Stable Soil Organic Carbon Pool Research Articles

Environmental factors controlling soil organic carbon stability in French forest soils

In temperate forests, soils contain a large part of the ecosystem carbon that can be partially lost or gained upon global change. Our aim was to identify the factors controlling soil organic carbon (SOC) stability in a wide part of French forests. Using a set of soils from 53 French forest sites, we assessed the effects of depth (up to 1 m), soil class (dystric Cambisol; eutric Cambisol; entic Podzol), vegetation types (deciduous; coniferous) and climate (continental influence; oceanic influence; mountainous influence) on SOC stability using indicators derived from laboratory incubation, physical fractionation and thermal analysis. Labile SOC pools decreased while stable SOC pool increased with depth. Soil class also significantly influenced SOC stability. Eutric Cambisols had less labile SOC in surface layers but had more labile SOC at depth (> 40 cm) than the other soil classes. Vegetation influenced thermal indicators of SOC pools mainly in topsoils (0–10 cm). Mountainous climate forest soils had a low thermal SOC stability. On top of the expected effect of depth, this study also illustrates the noticeable effect of soil class on SOC stability. It suggests that environmental variables should be included when mapping climate regulation soil service.

Can Carbon Fluxes Explain Differences in Soil Organic Carbon Storage under Aspen and Conifer Forest Overstories?

Climate- and management-induced changes in tree species distributions are raising questions regarding tree species-specific effects on soil organic carbon (SOC) storage and stability. Quaking aspen (Populus tremuloides Michx.) is the most widespread tree species in North America, but fire exclusion often promotes the succession to conifer dominated forests. Aspen in the Western US have been found to store more SOC in the mineral soil than nearby conifers, but we do not yet fully understand the source of this differential SOC accumulation. We measured total SOC storage (0–50 cm), characterized stable and labile SOC pools, and quantified above- and belowground litter inputs and dissolved organic carbon (DOC) fluxes during snowmelt in plots located in N and S Utah, to elucidate the role of foliage vs. root detritus in SOC storage and stabilization in both ecosystems. While leaf litterfall was twice as high under aspen as under conifers, input of litter-derived DOC with snowmelt water was consistently higher under conifers. Fine root (<2 mm) biomass, estimated root detritus input, and root-derived DOC fluxes were also higher under conifers. A strong positive relationship between root and light fraction C content suggests that root detritus mostly fueled the labile fraction of SOC. Overall, neither differences in above- and belowground detritus C inputs nor in detritus-derived DOC fluxes could explain the higher and more stable SOC pools under aspen. We hypothesize that root–microbe–soil interactions in the rhizosphere are more likely to drive these SOC pool differences.

Surveying the carbon pools of mountain soils: A comparison of physical fractionation and Rock-Eval pyrolysis

Accurate monitoring of soil organic carbon (SOC) pools with contrasted turnover times is necessary for better evaluation of the vulnerability of SOC stocks in the face of global changes. To obtain this information in heterogeneous landscapes such as mountain areas, fast and reliable approaches providing an assessment of SOC biogeochemical stability are needed. Recently, it has been shown that Rock-Eval (RE) pyrolysis enables a fast quantification of three SOC pools of different thermostabilities [Saenger, A., Cécillon, L., Sebag, D., & Brun, J.J. 2013. Soil organic carbon quantity, chemistry, and thermal stability in a mountainous landscape: A Rock-Eval pyrolysis survey. Organic Geochemistry 54, 101–114]. The aim of this study was to compare the RE pyrolysis method with the standardized and widely used technique of SOC physical fractionation for characterizing SOC pools at landscape-scale in a mountainous area of the calcareous French Alps. Physical fractionation revealed large quantities of particulate organic matter (POM, i.e. labile SOC) for all samples, suggesting an overall high vulnerability of mountain SOC stocks to environmental changes. Organic C stocks of mountain ridge topsoils (0–10cm) may be the most vulnerable with 76% consisting of POM, while alpine and subalpine grasslands developed on Cambisols presented more stabilized forms of SOC (64% of SOC stocks being associated with minerals, MaOM fraction). RE pyrolysis gave results similar to the physical fractionation method for the identification of stability patterns of the different ecological units and for estimating their dominant SOC pools (labile vs. stable). However, some discrepancies between the two techniques were observed for the absolute quantification of SOC pools, with slightly higher correlations between the physically and thermally derived stable SOC pool than for the labile pool (R2=0.59 and 0.50, respectively). These differences can be explained by the fact that each method has inherent biases and isolates somewhat different pools. Additionally, RE pyrolysis provided information about the chemistry of SOC pools, suggesting a greater maturity of SOC in the MaOM than in the POM fractions. Overall, this study confirms that the RE pyrolysis technique, being both time- and cost-effective, could become a standardized proxy for assessing SOC pools at landscape-scale although it requires further comparison with other methods for several soil and land-use types.

