SOC Decomposition Research Articles

ORCHIMIC (v1.0), a microbe-mediated model for soil organic matter decomposition

Abstract. The role of soil microorganisms in regulating soil organic matter (SOM) decomposition is of primary importance in the carbon cycle, in particular in the context of global change. Modeling soil microbial community dynamics to simulate its impact on soil gaseous carbon (C) emissions and nitrogen (N) mineralization at large spatial scales is a recent research field with the potential to improve predictions of SOM responses to global climate change. In this study we present a SOM model called ORCHIMIC, which utilizes input data that are consistent with those of global vegetation models. ORCHIMIC simulates the decomposition of SOM by explicitly accounting for enzyme production and distinguishing three different microbial functional groups: fresh organic matter (FOM) specialists, SOM specialists, and generalists, while also implicitly accounting for microbes that do not produce extracellular enzymes, i.e., cheaters. ORCHIMIC and two other organic matter decomposition models, CENTURY (based on first-order kinetics and representative of the structure of most current global soil carbon models) and PRIM (with FOM accelerating the decomposition rate of SOM), were calibrated to reproduce the observed respiration fluxes of FOM and SOM from the incubation experiments of Blagodatskaya et al. (2014). Among the three models, ORCHIMIC was the only one that effectively captured both the temporal dynamics of the respiratory fluxes and the magnitude of the priming effect observed during the incubation experiment. ORCHIMIC also effectively reproduced the temporal dynamics of microbial biomass. We then applied different idealized changes to the model input data, i.e., a 5 K stepwise increase of temperature and/or a doubling of plant litter inputs. Under 5 K warming conditions, ORCHIMIC predicted a 0.002 K−1 decrease in the C use efficiency (defined as the ratio of C allocated to microbial growth to the sum of C allocated to growth and respiration) and a 3 % loss of SOC. Under the double litter input scenario, ORCHIMIC predicted a doubling of microbial biomass, while SOC stock increased by less than 1 % due to the priming effect. This limited increase in SOC stock contrasted with the proportional increase in SOC stock as modeled by the conventional SOC decomposition model (CENTURY), which can not reproduce the priming effect. If temperature increased by 5 K and litter input was doubled, ORCHIMIC predicted almost the same loss of SOC as when only temperature was increased. These tests suggest that the responses of SOC stock to warming and increasing input may differ considerably from those simulated by conventional SOC decomposition models when microbial dynamics are included. The next step is to incorporate the ORCHIMIC model into a global vegetation model to perform simulations for representative sites and future scenarios.

Stronger ecosystem carbon sequestration potential of mangrove wetlands with respect to terrestrial forests in subtropical China

Mangrove wetlands and terrestrial forests are considered as important carbon sinks for alleviating climate changes, but the sequestration processes and regulations of climate factors on controlling the variability of carbon fluxes of these ecosystems may differ. In order to compare the different mechanisms of carbon sequestration in mangrove and terrestrial forest ecosystems, we analyzed ecosystem CO2 flux data measured by eddy covariance (EC) technique from four forests ecosystems in subtropical China: two mangrove wetlands and two terrestrial forests. Our results showed that the mangrove wetlands could sequester much more carbon than the nearby terrestrial forests because of significantly higher gross ecosystem production (GEP) and lower ecosystem respiration (Re) values. Moreover, our analysis of the responses of net ecosystem exchange (NEE) to photosynthetically active radiation showed that the mangrove wetlands had lower light compensation point but higher maximum photosynthesis rates than the terrestrial forests. Furthermore, the relationships between Re and air temperature (Tair) showed the ecosystem respiration rate (Reref) at 20°C values were lower but that the temperature sensitivity (Q10) values were higher in the mangrove wetlands than in the terrestrial forests, which might be caused by tides in the mangrove ecosystems. In addition, the relationships between the logarithmic values of soil organic carbon ln(SOC) and δ13C indicated that SOC decomposition rates were lower in mangrove forests than in terrestrial forests, which thus led to lower Re values compared to terrestrial forests. Our results imply that mangrove forests can sequester more CO2 from the atmosphere than nearby terrestrial forests due to relatively higher GEP and lower Re values. Moreover, the regulation of ecosystem carbon exchange by tides in mangrove wetlands should be investigated in more detail in future studies.

