qinghai-Tibet Plateau, viz. 1) household-based all year continuous grazing under the individual use of rangeland with fences demarcating boundaries; 2) community-based seasonal grazing under the common use of the whole rangeland. The objective was to examine the differences in greenhouse gas emission intensity between the two systems using life cycle assessment (LCA). The results showed that the transition from community-based seasonal grazing into household-based continuous grazing increased the GHG emissions intensity from -0.62 kgCO2-eq/kg meat to 10.51 kgCO2-eq/kg meat. The increase was primarily attributed to changes in soil carbon storage. Findings suggest that to minimize GHG emissions and environmental degradation, community-based seasonal grazing in the pastoral area of Qinghai-Tibet Plateau should be maintained. Enhancing soil carbon sequestration by adopting appropriate practices would further reduce the GHG emissions intensity arising from the livestock system.

Soil nutrient levels define herbage yield but not root biomass in a multispecies grass-legume ley

The response of above- and below-ground biomass to soil nutrient availability is crucial for estimating belowground carbon input and predicting changes in soil carbon storage. However, the response is far from clear at plant community level, especially for grassland systems. Using a long-term field experiment initiated 123 years ago with varying soil nutrient levels (deficient, sub-optimal, optimal and over-optimal) established by use of two nutrient sources (animal manure or mineral fertiliser), we examined the effects of soil nutrient level and source on herbage yield and composition, root biomass and root-to-shoot (R/S) ratio of an unfertilised multispecies grass-legume ley. Increased nutrient levels enhanced herbage yield, but did not affect root biomass. The R/S ratio decreased from deficient to sub-optimal level, but remained constant from optimal to over-optimal level. Nutrient source did not influence herbage yield, root biomass or R/S ratio, but the legume proportion increased in soils previously receiving mineral fertiliser. The R/S ratio decreased with herbage yield, but did not vary with herbage composition. We conclude that soil nutrient level and herbage yield rather than nutrient source and herbage composition determine biomass allocation between aboveground and belowground in temperate grassland leys.

Corrigendum to: Mapping change in key soil properties due to climate change over south-eastern Australia

Climate change will lead to altered soil conditions that will impact on plant growth in both agricultural and native ecosystems. Additionally, changes in soil carbon storage will influence carbon accounting schemes that may play a role in climate change mitigation programs. We applied a digital soil mapping approach to examine and map (at 100-m resolution) potential changes in three important soil properties – soil organic carbon (SOC), pH and sum-of-bases (common macro-nutrients) – resulting from projected climate change over south-eastern Australia until ~2070. Four global climate models were downscaled with three regional models to give 12 climate models, which were used to derive changes for the three properties across the province, at 0–30 and 30–100 cm depth intervals. The SOC stocks were projected to decline over the province, while pH and sum-of-bases were projected to increase; however, the extent of change varied throughout the province and with different climate models. The average changes primarily reflected the complex interplay of changing temperatures and rainfall throughout the province. The changes were also influenced by the operating environmental conditions, with a uniform pattern of change particularly demonstrated for SOC over 36 combinations of current climate, parent material and land use. For example, the mean decline of SOC predicted for the upper depth interval was 15.6 Mg ha–1 for wet–mafic–native vegetation regimes but only 3.1 Mg ha–1 for dry–highly siliceous–cropping regimes. The predicted changes reflected only those attributable to the projected climate change and did not consider the influence of ongoing and changing land management practices.

Mapping change in key soil properties due to climate change over south-eastern Australia

Climate change will lead to altered soil conditions that will impact on plant growth in both agricultural and native ecosystems. Additionally, changes in soil carbon storage will influence carbon accounting schemes that may play a role in climate change mitigation programs. We applied a digital soil mapping approach to examine and map (at 100-m resolution) potential changes in three important soil properties – soil organic carbon (SOC), pH and sum-of-bases (common macro-nutrients) – resulting from projected climate change over south-eastern Australia until ~2070. Four global climate models were downscaled with three regional models to give 12 climate models, which were used to derive changes for the three properties across the province, at 0–30 and 30–100 cm depth intervals. The SOC stocks were projected to decline over the province, while pH and sum-of-bases were projected to increase; however, the extent of change varied throughout the province and with different climate models. The average changes primarily reflected the complex interplay of changing temperatures and rainfall throughout the province. The changes were also influenced by the operating environmental conditions, with a uniform pattern of change particularly demonstrated for SOC over 36 combinations of current climate, parent material and land use. For example, the mean decline of SOC predicted for the upper depth interval was 15.6 Mg ha–1 for wet–mafic–native vegetation regimes but only 3.1 Mg ha–1 for dry–highly siliceous–cropping regimes. The predicted changes reflected only those attributable to the projected climate change and did not consider the influence of ongoing and changing land management practices.

