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• Environmental variables surrounding sample locations play an important role in DSM. • Landscape metrics can comprehensively characterize the surrounding environment. • The intricate relationship between landscape metrics and SOC was fully explored. • Landscape metrics can promote our understanding on the spatial variation of SOC. Soil organic carbon (SOC) mapping delivers invaluable information to the global carbon budget and climate change mitigation endeavour. Environmental variables at sample locations are frequently used as explanatory variables for simulating the spatial distribution of soil properties. However, these may not fully capture the spatial information generated by soil-forming processes. We applied multi-scale landscape metrics that can comprehensively characterize the surrounding landscape information of sample points. The metrics were extracted at two levels (landscape level and class level) and include the diversity, shape and area, fragmentation and connectivity with a buffer distance from 500 m to 5,000 m. We then investigated its effectiveness as environmental variables via recursive feature elimination, random forests, and quantile regression forests. The Jianghan Plain, China, was selected as the study area, where over 19,000 topsoil samples were collected. Results indicated that multi-scale landscape metrics enhanced the predictability of SOC mapping, with R2 increased by 43 %. Specifically, Shannon's diversity index, the percentage of landscape index, interspersion and juxtaposition index, and patch cohesion index outperformed environmental variables that were extracted at the sample location. In addition, the relationships between SOC and landscape metrics were found to be scale-dependent. Landscape metrics demonstrated significant explanatory capacity for SOC across various spatial scales. Notably, as the scale surpassed 3,000 m, there was a discernible improvement in the explanatory effectiveness of the landscape metrics for SOC. Our findings highlight that landscape metrics are effective in characterising the soil-landscape relationship that is generated by multi-scale natural and anthropogenic soil-forming processes. Meanwhile, knowledge of the intricate relationship between landscape characteristics and SOC is crucial for informing land management decisions aimed at enhancing carbon sequestration, mitigating climate change, and maintaining soil fertility. [ABSTRACT FROM AUTHOR]

The assessment of soil health and the determination of carbon stability in soils are current challenges that have gained momentum through initiatives at national as well as international scales. However, inferring universally valid soil health parameters that are directly linked to carbon permanence remains challenging. Our aim was to evaluate the potential of simultaneous thermal analysis (STA) for an improved monitoring of soil health advances in the context of carbon farming strategies. For this purpose, an on-farm comparison of soil health in ten conservation agricultural systems with adjacent conventional farms was performed based on STA and thermal measures were related to key biological and physical indicators for monitoring soil functions. Using an autocorrelation-based approach, we identified independent thermal indicators, revealing that carbon farming efforts predominantly increased labile soil organic matter fractions in the range of 300–400 ° C , while thermally more recalcitrant fractions did not respond to farming system transformation. Similarly, most soil health indicators revealed highest correlation with temperature ranges of mass loss of labile organic matter fractions. The relation of STA with commonly used soil health indicators was highest for soil organic carbon (SOC, r = 0. 971) and total nitrogen (TN, r = 0. 981). However, also inference on microbial activity parameters such as dimethylsulfoxid reduction, microbial biomass carbon and substrate-induced respiration could be demonstrated with r > 0. 663. The results thus highlight key temperature ranges of organic matter stability for future soil monitoring tasks of climate change mitigation potentials of carbon farming and related advances in soil health. • We tested simultaneous thermal analysis (STA) as a tool for soil health assessment. • We did so by an on-farm comparison of conventional and conservation farming systems. • Improvements by conservation farming were mainly detected for labile C fractions. • Specific STA mass loss ranges correlated well with several soil health indicators. [ABSTRACT FROM AUTHOR]

• Higher model performance for granulometry than SOC% using seedbed spectra. • PLSR outperformed other algorithms for SOC% estimation, CR for granulometry. • Low contribution of seedbed spectra to the DSM of SOC% using environmental covariates. Soil organic carbon (SOC) is central to the functioning of terrestrial ecosystems, has climate mitigation potential and provides several benefits for soil health. Understanding the spatial distribution of SOC can help formulate sustainable soil management practices. Digital soil mapping (DSM) uses advanced statistical and geostatistical methods to estimate soil properties across large areas. DSM integrates climate data, topographic features, geology, legacy soil maps, land management and remote sensing data. Bare soil spectra may reflect the presence of particular soil components, making satellite derived spectra suitable predictors of SOC. Bare soil spectra derived from Sentinel-2 were used to estimate SOC concentration (SOC%) and granulometric fractions in the plough layer (0–30 cm) of agricultural parcels in northern Belgium. Thereafter, the estimation performance of SOC% was compared for three DSM models: one with bare soil spectra, one with environmental covariates (topography, granulometry and vegetation), and a combined model with bare soil spectra and environmental covariates. The estimation performance of sand, silt and clay fractions using bare soil spectra from the spring seedbed (R2: 0.53–0.74; RPD: 1.49–2.05; RPIQ: 1.52–2.39) was higher than that of SOC% (R2: 0.16; RPD: 1.08; RPIQ: 1.32). The highest estimation performance of SOC% was obtained for a DSM model including all covariates (R2: 0.28; RPD: 1.18; RPIQ: 1.44), but the contribution of spring seedbed spectra to a model containing environmental covariates was small. The results provide valuable insights for refining soil property estimation using DSM with spectral and environmental covariates. [ABSTRACT FROM AUTHOR]

• The arable-to-grassland conversion caused an annual SOC accrual of 0.30 Mg C/ha. • The accrual was unaffected by pre-conversion soil C level. • A steady state in SOC accrual was not observed during the 26 years of grassland. Converting arable land to permanent grassland may favor SOC sequestration and thus climate change mitigation. We studied a field experiment on coarse sand soil that during 1894–1997 was under arable rotation with various nutrient treatments (unfertilized, mineral fertilizer, animal manure), leading to topsoils with different SOC contents. In 1998, the treatments stopped, and a semi-natural grassland has been established with topsoil sampled seven times during the following 26 years. Introduction of grassland increased topsoil SOC content linearly at a rate of 0.30 ± 0.01 Mg C ha−1 yr−1, unaffected by pre-conversion SOC level and with no sign of steady state. By mitigating the loss of SOC associated with continued arable cropping, the overall benefit of this land conversion becomes 0.39 Mg C ha−1 yr−1 during the period 1998 to 2023. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hedgerows and adjacent croplands at 23 sites across Germany were sampled. • Under hedgerows, 36 ± 49 % more soil organic carbon (SOC) was stored compared to cropland. • 41 % of absolute SOC stock increase was found in subsoil below 30 cm soil depth. • SOC stocks were also increased along the transect from the hedgerow to the cropland. • Planting hedgerows on 3 % of German cropland could increase SOC stocks by 13 Tg C. Carbon dioxide removal strategies are becoming increasingly important as a fundamental component of comprehensive climate policies. One strategy to increase carbon (C) sinks is the integration of hedgerows into agricultural landscapes. Besides additional C storage in above-ground and below-ground hedgerow biomass, the establishment of hedgerows has the potential to increase soil organic carbon (SOC) stocks. However, empirical data regarding the magnitude of SOC accrual with hedgerow establishment are still limited. We sampled 23 sites across Germany in a paired-plot approach with the aim of estimating SOC stock change with hedgerow establishment on cropland. At 21 sites, SOC stocks were higher beneath hedgerows than in the reference cropland. On average, SOC stock accrual was 29 ± 22 Mg ha−1 (36 ± 49 %) in 0–100 cm soil depth. SOC stocks were significantly different in both topsoil and subsoil. Subsoils below 30 cm depth contributed 41 % (12 ± 17 Mg C ha−1) of the total SOC stock change, stressing the importance of subsoil SOC for the total SOC stocks of hedgerow systems. The positive effect of hedgerows on SOC stocks extended laterally beyond the hedgerow area itself. SOC stock in the grassy hedgerow edge also increased significantly by 22 ± 22 Mg C ha−1 (28 ± 30 %) and SOC stock in the cropland directly adjacent to the hedgerow were 9 ± 19 Mg C ha−1 (12 ± 25 %) higher than in the reference cropland. Particularly high C stock differences between hedgerows and reference cropland soils were found in old hedgerows (>200 years) planted on hedgebanks, which are typically found in northern Germany. Our study confirmed that SOC stocks increase with hedgerow establishment on cropland throughout the whole soil profile. If hedgerows were to be established on 3 % of Germany's cropland area, SOC stocks would increase by 13 Tg C equivalent 48 Mio. t CO 2 , highlighting that hedgerows are a promising and multi-functional climate change mitigation option. [ABSTRACT FROM AUTHOR]

• Soil water regulation was improved by enrollment in stewardship programs. • Greater soil carbon was observed with enrollment in stewardship programs. • Grassland set-aside with winter cover cropping improved soil organic carbon storage potential. • Enrollment in stewardship programs can lead to regenerative outcomes. Agricultural stewardship programs that incentivize practices such as winter cover cropping (WCC), grassland set-asides (GLSA), and hedgerows (HR) for intensive annual agricultural production are often implemented to improve both soil conditions and provision of wildlife habitat in coastal agricultural systems of the Fraser River Delta, British Columbia (BC). Although these programs have been shown to support numerous ecosystem services, it is unclear how they contribute to regenerative agricultural outcomes. Regenerative agricultural practices aim to increase soil carbon inputs and reduce carbon dioxide losses which in turn improves soil structure, optimizes soil water regulation, and provides climate mitigation and resiliency of agricultural systems. The objectives of this study were to quantify the effects of agricultural stewardship programs for regenerative outcomes through indicators of climate mitigation and adaptation. Twenty-six fields in the southwestern, BC were grouped by those that participated in stewardship programs between 1992 and 2016 and those that did not. In the spring of 2018, soil samples were collected; soil organic carbon (SOC) concentrations were measured, and stocks were calculated; pedo-transfer functions were used to estimate soil water regulation indicators; and differences among stewardship program and non-program fields were statistically analyzed using a linear mixed effects model. The SOC (mg kg−1) from 0 to 15 cm was greater with implementation of stewardship programs. Topsoil equivalent soil mass SOC stocks (t/ha) were 71 % and 63 % greater in HR and WCC + GLSA program fields, respectively, compared to non-program fields. Soil workability threshold was 25 % and 14 % greater in HR and WCC + GLSA program fields, respectively, compared to non-program. Our results indicate that stewardship programs that include hedgerows and grassland set-asides promote accrual of soil carbon and improved soil water regulation and show that these programs can effectively enhance regenerative outcomes and improve agroecosystem resilience in this important agricultural region. [ABSTRACT FROM AUTHOR]