Large uncertainty in permafrost carbon stocks due to hillslope soil deposits

AbstractNorthern circumpolar permafrost soils contain more than a third of the global soil organic carbon pool (SOC). The sensitivity of this carbon pool to a changing climate is a primary source of uncertainty in simulation‐based climate projections. These projections, however, do not account for the accumulation of soil deposits at the base of hillslopes (hill toes) and the influence of this accumulation on the distribution, sequestration, and decomposition of SOC in landscapes affected by permafrost. Here we combine topographic models with soil profile data and topographic analysis to evaluate the quantity and uncertainty of SOC mass stored in perennially frozen hill toe soil deposits. We show that in Alaska this SOC mass introduces an uncertainty that is >200% the state‐wide estimates of SOC stocks (77 Pg C) and that a similarly large uncertainty may also pertain at a circumpolar scale. Soil sampling and geophysical imaging efforts that target hill toe deposits can help constrain this large uncertainty.

Priming effects in biochar enriched soils using a three-source-partitioning approach: 14C labelling and 13C natural abundance

The changes to soil properties due to biochar addition may affect both the direction and magnitude of priming effects. However, the mechanisms involved in biochar induced priming effects still remain largely unknown due to the limitation of methods to separate more than two carbon (C) sources (e.g. soil, biochar, substrate). We combined 14C labeling with 13C natural abundance to separate the total CO2 from i) native soil organic C (SOC, C3 signature), ii) added glucose (14C labelled) and iii) biochar (C4 signature). The primed soil CO2 emissions following a large addition of glucose (1000 mg glucose kg−1 soil) to one Chinese and one German Luvisol soil were much larger (140% and 53% respectively) in a soil recently amended with maize derived biochar (pyrolyzed at 400 °C), compared to non amended soil. Glucose addition at a lower rate (100 mg C kg−1 soil) produced no significant differences in priming effects of native soil organic matter between the biochar amended and non-amended soils. Glucose also caused priming of biochar decomposition, with an additional C4 biochar loss of between 270 μg CO2-C g−1 and 540 μg CO2-C g−1 depending on soils and glucose concentrations. Approaches using two stable isotopes (13C and12C) have previously been limited to partitioning two sources (biochar C and soil organic C). Here, for the first time, 14C labeling was combined with 13C natural abundance to partition three C sources in a biochar amended soil. By partitioning soil CO2 emissions derived from SOC, added biochar and glucose decomposition, this study provides a better understanding of the priming effects following addition of substrates to biochar amended soil, to approximate to the true complexity of biochar enriched soils.

Black carbon concentration and isotopic composition of surface sand from deserts and dune fields in Northern China

Black carbon (BC) is a continuum of incomplete combustion products of vegetation and fossil fuels. It can be used as a proxy for wildfires and paleovegetation reconstruction. Nevertheless, BC can be transported by wind and water via atmospheric and fluvial transport. To research the characteristics of BC in deserts and dune fields that are directly connected with the Chinese Loess Plateau (CLP) in Northern China, and to evaluate the possibility that BC in deserts and dune fields occurs as exogenous BC in the CLP, we investigated surface sand from the Mu Us dune field, the Tengger Desert, and the Badain Jaran Desert, and analyzed the stable carbon concentration and carbon isotopes of organic carbon (SOC%, δ13CSOC) and BC (BC%, δ13CBC). The δ13CSOC, δ13CBC values ranged from −27.4‰ to −22.5‰, and −28.2‰ to −23.0‰, respectively, which mainly reflected δ13C changes in surface C3 grasses. A positive correlation was observed between δ13CBC and δ13CSOC, suggesting that local vegetation mainly controls δ13CBC. The difference between δ13CBC and δ13CSOC ranged from −0.2‰ to +3.5‰, and the difference is likely to be due to carbon isotope discrimination occurring during burning and SOC decomposition. The SOC% and BC% in these deserts and dune fields are very small (averaging 0.048% and 0.011%). The BC% of surface sand in these areas is about 5.5% of BC% in the CLP surface soils, and the Δ-δ13СBC value between surface sand in deserts and surface soils in CLP ranged from −0.3‰ to −6.3‰, so the change of δ13СBC in the CLP surface soils, deduced by exogenous BC from deserts, is less than 0.35‰. All these findings suggest that BC in deserts has a limited impact on BC in the CLP and is unlikely to affect interpretations of the BC record of wildfires and paleovegetation information in the CLP.