Microbial biomass and enzymatic responses to temperate oak and larch forest thinning: Influential factors for the site-specific changes

Microbial biomass and enzyme activity are essential for ecosystem function in managed forests; however, uncertainty remains because microbial biomass and enzymatic responses to thinning highly differ with case studies. This study addressed the drivers for the site-specific responses of microbial biomass and enzyme activity to thinning. Study sites included two oak and three larch forests; each had un-thinned control, intermediate thinning (15–23% basal area reduction), and heavy thinning treatments (30–44% basal area reduction). Soil properties (temperature, water content, pH, total and inorganic nitrogen, and total carbon/nitrogen ratio), microbial biomass, enzyme (β-glucosidase, N-acetylglucosaminidase, leucyl aminopeptidase, acid phosphatase, and phenol oxidase) activity, and soil carbon storage were determined 6 years after thinning. Compared to the control, microbial biomass carbon and nitrogen were higher under the intermediate and the heavy thinning by 13.9 and 24.4% and 11.5 and 29.9% at one oak forests, respectively, and higher under the intermediate thinning by 53.7 and 70.7% at one larch forests. There were the post-thinning changes in leucyl aminopeptidase activity by −46.9% and by 150.0–210.0% at an oak and larch forest, respectively, acid phosphatase activity by 60.0% at one oak forest, and phenol oxidase activity by 355.0% at one oak forest. The effect sizes of thinning for soil properties explained 94% and 77% of variance of the effect sizes for microbial biomass and enzyme activity. Especially, the effect sizes for soil water content, NH4+, total carbon/nitrogen ratio, and temperature were the most influential. Furthermore, the effect size for soil carbon storage was parabolically related to the effect size for microbial biomass carbon (R2 = 0.66). These findings highlight that inconsistent thinning effects on soil properties varied microbial biomass and enzymatic responses to thinning, which differentiated the change in soil carbon storage across sites. Future studies should consider such inconsistencies when examining the effects of forest management.

Relationship between Soil Organic Carbon Stocks and Clay Content under Different Climatic Conditions in Central China

Understanding the association between soil organic carbon (SOC) and texture under different climatic conditions is important for assessing the effects of future climate changes on SOC stocks. In this study, we conducted a climatic gradient experiment covering three climate types (humid, sub-humid, and semi-arid) with a steep rainfall ranging from 345 to 910 mm, and specifically determined SOC dynamics, clay content, and vegetation and soil characteristics. The results showed that, from semi-arid to humid regions, SOC stocks, SOC, and clay content increased synchronously and were closely related in layers of depths of both 0–10 and 10–20 cm. In contrast, under similar climatic conditions, SOC dynamics were mainly affected by vegetation and soil characteristics, especially total nitrogen and total phosphorus dynamics, but not the soil clay content. Therefore, these results suggest that the relationship between SOC stocks and clay content depended on scale sizes. Specifically, on a larger scale with different climatic gradients, the climate may partly determine the changes in SOC and clay dynamics, whereas, at a smaller scale where climate type does not vary considerably, the changes in SOC stocks and clay content may be related to vegetation diversity and soil nutrient dynamics. These results may contribute to future model development and the projection of changes in soil carbon storage.

Mitigation of carbon using Atriplex nummularia revegetation

The use of abandoned or marginally productive land to mitigate greenhouse gas emissions may avoid competition with food and water production. Atriplex nummularia Lindl. is a perennial shrub commonly established for livestock forage on saline land, however, its potential for carbon mitigation has not been systematically evaluated. Similarly, although revegetation is an allowable activity to mitigate carbon within Article 3.4 of the United Nations Framework Convention on Climate Change’s Kyoto Protocol, there is a paucity of information on rates of carbon mitigation in soils and biomass through this mechanism. For six sites where A. nummularia had been established across southern Australia four were used to assess changes in soil carbon storage and four were used to develop biomass carbon sequestration estimates. A generalised allometric equation for above and below ground biomass was developed, with a simple crown volume index explaining 81% of the variation in total biomass. There were no significant differences in soil organic carbon storage to 0.3m or 2m depth compared to existing agricultural land-use. Between 2.2 and 8.3MgCha−1 or 0.2–0.6MgCha−1yr−1 was sequestered in above and below ground biomass and this translates to potential total sequestration of 1.1–3.6TgCyr−1 on saline land across Australia. Carbon income and forage grazing may thus provide a means to finance the stabilization of compromised land.

Assessing uncertainty in the profitability of prairie biomass production with ecosystem service compensation

Compensation for ecosystem services can encourage the management of agricultural systems for a broad range of benefits beyond crop production. Here we explore how payments for carbon sequestration and phosphorus retention affect the profitability and economic competitiveness of perennial herbaceous biomass. We consider the case of converting marginal land currently in corn and soy production in southern Minnesota, United States, to native diverse prairie grown as a biofuel feedstock. We estimate the resulting changes in soil carbon storage and water quality, and the economic value of both. To test the robustness of our results, we perform Monte Carlo simulations that incorporate variability and uncertainty in our model parameters. Our analyses show that prairie biomass production on marginal lands is 22% likely to be profitable when ecosystem service compensation is included, but only 5% when it is not. This suggests that the two ecosystem services modeled here may alone be insufficient to make prairie biomass production reliably profitable. Furthermore, by using ranges of model parameters rather than point estimates, this study shows that the profitability gap between conventional row crops and prairie is too large to be closed with the two services modeled here across a range of recent economic conditions.