The soil security concept has been put forward to maintain and improve soil resources inter alia to provide food, clean water, climate change mitigation and adaptation, and to protect ecosystems. A provisional framework suggested indicators for the soil security dimensions, and a methodology to achieve a quantification. In this study, we illustrate the framework for the function soil carbon storage and the two dimensions of soil capacity and soil condition. The methodology consists of (i) the selection and quantification of a small set of soil indicators for capacity and condition, (ii) the transformation of indicator values to unitless utility values via expert-generated utility graphs, and (iii) a two-level aggregation of the utility values by soil profile and by dimension. For capacity, we used a set of three indicators: total organic and inorganic carbon content and mineral associated organic carbon in the fine fraction (MAOC) estimated via their reference value using existing maps of pedogenons and current landuse to identify areas of remnant genosoils (total organic and inorganic carbon) and the 90th percentile for MAOC. For condition we used the same set of indicators, but this time using the estimated current value and comparing with their reference-state values (calculated for capacity). The methodology was applied to the whole of Australia at a spatial resolution of 90 m × 90 m. The results show that the unitless indicator values supporting the function varied greatly in Australia. Aggregation of the indicators into the two dimensions of capacity and condition revealed that most of Australia has a relatively low capacity to support the function, but that most soils are in a generally good condition relative to that capacity, with some exceptions in agricultural areas, although more sampling of the remnant genosoils is required for corroboration and improvement. The maps of capacity and condition may serve as a basis to estimate a spatially-explicit local index of Australia's soil resilience to the threat of decarbonization. • Methodology based on the soil security assessment framework. • Quantification of the capacity and condition of the soil to store carbon. • Spatial estimation of the utility to support carbon storage for three indicators. • Australia has low capacity, but overall good condition to store carbon. • Intensive up-to-date sampling is required to corroborate the results. [ABSTRACT FROM AUTHOR]

• Structural plant inputs contribute to POC, soluble plant inputs contribute to MAOC. • Disturbed soil with high aggregation capacity can form and stabilize POC and MAOC. • Surface area availability and microbial necromass drive MAOC formation. • Soils with high aggregation potential regenerate post disturbance and protect SOC. • Plant inputs can mitigate initial losses of SOC following disturbance. Efforts to increase soil organic carbon (SOC) are pursued as a viable climate change mitigation strategy. Boosting SOC stocks requires increasing plant carbon (C) inputs and promoting their persistence in SOC. In well aerated mineral soils, water soluble inputs are expected to stabilize through chemical binding to minerals, forming mineral-associated organic carbon (MAOC) before or after microbial transformation, while structural inputs are expected to stabilize as particulate organic carbon (POC) via protection in soil aggregates. Although ample research is centered on the effects of soil aggregation, its disturbance (e.g., tillage), and microbial processing on SOC cycling, we still lack mechanistic understanding of how plant C input type (i.e., soluble versus structural) and disturbance (i.e., aggregate disruption) independently and in interaction affect POC and MAOC formation and stabilization in soils with inherently different degrees of aggregation. To this end, using 13C enriched structural and soluble plant inputs, we traced SOC formation and stabilization in a lab incubation experiment in soils with differing levels of aggregation, and capacity to form aggregates after disturbance. Our results showed that soluble plant inputs contributed substantially to MAOC and that higher formation and persistence of MAOC occurred in the highly aggregated soil. Moreover, the highly aggregated soil retained more soluble inputs stabilized as MAOC when disturbed. Disturbance in this fine textured, organic carbon rich soil stimulated regeneration of aggregates around structural plant inputs leading to greater persistence of aggregate-occluded POC. Soluble plant inputs, specifically, as well as structural plant inputs in the highly aggregated disturbed soil, compensated for SOC lost due to disturbance alone. Overall, this study provides mechanistic evidence suggesting that management strategies for SOC accrual should consider soil type, aggregation potential, and plant attributes to inform decisions surrounding tillage frequency and cropping regimes. Our mechanistic findings require testing. If confirmed in the field, they would suggest that no disturbance in tandem with high structural plant C inputs would benefit most SOC accrual in systems with soils that have little capacity to form organo-mineral complexes, including large aggregates. On the contrary, with sustained plant inputs, occasional disturbance in systems with soils that have a high capacity to form organo-mineral complexes can promote both POC and MAOC formation and stabilization. [ABSTRACT FROM AUTHOR]

• A dataset of biochar incubations is made available along with code to analyse it. • Curve fitting and data selection are key steps in biochar persistence modelling. • The H/C ratio remains the main indicator of biochar persistence. • Non-linear models improved correlations between BC 100 estimates and properties. The climate change mitigation benefits of biochar systems arise largely from carbon storage in biochar. However, while biochar is increasingly recognized as a carbon dioxide removal technology, there are on-going scientific discussions on how to estimate the persistence of biochar carbon when biochar is used in soils. Estimates vary from decades to millennia, building on different modelling approaches and evidence. Here, we revisited the persistence estimates derived from extrapolation of biochar incubation experiments, with the aims of making incubation data available, modelling choices transparent, and results reproducible. An extensive dataset of biochar incubations, including 129 biochar decomposition time series, was compiled and is made available alongside code for its analysis. Biochar persistence correlations were sensitive to data selection procedures and to the curve fitting modelling step, while soil temperature adjustments methods had less impact. Biochar H/C ratio remained the main predictor of biochar persistence, in line with previous research, regardless of the extrapolation assumptions (multi-pool exponential functions or power function) used in curve fitting. The relation between H/C and percentage of biochar carbon remaining after 100 years (BC 100) was better explained by a power model than a linear model, with R2 values between 0.5 and 0.9. Using multi-pool exponential functions, estimated BC 100 varied between 90 % and 60 % for H/C from 0 to 0.7. However, using power functions, BC 100 was constrained between 90 % and 80 % for the same H/C range. Additional information about the biochar, the pyrolysis conditions or the environmental incubation conditions did not significantly increase explained variance. Notably, the dataset lacks observations at H/C ratios below 0.2, of biochar made from manure and biosolids, biochar from processes other than slow pyrolysis, field studies, and incubation temperatures below 10 °C, which should guide future experimental work. The detailed analysis performed in this study does not cast doubts on the longevity of biochar carbon storage; rather, it confirms previous knowledge by critically examining the modelling, elucidating the assumptions and limitations, and making the analysis fully reproducible. There is a need for further interdisciplinary work on integration of various theories and approaches to biochar persistence, ultimately leading to the formulation of policy-relevant conclusions. [ABSTRACT FROM AUTHOR]

We modelled and mapped the distribution of three principal soil organic carbon (SOC) fractions across New South Wales and gained insights into the factors controlling their distribution. Carbon fractions are important for modelling soil carbon dynamics for carbon accounting, and as a potential indicator of soil quality. We considered particulate organic carbon (POC), humic organic carbon (HOC) and resistant organic carbon (ROC), which represent fractions of increasing bio-chemical stability. A dataset of 427 NSW profiles with mid-infrared (MIR) derived carbon fractions and a set of 16 predictor variables were applied. Random Forest and multiple linear regression techniques were applied, for depth intervals to 30 cm. The derived models and maps were of moderate strength, with validation results revealing Lin's concordance values for the 0–30 cm depth interval of 0.79 for total SOC stocks and 0.60 to 0.74 for the fraction stocks. Maps of mean stocks, and upper and lower 95% prediction intervals are presented. Absolute stocks (in Mg ha−1) of each fraction have a strong linear relationship with total SOC and are controlled by similar environmental and land management factors, normally increasing in a systematic way with increasingly moist climate (considering rainfall and temperatures), increasing mafic lithology (associated with more fertile, clay rich soils) and less disturbed land uses with higher vegetation cover. The environmental factors influencing the relative proportions of each fraction (% of total SOC) are more complex, but climate and lithology appear to be dominant, in addition to depth in profile. An SOC vulnerability index is mapped over the State, identifying areas where the stored carbon is most vulnerable to land management or environmental change. Our findings add to the understanding of factors driving the distribution of soil carbon and its fractions, which may ultimately contribute to more effective climate change mitigation programs. Unlabelled Image • Digital soil maps of three SOC fractions (at 100 m resolution) prepared for NSW • Absolute stocks of SOC fractions have strong linear relationship with total SOC. • All stocks increase with moister climate, more mafic soil and higher vegetation cover. • Relative proportions (% of total SOC) influenced by climate, lithology and depth • SOC vulnerability index mapped over NSW [ABSTRACT FROM AUTHOR]

• Organically managed soils had greater warming-induced increases in N mineralization. • Q10-Nmin was likely driven by SOM degradation. • Compost did not affect warming-induced increases in N mineralization. Rising global temperatures have the potential to increase soil nitrogen (N) mineralization from soil organic matter (SOM). By increasing SOM over time, management practices that increase SOM through the addition of soil amendments, such as compost, have been recognized as effective strategies for mitigating the effects of climate change and building resilience in agricultural ecosystems. However, the effects of these strategies on temperature-induced changes to soil N cycling are unclear, particularly when soils are managed to increase SOM. To determine how agricultural management history and compost amendments affect net N mineralization, net nitrification, and nitrous oxide (N 2 O) production, we performed a laboratory incubation of soils with two distinct agricultural management histories under three incubation temperatures. Three compost treatments (green-waste compost, food-waste compost, and no compost) were applied, each with and without the addition of synthetic urea fertilizer. We found that organically managed soil exhibited higher rates of net N mineralization and nitrification than conventionally managed soil, leading to greater nitrate production. The rate of N mineralization in organically managed soil was also more sensitive to temperature increases. Although compost addition stimulated microbial activity, it did not affect the N -cycling processes measured in this study at any temperature. Therefore, the implementation of climate change resilience and mitigation strategies aimed at augmenting stocks of soil carbon may render agricultural soils more susceptible to increased N mineralization and subsequent losses under warming, particularly if plant uptake of the mineralized N does not occur concurrently. Moreover, the effects of compost application to stimulate the immobilization of excess N is likely limited in soils with low background C. [ABSTRACT FROM AUTHOR]

• In-field variation in soil C stocks is assumed to make change detection infeasible. • We test this 'infeasibility assumption' using soils data from 45 cropland fields. • Estimated management effects for individual fields are wildly inaccurate. • Yet project-scale (i.e., multi-field) effect sizes can be robust and accurate. • Empirical sampling with current methods can inform the soil carbon debate. There is disagreement about the potential for regenerative management practices to sequester sufficient soil organic carbon (SOC) to help mitigate climate change. Measuring change in SOC stocks following practice adoption at the grain of farm fields, within the extent of regional agriculture, could help resolve this disagreement. Yet sampling demands to quantify change are considered infeasible primarily because within-field variation in stock sizes is thought to obscure accurate quantification of management effects on incremental SOC accrual. We evaluate this 'infeasibility assumption' using high-density (e.g., 0.1 ha sample–1), within-field, sampling data from 45 cropland fields inventoried for SOC. We explore how more typical within-field sampling densities, as well as field numbers and magnitude of simulated change in SOC stocks, impacts the ability to accurately quantify management effects on SOC change. We find that (1) stock change estimates for individual fields are inaccurate and variable, where marked losses and gains in SOC stocks are frequently estimated even when no change has occurred. Higher sampling densities (e.g., 1.2 versus 4.0 ha sample–1) narrow the range of estimated stock changes but inaccuracies remain large. (2) The accuracy of stock change estimates at the project level (i.e., multiple fields) were similarly sensitive to sampling density. In contrast to individual fields, however, higher sampling densities, as well as a greater number of fields (e.g., 30), generated robust and accurate, mean project-level estimates of carbon accrual, with ∼ 80 % of the estimates falling within 20 % of the simulated stock change. Yet such monitoring designs do not account for dynamic baselines, which necessitates measurement of stock changes in control, non-regenerative fields. We find (3) that higher sampling densities (e.g., 1.2 versus 4.0 ha sample–1), field numbers (e.g., 30 versus 10 pairs of fields), and magnitudes of simulated SOC stock change (+3 and +5 versus +1 Mg C ha−1 10 y–1) are then collectively required to make accurate estimates of management effects on stock change at the project level. The simulated effect sizes that could be consistently detected under these conditions included rates of SOC accrual considered achievable and meaningful for climate mitigation (e.g., 3 Mg C ha−1 10 y–1), with field numbers and sampling densities that are reasonable given current sampling methods. Our findings reveal the potential to use empirical approaches to accurately quantify, at project scales, SOC stock responses to practice change. We provide recommendations for data that government, farmer and corporate entities should measure and share to build confidence in the effects of regenerative practices, freeing the SOC debate from overreliance on theory and data collected at scales mismatched with agricultural management. [ABSTRACT FROM AUTHOR]