Reproducibility of a soil organic carbon fractionation method to derive RothC carbon pools

SummaryFractionation of soil is undertaken to isolate organic carbon with distinct functional properties, such as stability and turnover times. Soil organic carbon (SOC) fractionation helps us to understand better the response of SOC to changes in land use, management or climate. However, fractionation procedures are often poorly defined and there is little information available on their reproducibility in different laboratories. In a ring trial, we assessed the reproducibility of a SOC fractionation method introduced by Zimmermann et al. (2007). The isolated fractions were linked to the model pool sizes of the Rothamsted carbon model (RothC). We found significant differences between six laboratories for all five defined fractions in three different soils with coefficients of variation ranging from 14 to 138%. During ultrasonic dispersion, the output power (energy per unit time) was identified as an important factor controlling the distribution of SOC within these five fractions, while commonly only the output energy is standardized. The amount of water used to wet‐sieve dispersed soil slurry significantly influenced the amount of extracted dissolved organic carbon (DOC). We therefore suggest using a fixed amount of power for ultrasonic dispersion (20 W) and a minimum amount of water for wet sieving (2000 ml). RothC pool sizes were predicted from the measured fractions and compared with RothC equilibrium pool size distributions. This model initialization using measured SOC fractions, however, led to an over‐estimation of stable RothC SOC pools when compared with pool size distributions derived from RothC equilibrium runs under a bare fallow soil model simulation. To improve the isolation of particulate organic matter from stable mineral‐bound organic matter, we suggest that the density should be increased from 1.8 to 2.0 g cm−3 in the density fractionation step. We formulated a modified fractionation procedure, which aims specifically to enhance reproducibility across laboratories and to improve the match of the isolated SOC fractions with RothC's SOC pools.

Mean Residence Time of Soil Organic Carbon in Aggregates Under Contrasting Land Uses Based on Radiocarbon Measurements

Radiocarbon is a useful tool for studying carbon dynamics in soil aggregates. The objective of the current study was to determine the mean residence time (MRT) of soil organic carbon (SOC) in macroaggregates and microaggregates under contrasting land uses. Contrasting land uses investigated at Alfisol (equivalent to Dermosol in Australian Soil Classification) sites were native pasture (NP), crop-pasture rotation (CP), and Eucalypt woodland (WL), whereas in Oxisol (Ferrosol in Australian Soil Classification) sites, land uses comprised improved pasture (IP), cropping (CR), and forest (FR). Soil aggregates were separated into macroaggregates (250–2000 μm) and microaggregates (53–250 μm) by wet-sieving, and their14C signatures were determined by accelerator mass spectrometry (AMS). The14C activity in both macro- and microaggregates was >100 pMC, indicating the presence of post-bomb carbon in the soil. The mean residence time (MRT) of SOC in macro- and microaggregates (MRTagg) was on average 68 yr longer in the Oxisol compared with that in the Alfisol. The MRTaggin microaggregates was 10 yr longer than that of macroaggregates in the Alfisol. However, the MRTaggin microaggregates was 50 yr shorter compared to macroaggregates in the Oxisol.The MRT of macro- and microaggregates can be separated into active, slow, and stable SOC pools. Among the 3 SOC pools, the MRT of the stable pool is of higher significance in terms of SOC stabilization in soil aggregates because of its longer MRT. However, isolation and direct MRT estimation of the stable SOC pool is difficult. The MRT of active and slow SOC pools associated with macro- and microaggregates was measured using a SOC mineralization experiment to estimate the MRT of the stable SOC pool under contrasting land uses by applying a mass balance criterion. The MRT of active (MRTA) and slow (MRTS) SOC pools in macro- and microaggregates varied between 1–50 days and 13–38 yr, respectively. The estimated MRT of the stable pool carbon (MRTP) in microaggregates was 897 yr longer compared to that of macroaggregates in the Alfisol. However, in the Oxisol, MRTPin microaggregates was 568 yr shorter than that of macroaggregates. Among the land uses, WL in Alfisol and CR in Oxisol had longer MRTaggand MRTPcompared to other land uses.