Applicability of soil column incubation experiments to measure CO2 efflux

Abstract Accurate measurements of CO2 efflux from soils are essential to understand dynamic changes in soil carbon storage. Column incubation experiments are commonly used to study soil water and solute transport; however, the use of column incubation experiments to study soil CO2 efflux has seldom been reported. In this study, a 150-day greenhouse experiment with two treatments (no-tillage and tillage soils) was conducted to evaluate the applicability of soil column incubation experiments to study CO2 efflux. Both the chamber measurement and the gradient method were used, and results from the two methods were consistent: tillage increased soil cumulative CO2 efflux during the incubation period. Compared with fieldwork, incubation experiments can create or precisely control experimental conditions and thus have advantages for investigating the influence of climate factors or human activities on CO2 efflux. They are superior to bottle incubation because soil column experiments maintain a soil structure that is almost the same as that in the field, and thus can facilitate analyses on CO2 behaviour in the soil profile and more accurate evaluations of CO2 efflux. Although some improvements are still required for column incubation experiments, wider application of this method to study soil CO2 behaviour is expected.

Soil carbon responses to past and future CO2 in three Texas prairie soils

Changes in soil carbon storage could affect and be affected by rising atmospheric CO2. However, it is unlikely that soils will respond uniformly, as some soils are more sensitive to changes in the amount and chemistry of plant tissue inputs whereas others are less sensitive because of mineralogical, textural, or microbial processes. We studied soil carbon and microbial responses to a preindustrial-to-future CO2 gradient (250–500 ppm) in a grassland ecosystem in the field. The ecosystem contains three soil types with clay fractions of 15%–55%: a sandy loam Alfisol, a silty clay Mollisol, and a black clay Vertisol. Soil and microbial responses to atmospheric CO2 are plant-mediated; and aboveground plant productivity in this ecosystem increased linearly with CO2 in the sandy loam and silty clay. Although total soil organic carbon (SOC) did not change with CO2 treatment after four growing seasons, fast-cycling SOC pools increased with CO2 in the two clay soils. Microbial biomass increased 18% and microbial activity increased 30% across the CO2 gradient in the black clay (55% clay), but neither factor changed with CO2 in the sandy loam (15% clay). Similarly, size fractionation of SOC showed that coarse POM-C, the youngest and most labile fraction, increased four-fold across the CO2 gradient in the black clay, but increased by only 50% across the gradient in the sandy loam. Interestingly, mineral-associated C, the oldest and most recalcitrant fraction, declined 23% across the gradient in the third soil type, a silty clay (45% clay). Our results provide evidence for priming in this soil type, as labile C availability and decomposition rate (measured as soil respiration and soil C mineralization) also increased across the CO2 gradient in the silty clay soil. In summary, CO2 enrichment in this grassland increased the fast-cycling SOC pool as in other CO2 studies, but only in the two high-clay soils. Priming in the silty clay could limit SOC accumulation after prolonged CO2 exposure. Because soil texture varies geographically, including data on soil types could enhance predictions of soil carbon and microbial responses to future CO2 levels.

A dual isotope approach to isolate soil carbon pools of different turnover times

Abstract. Soils are globally significant sources and sinks of atmospheric CO2. Increasing the resolution of soil carbon turnover estimates is important for predicting the response of soil carbon cycling to environmental change. We show that soil carbon turnover times can be more finely resolved using a dual isotope label like the one provided by elevated CO2 experiments that use fossil CO2. We modeled each soil physical fraction as two pools with different turnover times using the atmospheric 14C bomb spike in combination with the label in 14C and 13C provided by an elevated CO2 experiment in a California annual grassland. In sandstone and serpentine soils, the light fraction carbon was 21–54% fast cycling with 2–9 yr turnover, and 36–79% slow cycling with turnover slower than 100 yr. This validates model treatment of the light fraction as active and intermediate cycling carbon. The dense, mineral-associated fraction also had a very dynamic component, consisting of ∼7% fast-cycling carbon and ∼93% very slow cycling carbon. Similarly, half the microbial biomass carbon in the sandstone soil was more than 5 yr old, and 40% of the carbon respired by microbes had been fixed more than 5 yr ago. Resolving each density fraction into two pools revealed that only a small component of total soil carbon is responsible for most CO2 efflux from these soils. In the sandstone soil, 11% of soil carbon contributes more than 90% of the annual CO2 efflux. The fact that soil physical fractions, designed to isolate organic material of roughly homogeneous physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations of rapid isotope incorporation into seemingly stable fractions and with emerging evidence for hot spots or micro-site variation of decomposition within the soil matrix. Predictions of soil carbon storage using a turnover time estimated with the assumption of a single pool per density fraction would greatly overestimate the near-term response to changes in productivity or decomposition rates. Therefore, these results suggest a slower initial change in soil carbon storage due to environmental change than has been assumed by simpler (one-pool) mass balance calculations.