• Arable measures bear high potential to affect topsoil SOC stocks. • Mineral and organic fertilization, straw incorporation caused major SOC gain. • Liming and reduced tillage had little or even negative effects on SOC storage. • Sandy soils exhibited the largest relative response to management options. • Up to 10‰ SOC accrual per year were reached by combination of measures. Crop production often leads to soil organic carbon (SOC) losses. However, under good management practice it is possible to maintain and even re-accumulate SOC. We evaluated how different cropland management techniques affected SOC stocks in the topsoil (0–30 cm depth) of 10 long-term experiments (LTE) in Germany. We found that SOC stocks were particularly enhanced by mineral fertilization and organic amendments like straw incorporation and to a smaller degree by irrigation, but only slightly affected by the choice of preceding crops. In agreement with global meta -analyses, liming and reduced tillage had little or even negative effects on SOC storage, but effects also depended on fertilization. Management effects on SOC stocks were dependent on soil texture: sandy soils showed the lowest SOC stocks of 20.9 ± 2.3 (standard error of the mean) Mg ha−1, but exhibited the largest relative response to different management options. Annual changes in SOC stocks ranged from −3.0 ‰ with no mineral N fertilization, to + 6.1 ‰ with farmyard manure application, using the mineral-fertilized and limed treatment as reference. Even higher rates of up to + 10.6 ‰ yr−1 were reached with the combination of irrigation and straw incorporation. Note that the contribution of organic amendments to SOC accrual and thus to climate change mitigation must be adjusted for reduction in SOC at sites from which straw was removed. Overall, the potential of agricultural management to influence and enhance SOC stocks is significant. This potential is controlled by soil type and land-use duration, is largest for sandy soils with overall lowest SOC stocks, and is characterized by antagonistic and synergistic effects of different management practices. [ABSTRACT FROM AUTHOR]

Abstract Emissions of greenhouse gases (GHG) from paddy rice are significant, so reducing these emissions has significant potential for climate change mitigation. We investigated alternate wetting and drying (AWD) as part of an integrated management approach to enhance mitigation, together with combinations of mineral nitrogen (N), reduced tillage, a suitable combination of plant residues and well decomposed manure. To quantify GHG emissions, and the potential for mitigation without yield decline, a process-based model, DayCent was used to simulate methane (CH 4) and nitrous oxide (N 2 O) emissions from paddy rice (Oryza sativa L.) in Bangladesh. The four test sites selected were amended with mineral N fertilizer or an organic amendment (rice straw). A good agreement (p < 0.05) was observed between model simulated and measured daily CH 4 flux at most of these test sites with no significant bias. The seasonal CH 4 emission from a site receiving mineral N fertilizer at a rate of 110 kg N ha−1 was predicted by the model to be 210 and 150 kg ha−1 for the water management scenarios of continuous flood (CF) and AWD, respectively. These values compare well with estimates of CH 4 emissions using Intergovernmental Panel on Climate Change tier 1 methods for the different water regimes. Our model results suggest emission factors for N 2 O of 0.4% and 0.6% of applied fertilizer under CF and AWD water regimes, respectively. Based on modelling studies, AWD was found to be an important strategy not only with respect to reducing GHG emissions, but also in terms of cost effectiveness. We also found that integrated management is a promising option for farmers and policy makers interested in either yield increase, GHG mitigation or both. Yield scaled emissions intensity under AWD was found to be about 24% lower than under CF, followed by integrated management. Highlights • Greenhouse gas (GHG) emissions from rice were simulated with the DayCent model. • DayCent simulates methane and nitrous oxide emissions well in paddy rice. • Alternate wet and drying is the most effective mitigation option to mitigate GHGs. • Integrated management is promising for increasing yield and for GHG mitigation. [ABSTRACT FROM AUTHOR]

Abstract Soil organic carbon (SOC) in agricultural lands is vital for global food production and greenhouse gas (GHG) mitigation. Accurate quantification of the change in SOC stocks at regional or national scales, which depends heavily on reliable spatiotemporal carbon (C) input data, remains a big challenge. Here we use the process-based RothC model to estimate change in SOC stocks across Canada for 1971 to 2015, based on calculated annual C flows between cropland and livestock sectors. Total C input to 0–20 cm soils from crops, manure, and biosolids in Canada increased by 81% from 1971 to 2015, which shifted Canadian agricultural lands from a CO 2 source before 1990 (−1.1 Tg C yr−1) to a small sink during 1990–2005 (4.6 Tg C yr−1), and a larger sink thereafter (10.6 Tg C yr−1). The increasing trend of SOC stocks is mainly driven by the increases in crop yield; the enhanced C sink since 2005 reflects increasing C input largely driven by the increasing area and yield of canola. SOC sequestration showed a potential to offset ~34% or more of agricultural GHG emission since 1990. Increasing crop yields and adopting crop mixes that input proportionately more below-ground C, such as canola and oat, showed potential additional opportunity to sequester SOC, estimated at 1.7 Tg C yr−1 for 2016–2030 in Canada. This study illustrates that SOC sequestration is driven largely by plant C inputs, and shows that agronomic measures which augment C input through crop choices and yield-enhancing practices can profoundly benefit climate mitigation strategies. Graphical abstract Unlabelled Image Highlights • Increasing crop production affects agricultural SOC and thereby GHG mitigation. • A whole-system (cropland and livestock) C fluxes analysis was performed. • Total carbon input to 0–20 cm soils increased by 81% from 1971 to 2015 in Canada. • Canadian agricultural land shifted from a CO 2 source before 1990 to a sink thereafter. • Increasing C input through crop choices and yield-enhance benefit SOC sequestration. [ABSTRACT FROM AUTHOR]

Biochar production for soil amendment was recently proposed as a tool to mitigate climate change, reducing soil greenhouse gas (GHG) emissions and sequestrating carbon (C) in soil. The aim of this research project was to test the hypothesis that only biochars with specific requirements (low H/C org and O/C org ratios, high C/N ratio) can reduce soil N 2 O emissions without increasing CO 2 emissions in the short term. A 45-days incubation study was carried out, in which six engineered biochars made from the pyrolysis of wood, switchgrass and the solid fraction of pig manure (SFPM), were amended to two agricultural soils (loamy sand and silt loam) at a dose of 2% (w/w) in 1-liter jars. Soil moisture content was adjusted at 80% of water-filled pore space with a solution of ammonium nitrate that corresponds to 170 kg of nitrogen per hectare. N 2 O and CO 2 emissions were analysed on days 2, 3, and then weekly. Soil chemical properties and bacterial richness, composition and taxonomy were analysed after the incubation period. When compared to the control soils without biochar, N 2 O emissions were decreased by 42 to 90%, but only in the silt loam amended with biochars made from wood and switchgrass, these biochars having a high C/N ratio (>30). Lower N-NH 4 + and N-NO 3 − concentrations were observed in these biochar treatments than in control soil. Moreover, two bacterial classes ( Deltaproteobacteria and Thermoleophilia ) were correlated with a decrease in N 2 O emissions. For each type of biochar, those produced at the highest temperature with low O/C org and H/C org ratios resulted in the lowest increase in CO 2 emissions, which could indicate a higher biochar carbon stability. Overall, results of this study demonstrated that biochar can either increase or decrease soil GHG emissions depending on its properties, and that the effect can differ according to soil properties. Future long-term studies in the field in the presence of crop should be carried out in order to validate the conclusions of this study. [ABSTRACT FROM AUTHOR]