Chemical composition of organic matter in a deep soil changed with a positive priming effect due to glucose addition as investigated by 13C NMR spectroscopy

Fresh organic carbon becomes more accessible to deep soil following losses of surface soil and deep intentional incorporation of crop residues, which can cause the priming effect and influence the quality and quantity of SOC in deep soil. This study determined the priming effect due to addition of water-dissolved 13C-labeled glucose (0.4 g C kg−1 soil) to a soil taken from 1.00 to 1.20 m depth. The changes in chemical compositions of SOC in soils without (G0) and with (G0.4) glucose addition during a 31-d incubation were investigated with solid-state 13C cross polarization/total sideband suppression (13C-CP/TOSS) and CP/TOSS with dipolar dephasing nuclear magnetic resonance (NMR) techniques. No glucose remained in the soil after 21 days of incubation, with 48% being completely mineralized into CO2 emission and 52% being incorporated into SOC. The native SOC was decomposed by 0.23% more in G0.4 than in G0. The NMR spectra demonstrated that both labile and recalcitrant organic compounds in SOC changed during the incubation, but in different manners in G0 and G0.4. During the incubation, the -(CH2)n-abundance in G0 did not change over time, but in G0.4 it decreased from Day 0 to Day 21 and then increased from Day 21 to Day 31, suggesting shifts of soil microbial communities only in G0.4. After the incubation, in G0 the abundances of ketones/aldehydes and nonpolar alkyl C increased, but those of aromatic C–C and protonated O-alkyl C (OCH) decreased; In G0.4, the abundances of NCH and protonated O-alkyl C (OCH) increased, but those of nonpolar alkyl C and nonprotonated aromatic C–O and ketones/aldehydes decreased. Such inconsistent changes in recalcitrant compounds between G0 and G0.4 indicated that glucose addition likely primed the decomposition of aromatic C–O and suppressed the formation of ketones/aldehydes. We have demonstrated for the first time that the priming effect of SOC decomposition in the deep soil was involved with larger notable changes in both labile and recalcitrant structures of native SOC due to glucose addition compared with that without glucose addition.

Dynamics of soil organic carbon pools after agricultural abandonment

Abandonment of agricultural land and the subsequent recolonization by natural vegetation is known to cause increases in C contents, contributing to reduction in atmospheric CO2 concentrations. Assessment of the possible mitigation of CO2 excess requires understanding the SOC dynamics, the origin of C pools and the pathways of their transformation. The aims of this work were to assess, by using the δ13C signature, the changes of old and new organic C in total (soil organic carbon, SOC) and labile (microbial biomass C, MBC, dissolved organic C, DOC, CO2 efflux from soil) pools after vegetation change from vineyard (C3) to grassland (C4) under semiarid Mediterranean climate. Colonization of abandoned vineyard by the perennial C4-grass Hyparrhenia hirta after 15 or 35years increased topsoil C stocks by 13% and 16%, respectively. Such an increase was attributed to new above- and below-ground biomass C input from H. hirta. The maximal incorporation of new C was observed in MBC, whereas the DOC derived mainly from old SOC. Based on δ13C values of SOC, MBC, DOC and CO2 in C3 soil and in soils after 15 and 35years of C4 grass colonization, 13C fractionation per se from changes in isotopic composition by preferential utilization of substrates with different availability was separated. MBC in C3–C4 soil used more recent (13C-enriched) versus old C (relatively 13C-depleted) sources. The Δ13C by decomposition of SOC to CO2 (δ13C of CO2 minus δ13C of SOC) was higher than Δ13C by microbial respiration (CO2 minus MBC), demonstrating that under semiarid climate, soil microorganisms do not always preferentially decompose the most available SOC pools. The use of δ13C signature of SOC after C3–C4 vegetation change combined with soil incubation is a powerful tool to assess the exchange between old and new C in pools of various availability.

