Reconciling carbon quality with availability predicts temperature sensitivity of global soil carbon mineralization.

Soil organic carbon (SOC) mineralization is a key component of the global carbon cycle. Its temperature sensitivity Q10 (which is defined as the factor of change in mineralization with a 10 °C temperature increase) is crucial for understanding the carbon cycle-climate change feedback but remains uncertain. Here, we demonstrate the universal control of carbon quality-availability tradeoffs on Q10. When carbon availability is not limited, Q10 is controlled by carbon quality; otherwise, substrate availability controls Q10. A model driven by such quality-availability tradeoffs explains 97% of the spatiotemporal variability of Q10 in incubations of soils across the globe and predicts a global Q10 of 2.1 ± 0.4 (mean ± one SD) with higher Q10 in northern high-latitude regions. We further reveal that global Q10 is predominantly governed by the mineralization of high-quality carbon. The work provides a foundation for predicting SOC dynamics under climate and land use changes which may alter soil carbon quality and availability.

Carbon sequestration in the subsoil and the time required to stabilize carbon for climate change mitigation.

Soils store large quantities of carbon in the subsoil (below 0.2 m depth) that is generally old and believed to be stabilized over centuries to millennia, which suggests that subsoil carbon sequestration (CS) can be used as a strategy for climate change mitigation. In this article, we review the main biophysical processes that contribute to carbon storage in subsoil and the main mathematical models used to represent these processes. Our guiding objective is to review whether a process understanding of soil carbon movement in the vertical profile can help us to assess carbon storage and persistence at timescales relevant for climate change mitigation. Bioturbation, liquid phase transport, belowground carbon inputs, mineral association, and microbial activity are the main processes contributing to the formation of soil carbon profiles, and these processes are represented in models using the diffusion-advection-reaction paradigm. Based on simulation examples and measurements from carbon and radiocarbon profiles across biomes, we found that advective and diffusive transport may only play a secondary role in the formation of soil carbon profiles. The difference between vertical root inputs and decomposition seems to play a primary role in determining the shape of carbon change with depth. Using the transit time of carbon to assess the timescales of carbon storage of new inputs, we show that only small quantities of new carbon inputs travel through the profile and can be stabilized for time horizons longer than 50 years, implying that activities that promote CS in the subsoil must take into consideration the very small quantities that can be stabilized in the long term.

Significant accrual of soil organic carbon through long-term rice cultivation in paddy fields in China.

Paddy fields serve as significant reservoirs of soil organic carbon (SOC) and their potential for terrestrial carbon (C) sequestration is closely associated with changes in SOC pools. However, there has been a dearth of comprehensive studies quantifying changes in SOC pools following extended periods of rice cultivation across a broad geographical scale. Using 104 rice paddy sampling sites that have been in continuous cultivation since the 1980s across China, we studied the changes in topsoil (0-20 cm) labile organic C (LOC I), semi-labile organic C (LOC II), recalcitrant organic C (ROC), and total SOC. We found a substantial increase in both the content (48%) and density (39%) of total SOC within China's paddy fields between the 1980s to the 2010s. Intriguingly, the rate of increase in content and density of ROC exceeded that of LOC (I and II). Using a structural equation model, we revealed that changes in the content and density of total SOC were mainly driven by corresponding shifts in ROC, which are influenced both directly and indirectly by climatic and soil physicochemical factors; in particular temperature, precipitation, phosphorous (P) and clay content. We also showed that the δ13 CLOC were greater than δ13 CROC , independent of the rice cropping region, and that there was a significant positive correlation between δ13 CSOC and δ13 Cstraw . The δ13 CLOC and δ13 CSOC showed significantly negative correlation with soil total Si, suggesting that soil Si plays a part in the allocation of C into different SOC pools, and its turnover or stabilization. Our study underscores that the global C sequestration of the paddy fields mainly stems from the substantial increase in ROC pool.

A field incubation approach to evaluate the depth dependence of soil biogeochemical responses to climate change.