• Temperature was most important regulator of plant litter decay and retention of C in soil. • Precipitation plays an interactive role with temperature. • More than ½ of litter C was lost <1 yr and only about 5–12% remained after 8–10 yr. • Soil properties did not play a discernible role in these processes. • No-till slowed plant litter decay, but only initially in arid environments. Identifying the primary regulators of plant litter decay in agricultural soils is important for understanding ecosystem function now and for managing post-harvest litter in a warmer world. We conducted a litter decay study at 11 sites in Canada and New Zealand with diverse soils and climates. We incorporated 13C-labelled barley litter into the surface 10 cm of soil and monitored amount remaining over 8–10 years; at five sites litter was also applied to the soil surface to simulate no-tillage. Our objectives were to evaluate litter decay as influenced by soil type, tillage practice, and environmental conditions, and quantify the most important factors controlling C retention in soil. Loss of C via decomposition occurred quickly – more than half was lost within 1 year and only about 5–12% remained at the end of the experiment. A double exponential decay model, based on temperature and developed from the first 5 years of the study, accurately described litter decay, but only after re-tuning using measurements from the full experimental period. Including precipitation in the model further improved its fit. Soil properties exerted minimal discernible influence on the amount of litter C remaining, implying that properties such as the amount and surface area of clay minerals, were less important than climate in limiting litter decay or enhancing retention of C in soil. Comparison of litter application treatments showed that no-till slowed decay of plant litter, but only for a short time (∼1 yr) and only in environments with a mean annual precipitation of <1000 mm. These findings have implications for the role of soils in climate mitigation. If only 10% of added plant litter C remains in soil beyond a few years, regardless of climate, residue placement, or soil type, then rates of soil gain are limited without substantive increases in residue inputs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbon sequestration potential of the clay fraction. • Determination of the carbon saturation deficit of each mineral of the clay fraction. • Identification of the equation for the prediction of carbon sequestration potential. • Identification of land uses with the greatest carbon sequestration potential. The capacity of soil to sequester carbon (C) is a key process that promotes the reduction of CO 2 in the atmosphere. Soils can absorb as much as 20% of anthropogenic carbon emissions, which can contribute to mitigate climate change. This capacity relies on the organo-mineral association, which includes different minerals, Fe and Al oxides, which have a critical soil organic carbon (SOC) sorption surface. Based on an equation of the potential C saturation deficit of fine soil particles (<20 μm/silt and clay fractions) for tropical regions, this study investigated the SOC sequestration potential of the clay fraction for soils in Piracicaba region, São Paulo State, Brazil as influenced by the clay minerals. This potential was fitted to a spatial regression model for soil depths 0–20 cm and 80 to 100 cm. In the surface layer, the sequestration potential was mostly explained by the relative abundance of soil minerals (Kaolinite, Hematite, Goethite and Gibbsite) determined using vis-NIR-SWIR spectroscopy. A direct relationship was observed with Goethite and Gibbsite, indicating that low concentrations would reduce the sequestration potential. At 80 to 100 cm depth, Kaolinite and Hematite explained most variation in SOC sequestration potential. Additionally, the C associated with the mineral fraction and the C saturation potential as a function of minerals were modeled and a strong importance of hematite in the C sequestration and stabilization cycle was identified at both depths. The individual mineral contribution to SOC sequestration potential was also mapped, which identified high contributions of goethite and gibbsite for deep soil layers. The influence of land use on the carbon sequestration potential of minerals was observed, with the greatest potential being found in areas with pasture and cropping mosaics and grassland and forest mosaics, with a high presence of kaolinite and hematite. These minerals have a greater potential for carbon sequestration at greater depths and, therefore, could be critical in climate change mitigation strategies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biochar decreased Acidobacteria abundance while increasing Gemmatimonadetes. • Biochar increased the genetic richness and diversity of soil microbes. • Experiment types played a role in shaping microbial response to biochar addition. • Biochar and soil properties were related to the changes of phyla abundance. • Selecting key biochar properties can improve soil microbial functions. Biochar has gained global attention due to its potential for climate change mitigation and soil quality improvement. Yet, the consequences of biochar additions for soil microbes -the major biotic drivers of soil function- remain unknown across global environmental gradients. We aimed to explore the responses of soil bacterial communities to biochar addition, and further investigate how biochar and soil properties impact these responses. We conducted a global meta -analysis and found that, in general, biochar has a limited impact on the proportion of major bacterial phyla, with only Acidobacteria and Gemmatimonadetes being largely impacted: the relative abundance of Acidobacteria decreased by 14.6%, while that of Gemmatimonadetes increased by 19.8%. Also, the experimental type played a role in shaping the response of microbial community to biochar application. In addition, biochar significantly promoted the diversity of soil bacteria, i.e., genetic richness and diversity. These changes were significantly associated with biochar load, C/N ratio, pyrolysis temperature, biochar pH, as well as soil C/N ratio and pH. We further found that the impacts of biochar on functional diversity, i.e., C substrate richness consumed by soil microbes increased with the biochar load, which might relate to increased genetic richness. Our work suggests that selecting key biochar properties can improve soil quality, microbial function, and climate change mitigation while maintaining the positive impacts of biochar on soil microbial diversity. Further research is needed to link the response of soil microbial composition at the genus level to biochar addition, with microbial functions. [ABSTRACT FROM AUTHOR]

• SOC sequestration beneath hedgerows continues for decades but at declining annual rates. • Shallow topsoils do not contribute to sequestration. • Hedgerow age and ammonium oxalate extractable Al (Alo) are master variables of sequestration. • Expanding hedgerows to 10% of the agricultural land (EU Biodiversity Strategy) is relevant for climate change mitigation. Hedgerows supply multiple ecosystem services in agricultural landscapes and have been advocated to provide a relevant carbon sink for climate change mitigation. Using a space-for-time approach, we investigated carbon sequestration and fractionation in soils beneath hedgerows 1 to 70 yr after planting on cultivated land in the humid continental climate of Eastern Austria. We sampled 54 pairs of hedgerow and adjacent cultivated soil volumetrically to 40 cm depth and analysed them for bulk soil (< 2 mm) mass, SOC fractions of differential stability and related soil properties. Total SOC stocks to 40 cm depth increased significantly (p < 0.05) by 34.7 ± 4.70 Mg C ha−1 to 119 ± 6.77 Mg C ha−1 beneath 31–70 yr old hedgerows, and by 15.6 ± 5.94 Mg C ha−1 to 106 ± 8.98 Mg C ha−1 beneath 1–30 yr old hedgerows. Carbon sequestration was limited to the organic and mineral topsoil layer (0–20 cm), SOC changes (ΔSOC) at 20–40 cm depth were small and insignificant (p < 0.05). Linear regression of ΔSOC on hedgerow age (1–70 yr) yields a mean SOC sequestration rate of 0.65 ± 0.10 Mg ha−1 yr−1 down to 40 cm depth. Combining our data with published work we found hedgerow age explaining ∼65 % of the variation of ΔSOC in topsoils, and ∼68 % in shallow subsoils; ΔSOC increases with hedgerow age in topsoils, but declines in subsoils. Annual sequestration rates decline with hedgerow age in both soil layers. Up to 30 years after conversion, SOC is preferably sequestered in labile particulate organic matter (POM; > 20 µm), and stabilised in the mineral-associated organic matter (MAOM; < 20 µm fraction) thereafter. In the bulk soil and MAOM fraction, SOC strongly increases with oxalate-extractable Al (Al o). We conclude that SOC sequestration beneath hedgerows offers a relevant CO 2 sequestration potential which is mainly limited to topsoils and primarily controlled by hedgerow age and Al o. [ABSTRACT FROM AUTHOR]

[Display omitted] • Topsoil SOC stocks and soil structure recovered 26 years after topsoil removal. • Topsoil buried under the plough layer non-linearly lost 30–50% of SOC in 55 years. • Topsoil SOC recovery and stability of buried SOC was dependent on N fertilisation. Erosion of fertile topsoil means severe land degradation and yield loss. Here, we show that reformation of the fertile topsoil and crop yield increase is possible by good agricultural management within a few decades. Evidence relies on a unique long-term field experiment in NE Germany, where in 1964 the sandy soil (Luvisol) had been excavated to 52 cm and refilled with subsoil material of 52 cm thickness. Undisturbed trials served as controls, these were all combined with either no fertilisation, mineral fertilisation or a combined mineral and organic fertilisation. It took only 26 years until new topsoil had been formed. Its soil organic carbon (SOC) and total N stocks and the aggregate size distributions equalled that of the other treatments; SOC sequestration corresponded to 28 Mg SOC ha−1. The former topsoil buried below the plough layer lost 30–50 percent of its SOC during 5 decades in a non-linear manner. We conclude that our current agricultural tools can restore degraded arable land rather fast and contribute to climate change mitigation through a significant SOC sequestration as well. [ABSTRACT FROM AUTHOR]

• MAOM-C content was modeled for arable fields with different management. • Organic fertilization, crop rotation, and soil texture define MAOM-C content. • A POM-C/MAOM-C ratio indicator was developed and tested. • This indicator contains information on MAOM-C saturation deficits. • POM-C/MAOM-C ratio indicators might support climate mitigation strategies. Soil organic matter (SOM) is indispensable for soil health and, in the context of climate change, is considered a significant CO 2 sink. Improving agricultural management to increase long-term soil organic carbon (SOC) stocks for mitigating climate change requires tools that estimate short and long-cycling SOM pools. In this study, we analyzed changes in fast-cycling particulate organic matter (POM) and slow-cycling mineral-associated organic matter (MAOM) induced by common management practices, i.e., fertilization and crop rotation in topsoils from 25 Central European long-term field experiments. When relating MAOM-C contents to recent MAOM-C saturation levels, estimated sequestration potentials were only met in coarse-textured soils under appropriate agricultural management or fine-textured soils under extreme organic fertilization. Soil texture, organic fertilization, and below-ground OC inputs through root exudates and root biomass were decisive for estimating MAOM-C, allowing for calibration of a mixed-effects model (Nakagawa's: marginal R2 m = 0.6, conditional R2 c = 0.89). While the models containing soil texture and organic fertilization parameters can be validated and generalized (R2 = 0.43), the below-ground OC input predictor substantially decreases the generalizability of the validated models (R2 = 0.14). According to quantile regression models, we estimate the average difference in MAOM-C concentration between well-managed and control site (without organic fertilization) topsoils to 4.1 mg g−1 soil. In dependence on the soil bulk density, this amounts to 1.38 – 1.84 t ha−1 MAOM-C stocks or 5.06 – 10.1 t ha−1 CO 2 -equivalents. POM-C was difficult to predict (R2 = 0.28), presumably due to strong POM dynamics. The POM-C / MAOM-C ratio can inform on the effects of agricultural practices in before/after management change comparisons. Under increasing SOC concentration, an increasing POM-C / MAOM-C ratio indicates that the effects of organic fertilization do not transfer to real effects on long-term SOC sequestration. Because MAOM-C depends on soil texture, this ratio is also a covariate of soil texture, limiting it for comparisons between sites with different textures. However, our data indicate that agricultural long-term field experiment soils constantly approximate MAOM-C saturation when the POM-C/MAOM-C ratio is >0.35. This ratio might be used as a management goal to prevent organic over-fertilization and N loss, especially on coarse-textured soils. Thereby, the POM-C / MAOM-C ratio can help to optimize SOC management and sequestration on agricultural soils and support climate change mitigation strategies in Central Europe. [ABSTRACT FROM AUTHOR]

• We evaluated on-farm management and site predictors for soil organic carbon storage. • Soil texture strongly predicted soil organic carbon sequestration potentials. • Conservation management practices were overall of less importance. • Future carbon farming schemes need to properly account for biophysical constraints. Although conservation agriculture practices evidently facilitate the build-up of soil organic carbon (SOC), the sequestration potential of arable soils is strongly mediated by edaphic attributes; so far, their interplay is not well understood. Deciphering these drivers is however important to correctly estimate SOC storage potentials in arable soils and to derive effective strategies for the implementation of successful measures. By using an on-farm approach, we conducted a pairwise comparison of 21 conventional and highly innovative 'pioneer' farms across a wide range of arable soil types and evaluated the leverage of site attributes and management practices such as crop diversity, reduced tillage, organic fertilization, cover cropping and inter cropping on the SOC sequestration potential. While most pioneer management practices proved beneficial for the sequestration of SOC – particularly cover cropping and crop diversity – our results clearly show that soil texture was the most significant shaping factor. Coarse-textured soils had a significantly higher potential for SOC accrual compared to medium- and fine-textured soils. The initial SOC content also had a significant effect on prevalent sequestration potentials. Based on the fact of a clear predominance of natural site conditions over management impacts in enhancing SOC storage of arable soils, we call for a critical discussion of carbon farming schemes. As similar efforts and costs of implementing carbon farming measures will have distinctive carbon gains, dependent on environmental constraints beyond farmers' influence, we advocate for strategies harmonizing both activity- and results-based approaches to maximize the ecological effectiveness and the spatial dissemination of soil health innovations. Carbon farming schemes thus need reconsideration within the state-of-the-art scientific framework of carbon saturation behaviour in order to properly account for biophysical constraints when formulating soil-related climate change mitigation policies. [ABSTRACT FROM AUTHOR]