Soil organic carbon dynamics within density and particle-size fractions of Aquic Cambisols under different land use in northern China

A long-term crop residue incorporation and tillage experiment was initiated in 1999 on cropland that was converted from natural grassland in 1974. Three management factors included in the experiment were land use, tillage systems (tillage/no-till) and methods of maize (Zea mays L.) straw incorporation (whole straw, and pulverized straw and no straw). Soil samples were obtained in 2010, and 13C natural abundance analysis was used to estimate the maize-derived C in density fractions (light fraction, LF; heavy fraction, HF), particle-size fractions (>50μm, 50–5μm, 5–2μm, 2–0.2μm and <0.2μm), and original SOC decomposition. There occurred a 14–23% decrease in the SOC concentration or a loss of 0.166–0.225MgCha−1yr−1 upon conversion of natural grassland to cropland. Organic carbon (OC) in the LF was a sensitive indicator reflecting this reduction in SOC concentration. Conversion to no-till did not significantly increase the SOC concentration, compared with tillage systems. The data on OC concentration in particle size fractions and those on maize-derived OC in clay and silt fractions indicate that the most stable OC was associated with the silt fraction, while OC bound to clay was important in the short to medium term. Incorporation of pulverized straw over 11years contributed 2 to 2.4Mg of maize-derived Cha−1 into the SOC stock, significantly higher than that by incorporation of whole straw. However, because the whole straw incorporation retained more of the antecedent SOC, it effectively increased the SOC stock. These data suggest that incorporation of whole straw combined with tillage could be an effective practice for improving the SOC stock in croplands of the North China Plain.

Implication of erosion on the assessment of decomposition and humification of soil organic carbon after land use change in tropical agricultural systems

Soil organic carbon decline after land use change from forest to maize usually leads to soil degradation and elevated CO2 emissions. However, limited knowledge is available on the interactions between rates of SOC change and soil erosion and how SOC dynamics vary with soil depth and clay contents. The 13C isotope based CIDE approach (Carbon Input, Decomposition and Erosion) was developed to determine SOC dynamics on erosion prone slopes. The aims of the present study were: (1) to test the applicability of the CIDE approach to determine rates of decomposition and SOC input under particular considerations of concurrent erosion events on three soil types (Alisol, Luvisol, Vertisol), (2) to adapt the CIDE approach to deeper soil layers (10–20 and 20–30 cm) and (3) to determine the variation of decomposition and SOC input with soil depth and soil texture. SOC dynamics were determined for bulk soil and physically separated SOC fractions along three chronosequences after land use change from forest to maize (up to 21 years). Consideration of the effects of soil erosion on SOC dynamics by the CIDE approach yielded a higher total SOC loss (6–32%), a lower decomposition (13–40%) and a lower SOC input (14–31%) relative to the values derived from a commonly applied 13C isotope based mass balance approach. Comparison of decomposition between depth layers revealed that tillage accelerated decomposition in the plough layer (0–10 cm), accounting for 3–34% of total decomposition. With increasing clay contents SOC input and mass of sand sized stable aggregates increased. In addition, decomposition increased with increasing clay contents, too, being attributed to decomposition of labile SOC which was attached to clay particles in the sand sized stable aggregate fraction. In conclusion, this study suggests that in situ SOC dynamics on erosion prone slopes are commonly misrepresented by erosion unadjusted approaches leading to an overestimation of SOC input and underestimation of SOC loss.