Soil biogeochemical processes may present depth-dependent responses to climate change, due to vertical environmental gradients (e.g., thermal and moisture regimes, and the quantity and quality of soil organic matter) along soil profile. However, it is a grand challenge to distinguish such depth dependence under field conditions. Here we present an innovative, cost-effective and simple approach of field incubation of intact soil cores to explore such depth dependence. The approach adopts field incubation of two sets of intact soil cores: one incubated right-side up (i.e., non-inverted), and another upside down (i.e., inverted). This inversion keeps soil intact but changes the depth of the soil layer of same depth origin. Combining reciprocal translocation experiments to generate natural climate shift, we applied this incubation approach along a 2200 m elevational mountainous transect in southeast Tibetan Plateau. We measured soil respiration (Rs) from non-inverted and inverted cores of 1 m deep, respectively, which were exchanged among and incubated at different elevations. The results indicated that Rs responds significantly (p < .05) to translocation-induced climate shifts, but this response is depth-independent. As the incubation proceeds, Rs from both non-inverted and inverted cores become more sensitive to climate shifts, indicating higher vulnerability of persistent soil organic matter (SOM) to climate change than labile components, if labile substrates are assumed to be depleted with the proceeding of incubation. These results show in situ evidence that whole-profile SOM mineralization is sensitive to climate change regardless of the depth location. Together with measurements of vertical physiochemical conditions, the inversion experiment can serve as an experimental platform to elucidate the depth dependence of the response of soil biogeochemical processes to climate change.

Younger carbon dominates global soil carbon efflux.

Soil carbon (C) is comprised of a continuum of organic compounds with distinct ages (i.e., the time a C atom has experienced in soil since the C atom entered soil). The contribution of different age groups to soil C efflux is critical for understanding soil C stability and persistence, but is poorly understood due to the complexity of soil C pool age structure and potential distinct turnover behaviors of age groups. Here, we build upon the quantification of soil C transit times to infer the age of C atoms in soil C efflux (aefflux ) from seven sequential soil layer depths down to 2 m at a global scale, and compare this age with radiocarbon-inferred ages of C retained in corresponding soil layers (asoil ). In the whole 0-2 m soil profile, the mean aefflux is 194 21 1021 (mean with 5%-95% quantiles) year and is just about one-eighth of asoil ( 1476 717 2547 year), demonstrating that younger C dominates soil C efflux. With increasing soil depth, both aefflux and asoil are increased, but their disparities are markedly narrowed. That is, the proportional contribution of relatively younger soil C to efflux is decreased in deeper layers, demonstrating that C inputs (new and young) stay longer in deeper layers. Across the globe, we find large spatial variability of the contribution of soil C age groups to C efflux. Especially, in deep soil layers of cold regions (e.g., boreal forests and tundra), aefflux may be older than asoil , suggesting that older C dominates C efflux only under a limited range of conditions. These results imply that most C inputs may not contribute to long-term soil C storage, particularly in upper layers that hold the majority of new C inputs.

Decoupling environmental impact from economic growth to achieve Sustainable Development Goals in China.

Developing countries, such as China, have achieved unprecedented success in a single Sustainable Development Goal (SDG), which usually leads to trade-offs between the three pillars of sustainability, and even destroys sustainability. Quantifying the degrees of coupling among the pillars is essential to support policymakers' systematic actions to minimize trade-offs and maximize co-benefits between the pillars, and simultaneously achieve all SDGs. However, assessing the degrees of coupling among the pillars for the full SDGs is lacking. Here, we evaluate the progress of the pillars towards the SDGs and quantify the degrees of coupling among them at both national and sub-national levels in China from 2000 to 2015. The results indicate that the degrees of coupling among the pillars were almost constant while the degrees of coupling between the pillars and economic growth declined over time. The degrees of coupling between environmental impact and economic growth accounted for 52%-83% of the SDGs' progress. Reducing the degrees of coupling helps achieve simultaneously economic growth and environmental protection. The higher the degrees of coupling, the lower progress. This trend was universal among all provinces (sub-national level) regardless of their development levels. Our study highlights not only the necessity to track the degrees of coupling among the pillars, but also decoupling environmental impact from economic growth to achieve the SDGs.

Warming-induced global soil carbon loss attenuated by downward carbon movement.