• Soil organic carbon (SOC) stabilities were studied in two agroforestry systems. • We studied hedgerow and shelterbelt systems and their component land-uses (cropland vs forest). • The biological stability of SOC was higher in hedgerow than in shelterbelt system. • The biological stability of SOC was higher in cropland than in forest land-use. • Agroforestry system by land-use interactively affected the thermal stability of SOC. Agroforestry systems play important roles in carbon (C) sequestration in western Canada. However, the effect of agroforestry system type and their component land-uses (forested area and cropland) on soil organic C (SOC) stability is poorly understood. We studied SOC stability in surface soils from two agroforestry systems (shelterbelt system and hedgerow system) and their component land-uses (forested area and cropland) in western Canada. A 61-day laboratory incubation was used to assess SOC biological stability, and compared to thermal stability measured during ramped combustion. The shelterbelt system had a lower biological stability than the hedgerow system as indicated by the higher cumulative soil respiration and SOC loss during incubation. Cumulative respiration and SOC loss during incubation in the forested area was 1.63 and 1.57 times that in the cropland, indicating a lower SOC biological stability in the former. Thermal stability indicators, CO 2 -T 50 (the temperature at which half of the CO 2 is produced), DSC-T 50 (the temperature at which half of the exothermic energy of the SOC is released) and ROI (return on energy investment) were influenced by the interactive effects of the agroforestry system by land-use, while TG-T 50 (the temperature at which half of the exothermic mass is lost) and Ed (energy density) were influenced by land-use and the interactive effects of agroforestry system by land-use. Hedgerow cropland had a higher SOC thermal stability (higher TG-T 50) than the hedgerow forested area, while there was no difference among shelterbelt cropland, and the forested areas of shelterbelt and hedgerow system. We conclude that planting trees on agricultural land to form agroforestry systems alters SOC biological and thermal stabilities, and the hedgerow system may have a more stable SOC than the shelterbelt system, thus, maintaining the hedgerow system may enhance C stability, promote C sequestration and climate change mitigation. [ABSTRACT FROM AUTHOR]

• The drought-induced AGB decline occurred only at aridity levels > 0.37. • Higher SOM content led to increased AGB, with greater gains below 2.5% SOM. • SOM alleviates the adverse effects of drought on AGB by improving soil structure. Increasing the resilience of plant productivity to drought is crucial for provisioning ecosystem services. Although soil organic matter (SOM) promotes plant growth, whether SOM mitigates aboveground biomass (AGB) loss due to drought in alpine grasslands remains unclear. Here, we evaluated the link between AGB and SOM across 209 alpine grassland sites along a 3,500-km aridity gradient on the Tibetan Plateau. We observed that AGB decreased in response to increasing aridity severity only when the aridity level was above a threshold of 0.37. We further found a stronger positive relationship between AGB and SOM in more arid conditions than in less arid ones, with an abrupt increase beyond an aridity threshold of 0.64. Our results confirm that SOM alleviates drought stress on AGB, mainly by reducing bulk density, increasing fine fraction aggregates, and increasing cation exchange capacity. The identified aridity thresholds and mechanisms emphasise the importance of soil carbon sequestration strategies for biomass production and climate change mitigation in arid areas. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tillage versus no tillage and the use of DMPSA tested in rainfed semiarid barley. • DMPSA decreased 15N 2 O emissions during the preharvest period. • Remarkable postharvest N 2 O peak after rewetting derived from endogenous N. • No tillage and DMPSA increased crop recovery of exogenous and endogenous N, respectively. • No tillage reduced soil residual N, while DMPSA increased topsoil N retention. The sustainability of rainfed crops under semiarid conditions is threatened by low plant nitrogen (N) recovery as well as the potential loss of reactive N to the environment. A field 15N tracing experiment on barley (Hordeum vulgare L.) under rainfed conditions was carried out to study how different tillage management practices and the use of the nitrification inhibitor DMPSA affected the fate of N. The experiment consisted of a factorial combination of tillage (i.e., no tillage, NT, and conventional tillage, T) and fertilizer treatments (unfertilized control and ammonium nitrate, AN, with or without DMPSA). Single-labelled ammonium nitrate (15NH 4 NO 3 , 15AN, or NH 4 15NO 3 , A15N) was applied at top-dressing to microplots at a rate of 80 kg N ha−1. Our results show out that DMPSA modulates the nitrification process from both fertilizer-N and endogenous soil-N (which was the main contributor to plant N uptake and N 2 O emissions), affecting soil residual N at the end of the cropping period (i.e., higher topsoil retention of 15AN in DMPSA-amended plots). Generally, cumulative N 2 O emissions from fertilizer were derived from 15AN rather than from A15N, thus confirming the site-specific choice of the source of synthetic N as an effective N 2 O mitigation strategy. Two months after harvest, a rewetting event produced a remarkable N 2 O emission peak that drove total cumulative N 2 O emissions and was also mainly derived from endogenous N. These results suggest that dry seasons could decrease N 2 O losses after fertilization while causing critical peaks after rewetting, thus potentially limiting the effectiveness of mitigation strategies. The average plant N recovery from the synthetic fertilizer was 22.6%, while the use of DMPSA combined with NT enhanced plant N uptake from endogenous soil-N. This could be a result of the improved crop development and plant N acquisition under NT, consistent with the decrease of soil N retention for A15N in the deeper layer at the end of the experiment in the nontilled plots. This study contributes to the mechanistic understanding of the effect of nitrification inhibitors and tillage on N 2 O emissions, soil N dynamics and N plant recovery, revealing relevant effects of both management strategies and a critical role of endogenous soil-N under dry rainfed conditions. It can be concluded that, under the conditions of our study, combining DMPSA with NT could help to improve plant N recovery, thus resulting in positive impacts on reactive N loss and climate change mitigation and adaptation. [ABSTRACT FROM AUTHOR]

• Several challenges limit SOC measurement accuracy and reliability. • Spatial heterogeneity, especially on rangelands, drives SOC measurement uncertainty. • Small sample sizes are unreliable for detecting and quantifying expected SOC changes. • Non-parametric statistical methods are more reliable than t-tests in realistic conditions. • Common SOC verification methods must improve to provide robust reliability. Using soil organic carbon (SOC) to generate carbon offsets requires reliably quantifying SOC sequestration. However, accuracy of SOC measurement is limited by inherent spatial heterogeneity, variability of laboratory assays, unmet statistical assumptions, and the relatively small magnitude of SOC changes over time, among other things. Most SOC measurement protocols currently used to generate offsets for C markets do not adequately address these issues, threatening to undermine climate change mitigation efforts. Using analyses and simulations from 1,117 soil samples collected from California crop and rangelands, we quantified measurement errors and sources of uncertainty to optimize SOC measurement. We demonstrate that (1) spatial heterogeneity is a primary driver of uncertainty; (2) dry combustion assays contribute little to uncertainty, although inorganic C can increase error; (3) common statistical methods—Student's t -test and its relatives—can be unreliable for SOC (e.g. at low to medium sample sizes or when the distribution of SOC is skewed), which can lead to incorrect interpretations of SOC sequestration; and (4) common sample sizes (10–30 cores) are insufficiently powered to detect the modest SOC changes expected from management in heterogeneous agricultural landscapes. To reduce error and improve the reliability of future SOC offsets, protocols should: (1) require power analyses that include spatial heterogeneity to determine minimum sample sizes, rather than allowing arbitrarily small sample sizes; (2) minimize the use of compositing; (3) require dry combustion analysis, by the same lab for all assays; and (4) use nonparametric statistical tests and confidence intervals to control Type I error rates. While these changes might increase costs, they will make SOC estimates more accurate and more reliable, adding credibility to soil management as a climate change mitigation strategy. [ABSTRACT FROM AUTHOR]

• Sizable discrepancy exist in modeled carbon budgets using gSSURGO and RaCA datasets. • The simulated carbon budget was more sensitive to SOC% than to bulk density. • Modeled ΔSOC uncertainty correlates with SOC% negatively and temperature positively. • SOC stock uncertainty has small impacts on simulated carbon credit. Cropland carbon budget depicts the amount of carbon flowing in and out of agroecosystems and the changes in carbon stocks of soil and living biomass during the same period. Soil carbon credit is the additional change in soil carbon stock under certain farming practices compared with the business-as-usual practices. Accurately calculating cropland carbon budget and soil carbon credit is critical to assessing climate change mitigation potential in agroecosystems. The calculation of cropland carbon budget and soil carbon credit is sensitive to local soil and climatic conditions, especially initial soil organic carbon (SOC) stock, which is determined by both SOC concentration (SOC%) and bulk density (Bulk_Density). SOC stock data are either from soil sampling or gridded public survey data. In agroecosystem models, SOC stock data are a key model input for quantifying cropland carbon budget and soil carbon credit. However, various types and degrees of uncertainties exist in SOC stock datasets, which propagate to the quantification of SOC stock change. In particular, a large discrepancy is found in two widely used SOC stock datasets — Rapid Carbon Assessment dataset (RaCA) and Gridded Soil Survey Geographic Database (gSSURGO) — in the U.S. Midwest, with a relative difference (quantified using Normalized Root Mean Square Error, NRMSE) of 48.0% for 0–30 cm SOC stock between the two datasets. It remains largely unclear how uncertainty in SOC stocks affects the calculation of cropland carbon budget and soil carbon credit. To address this question, we used a well-validated process-based agroecosystem model, ecosys , to assess the impacts of SOC stock uncertainty on carbon budget and soil carbon credit calculation in the U.S. Midwestern corn-soybean rotation systems. Our results reveal the following findings: (1) A sizable discrepancy exists in simulated cropland carbon budget between using gSSURGO and using RaCA for their SOC% and Bulk_Density as model inputs, with a Pearson correlation coefficient (r) of only 0.4 for simulated change of SOC stock (ΔSOC) using these two different soil datasets. (2) Simulated cropland carbon budget components were more sensitive to initial SOC% than to Bulk_Density. For example, the upper and lower quartiles of multi-year averaged ΔSOC were −29.8 and 4.8 gC/m2/year for the selected counties respectively, with an uncertainty of 13.7 and 0.7 gC/m2/year induced by uncertainties in initial SOC% and Bulk_Density, respectively. (3) Both simulated ΔSOC and its uncertainty were negatively correlated with initial SOC%, whereas ΔSOC was negatively correlated with air temperature, and ΔSOC uncertainty was positively correlated with air temperature. (4) The uncertainty of calculated soil carbon credits was much smaller compared with the uncertainty of calculated absolute carbon budgets assuming the same SOC stock uncertainty level in the inputs. Specifically, in our assessment comparing planting cover crops vs no cover crop, the uncertainty of calculated soil carbon credits induced by initial SOC% uncertainty was less than 4% (relative to the quantified value of the soil carbon credits) for 90% of the cases. Our analysis highlights that high accuracy measurement of SOC% as inputs is needed for the calculation of cropland carbon budgets; however, soil carbon credit quantification is much less sensitive to the initial SOC% inputs, and the current publicly available soil datasets (e.g., gSSURGO) are largely suitable for the calculation of soil carbon credits. [ABSTRACT FROM AUTHOR]