Mean Residence Time of Soil Organic Carbon in Aggregates Under Contrasting Land Uses Based on Radiocarbon Measurements

Radiocarbon is a useful tool for studying carbon dynamics in soil aggregates. The objective of the current study was to determine the mean residence time (MRT) of soil organic carbon (SOC) in macroaggregates and microaggregates under contrasting land uses. Contrasting land uses investigated at Alfisol (equivalent to Dermosol in Australian Soil Classification) sites were native pasture (NP), crop-pasture rotation (CP), and Eucalypt woodland (WL), whereas in Oxisol (Ferrosol in Australian Soil Classification) sites, land uses comprised improved pasture (IP), cropping (CR), and forest (FR). Soil aggregates were separated into macroaggregates (250–2000 μm) and microaggregates (53–250 μm) by wet-sieving, and their 14C signatures were determined by accelerator mass spectrometry (AMS). The 14C activity in both macro- and microaggregates was >100 pMC, indicating the presence of post-bomb carbon in the soil. The mean residence time (MRT) of SOC in macro- and microaggregates (MRTagg) was on average 68 yr longer in the Oxisol compared with that in the Alfisol. The MRTagg in microaggregates was 10 yr longer than that of macroaggregates in the Alfisol. However, the MRTagg in microaggregates was 50 yr shorter compared to macroaggregates in the Oxisol.The MRT of macro- and microaggregates can be separated into active, slow, and stable SOC pools. Among the 3 SOC pools, the MRT of the stable pool is of higher significance in terms of SOC stabilization in soil aggregates because of its longer MRT. However, isolation and direct MRT estimation of the stable SOC pool is difficult. The MRT of active and slow SOC pools associated with macro- and microaggregates was measured using a SOC mineralization experiment to estimate the MRT of the stable SOC pool under contrasting land uses by applying a mass balance criterion. The MRT of active (MRTA) and slow (MRTS) SOC pools in macro- and microaggregates varied between 1–50 days and 13–38 yr, respectively. The estimated MRT of the stable pool carbon (MRTP) in microaggregates was 897 yr longer compared to that of macroaggregates in the Alfisol. However, in the Oxisol, MRTP in microaggregates was 568 yr shorter than that of macroaggregates. Among the land uses, WL in Alfisol and CR in Oxisol had longer MRTagg and MRTP compared to other land uses.