Environmental Controls on Fallow Carbon Dioxide Flux in a Single‐Crop Rice Paddy, Japan

AbstractSoil organic carbon (SOC) is one of the important measures of soil fertility and sustainability in arable lands. With continuous CO2 flux measurements, this study assessed the SOC decomposition and its environmental controls at both half‐hourly and season‐long scales in a single‐crop rice (Oryza sativa L.) paddy during three fallow periods between 2004 and 2007. Measurements were made on a gray lowland soil sited in eastern Japan using the eddy covariance method. Ecosystem respiration was strongly affected by soil water content measured at 0–0·1 m depth. At 0·5 m3 m− 3 or more of soil water content, the baseline of ecosystem respiration decreased by 50% compared with that at 0·2 m3 m− 3 . The effect was quantified at half‐hourly scale using an empirical multiple regression model, together with the soil surface temperature and the time after residue incorporation. At season‐long scale, net biome production, which is equivalent to the change in the SOC pool during the fallow period, was estimated from the flux and ancillary data at 150 g C m− 2 in 2004–2005, 70 g m− 2 in 2005–2006, and 270 g C m− 2 in 2006–2007. Apparently, as much as 46 to 79% of the soil organic matter incorporated (crop residues, ratoon, and stable manure) was decomposed during the fallow period. Precipitation, or associated soil water content, was important for the carbon balance of the field at season‐long scale because of its large interannual variability and relatively low permeability of the paddy soil. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Variation in root architecture among switchgrass cultivars impacts root decomposition rates

Roots regulate soil carbon (C) input, but fine root decomposition rates and root impacts on soil organic C turnover (SOC) are uncertain. This uncertainty is, partly, caused by the heterogeneity of root systems, which vary in diameter distributions and tissue chemistry. Here, we evaluated how root diameter distributions affect root and SOC decomposition. Roots from eight Panicum virgatum (switchgrass) cultivars were analyzed for root diameter size-class distribution and C:N ratio. Roots from each cultivar were mixed with C3 soil according to five root diameter treatments: (1) 0–0.5 mm, (2) 0.5–1 mm, (3) 1–2.5 mm, (4) a 1:1:1 mixture of roots from each diameter size class, and (5) a mixture combining diameter classes in proportions representing measured size distributions for each cultivar. All treatments were incubated for 90 days under laboratory conditions. Respired CO2 was measured throughout and the microbial community structure was measured at termination of the experiment. Carbon-13 isotope techniques were used to partition respiration into root-derived C versus native SOC-derived C. Results indicated: (1) specific root length differed among the cultivars, (2) root decomposition rates within the three size classes varied by cultivar, but were not correlated with cultivar differences in root C:N ratios, (3) root diameter size class affected root and SOC decomposition, and (4) mixing roots of different diameters did not lead to synergistic increases in decomposition. We conclude that intraspecific variation in root architecture is significant and that fine root diameter size class distribution is an important trait for shaping decomposition processes.

Assessment of soil organic carbon and total nitrogen under conservation management practices in the Central Claypan Region, Missouri, USA

Conservation management practices including upland vegetative filter strips (VFS) and no-till cultivation have the potential to enhance soil carbon sequestration and other ecosystem services in agroecosystems. A modified two-factor analysis of variance (ANOVA) with subsamples was used to compare SOC and TN on a concentration, soil volume and soil mass basis in claypan soils planted to different conservation management practices and as a function of landscape position. The three conservation management practices (no-till cultivation, grass VFS and agroforestry VFS) and four landscape positions (summit, shoulder, backslope and footslope) investigated were compared 10 years after VFS establishment in a no-till system planted to maize ( Zea mays. L.)–soybean ( Glycine max (L.) Merr.) rotation. Two soil depth increments (0–5 cm and 5–13 cm) were modeled separately to test for treatment effects. In the surface layer, mean SOC concentration was significantly greater in the VFS soils compared to no-till. On a soil volume or mass basis, no significant differences in SOC stocks were found among treatment means. Concentration and mass based TN values were significantly greater in the grass VFS relative to no-till in the surface layer. A rapid slaking stability test, developed to separate particulate, adsorbed and occluded organic carbon (PAO-C) and nitrogen (PAO-N), showed that VFS soils had significantly greater mean PAO-C and PAO-N concentrations, soil volume and soil mass based stocks than no-till. In addition, comparison of SOC:TN and PAO-C:PAO-N ratios suggest reduced decomposition and mineralization of SOC in the PAO fraction. No significant treatment effects were detected in total or PAO soil fractions in the subsurface layer or among landscape position in either depth increment. Study results emphasize the need to compare soil carbon and nitrogen stocks on a soil volume and/or soil mass basis using bulk density measurements. Additionally, the rapid PAO separation technique was found to be a good indicator of early changes in SOC and TN in the systems studied. Overall, this research indicates that grass VFS may sequester TN more rapidly than agroforestry VFS and that a greater proportion of SOC and TN may be stabilized in VFS soils compared to no-till.