• The CH 4 fluxes from global forest soils are estimated. • MAT, MAP, soil BD, SOC, and TN were the key drivers of the forest soil CH 4 fluxes. • The CH 4 budget of global forest soils is 14.98 [7.96–21.73] Tg CH 4 yr−1. • Nearly 3.17% of the total area of global forest soils was a net CH 4 source. • The global mean CH 4 uptake in forest soils was 3.95 kg CH 4 ha−1 yr−1. Forest ecosystems play an important role in the global CH 4 cycle. Understanding and quantifying the contribution and distribution of CH 4 sinks and sources in global forest soils is vital for assessing realistic approaches to climate change mitigation. Here, we compiled a dataset of in situ global forest soil CH 4 fluxes from published data, incorporating 772 case studies covering boreal (n = 12), temperate (n = 369), subtropical (n = 208), and tropical (n = 183) forests and spanning 1991–2020 as a basis to build the mixed-effect model. Using the screened best model, we identified the main drivers and predicted the global distribution of the forest soil CH 4 flux. Our research revealed that global forest soil CH 4 uptake decreased significantly with increasing mean annual temperature (MAT), soil bulk density (BD), soil organic carbon (SOC), and soil total nitrogen (TN) but increased significantly with increasing mean annual precipitation (MAP). The global mean CH 4 uptake rate in forest soils was 3.95 ± 1.78 kg CH 4 ha−1 yr−1, with the total sink of 14.98 ± 6.75 Tg CH 4 yr−1. The soil CH 4 sinks in temperate and tropical forests contributed 84 % to the total sink of global forests. The CH 4 emission rate in global forest soils averaged 1.12 ± 1.11 kg CH 4 ha−1 yr−1, with the total source of 0.14 ± 0.14 Tg CH 4 yr−1. Nearly 3 % of the total area of global forest soils was a net CH 4 source. In summary, we identified the key drivers of forest soil CH 4 flux and improved previous estimates of the global CH 4 budget in forest soils. These findings can support decision-making related to forest management and greenhouse gas restrictions. [ABSTRACT FROM AUTHOR]

• Sizable discrepancy exist in modeled carbon budgets using gSSURGO and RaCA datasets. • The simulated carbon budget was more sensitive to SOC% than to bulk density. • Modeled ΔSOC uncertainty correlates with SOC% negatively and temperature positively. • SOC stock uncertainty has small impacts on simulated carbon credit. Cropland carbon budget depicts the amount of carbon flowing in and out of agroecosystems and the changes in carbon stocks of soil and living biomass during the same period. Soil carbon credit is the additional change in soil carbon stock under certain farming practices compared with the business-as-usual practices. Accurately calculating cropland carbon budget and soil carbon credit is critical to assessing climate change mitigation potential in agroecosystems. The calculation of cropland carbon budget and soil carbon credit is sensitive to local soil and climatic conditions, especially initial soil organic carbon (SOC) stock, which is determined by both SOC concentration (SOC%) and bulk density (Bulk_Density). SOC stock data are either from soil sampling or gridded public survey data. In agroecosystem models, SOC stock data are a key model input for quantifying cropland carbon budget and soil carbon credit. However, various types and degrees of uncertainties exist in SOC stock datasets, which propagate to the quantification of SOC stock change. In particular, a large discrepancy is found in two widely used SOC stock datasets — Rapid Carbon Assessment dataset (RaCA) and Gridded Soil Survey Geographic Database (gSSURGO) — in the U.S. Midwest, with a relative difference (quantified using Normalized Root Mean Square Error, NRMSE) of 48.0% for 0–30 cm SOC stock between the two datasets. It remains largely unclear how uncertainty in SOC stocks affects the calculation of cropland carbon budget and soil carbon credit. To address this question, we used a well-validated process-based agroecosystem model, ecosys , to assess the impacts of SOC stock uncertainty on carbon budget and soil carbon credit calculation in the U.S. Midwestern corn-soybean rotation systems. Our results reveal the following findings: (1) A sizable discrepancy exists in simulated cropland carbon budget between using gSSURGO and using RaCA for their SOC% and Bulk_Density as model inputs, with a Pearson correlation coefficient (r) of only 0.4 for simulated change of SOC stock (ΔSOC) using these two different soil datasets. (2) Simulated cropland carbon budget components were more sensitive to initial SOC% than to Bulk_Density. For example, the upper and lower quartiles of multi-year averaged ΔSOC were −29.8 and 4.8 gC/m2/year for the selected counties respectively, with an uncertainty of 13.7 and 0.7 gC/m2/year induced by uncertainties in initial SOC% and Bulk_Density, respectively. (3) Both simulated ΔSOC and its uncertainty were negatively correlated with initial SOC%, whereas ΔSOC was negatively correlated with air temperature, and ΔSOC uncertainty was positively correlated with air temperature. (4) The uncertainty of calculated soil carbon credits was much smaller compared with the uncertainty of calculated absolute carbon budgets assuming the same SOC stock uncertainty level in the inputs. Specifically, in our assessment comparing planting cover crops vs no cover crop, the uncertainty of calculated soil carbon credits induced by initial SOC% uncertainty was less than 4% (relative to the quantified value of the soil carbon credits) for 90% of the cases. Our analysis highlights that high accuracy measurement of SOC% as inputs is needed for the calculation of cropland carbon budgets; however, soil carbon credit quantification is much less sensitive to the initial SOC% inputs, and the current publicly available soil datasets (e.g., gSSURGO) are largely suitable for the calculation of soil carbon credits. [ABSTRACT FROM AUTHOR]

• The CH 4 fluxes from global forest soils are estimated. • MAT, MAP, soil BD, SOC, and TN were the key drivers of the forest soil CH 4 fluxes. • The CH 4 budget of global forest soils is 14.98 [7.96–21.73] Tg CH 4 yr−1. • Nearly 3.17% of the total area of global forest soils was a net CH 4 source. • The global mean CH 4 uptake in forest soils was 3.95 kg CH 4 ha−1 yr−1. Forest ecosystems play an important role in the global CH 4 cycle. Understanding and quantifying the contribution and distribution of CH 4 sinks and sources in global forest soils is vital for assessing realistic approaches to climate change mitigation. Here, we compiled a dataset of in situ global forest soil CH 4 fluxes from published data, incorporating 772 case studies covering boreal (n = 12), temperate (n = 369), subtropical (n = 208), and tropical (n = 183) forests and spanning 1991–2020 as a basis to build the mixed-effect model. Using the screened best model, we identified the main drivers and predicted the global distribution of the forest soil CH 4 flux. Our research revealed that global forest soil CH 4 uptake decreased significantly with increasing mean annual temperature (MAT), soil bulk density (BD), soil organic carbon (SOC), and soil total nitrogen (TN) but increased significantly with increasing mean annual precipitation (MAP). The global mean CH 4 uptake rate in forest soils was 3.95 ± 1.78 kg CH 4 ha−1 yr−1, with the total sink of 14.98 ± 6.75 Tg CH 4 yr−1. The soil CH 4 sinks in temperate and tropical forests contributed 84 % to the total sink of global forests. The CH 4 emission rate in global forest soils averaged 1.12 ± 1.11 kg CH 4 ha−1 yr−1, with the total source of 0.14 ± 0.14 Tg CH 4 yr−1. Nearly 3 % of the total area of global forest soils was a net CH 4 source. In summary, we identified the key drivers of forest soil CH 4 flux and improved previous estimates of the global CH 4 budget in forest soils. These findings can support decision-making related to forest management and greenhouse gas restrictions. [ABSTRACT FROM AUTHOR]

• SOC sequestration potentials of grass-clover leys and cattle slurry were assessed. • No interaction between grassland proportion and cattle slurry input was found. • SOC stocks increased with an increasing share of grass-clover in the crop rotation. • Of applied slurry-C, 11% was retained in the soil. Grass-clover leys in crop rotations on dairy farms may contribute to mitigation of climate change through soil organic carbon (SOC) sequestration. However, accurately quantifying SOC sequestration potential of grass-clover leys is a challenge as long-term experiments with regular soil sampling and crop rotations with varying grassland proportion are extremely rare. Based on a long-term organic dairy crop rotation experiment at Foulumgaard Experimental Station (Denmark) initiated in 1987 with contrasting crop rotations (2 or 4 years of grass-clover in a six-course rotation) and cattle slurry input from 2006 to 2018, we examined the effect of grassland proportion (31 to 69% grass-clover) and slurry C-input (0 to 1.45 Mg C ha−1 yr−1) on SOC storage in the top soil layer (0–20 cm). At steady-state conditions, the SOC stock based on equivalent soil mass (SOC stock FM) increased by 5.0 Mg C ha−1 when converting soil with a prehistory of cereal dominated cropping to a crop rotation with 1/3 grass-clover. The steady-state condition was established after 20 years, and the rate of change was largest in the initial years. A multiple regression model including grassland proportion, slurry C-input and the initial SOC stock FM explained 47% of the variation in the SOC stock from 2005 to 2018. The SOC stock FM increased 4.2 Mg C ha−1 when increasing the grassland proportion from 1/3 to 2/3 during the 13 years (0.32 Mg C ha−1 yr−1). Of the applied slurry-C, 11% was retained in soil, which means that an annual increase in cattle slurry input of 1.5 Mg C ha−1 (150 kg total-N ha−1) will increase SOC stock FM by 2.2 Mg C ha−1 (0.17 Mg C ha−1 yr−1) during the 13 years. Further, our study showed that soil nitrogen is stored along with SOC in equal proportions. [ABSTRACT FROM AUTHOR]

• Peatland degradation caused a significant loss of testate amoeba (TA) biodiversity. • Unexpectedly, this loss was accompanied by an increase in TA biosilicification. • TA biodiversity was not coupled to protozoic biogeochemical silicon (Si) cycling. • New insights into microbial biogeochemical Si cycling on a continental scale. Anthropogenic peatland degradation is a global threat. As peatlands store large amounts of carbon (C) their potential for mitigation of climate change has been emphasized recently. Global C cycling is linked to silicon (Si) fluxes from the continents into the oceans. These fluxes in turn are driven by biosilicification, the incorporation of inorganic Si into living organisms, in terrestrial ecosystems. Biosilicification by testate amoeba (TA) communities and its potential for Si cycling has been highlighted since the beginning of the 21st century. However, the effects of peatland degradation on TA biodiversity and corresponding protozoic biosilicification on a continental scale remained unresolved so far. We show that TA biodiversity in Asian peatlands is strongly affected by the grade of human impact. This biodiversity decline was accompanied by an unexpected increase in protozoic biosilicification. Our findings provide new insights into the interactions between the biodiversity of soil microorganisms and biogeochemical Si cycling. [ABSTRACT FROM AUTHOR]