The fate of soil organic carbon (SOC) under warming is poorly understood, particularly across large extents and in the whole-soil profile. Using a data-model integration approach applied across the globe, we find that downward movement of SOC along the soil profile reduces SOC loss under warming. We predict that global SOC stocks (down to 2 m) will decline by 4% (~80 Pg) on average when SOC reaches the steady state under 2°C warming, assuming no changes in net primary productivity (NPP). To compensate such decline (i.e. maintain current SOC stocks), a 3% increase of NPP is required. Without the downward SOC movement, global SOC declines by 15%, while a 20% increase in NPP is needed to compensate that loss. This vital role of downward SOC movement in controlling whole-soil profile SOC dynamics in response to warming is due to the protection afforded to downward-moving SOC by depth, indicated by much longer residence times of SOC in deeper layers. Additionally, we find that this protection could not be counteracted by promoted decomposition due to the priming of downward-moving new SOC from upper layers on native old SOC in deeper layers. This study provides the first estimation of whole-soil SOC changes under warming and additional NPP required to compensate such changes across the globe, and reveals the vital role of downward movement of SOC in reducing SOC loss under global warming.

Distinct controls over the temporal dynamics of soil carbon fractions after land use change.

Soil organic carbon (SOC), the largest terrestrial carbon pool, plays a significant role in soil-related ecosystem services such as climate regulation, soil fertility and agricultural production. However, its fate under land use change is difficult to predict. A major issue is that SOC comprised of numerous organic compounds with potentially distinct and poorly understood turnover properties. Here we use spatiotemporal measurements of the particulate (POC), mineral-associated (MOC) and charred SOC (COC) fractions from 176 trials involving changes in land use to assess their underlying controls. We find that the initial pool sizes of each of the three fractions consistently and dominantly control their temporal dynamics after changes in land use (i.e. the baseline effects). The effects of climate, soil physicochemical properties and plant residues, however, are fraction- and time-dependent. Climate and soil properties show similar importance for controlling the dynamics of MOC and COC, while plant residue inputs (in term of their quantity and quality) are much less important. For POC, plant residues and management practices (e.g. the frequency of pasture in crop-pasture rotation systems) are substantially more important, overriding the influence of climate. These results demonstrate the pivotal role of measuring SOC composition and considering fraction-specific stabilization and destabilization processes for effective SOC management and reliable SOC predictions.

Neighbouring crop diversity mediates the effect of Bt cotton on insect community and leaf damage in fields.

Effects of large-scale cultivation of transgenic crops on agricultural biodiversity remain unclear, particularly in the context of complex ecological interactions between transgenic crops and other organisms. Here we conducted a comprehensive survey to investigate the number of species, population abundance, community evenness and dominance of insects and weeds as well as leaf damage to weeds in Bt and non-Bt cotton fields at 27 sites across northern China. The role of neighbouring crop diversity around cotton fields in controlling insects and weeds in the cotton fields was also assessed. In addition, we conducted a 3-year field experiment to verify the results of the survey. Weed diversity in Bt and non-Bt cotton fields was similar, but the species number and diversity indices of insects are significantly decreased in Bt fields aligning with reduced leaf damage to broadleaf plant species including cotton as well as crops in neighbouring plots. The leaf damage to Bt and non-Bt cotton negatively associates with the diversity of neighbouring crops in cotton fields. Our study demonstrates the neighbouring crop diversity mediates the effects of Bt crops on agricultural diversity in complex interactions among transgenic crops, in-field weed and insect communities, and neighbouring crops.

Modelling the dynamic physical protection of soil organic carbon: Insights into carbon predictions and explanation of the priming effect.

