Microbial models with data‐driven parameters predict stronger soil carbon responses to climate change

Long‐term carbon (C) cycle feedbacks to climate depend on the future dynamics of soil organic carbon (SOC). Current models show low predictive accuracy at simulating contemporary SOC pools, which can be improved through parameter estimation. However, major uncertainty remains in global soil responses to climate change, particularly uncertainty in how the activity of soil microbial communities will respond. To date, the role of microbes in SOC dynamics has been implicitly described by decay rate constants in most conventional global carbon cycle models. Explicitly including microbial biomass dynamics into C cycle model formulations has shown potential to improve model predictive performance when assessed against global SOC databases. This study aimed to data‐constrained parameters of two soil microbial models, evaluate the improvements in performance of those calibrated models in predicting contemporary carbon stocks, and compare the SOC responses to climate change and their uncertainties between microbial and conventional models. Microbial models with calibrated parameters explained 51% of variability in the observed total SOC, whereas a calibrated conventional model explained 41%. The microbial models, when forced with climate and soil carbon input predictions from the 5th Coupled Model Intercomparison Project (CMIP5), produced stronger soil C responses to 95 years of climate change than any of the 11 CMIP5 models. The calibrated microbial models predicted between 8% (2‐pool model) and 11% (4‐pool model) soil C losses compared with CMIP5 model projections which ranged from a 7% loss to a 22.6% gain. Lastly, we observed unrealistic oscillatory SOC dynamics in the 2‐pool microbial model. The 4‐pool model also produced oscillations, but they were less prominent and could be avoided, depending on the parameter values.     

Evaluating litter decomposition in earth system models with long‐term litterbag experiments: an example using the Community Land Model version 4 (CLM4)

Decomposition is a large term in the global carbon budget, but models of the earth system that simulate carbon cycle‐climate feedbacks are largely untested with respect to litter decomposition. We tested the litter decomposition parameterization of the community land model version 4 (CLM4), the terrestrial component of the community earth system model, with data from the long‐term intersite decomposition experiment team (LIDET). The LIDET dataset is a 10‐year study of litter decomposition at multiple sites across North America and Central America. We performed 10‐year litter decomposition simulations comparable with LIDET for 9 litter types and 20 sites in tundra, grassland, and boreal, conifer, deciduous, and tropical forest biomes using the LIDET‐provided climatic decomposition index to constrain temperature and moisture effects on decomposition. We performed additional simulations with DAYCENT, a version of the CENTURY model, to ask how well an established ecosystem model matches the observations. The results show large discrepancy between the laboratory microcosm studies used to parameterize the CLM4 litter decomposition and the LIDET field study. Simulated carbon loss is more rapid than the observations across all sites, and nitrogen immobilization is biased high. Closer agreement with the observations requires much lower decomposition rates, obtained with the assumption that soil mineral nitrogen severely limits decomposition. DAYCENT better replicates the observations, for both carbon mass remaining and nitrogen, independent of nitrogen limitation. CLM4 has low soil carbon in global earth system simulations. These results suggest that this bias arises, in part, from too rapid litter decomposition. More broadly, the terrestrial biogeochemistry of earth system models must be critically tested with observations, and the consequences of particular model choices must be documented. Long‐term litter decomposition experiments such as LIDET provide a real‐world process‐oriented benchmark to evaluate models.     

Increasing microbial carbon use efficiency with warming predicts soil heterotrophic respiration globally