• 394 trials worldwide reporting CO 2 emissions from residues-added were considered. • Legumes exhibited the highest emissions followed by grasses, oilseed crops and cereals. • Emissions positively correlated with residue C and lignin content. • Emissions negatively correlated to residue N content. Decomposition of crop residues may affect soil organic carbon (C) stocks, which are key for soil fertility improvement and mitigation of climate change. Numerous independent studies across the world point to contradictory results but their existence provides an opportunity to conduct a comprehensive analysis of the impact of crop type on residue decomposition. In the present study, data from 394 trials from across the world were used to assess cumulative CO 2 emissions from residues of 17 crops during 0–30, 0–90 and 0–120 days (i.e. C R 30, C R 90 and C R 120; 1-[C R 30/C R 120] ratio as a stability index of C emissions) and to relate the results with residue quality (C, N and lignin concentrations) and selected soil properties (texture, pH, soil organic carbon concentration). At all durations, legumes exhibited the highest CO 2 emissions per gram of C added (1003 mg CO 2 -C g−1C after 120 days) followed by grasses (9 4 7), oilseed crops (9 4 4) and cereals (8 4 6), with the legumes and grasses showing the lowest temporal stability of C emission as pointed out by a 1-[C R 30/C R 120] of 0.78 and 0.79, respectively, versus 0.82 and 0.83 for cereals and oilseed crops. At all durations, maize residues emitted the least C-CO 2 (86, 275 and 495 mg CO 2 -C g−1C), followed by two other lignin rich crops (cotton and sunflower), while the highest emissions were from Alfalfa residues that produced about 4 times more CO 2 (e.g. 359 at C R 30 and 1319 at C R 120) than maize. Overall, CO 2 emissions were positively correlated with soil clay concentration (r > 0.22), residue C concentration (e.g. r = 0.46 at C R 90 and r = 0.37 with emission stability, P < 0.05) but negatively to residue N concentration (r = −0.26 at C R 120, P < 0.05). The global trend pointed to decreased CO 2 emissions with increasing residue lignin. Contrary to what is generally believed, providing the soil with high lignin and high N concentration may foster C stabilization into soils by soil microbes. [ABSTRACT FROM AUTHOR]

• MLR and RF were used to estimate current SOC stocks in south-eastern Australia. • Rainfall and minimum temperature were major drivers in determining SOC stocks. • The multi-GCM ensemble means suggested an overall decrease in SOC stocks. • The largest mean decrease of SOC stocks occurs in the high-rainfall alpine regions. Soil organic carbon (SOC) plays a key role in the sequestration of carbon that could otherwise be warming the atmosphere. Climate change including increased temperature and changed rainfall will greatly impact the global SOC cycle. There are still significant gaps in our knowledge of the size of the global SOC pool and how future climate will affect SOC stocks and flows in many parts of the world, including Australia. In this study, we used SOC data in a Digital Soil Mapping framework to predict current and future SOC stocks across the state of New South Wales (NSW) in south-eastern Australia. In the first phase of the study we estimated the current SOC stock using multiple linear regression (MLR) and random forest (RF) modelling, and in the second phase we projected the change of SOC stocks in the near future (2050s) and far future (2090s) under two Shared Socio-economic Pathways (SSPs) scenarios based on 25 global climate models (GCMs) from the Coupled Model Inter-comparison Project Phase 6 (CMIP6). Our spatial modelling showed that estimated current SOC stocks in NSW decreased from east to west. Multi-GCM ensemble means suggested SOC stocks would decrease by 7.6–12.9% under SSP2-4.5 and 9.1–20.9% under SSP5-8.5 across NSW under future climate. The extent of change in SOC stocks varied spatially with the largest mean decrease of SOC stocks occurring in the North Coast and South East (alpine) regions of NSW. Our findings can support decision-making in land management and climate change mitigation strategies in NSW at the regional level. Furthermore, the modelling methods can be applied to other areas where edaphic and landscape properties, land use, and climate data are available. [ABSTRACT FROM AUTHOR]

• Use of geostatistics is indispensable when spatial aggregation with quantified uncertainty is targeted. • Spatial aggregation decreases uncertainty and supports detection of statistically significant SOC stock change. • Many counties in Hungary had a statistically significant topsoil SOC stock increase between 1992 and 2010. • The total topsoil SOC stock increase in Hungary between 1992 and 2010 was predicted as 14.79 Tg C. Many national and international initiatives rely on spatially explicit information on soil organic carbon (SOC) stock change at multiple scales to support policies aiming at land degradation neutrality and climate change mitigation. In this study, we used regression cokriging with random forest and spatial stochastic cosimulation to predict the SOC stock change between two years (i.e. 1992 and 2010) in Hungary at multiple aggregation levels (i.e. point support, 1 × 1 km, 10 × 10 km square blocks, Hungarian counties and entire Hungary). We also quantified the uncertainty associated with these predictions in order to identify and delimit areas with statistically significant SOC stock change. Our study highlighted that prediction of spatial totals and averages with quantified uncertainty requires a geostatistical approach and cannot be solved by machine learning alone, because it does not account for spatial correlation in prediction errors. The total topsoil SOC stock for Hungary was predicted to increase between 1992 and 2010 with 14.9 Tg, with lower and upper limits of a 90% prediction interval equal to 11.2 Tg and 18.2 Tg, respectively. Results also showed that both the predictions and uncertainties of the average SOC stock change were smaller for larger spatial supports, while spatial aggregation also made it easier to obtain statistically significant SOC stock changes. The latter is important for carbon accounting studies that need to prove in Measurement, Reporting and Verification protocols that observed SOC stock changes are not only practically but also statistically significant. [ABSTRACT FROM AUTHOR]

• We studied 32,000 Soil Organic Carbon content analyses of Swiss cropland topsoil from 1993 to present. • Annual SOC content change rates ranged from −30‰ to +30‰. • Annual SOC content change rates significantly increased from −5‰ in 1995 to +6‰ in 2015. • This change was related to the agri-environmental schemes adopted over the period. • On-farm observations contradict those from long-term experiments of the same region. Increasing cropland topsoil organic carbon (SOC) content is a key goal for soil improving quality and adaptating soils to climate change. Moreover, the short term potential of climate mitigation by carbon sequestration is mostly attributed to increasing topsoil SOC content (Balesdent and Arrouays, 1999; Chambers et al., 2016 ; Minasny et al., 2017; Balesdent et al., 2018). However, the possibility to increase SOC content is highly disputed in current literature which is mostly based on field experiments. We quantified the on-farm SOC content deficit and SOC content change rate of cropland topsoil (0–20 cm) from western Switzerland using the data bases of Geneva and Vaud cantons containing more than 30,000 topsoil analyses, performed every ten years on every cultivated field of the region since 1993. SOC deficit was estimated as the amount of SOC necessary to reach the 0.1 SOC :clay ratio considered as the minimum required SOC amount for acceptable soil quality. Cropland topsoils of the Vaud and Geneva cantons displayed a 20% and 70% SOC content deficit, respectively. In both cantons, the range of observed rates of change in SOC content from 1993 to present was very large, from −30 to +30‰ per year, with a median value of 0. However, the time trends showed a highly significant linear increase of rates from −5‰ to +6‰ per year on average, in 1995 and 2015, respectively, with no change in SOC content reached by 2005–2007. These trends were attributed to the Swiss agri-environmental schemes applied at the end of 20th century, namely mandatory cover crops and minimum rotations of 4 crops. Further, SOC content increase was accordant with the continuing adoption of minimum tillage, conservation agriculture and multi-species intense cover crops. These findings oppose to those obtained in Swiss long-term experiments, which emphasizes the need to use on-farm information when adressing agriculture policy, climate mitigation or soil quality management issues. [ABSTRACT FROM AUTHOR]

• Spatial variability of carbon stocks is overlooked by global datasets. • Integration of local data is needed to obtain adequate assessment of carbon stocks. • Care is needed when carbon stocks mapping is used to inform decision-making. • Global datasets can be used to rank and prioritise areas for restoration purposes. Land degradation leads to ecosystem degradation, reducing ecosystem functioning and depleting ecosystems' resilience. The majority of factors linked to land degradation are closely related with the depletion of below- and above-ground stocks of organic carbon. Organic carbon stock is important for climate change mitigation and for restoring soil functions such as those crucial to support food security. In this study, we mapped carbon stocks to infer land degradation in a small area in the Ethiopian Great Rift Valley. The study aimed to assess carbon stock status and to identify limitations and advantages of using global data in mapping at local scale relative to using local data. Two different datasets were developed; i) a "global dataset" characterised by data from datasets with global coverage data, and ii) a "hybrid dataset" that coupled data from global datasets, soil data derived from a local survey, and land cover data derived from a supervised classification of satellite images. The results showed that i) global datasets introduced inaccuracy that must be taken into account for advocating interventions at a local scale, and ii) global datasets could be used at a small catchment level for decision-making, if a simple rank of values is sufficient, but they might provide an optimistic picture of land degradation because they overestimate stocks. [ABSTRACT FROM AUTHOR]

• Straw addition increased CH 4 emissions and decreased CO 2 uptake, but decreased N 2 O emissions. • Biochar addition increased CH 4 emissions, but increased CO 2 uptake and decreased N 2 O emissions. • Straw addition decreased the GWP mainly by decreasing N 2 O emissions. • Biochar addition decreased the GWP mainly by increasing CO 2 uptake. • Climate change mitigation capacity of straw may be increased by improving management. The effects of incorporating rice straw or additions of biochar to rice paddies on climate change are uncertain. Both practices may have multiple roles in regulating greenhouse gas (GHG) emissions, and carbon (C) and nitrogen (N) cycling may be re-coupled. A mesocosm experiment was conducted to evaluate the effects of rice straw and straw-derived biochar amendments to rice paddies on methane (CH 4), carbon dioxide (CO 2), and nitrous oxide (N 2 O) emissions. Applying urea with straw increased total CH 4 emissions during the rice season by more than 20 times compared with urea alone and decreased total CO 2 uptake by 27.1%, but decreased total N 2 O emissions by 49.1%. Applying urea with biochar increased total CH 4 emissions by more than 10 times compared with urea alone, but increased total CO 2 uptake by 43.5% and decreased total N 2 O emissions by 8.4%. On a 100-year time horizon, N 2 O emissions dominated the global warming potential (GWP) of the three GHGs for all the N treatments, resulting in a positive GWP over the observation period. Applying urea with straw decreased the GWP by 436.9 g CO 2 -eq m−2 season−1 relative to urea alone, as the decrease in N 2 O emissions exceeded the increase in CH 4 emissions as well as the decrease in CO 2 uptake. Biochar amendment decreased the GWP by 375.6 g CO 2 -eq m−2 season−1, primarily due to increased CO 2 uptake. Based on our results, the climate change mitigation capacity of straw amendment may be further improved by decreasing N 2 O emissions during the basal fertilization period via changing plant residue species combined with improved N management. Another improvement was to control CO 2 emissions after the second topdressing through changing residue species and co-application of straw and its biochar. However, these measures require further evaluated to develop specific practices of crop residue management to address climate change. [ABSTRACT FROM AUTHOR]