The role and significance of physically protected soil organic carbon (SOC) in regulating SOC dynamics remains unclear. Here, we developed a simple theoretical model (DP model) considering dynamic physical protection to simulate the dynamics of protected (Cp ) and unprotected SOC (Cu ), and compared the modelling results with a conventional two-pool (fast vs. slow) model considering chemical recalcitrance. The two models were first constrained using extensive SOC data collected from soils with and without fresh carbon (C) inputs under incubation conditions, and then applied to project SOC dynamics and explore mechanisms underpinning the priming effect (PE). Overall, both models explained more than 99% of the variances in observed SOC dynamics. The DP model predicted that Cp accounted for the majority of total SOC. As decomposition proceeds, the proportion of Cp reached >90% and kept relatively constant. Although the similar performance of the two models in simulating observed total SOC dynamics, their predictions of future SOC dynamics were divergent, challenging the predictions of widely used pool-based models. The DP model also suggested alternative mechanisms underpinning the priming of SOC decomposition by fresh C inputs. The two-pool model suggested that the PE was caused by the stimulated decomposition rates, especially for the slow recalcitrant pool, while the DP model suggested that the PE might be the combined consequence of stimulated Cu decomposition, the liberation of Cp to decomposition and the inhibition of the protection of unprotected SOC. The model-data integration provided a new explanation for the PE, highlighting the importance of liberation of initially physically protected SOC to decomposition by new C inputs. Our model-data integration demonstrated the importance of simulating physical protection processes for reliable SOC predictions, and provided new insights into mechanistic understanding of the priming effect.

Soil organic carbon dynamics jointly controlled by climate, carbon inputs, soil properties and soil carbon fractions.

Soil organic carbon (SOC) dynamics are regulated by the complex interplay of climatic, edaphic and biotic conditions. However, the interrelation of SOC and these drivers and their potential connection networks are rarely assessed quantitatively. Using observations of SOC dynamics with detailed soil properties from 90 field trials at 28 sites under different agroecosystems across the Australian cropping regions, we investigated the direct and indirect effects of climate, soil properties, carbon (C) inputs and soil C pools (a total of 17 variables) on SOC change rate (rC , Mg C ha-1  yr-1 ). Among these variables, we found that the most influential variables on rC were the average C input amount and annual precipitation, and the total SOC stock at the beginning of the trials. Overall, C inputs (including C input amount and pasture frequency in the crop rotation system) accounted for 27% of the relative influence on rC , followed by climate 25% (including precipitation and temperature), soil C pools 24% (including pool size and composition) and soil properties (such as cation exchange capacity, clay content, bulk density) 24%. Path analysis identified a network of intercorrelations of climate, soil properties, C inputs and soil C pools in determining rC . The direct correlation of rC with climate was significantly weakened if removing the effects of soil properties and C pools, and vice versa. These results reveal the relative importance of climate, soil properties, C inputs and C pools and their complex interconnections in regulating SOC dynamics. Ignorance of the impact of changes in soil properties, C pool composition and C input (quantity and quality) on SOC dynamics is likely one of the main sources of uncertainty in SOC predictions from the process-based SOC models.

Uncertain future soil carbon dynamics under global change predicted by models constrained by total carbon measurements.

Pool-based carbon (C) models are widely applied to predict soil C dynamics under global change and infer underlying mechanisms. However, it is unclear about the credibility of model-predicted C pool size, decay rate (k), and/or microbial C use efficiency (e) as only data on bulked total C is usually available for model constraining. Using observing system simulation experiments (OSSE), we constrained a two-pool model using simulated data sets of total soil C dynamics under topical hypotheses on responses of soil C dynamics to warming and elevated CO2 (i.e., global change scenarios). The results indicated that the model predicted great uncertainties in C pool size, k, and e under all global change scenarios, resulting in the difficulty to correctly infer the presupposed "real" values of those parameters that are used to generate the simulated total soil C for constraining the model. Furthermore, the model using the constrained parameters generated divergent future soil C dynamics. Compared with the predictions using the presupposed real parameters (i.e., the real future C dynamics), the percentage uncertainty in 100-yr predictions using the constrained parameters was up to 45% depending on global change scenarios and data availability for model-constraining. Such great uncertainty was mainly due to the high collinearity among the model parameters. Using pool-based models, we argue that soil C pool size, k, and/or e and their responses to global change have to be estimated explicitly and empirically, rather than through model-fitting, in order to accurately predict C dynamics and infer underlying mechanisms. The OSSE approach provides a powerful way to identify data requirement for the new generation of model development and test model performance.

Fresh carbon input differentially impacts soil carbon decomposition across natural and managed systems.