Species-specific effects of native trees on soil organic carbon in biodiverse plantings across north-central Victoria, Australia

Plantings of diverse native tree species aim to reverse negative effects of widespread vegetation clearing in temperate Australia. Studies of the environmental outcomes of these biodiverse plantings currently provide little information on carbon sequestration, which could limit their integration into emerging carbon markets. This paper presents soil organic carbon (SOC) data from four biodiverse native plantings at age 9 to 17 years across north-central Victoria in south-eastern Australia. Effects of site and tree species on SOC (0–10 cm depth) were examined using soils from under four species common to all plantings ( Acacia implexa, Acacia mearnsii, Allocasuarina verticillata and Eucalyptus melliodora) that were analysed for aggregate stability, and for SOC and δ 13C in whole soil and in aggregate fractions. Tree establishment had no effect on soil aggregate stability despite significant effects on SOC. Weak associations between aggregation and SOC concentrations were presumably partly due to the clay mineralogy of the study soils, in which aggregate formation is more dependent on electrostatic bridging between 1:1 clay particles than on organic matter. This influence of clay mineralogy might also partly explain poor correlations between SOC and clay content across sites. Contrary to findings elsewhere of decreases in SOC to about 30 years after afforestation, effects of tree establishment on SOC in this study ranged from negligible to significant increases. Effects were species-specific and were largely limited to soils under A. mearnsii, which contained significantly more SOC in whole soil and in the macroaggregates (2000–250 μm) than non-planted soils. Mean rates of SOC accumulation in whole soils were 0.8 to 1.6 Mg C ha − 1 year − 1 greater under the N-fixing A. mearnsii than the non-N-fixing E. melliodora. At two of the sites, greater contribution of A. mearnsii than other tree species to new SOC was indicated by δ 13C soil and litter data. δ 13C data also pointed to greater responsiveness to tree establishment of SOC pools in mega- (8000–2000 μm) and macroaggregates than finer-sized fractions (microaggregate 250–53 μm; silt and clay < 53 μm). Negligible changes in the more stable SOC pools presumably contained within these finer-sized fractions suggested little impact of tree establishment on long-term SOC sequestration to ages 9 to 17 years. Continued absence of disturbance from these and similar biodiverse plantings should ensure slow macroaggregate turnover and more secure contributions to soil C sequestration as the trees age. Given that greatest SOC effects in this study were under a species of relatively short life span ( A. mearnsii, ~ 10–20 years), soil C sequestration outcomes from biodiverse plantings might also be enhanced by the inclusion of a suite of N-fixing trees that encompass a range of life expectancies to ensure sustained SOC accumulation over many years.

SOC Turnover and Lime-CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Evolution during Liming of an Acid Andisol and Ultisol

Agricultural liming contributes significantly to atmospheric CO2 emission from soils but data on magnitude of lime- contributed CO2 in a wide range of acid soils are still few. Data on lime-contributed CO2 and SOC turnover for global acid soils are needed to estimate the potential contribution of agricultural liming to atmospheric CO2. Using Ca13CO3 (13C 99%) as lime and tracer, here we separated lime-contributed and SOC-originated CO2 evolution in an acidic Kuroboku Andisol from Tanashi, Tokyo Prefecture (35°44′ N, 139°32′ E) and Kunigami Mahji Ultisol of Nakijin, Okinawa Prefecture, Japan (26°38′ N, 127°58′ E). On the average, lime-CO2 was 76.84% (Kuroboku Andisol) and 66.36% (Kunigami Mahji Ultisol) of overall CO2 emission after 36 days. There was increased SOC turnover in all limed soils, confirming priming effect (PE) of liming. The calculated PE of lime (Kuroboku Andisol, 51.97% - 114.95%; Kunigami Mahji Ultisol, 10.13% - 35.61%) was entirely 12C turnover of stable soil organic carbon (SOC) since SMBC, a labile SOC pool, was suppressed by liming in our experiment. Our results confirmed that mineralization of lime-carbonates is the major source of CO2 emission from acid soils during agricultural liming. Liming can influence the size of CO2 evolution from agricultural ecosystems considering global extent of acid soils and current volume of lime utilization. We propose the inclusion of liming in simulating carbon dynamics in agricultural ecosystems.