The degree to which climate warming will stimulate soil organic carbon (SOC) losses via heterotrophic respiration remains uncertain, in part because different or even opposite microbial physiology and temperature relationships have been proposed in SOC models. We incorporated competing microbial carbon use efficiency (CUE)–mean annual temperature (MAT) and enzyme kinetic–MAT relationships into SOC models, and compared the simulated mass‐specific soil heterotrophic respiration rates with multiple published datasets of measured respiration. The measured data included 110 dryland soils globally distributed and two continental to global‐scale cross‐biome datasets. Model–data comparisons suggested that a positive CUE–MAT relationship best predicts the measured mass‐specific soil heterotrophic respiration rates in soils distributed globally. These results are robust when considering models of increasing complexity and competing mechanisms driving soil heterotrophic respiration–MAT relationships (e.g., carbon substrate availability). Our findings suggest that a warmer climate selects for microbial communities with higher CUE, as opposed to the often hypothesized reductions in CUE by warming based on soil laboratory assays. Our results help to build the impetus for, and confidence in, including microbial mechanisms in soil biogeochemical models used to forecast changes in global soil carbon stocks in response to warming.     

Carbon cycle confidence and uncertainty: Exploring variation among soil biogeochemical models

Emerging insights into factors responsible for soil organic matter stabilization and decomposition are being applied in a variety of contexts, but new tools are needed to facilitate the understanding, evaluation, and improvement of soil biogeochemical theory and models at regional to global scales. To isolate the effects of model structural uncertainty on the global distribution of soil carbon stocks and turnover times we developed a soil biogeochemical testbed that forces three different soil models with consistent climate and plant productivity inputs. The models tested here include a first‐order, microbial implicit approach (CASA‐CNP), and two recently developed microbially explicit models that can be run at global scales (MIMICS and CORPSE). When forced with common environmental drivers, the soil models generated similar estimates of initial soil carbon stocks (roughly 1,400 Pg C globally, 0–100 cm), but each model shows a different functional relationship between mean annual temperature and inferred turnover times. Subsequently, the models made divergent projections about the fate of these soil carbon stocks over the 20^th century, with models either gaining or losing over 20 Pg C globally between 1901 and 2010. Single‐forcing experiments with changed inputs, temperature, and moisture suggest that uncertainty associated with freeze‐thaw processes as well as soil textural effects on soil carbon stabilization were larger than direct temperature uncertainties among models. Finally, the models generated distinct projections about the timing and magnitude of seasonal heterotrophic respiration rates, again reflecting structural uncertainties that were related to environmental sensitivities and assumptions about physicochemical stabilization of soil organic matter. By providing a computationally tractable and numerically consistent framework to evaluate models we aim to better understand uncertainties among models and generate insights about factors regulating the turnover of soil organic matter.     

Effects of plant diversity on soil carbon in diverse ecosystems: a global meta‐analysis

Soil organic carbon (SOC) is a valuable resource for mediating global climate change and securing food production. Despite an alarming rate of global plant diversity loss, uncertainties concerning the effects of plant diversity on SOC remain, because plant diversity not only stimulates litter inputs via increased productivity, thus enhancing SOC, but also stimulates microbial respiration, thus reducing SOC. By analysing 1001 paired observations of plant mixtures and corresponding monocultures from 121 publications, we show that both SOC content and stock are on average 5 and 8% higher in species mixtures than in monocultures. These positive mixture effects increase over time and are more pronounced in deeper soils. Microbial biomass carbon, an indicator of SOC release and formation, also increases, but the proportion of microbial biomass carbon in SOC is lower in mixtures. Moreover, these species‐mixture effects are consistent across forest, grassland, and cropland systems and are independent of background climates. Our results indicate that converting 50% of global forests from mixtures to monocultures would release an average of 2.70 Pg C from soil annually over a period of 20 years: about 30% of global annual fossil‐fuel emissions. Our study highlights the importance of plant diversity preservation for the maintenance of soil carbon sequestration in discussions of global climate change policy.     