The amount of fresh carbon input into soil is experiencing substantial changes under global change. It is unclear what will be the consequences of such input changes on native soil carbon decomposition across ecosystems. By synthesizing data from 143 experimental comparisons, we show that, on average, fresh carbon input stimulates soil carbon decomposition by 14%. The response was lower in forest soils (1%) compared with soils from other ecosystems (> 24%), and higher following inputs of plant residue-like substrates (31%) compared to root exudate-like substrates (9%). The responses decrease with the baseline soil carbon decomposition rate under no additional carbon input, but increase with the fresh carbon input rate. The rates of these changes vary significantly across ecosystems and with the carbon substrates being added. These findings can be applied to provide robust estimates of soil carbon balance across ecosystems under changing aboveground and belowground inputs as consequence of climate and land management changes.

Spatiotemporal co-optimization of agricultural management practices towards climate-smart crop production.

Co-optimization of multiple management practices may facilitate climate-smart agriculture, but is challenged by complex climate-crop-soil management interconnections across space and over time. Here we develop a hybrid approach combining agricultural system modelling, machine learning and life cycle assessment to spatiotemporally co-optimize fertilizer application, irrigation and residue management to achieve yield potential of wheat and maize and minimize greenhouse gas emissions in the North China Plain. We found that the optimal fertilizer application rate and irrigation for the historical period (1995-2014) are lower than local farmers' practices as well as trial-derived recommendations. With the optimized practices, the projected annual requirement of fertilizer, irrigation water and residue inputs across the North China Plain in the period 2051-2070 is reduced by 16% (14-21%) (mean with 95% confidence interval), 19% (7-32%) and 20% (16-26%), respectively, compared with the current supposed optimal management in the historical reference period, with substantial greenhouse gas emission reductions. We demonstrate the potential of spatiotemporal co-optimization of multiple management practices and present digital mapping of management practices as a benchmark for site-specific management across the region.

The adsorption-diffusion model and biomimetic simulation reveal the switchable roles of silicon in regulating toxic metal uptake in rice roots.

Soil contamination by heavy metals has become a serious threat to global food security. The application of silicon (Si)-based materials is a simple and economical method for producing safe crops in contaminated soil. However, the impact of silicon on the heavy-metal concentration in plant roots, which are the first line in the chain of heavy-metal entering plants and causing stress and the main site of heavy-metal deposition in plants, remains puzzling. We proposed a process-based model (adsorption-diffusion model) to explain the results of a collection of 28 experiments on alleviating toxic metal stress in plants by Si. Then we evaluated the applicability of the model in Si-mitigated trivalent chromium (Cr[III]) stress in rice, taking into account variations in experimental conditions such as Cr(III) concentration, stress duration, and Si concentration. It was found that the adsorption-diffusion model fitted the experimental data well (R2 > 0.9). We also verified the binding interaction between Si and Cr in the cell wall using SEM-EDS and XPS. In addition, we designed a simplified biomimetic device that simulated the Si in cell wall to analyze the dual-action switch of Si from increasing Cr(III) adsorption to blocking Cr(III) diffusion. We found that the adsorption of Cr(III) by Si decreased from 58% to 7% as the total amount of Cr(III) increased, and finally the diffusion blocking effect of Si dominated. This study deepens our understanding of the role of Si in mitigating toxic metal stress in plants and is instructive for the research and use of Si-based materials to improve food security.

Global turnover of soil mineral-associated and particulate organic carbon.

Soil organic carbon (SOC) persistence is predominantly governed by mineral protection, consequently, soil mineral-associated (MAOC) and particulate organic carbon (POC) turnovers have different impacts on the vulnerability of SOC to climate change. Here, we generate the global MAOC and POC maps using 8341 observations and then infer the turnover times of MAOC and POC by a data-model integration approach. Global MAOC and POC storages are 975 964 987 Pg C (mean with 5% and 95% quantiles) and 330 323 337 Pg C, while global mean MAOC and POC turnover times are 129 45 383 yr and 23 5 82 yr in the top meter, respectively. Climate warming-induced acceleration of MAOC and POC decomposition is greater in subsoil than that in topsoil. Overall, the global atlas of MAOC and POC turnover, together with the global distributions of MAOC and POC stocks, provide a benchmark for Earth system models to diagnose SOC-climate change feedback.

Meta-modeling soil organic carbon sequestration potential and its application at regional scale.