Integrating physical and chemical methods for isolating stable soil organic carbon

A better understanding of the effects of climate and management on long-term soil organic carbon (SOC) sequestration necessitates separation of the stable from the labile SOC fraction. An array of physical and chemical methods is currently used for the separation of SOC fractions, but these methods have not been subjected to a comparative evaluation to understand their effectiveness. Therefore, the present study was conducted with the objective to evaluate the pool size and 13C isotopic signatures of stable SOC fractions derived from physical and chemical approaches applied separately and in combination on a Typic Fragiudalf of Ohio, USA. Five methods were evaluated: recovery of silt-associated SOC by physical method (method 1a), clay-associated SOC by physical method (method 1b), bulk soil-associated SOC by chemical method (method 2), chemically stable silt-associated SOC by combined physical and chemical methods (method 3a), and chemically stable clay-associated SOC by combined physical and chemical methods (method 3b). The corn-derived fraction of the stable SOC pool was calculated using 13C natural abundance technique because there was a clear vegetation shift from C 3 (forest) to C 4 species (corn, Zea mays L.) at the study site. Across land use and soil depth increments, the pool sizes of stable SOC isolated by chemical treatment, individually and in combination with physical method (methods 2, 3a, and 3b), were substantially lower (1.5% to 29.4% of total SOC) than that of standalone physical method (methods 1a and 1b) (17.3% to 64.8% of total SOC). However, among the chemical methods (method 2, 3a and 3b), the pool size of chemically resistant SOC was generally comparable. In the surface layer, the proportion of young, corn-derived SOC was lower after chemical treatment than after individual physical method. But, the chemically stabilized SOC in subsoil showed an enrichment of 13C signatures and increased proportion of C 4–C than that of surface soil, and it is mostly similar to that derived from physical methods. The data obtained from this study indicate that integrating physical and chemical methods is effective in isolating the SOC fraction with greatest stability in the surface soil layer, but its effectiveness to separating the stable SOC fractions in the subsoil was comparable to individual physical and chemical methods.

Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO 2

Elevated atmospheric CO 2 tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO 2 concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO 2. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO 2 plots at 0–10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO 2 stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO 2 increased net N mineralization in the 10–30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO 2. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO 2 studies. Increased mineralization of stable soil organic carbon by a CO 2-induced priming effect may diminish the terrestrial carbon sink globally.

Modeling consequences of straw residues export on soil organic carbon

Cereal straw, which is most often returned to the soil in arable cropping systems, is of renewed interest as a potential source of bioenergy. However, the sustainability of this practice which implies systematic removal of aerial biomass of cereal crops is a controversial issue, particularly in soils having a low soil organic carbon (SOC) content. This study aims at evaluating a simple model (AMG) to predict the consequences of straw export on SOC evolution in various cropping and pedoclimatic conditions. The model was tested on nine long-term field experiments (18–35 yr) dominated by cereal crops and differing in climate, soil type and carbon inputs. The model was able to provide satisfactory simulations of the evolution of SOC in most experiments with a unique set of parameters. The sensitivity analysis indicated that the quality of fit was very sensitive to humification coefficient, moderately sensitive to the size of the stable SOC pool and weakly affected by the ratio of belowground: aerial C input. The dependence of model parameters (humification and mineralization rates) on pedoclimatic conditions (soil clay content and temperature) was analyzed and compared to those proposed in other models (DAISY, CENTURY, ROTHC, CN-SIM) since they vary widely between models. AMG functions provided the best fit in seven out of nine experiments. More generally, the best fit was obtained by assuming that clay content had a small or no effect on humification coefficient and a marked effect on mineralization rate, in accordance with incubation studies in literature. The AMG model was used to simulate the impact of a straw export scenario in nine experiments considering a systematic straw removal one year out of two. With this scenario, straw removal vs. incorporation would reduce carbon stocks by 2.5–10.9% of the initial SOC after 50 yr, depending primarily on the experiment (soil, climate, productivity) and secondarily on the size of the stable C pool (varying from 10% to 65%).