Simulating effects of changing climate and CO2 emissions on soil carbon pools at the Hubbard Brook experimental forest

Carbon (C) sequestration in forest biomass and soils may help decrease regional C footprints and mitigate future climate change. The efficacy of these practices must be verified by monitoring and by approved calculation methods (i.e., models) to be credible in C markets. Two widely used soil organic matter models – CENTURY and RothC – were used to project changes in SOC pools after clear‐cutting disturbance, as well as under a range of future climate and atmospheric carbon dioxide (CO₂) scenarios. Data from the temperate, predominantly deciduous Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA, were used to parameterize and validate the models. Clear‐cutting simulations demonstrated that both models can effectively simulate soil C dynamics in the northern hardwood forest when adequately parameterized. The minimum postharvest SOC predicted by RothC occurred in postharvest year 14 and was within 1.5% of the observed minimum, which occurred in year 8. CENTURY predicted the postharvest minimum SOC to occur in year 45, at a value 6.9% greater than the observed minimum; the slow response of both models to disturbance suggests that they may overestimate the time required to reach new steady‐state conditions. Four climate change scenarios were used to simulate future changes in SOC pools. Climate‐change simulations predicted increases in SOC by as much as 7% at the end of this century, partially offsetting future CO₂ emissions. This sequestration was the product of enhanced forest productivity, and associated litter input to the soil, due to increased temperature, precipitation and CO₂. The simulations also suggested that considerable losses of SOC (8–30%) could occur if forest vegetation at HBEF does not respond to changes in climate and CO₂ levels. Therefore, the source/sink behavior of temperate forest soils likely depends on the degree to which forest growth is stimulated by new climate and CO₂ conditions.     

Temporal response of soil organic carbon after grassland‐related land‐use change

The net flux of CO₂ exchanged with the atmosphere following grassland‐related land‐use change (LUC) depends on the subsequent temporal dynamics of soil organic carbon (SOC). Yet, the magnitude and timing of these dynamics are still unclear. We compiled a global data set of 836 paired‐sites to quantify temporal SOC changes after grassland‐related LUC. In order to discriminate between SOC losses from the initial ecosystem and gains from the secondary one, the post‐LUC time series of SOC data was combined with satellite‐based net primary production observations as a proxy of carbon input to the soil. Globally, land conversion from either cropland or forest into grassland leads to SOC accumulation; the reverse shows net SOC loss. The SOC response curves vary between different regions. Conversion of cropland to managed grassland results in more SOC accumulation than natural grassland recovery from abandoned cropland. We did not consider the biophysical variables (e.g., climate conditions and soil properties) when fitting the SOC turnover rate into the observation data but analyzed the relationships between the fitted turnover rate and these variables. The SOC turnover rate is significantly correlated with temperature and precipitation (p < 0.05), but not with the clay fraction of soils (p > 0.05). Comparing our results with predictions from bookkeeping models, we found that bookkeeping models overestimate by 56% of the long‐term (100 years horizon) cumulative SOC emissions for grassland‐related LUC types in tropical and temperate regions since 2000. We also tested the spatial representativeness of our data set and calculated SOC response curves using the representative subset of sites in each region. Our study provides new insight into the impact grassland‐related LUC on the global carbon budget and sheds light on the potential of grassland conservation for climate mitigation.     

Biotic responses buffer warming‐induced soil organic carbon loss in Arctic tundra

Climate warming can result in both abiotic (e.g., permafrost thaw) and biotic (e.g., microbial functional genes) changes in Arctic tundra. Recent research has incorporated dynamic permafrost thaw in Earth system models (ESMs) and indicates that Arctic tundra could be a significant future carbon (C) source due to the enhanced decomposition of thawed deep soil C. However, warming‐induced biotic changes may influence biologically related parameters and the consequent projections in ESMs. How model parameters associated with biotic responses will change under warming and to what extent these changes affect projected C budgets have not been carefully examined. In this study, we synthesized six data sets over 5 years from a soil warming experiment at the Eight Mile Lake, Alaska, into the Terrestrial ECOsystem (TECO) model with a probabilistic inversion approach. The TECO model used multiple soil layers to track dynamics of thawed soil under different treatments. Our results show that warming increased light use efficiency of vegetation photosynthesis but decreased baseline (i.e., environment‐corrected) turnover rates of SOC in both the fast and slow pools in comparison with those under control. Moreover, the parameter changes generally amplified over time, suggesting processes of gradual physiological acclimation and functional gene shifts of both plants and microbes. The TECO model predicted that field warming from 2009 to 2013 resulted in cumulative C losses of 224 or 87 g/m², respectively, without or with changes in those parameters. Thus, warming‐induced parameter changes reduced predicted soil C loss by 61%. Our study suggests that it is critical to incorporate biotic changes in ESMs to improve the model performance in predicting C dynamics in permafrost regions.     