Upscaling the results from process-based soil-plant models to assess regional soil organic carbon (SOC) change and sequestration potential is a great challenge due to the lack of detailed spatial information, particularly soil properties. Meta-modeling can be used to simplify and summarize process-based models and significantly reduce the demand for input data and thus could be easily applied on regional scales. We used the pre-validated Agricultural Production Systems sIMulator (APSIM) to simulate the impact of climate, soil, and management on SOC at 613 reference sites across Australia's cereal-growing regions under a continuous wheat system. We then developed a simple meta-model to link the APSIM-modeled SOC change to primary drivers, i.e., the amount of recalcitrant SOC, plant available water capacity of soil, soil pH, and solar radiation, temperature, and rainfall in the growing season. Based on high-resolution soil texture data and 8165 climate data points across the study area, we used the meta-model to assess SOC sequestration potential and the uncertainty associated with the variability of soil characteristics. The meta-model explained 74% of the variation of final SOC content as simulated by APSIM. Applying the meta-model to Australia's cereal-growing regions reveals regional patterns in SOC, with higher SOC stock in cool, wet regions. Overall, the potential SOC stock ranged from 21.14 to 152.71 Mg/ha with a mean of 52.18 Mg/ha. Variation of soil properties induced uncertainty ranging from 12% to 117% with higher uncertainty in warm, wet regions. In general, soils in Australia's cereal-growing regions under continuous wheat production were simulated as a sink of atmospheric carbon dioxide with a mean sequestration potential of 8.17 Mg/ha.

Impacts of Heat Stress around Flowering on Growth and Development Dynamic of Maize (Zea mays L.) Ear and Yield Formation.

Heat stress around flowering is harmful to maize growth and yield. Ear traits are closely related to yield; however, the effects of heat stress before and after flowering on ear development and yield traits remain unclear for different heat-tolerant cultivars. In this study, field experiments were conducted in 2020 and 2021, including (i) three sowing dates, (ii) three temperature regimes: control (CK), heated before silking (V9-R1, TBS) and heated after silking (R1-R1 + 15 d, TAS), and (iii) two hybrids (ZD958: heat-tolerant; DH605: heat-sensitive). The results showed that heating had negative effects on all surveyed ear and yield traits except for increased ear length under TBS. The negative effects were larger (i) for TAS than for TBS, (ii) for DH605 than for ZD958, and (iii) for kernel number per plant (KNP) than for kernel weight (KW). The decreased ear traits were a result of a decreased growth rate during rapid ear growth periods. Floret pollination failure and kernel abortion were the main reasons for the decrease in KNP, mainly depending on the daily maximum temperature during V15-R1 + 7 d. The strong linear relationships between ear and yield traits suggested that ear traits could be used as important indicators for breeding heat-resistant varieties in the future.

Global pattern of soil priming effect intensity and its environmental drivers.

The microbial priming effect-the decomposition of soil organic carbon (SOC) induced by plant inputs-has long been considered an important driver of SOC dynamics, yet we have limited understanding about the direction, intensity, and drivers of priming across ecosystem types and biomes. This gap hinders our ability to predict how shifts in litter inputs under global change can affect climate feedbacks. Here, we synthesized 18,919 observations of CO2 effluxes in 802 soils across the globe to test the relative effects (i.e., log response ratio [RR]) of litter additions on native SOC decomposition and identified the dominant environmental drivers in natural ecosystems and agricultural lands. Globally, litter additions enhanced native SOC decomposition (RR = 0.35, 95% CI: 0.32-0.38), with greater priming effects occurring with decreasing latitude and more in agricultural soils (RR = 0.43) than in uncultivated soils (RR = 0.28). In natural ecosystems, soil pH and microbial community composition (e.g., bacteria: fungi ratio) were the best predictors of priming, with greater effects occurring in acidic, bacteria-dominated sandy soils. In contrast, the substrate properties of plant litter and soils were the most important drivers of priming in agricultural systems since soils with high C:N ratios and those receiving large inputs of low-quality litter had the highest priming effects. Collectively, our results suggest that, though different factors may control priming effects, the ubiquitous nature of priming means that alterations of litter quality and quantity owing to global changes will likely have consequences for global C cycling and climate forcing.