Comparison of chemical fractionation methods for isolating stable soil organic carbon pools

SummaryStable soil organic matter (SOM) is important for long‐term sequestration of soil organic carbon (SOC), but the usefulness of different fractionation methods to isolate stable SOM is open to question. We assessed the suitability of five chemical fractionation methods (stepwise hydrolysis, treatment with H2O2, Na2S2O8, NaOCl, and demineralization of the NaOCl‐resistant fraction (NaOCl + HF)) to isolate stable SOM from soil samples of a loamy sand and a silty loam under different land use regimes (grassland, forest and arable crops). The apparent C turnover time and mean age of SOC before and after fractionation was determined by 13C and 14C analysis. Particulate organic matter was removed by density fractionation before soils were exposed to chemical fractionation. All chemical treatments induced large SOC losses of 62–95% of the mineral‐associated SOC fraction. The amounts of H2O2‐ and Na2S2O8‐resistant SOC were independent from land use, while those of NaOCl‐ (NaOCl + HF)‐ and hydrolysis‐resistant SOC were not. All chemical treatments caused a preferential removal of young, maize‐derived SOC, with Na2S2O8 and H2O2 being most efficient. The mean 14C age of SOC was 1000–10000 years greater after chemical fractionation than that of the initial, mineral‐associated SOC and mean 14C ages increased in the order: NaOCl &lt; NaOCl + HF ≤ stepwise hydrolysis ≪ H2O2≈ Na2S2O8. None of the methods appeared generally suitable for the determination of the inert organic matter pool of the Rothamsted Carbon Model. Nonetheless, our results indicate that all methods are able to isolate an older, more stable SOC fraction, but treatments with H2O2 and Na2S2O8 were the most efficient ones in isolating stable SOM.

Soil Organic Carbon Pools of Two Tropical Soils Inferred by Carbon Signatures

AbstractLabile and stable soil organic carbon (SOC) pools of two tropical soils of Belize, Central America, as influenced by land use management, were investigated using a combination of 14C and 13C techniques. The sizes and mean residence times (MRT) of the labile pools were found to be in the range of 1.5 to 59.8 g/kg and 5 to 8 yr, respectively, and those of the stable SOC pools, 25.1 to 65.2 g/kg and 250 to 388 yr, respectively. Both the labile and stable SOC pools have significantly shorter MRT than those of the temperate soils. The labile SOC pools of sugarcane (Saccharum officinarum L.) in continuous management averaged 12.9 and 19.8 g/kg, respectively, for Xaibe (Lithic Ustropept) and Louisville (Typic Pellustert) soils. Continuous sugarcane management reduced the labile pool to about one‐third the size of the corresponding secondary forests. The labile SOC pools is sensitive to changes in management practices and its size represents the intrinsic sustainable soil productivity. A radiocarbon single‐plot approach to study the dynamics of SOC pools was developed using the 14C and 13C signatures of the soil and those of the CO2 evolved. The results of the single‐plot method and that of the paired‐plot method agree well within the range of the experimental errors.

Pool Size and Mean Age of Stable Soil Organic Carbon in Cropland

AbstractMost modern surface soils are believed to contain a pool of very stable soil organic carbon (SOC) with a mean age (MA) that may reach several thousand years. The remaining SOC pools are much younger, perhaps no older than a few decades. The SOC can thus be divided into two major classes: very old (stable pool) and relatively young (active pool). Because active and stable SOC play very different roles in soil organic‐matter dynamics and nutrient cycling, it is desirable to know the pool size and turnover time of the two fractions in an ecosystem. Unfortunately, none of the physical or chemical fractionation methods can satisfactorily separate active from stable SOC. The pool size and MA of the stable SOC pool in the surface soil of the Morrow Plots (University of Illinois) and Sanborn Field (University of Missouri) were determined using the concept of management‐induced differential decomposition of active SOC in a pair of soil samples and the principle of radiocarbon dating. Pool size and MA of the stable SOC in the surface soil (0–20 cm) was estimated at 11.0 g/kg and 2973 yr for the Morrow Plots and 7.3 g/kg and 853 yr for Sanborn Field, respectively.