Grazing enhances belowground carbon allocation,microbial biomass,and soil carbon in a subtropical grassland

Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant‐derived SOC following microbial decomposition. Here, we used in situ isotope pulse‐chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long‐term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4‐dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.     

Enhanced decomposition of stable soil organic carbon and microbial catabolic potentials by long‐term field warming

Quantifying soil organic carbon (SOC) decomposition under warming is critical to predict carbon–climate feedbacks. According to the substrate regulating principle, SOC decomposition would decrease as labile SOC declines under field warming, but observations of SOC decomposition under warming do not always support this prediction. This discrepancy could result from varying changes in SOC components and soil microbial communities under warming. This study aimed to determine the decomposition of SOC components with different turnover times after subjected to long‐term field warming and/or root exclusion to limit C input, and to test whether SOC decomposition is driven by substrate lability under warming. Taking advantage of a 12‐year field warming experiment in a prairie, we assessed the decomposition of SOC components by incubating soils from control and warmed plots, with and without root exclusion for 3 years. We assayed SOC decomposition from these incubations by combining inverse modeling and microbial functional genes during decomposition with a metagenomic technique (GeoChip). The decomposition of SOC components with turnover times of years and decades, which contributed to 95% of total cumulative CO₂ respiration, was greater in soils from warmed plots. But the decomposition of labile SOC was similar in warmed plots compared to the control. The diversity of C‐degradation microbial genes generally declined with time during the incubation in all treatments, suggesting shifts of microbial functional groups as substrate composition was changing. Compared to the control, soils from warmed plots showed significant increase in the signal intensities of microbial genes involved in degrading complex organic compounds, implying enhanced potential abilities of microbial catabolism. These are likely responsible for accelerated decomposition of SOC components with slow turnover rates. Overall, the shifted microbial community induced by long‐term warming accelerates the decomposition of SOC components with slow turnover rates and thus amplify the positive feedback to climate change.     

Carbon quality and soil microbial property control the latitudinal pattern in temperature sensitivity of soil microbial respiration across Chinese forest ecosystems

Understanding the temperature sensitivity (Q₁₀) of soil organic C (SOC) decomposition is critical to quantifying the climate–carbon cycle feedback and predicting the response of ecosystems to climate change. However, the driving factors of the spatial variation in Q₁₀ at a continental scale are fully unidentified. In this study, we conducted a novel incubation experiment with periodically varying temperature based on the mean annual temperature of the soil origin sites. A total of 140 soil samples were collected from 22 sites along a 3,800 km long north–south transect of forests in China, and the Q₁₀ of soil microbial respiration and corresponding environmental variables were measured. Results showed that changes in the Q₁₀ values were nonlinear with latitude, particularly showing low Q₁₀ values in subtropical forests and high Q₁₀ values in temperate forests. The soil C:N ratio was positively related to the Q₁₀ values, and coniferous forest soils with low SOC quality had higher Q₁₀ values than broadleaved forest soils with high SOC quality, which supported the “C quality temperature” hypothesis. Out of the spatial variations in Q₁₀ across all ecosystems, gram‐negative bacteria exhibited the most importance in regulating the variation in Q₁₀ and contributed 25.1%, followed by the C:N ratio (C quality), fungi, and the fungi:bacteria ratio. However, the dominant factors that regulate the regional variations in Q₁₀ differed among the tropical, subtropical, and temperate forest ecosystems. Overall, our findings highlight the importance of C quality and microbial controls over Q₁₀ value in China's forest ecosystems. Meanwhile, C dynamics in temperate forests under a global warming scenario can be robustly predicted through the incorporation of substrate quality and microbial property into models.     