Landscape-level variation in temperature sensitivity of soil organic carbon decomposition

We examined landscape-level variation in temperature sensitivity of labile SOC across 71 sites at a central North American grassland. The observed range in activation energy of decomposition ( E a), an index of temperature sensitivity, was as great at the landscape scale as has been observed at the continental scale. E a was lower for soils with more labile C, consistent with the ‘Carbon quality-temperature’ hypothesis. Soil pH explained 67% of the variation in E a. Although there are strong environmental correlates with the E a of SOC decomposition at landscape scales, the amount of variation within landscapes could confound regional- to global-scale predictions of the response of soil C to warming.

Biogeochemical factors contributing to enhanced carbon storage following afforestation of a semi-arid shrubland

Abstract. Ecosystems in dry regions are generally low in productivity and carbon (C) storage. We report, however, large increases in C sequestration following afforestation of a semi-arid shrubland with Pinus halepensis trees. Using C and nitrogen (N) inventories, based in part on site-specific allometric equations, we measured an increase in the standing ecosystem C stock from 2380 g C m−2 in the shrubland to 5840 g C m−2 in the forest after 35 years, with no significant change in N stocks. Carbon sequestration following afforestation was associated with increased N use efficiency as reflected by an overall increase in C/N ratio from 7.6 in the shrubland to 16.6 in the forest. The C accumulation rate in the forest was particularly high for soil organic C (SOC; increase of 1760 g C m−2 or 50 g C m−2 yr−1), which was associated with the following factors: 1) Analysis of a small 13C signal within this pure C3 system combined with size fractionation of soil organic matter indicated a significant addition of new SOC derived from forest vegetation (68% of total forest SOC) and a considerable portion of the old original shrubland SOC (53%) still remaining in the forest. 2) A large part of both new and old SOC appeared to be protected from decomposition as about 60% of SOC under both land-use types were in mineral-associated fractions. 3) A short-term decomposition study indicated decreased decomposition of lower-quality litter and SOC in the forest, based on reduced decay rates of up to 90% for forest compared to shrubland litter. 4) Forest soil included a significant component of live and dead roots (12% of total SOC). Our results suggest a role for increased N use efficiency, enhanced SOC protection and reduced decomposition rates in the large C sequestration potential following afforestation in semi-arid regions. These results are particularly relevant in light of persistent predictions of drying trends in the Mediterranean and other regions.

The potential impact of climate change on Australia's soil organic carbon resources.

BackgroundSoil organic carbon (SOC) represents a significant pool of carbon within the biosphere. Climatic shifts in temperature and precipitation have a major influence on the decomposition and amount of SOC stored within an ecosystem and that released into the atmosphere. We have linked net primary production (NPP) algorithms, which include the impact of enhanced atmospheric CO2 on plant growth, to the SOCRATES terrestrial carbon model to estimate changes in SOC for the Australia continent between the years 1990 and 2100 in response to climate changes generated by the CSIRO Mark 2 Global Circulation Model (GCM).ResultsWe estimate organic carbon storage in the topsoil (0–10 cm) of the Australian continent in 1990 to be 8.1 Gt. This equates to 19 and 34 Gt in the top 30 and 100 cm of soil, respectively. By the year 2100, under a low emissions scenario, topsoil organic carbon stores of the continent will have increased by 0.6% (49 Mt C). Under a high emissions scenario, the Australian continent becomes a source of CO2 with a net reduction of 6.4% (518 Mt) in topsoil carbon, when compared to no climate change. This is partially offset by the predicted increase in NPP of 20.3%ConclusionClimate change impacts must be studied holistically, requiring integration of climate, plant, ecosystem and soil sciences. The SOCRATES terrestrial carbon cycling model provides realistic estimates of changes in SOC storage in response to climate change over the next century, and confirms the need for greater consideration of soils in assessing the full impact of climate change and the development of quantifiable mitigation strategies.