How to measure,report and verify soil carbon change to realize the potential of soil carbon sequestration for atmospheric greenhouse gas removal

There is growing international interest in better managing soils to increase soil organic carbon (SOC) content to contribute to climate change mitigation, to enhance resilience to climate change and to underpin food security, through initiatives such as international ‘4p1000’ initiative and the FAO's Global assessment of SOC sequestration potential (GSOCseq) programme. Since SOC content of soils cannot be easily measured, a key barrier to implementing programmes to increase SOC at large scale, is the need for credible and reliable measurement/monitoring, reporting and verification (MRV) platforms, both for national reporting and for emissions trading. Without such platforms, investments could be considered risky. In this paper, we review methods and challenges of measuring SOC change directly in soils, before examining some recent novel developments that show promise for quantifying SOC. We describe how repeat soil surveys are used to estimate changes in SOC over time, and how long‐term experiments and space‐for‐time substitution sites can serve as sources of knowledge and can be used to test models, and as potential benchmark sites in global frameworks to estimate SOC change. We briefly consider models that can be used to simulate and project change in SOC and examine the MRV platforms for SOC change already in use in various countries/regions. In the final section, we bring together the various components described in this review, to describe a new vision for a global framework for MRV of SOC change, to support national and international initiatives seeking to effect change in the way we manage our soils.     

The Dual Arrhenius and Michaelis–Menten kinetics model for decomposition of soil organic matter at hourly to seasonal time scales

Decomposition of soil carbon stocks is one of the largest potential biotic feedbacks to climate change. Models of decomposition of soil organic matter and of soil respiration rely on empirical functions that relate variation in temperature and soil water content to rates of microbial metabolism using soil‐C substrates. Here, we describe a unifying modeling framework to combine the effects of temperature, soil water content, and soluble substrate supply on decomposition of soluble soil‐C substrates using simple functions based on process concepts. The model's backbone is the Michaelis–Menten equation, which describes the relationship between reaction velocity and soluble organic‐C and O₂ substrate concentrations at an enzyme's reactive site, which are determined by diffusivity functions based on soil water content. Temperature sensitivity is simulated by allowing the maximum velocity of the reaction (V_max) to vary according to Arrhenius function. The Dual Arrhenius and Michaelis–Menten kinetics (DAMM) model core was able to predict effectively observations from of laboratory enzyme assays of β‐glucosidase and phenol‐oxidase across a range of substrate concentrations and incubation temperatures. The model also functioned as well or better than purely empirical models for simulating hourly and seasonal soil respiration data from a trenched plot in a deciduous forest at the Harvard Forest, in northeastern United States. The DAMM model demonstrates that enzymatic processes can be intrinsically temperature sensitive, but environmental constrains of substrate supply under soil moisture extremes can prevent that response to temperature from being observed. We discuss how DAMM could serve as a core module that is informed by other modules regarding microbial dynamics and supply of soluble‐C substrates from plant inputs and from desorption of physically stabilized soil‐C pools. Most importantly, it presents a way forward from purely empirical representation of temperature and moisture responses and integrates temperature‐sensitive enzymatic processes with constraints of substrate supply.     

Decadally cycling soil carbon is more sensitive to warming than faster‐cycling soil carbon

The response of soil organic carbon (SOC) pools to globally rising surface temperature crucially determines the feedback between climate change and the global carbon cycle. However, there is a lack of studies investigating the temperature sensitivity of decomposition for decadally cycling SOC which is the main component of total soil carbon stock and the most relevant to global change. We tackled this issue using two decadally ¹³C‐labeled soils and a much improved measuring system in a long‐term incubation experiment. Results indicated that the temperature sensitivity of decomposition for decadally cycling SOC (>23 years in one soil and >55 years in the other soil) was significantly greater than that for faster‐cycling SOC (<23 or 55 years) or for the entire SOC stock. Moreover, decadally cycling SOC contributed substantially (35–59%) to the total CO₂ loss during the 360‐day incubation. Overall, these results indicate that the decomposition of decadally cycling SOC is highly sensitive to temperature change, which will likely make this large SOC stock vulnerable to loss by global warming in the 21st century and beyond.     

气候和林分类型对土壤团聚体有机碳的影响

该研究选择我国分布于亚热带、暖温带和寒温带的三个样点8种林分(包括阔叶林、混交林和针叶林)下表层0~20 cm的土壤为研究对象,利用干筛法进行大团聚体和微团聚体分级,测定了各团聚体组分的有机碳量和有机碳百分比,并分析他们与气候、植被和土壤环境变量之间的关系。结果表明:土壤大团聚体和微团聚体有机碳量都受到气候的显著影响,表现为土壤大团聚体和微团聚体有机碳量随年均温的增高而降低,经分析这与低温抑制土壤微生物分解活动有关。土壤团聚体有机碳百分比受到林分类型的影响显著,表现为阔叶林土壤团聚体有机碳百分比高于针叶林,这与林分凋落物的质量有关。此外,土壤pH值和土壤质地也影响土壤团聚体有机碳百分比。这表明气温上升和人为干扰导致的林分类型改变都可能引起土壤团聚体有机碳的下降,加剧气候变化。该研究结果有助于了解土壤团聚体有机碳的变异规律,为预测全球变化下土壤有机碳响应提供数据支持。     

The responses of soil organic carbon mineralization and microbial communities to fresh and aged biochar soil amendments

While biochar soil amendment has been widely proposed as a soil organic carbon (SOC) sequestration strategy to mitigate detrimental climate changes in global agriculture, the SOC sequestration was still not clearly understood for the different effects of fresh and aged biochar on SOC mineralization. In the present study of a two‐factorial experiment, topsoil samples from a rice paddy were laboratory‐incubated with and without fresh or aged biochar pyrolyzed of wheat residue and with and without crop residue‐derived dissolved organic matter (CRM) for monitoring soil organic matter decomposition under controlled conditions. The six treatments included soil with no biochar, with fresh biochar and with aged biochar treated with CRM, respectively. For fresh biochar treatment, the topsoil of a same rice paddy was amended with wheat biochar directly from a pyrolysis wheat straw, the soil with aged biochar was collected from the same soil 6 years following a single amendment of same biochar. Total CO₂ emission from the soil was monitored over a 64 day time span of laboratory incubation, while microbial biomass carbon and phospholipid fatty acid (PLFA) were determined at the end of incubation period. Without CRM, total organic carbon mineralization was significantly decreased by 38.8% with aged biochar but increased by 28.9% with fresh biochar, compared to no biochar. With CRM, however, the significantly highest net carbon mineralization occurred in the soil without biochar compared to the biochar‐amended soil. Compared to aged biochar, fresh biochar addition significantly increased the total PLFA concentration by 20.3%–33.8% and altered the microbial community structure by increasing 17:1ω8c (Gram‐negative bacteria) and i17:0 (Gram‐positive bacteria) mole percentages and by decreasing the ratio of fungi/bacteria. Furthermore, biochar amendment significantly lowered the metabolic quotient of SOC decomposition, thereby becoming greater with aged biochar than with fresh biochar. The finding here suggests that biochar amendment could improve carbon utilization efficiency by soil microbial community and SOC sequestration potential in paddy soil can be enhanced by the presence of biochar in soil over the long run.     

Networking our science to characterize the state,vulnerabilities, and management opportunities of soil organic matter

Soil organic matter (SOM) supports the Earth's ability to sustain terrestrial ecosystems, provide food and fiber, and retains the largest pool of actively cycling carbon. Over 75% of the soil organic carbon (SOC) in the top meter of soil is directly affected by human land use. Large land areas have lost SOC as a result of land use practices, yet there are compensatory opportunities to enhance productivity and SOC storage in degraded lands through improved management practices. Large areas with and without intentional management are also being subjected to rapid changes in climate, making many SOC stocks vulnerable to losses by decomposition or disturbance. In order to quantify potential SOC losses or sequestration at field, regional, and global scales, measurements for detecting changes in SOC are needed. Such measurements and soil‐management best practices should be based on well established and emerging scientific understanding of processes of C stabilization and destabilization over various timescales, soil types, and spatial scales. As newly engaged members of the International Soil Carbon Network, we have identified gaps in data, modeling, and communication that underscore the need for an open, shared network to frame and guide the study of SOM and SOC and their management for sustained production and climate regulation.     

Carbon sequestration potential of soils in southeast Germany derived from stable soil organic carbon saturation

Sequestration of atmospheric carbon (C) in soils through improved management of forest and agricultural land is considered to have high potential for global CO₂ mitigation. However, the potential of soils to sequester soil organic carbon (SOC) in a stable form, which is limited by the stabilization of SOC against microbial mineralization, is largely unknown. In this study, we estimated the C sequestration potential of soils in southeast Germany by calculating the potential SOC saturation of silt and clay particles according to Hassink [Plant and Soil 191 (1997) 77] on the basis of 516 soil profiles. The determination of the current SOC content of silt and clay fractions for major soil units and land uses allowed an estimation of the C saturation deficit corresponding to the long‐term C sequestration potential. The results showed that cropland soils have a low level of C saturation of around 50% and could store considerable amounts of additional SOC. A relatively high C sequestration potential was also determined for grassland soils. In contrast, forest soils had a low C sequestration potential as they were almost C saturated. A high proportion of sites with a high degree of apparent oversaturation revealed that in acidic, coarse‐textured soils the relation to silt and clay is not suitable to estimate the stable C saturation. A strong correlation of the C saturation deficit with temperature and precipitation allowed a spatial estimation of the C sequestration potential for Bavaria. In total, about 395 Mt CO₂‐equivalents could theoretically be stored in A horizons of cultivated soils – four times the annual emission of greenhouse gases in Bavaria. Although achieving the entire estimated C storage capacity is unrealistic, improved management of cultivated land could contribute significantly to CO₂ mitigation. Moreover, increasing SOC stocks have additional benefits with respect to enhanced soil fertility and agricultural productivity.     

Simulation of management and soil interactions impacting SOC dynamics in sugarcane using the CENTURY Model

Newer methods of management and harvesting of sugarcane are being considered to improve soil and water conservation in Brazil. Our aim in this study was to evaluate soil C dynamics under sugarcane cultivation as influenced by the use of conservation management, using measurements from four different management systems and land use histories, i.e. conventional management with preharvest burning, no burning with residue retention and two systems without burning plus additional organic amendments. Field sites also differed in terms of soil texture. We compared field measurements of soil C stocks, ¹³C and microbial biomass with simulated results from the Century ecosystem model for each of the sites and management histories. We also did long-term simulations of the management treatments and sites to approximate steady-state SOC levels, to explore potential management-induced differences in SOC stocks and interactions with soil texture. The model accurately represented treatment and site differences for total SOC stocks, in which SOC stocks were strongly affected by both rates of organic matter input to soil and soil clay content. However, the model tended to underestimate the relative contribution of sugarcane-derived C to total SOC for sites with high residue and external organic matter amendments. Measured microbial biomass C across the sites was closely aligned with relative amounts of organic matter input but did not appear to be strongly affected by soil texture, whereas the model predicted that both texture and organic matter input rate would impact microbial biomass C. Long-term simulations of the conservation management alternatives suggested that SOC stocks could be maintained at or above levels in the original native Cerradão vegetation, whereas conventional practices using residue burning would result in a reduction of SOC to ca. 60% of native levels. Our results support the use of the CENTURY model as an aid to assess the impacts of different soil management practices on SOC stocks under sugarcane in Brazil.