• Total carbon stocks (TCS) in Sri Lankan mangroves varied with climatic zone. • TCS significantly varied across an intertidal gradient for most forests examined. • Soil carbon comprised 83–90% of the total mangrove carbon stocks at all sites. • SOC varied with depth and significantly contributed for below ground carbon stocks. • Sampling methodology does matter in soil carbon estimation. Sri Lanka is at the forefront of global mangrove conservation. It is the first country to officially protect all its remaining mangrove forests and has embarked on an ambitious plan to restore 10,000 ha of wetland during the United Nations Decade of Ecosystem Restoration. One incentive for this conservation effort is a recognition, based on research mostly done elsewhere, of the importance of mangroves for carbon sequestration and storage. However, a lack of data on Sri Lankan mangrove carbon pools, especially on soil organic carbon, has been recognized as a major impediment to national climate change mitigation strategies. The current work examined both above and below-ground carbon stocks of five important mangrove forests in Sri Lanka (Rekawa, Puttalam-Kalpitiya, Pambala-Chilaw, Batticaloa and Negombo) which are situated in the three major climate zones (dry, intermediate and wet) and therefore sample the main climatic drivers of spatial variability. Above-ground carbon, below-ground root carbon and soil carbon stocks of mangroves in Sri Lanka ranged from 75.5 to 189.1 Mg C ha−1, 7.9 to 14.3 Mg C ha−1 and 643.6 to 1253.6 Mg C ha−1, respectively. The highest total mangrove carbon stock was recorded from the Rekawa lagoon which is in the intermediate climate zone (1455.4 Mg C ha−1) while the lowest was found in the Batticaloa lagoon in the dry zone (734.7 Mg C ha−1). Soil carbon stocks were substantially higher in the places where vegetation biomass and stand densities are high. Soil comprised 83–90% of the total mangrove carbon stocks at all sites, highlighting the large potential for release into the atmosphere as carbon dioxide if these habitats are disturbed. Overall, our study contributes important data that broadens our current understanding of how mangrove organic carbon pools vary spatially and with climatic zone. [ABSTRACT FROM AUTHOR]

• vis–NIR spectroscopy was efficient to classify and predict forest SOC in different horizons. • The SVM correctly classified forest soil horizons based on their spectra. • Models performed better for predicting SOC concentrations of organic horizons compared to mineral horizons. Soil Carbon (C) is central to the functioning of ecosystems and climate change mitigation. It represents the largest terrestrial pool and much of it, is stored in forest soils. Soil Organic Carbon (SOC) in a forest varies not only laterally, but also vertically (i.e., with depth). However, the SOC content of forest soil horizons has not been investigated over large scales, despite its importance for framing our understanding of soil function. Visible–Near Infrared (vis–NIR) reflectance spectroscopy enables rapid and cost-effective examination of forest SOC distribution, both laterally and vertically. This study aims to evaluate the potential of vis–NIR spectroscopy for classifying and predicting the SOC concentration of organic and mineral horizons in forests of the Czech Republic. We investigated 1080 forest sites across the country, each with five soil horizons, representing the Litter (L), Fragmented (F), and Humus (H) organic horizons, as well as the A 1 (depth of 2–10 cm) and A 2 (depth of 10–40 cm) mineral horizons. We, then, used Support Vector Machines (SVMs) to classify the soil horizons based on their spectra and also to model the SOC concentration of (i) the profile (organic and mineral horizons together), (ii) only the organic horizons, (iii) only the mineral horizons, and (iv) each individual horizon separately. The models were validated using 10-repeated 10-fold cross-validation. Results show that the SVM with radial basis kernel could accurately classify the soil horizons (Correct Classification Rate (CCR) of 70% and Kappa coefficient of 0.63). The SOC model developed for the soil profile performed well (R2 = 0.76 and RMSE = 1.63%). The model of the combined organic horizons was considerably more accurate than that of the combined mineral horizons (R2 = 0.78 and R2 = 0.53, respectively). Estimates of SOC in the individual soil horizons had R2 values greater than 0.63 but those of the F and A 1 models were better with R2 > 0.70. The study indicates that vis–NIR spectroscopy can effectively characterize the SOC concentration of the highly variable forest soil horizons in the Czech Republic. [ABSTRACT FROM AUTHOR]

• The governance of soil to achieve global solutions is a long-standing challenge. • Binding international soil policy is lacking. • A complex web of institutions exhibit emergent properties that may lead to change. • Soil management problems are chronic problems requiring natural/social science partnerships. The global soilscape is a resource commons that is essential to society. Social science has been engaged in theoretical and empirical research as to how commons can be effectively managed, especially at a global scale, for more than 50 years. This has provided some rules and guideposts for their sustainable management, rules relevant to the soil commons. Presently, there is an international soil policy vacuum. The absence of binding policy helps explain the inability to implement scientific tools, such as soil C climate mitigation strategies, at significant levels over the past two decades. At national and subnational scales, especially in the USA which is considered here, there are a multitude of public and private soil health policies and programs, with vastly different scales and funding resources. These are largely not well coordinated, but according to polycentric management theory, may be viewed as a complex system, with evolving emergent properties and resulting impacts. Soil science must connect with business leaders, social scientists, policy implementers, politicians and others to develop a more coherent and effective policy web, one which might eventually lead to binding international agreements. This is likely to be a multi-generational effort, but will provide intellectually stimulating challenges for present and future generations of scholars. [ABSTRACT FROM AUTHOR]

• New methodology for regional objectives of 4perMille Initiative through No Tillage. • Based on modelling in Carbon Benefits Project online tool for 15 Spanish regions. • Carbon sequestration modelled was compared to organic carbon contained in the soil. • New geographical data useful for policy makers involved in climate change mitigation. • This methodology can be easily replicated in regions and countries around the world. The 4perMille Initiative aims to mitigate anthropogenic CO 2 emissions through carbon sequestration in soil organic carbon (SOC). Its main objective is to increase the annual amount of C in soils of the world by 0.4%. A new study methodology focused on achieving the regional objectives of 4perMille Initiative through No Tillage (NT) potential application in annual crops has been presented in this paper. It has been based on the modelling of Carbon Sequestration (CS) in Carbon Benefits Project (CBP), an online free tool. For the entire field of study (15 regions in Spain), the annual average is 0.38 Mg of C ha−1 yr−1. And the potential CS amounts are almost up to 3 Tg of C per year. CBP is a digital tool which allows to make an online modelling of the evolution of C stocks and Greenhouse Gas (GHG) emissions, derived from using different soil management systems. The comparison between the annual CS through NT and the contents in SOC in annual crops, has allowed to calculate the potential importance of NT, within the regional framework, of the 4perMille Initiative. There are large areas where the stored C could be 2 and up to 3 times over the reference values. What entails that the potential application of NT would represent a significant percentage of the CS objectives of the 4perMille Initiative in Navarra (52%), Castilla-La Mancha (51%), Castilla y León (43%), Aragón (35%), La Rioja (35%), Andalucía (35%) and Cataluña (29%). This methodology for calculating the NT potential over the 4perMille Initiative can be easily replicated in regions and countries around the world. [ABSTRACT FROM AUTHOR]

• Estimation of the C sequestration potential in Bavaria according to the 4‰ initiative. • Analysis of current (2012–2015) soil management for each agricultural field. • Development of C sequestration scenarios for five promising management practices. • Annual C sequestration potential of 0.3–0.4 Mt C, around 1‰ of SOC stocks. • Cover crops and agroforestry as most effective management options. An increase of soil organic carbon (SOC) stocks in agricultural soils does not only have positive effects on soil quality and soil resilience but may also contribute to climate change mitigation. The '4 per 1000′ (4p1000) initiative launched at the 2015 United Nations Climate Change Conference in Paris aims at increasing global SOC stocks in 0–40 cm depth by annually 4‰ in order to compensate the increase of anthropogenic CO 2 emissions. In this study we analysed the feasibility of this target for agricultural soils in Bavaria, Southeast Germany. Assuming a total organic carbon (OC) amount of 276 Tg currently stored in the upper 40 cm of agricultural soils in Bavaria (cropland and grassland), the 4p1000 goal corresponds to an annual carbon (C) sequestration of 1.1 Tg. Based on a site-specific analysis of present soil management, we developed spatially explicit C sequestration scenarios including five promising management practices (cover cropping, improved crop rotation, organic farming, agroforestry and conversion of arable land to grassland). The results revealed that the 4p1000 target is not feasible for Bavaria. The total potential of the five practices to sequester C resulted in increases in 0.3 to 0.4 Tg OC per year corresponding to around 1‰ of the present SOC stocks. Expansion of cover crops and agroforestry were identified as most promising options to increase SOC in agricultural soils. Although only around 1.5% of Bavaria's yearly GHG emissions would be compensated, this represents an essential contribution to climate change mitigation. Besides the need to develop new incentive systems (particularly for agroforestry), implementation of networks including farms and/or field trials that demonstrate improved soil management practices would be required to inform farmers and other stakeholders about the benefits of such practices. To maintain a resilient agriculture that withstands more extreme weather conditions in the future, healthy soils are needed. We therefore conclude that expected positive effects of a SOC stock increase on nutrient and water storage, soil erosion, biodiversity and food security are crucial for climate change adaptation. [ABSTRACT FROM AUTHOR]

• About 5.7 Pg SOC are stored in the top 1 m of soils in Cameroon. • There is a gap between globals and the national estimate of SOCS in Cameroon. • SOCS distribution is consistent with the agroecological zones pattern in Cameroon. • IDW and OK hybridization with machine learning algorithms showed similar performances. • These findings are vital for sustainable land management strategies in Cameroon. Countrywide estimates of soil organic carbon stock (SOCS) are useful to set up national strategies for sustainable land use management as well as to enhance the accuracy of global SOCS inventories. We appraised the spatial distribution of SOCS at five depth layers (0–15 cm, 15–30 cm, 30–100 cm, 0–30 cm and 0–100 cm) in Cameroon at 100 m spatial resolution, using a national harmonized legacy soil database (Camsodat 0.1) with 1432 georeferenced soil profiles. We assessed the prediction performances of random forest (RF) and generalized boosted regression (GBR), combined with two hybridization approaches of spatial interpolation of the residuals using ordinary kriging (OK) and inverse distance weighting (IDW). The estimates were compared to two global estimates derived from the Harmonized World Soil Database (HWSD) and SoilGrids250m. The SOCS distribution across the country showed a moderate spatial heterogeneity at all depth layers with coefficients of variation between 35% and 47%, and values ranging from 6 to 108 Mg C ha−1 at 0–15 cm, from 4 to 107 Mg C ha−1 at 15–30 cm, from 10 to 276 Mg C ha−1 at 30–100 cm, from 11 to 210 Mg C ha−1 at 0–30 cm and from 21 to 468 Mg C ha−1 at the 0–100 cm layer. Of the selected environmental covariates, terrain and climate attributes were the most relevant to predict the SOCS spatial distribution at country level. The RF model outperformed the GBR model, with about 10% improvement on prediction performance (R2) for most soil depths. The hybridization further slightly improved performance. However, OK was only slightly better than IDW in the overall assessment. Compared to national estimates, SoilGrids overestimated the SOCS by 15% at 0–30 cm depth, while HWSD underestimated SOCS by 26% at the same depth. Overall, about 5.7 Pg C are stored in the top 1 m of soils in Cameroon, with about 50% of that in the top 30 cm. The national distribution of SOCS is consistent with the pattern of agro-ecological zones. Our assessment provides baseline information for sustainable land management and climate change mitigation, as well as for improving the understanding of the spatial distribution of SOCS in Cameroon. [ABSTRACT FROM AUTHOR]

• Urban gardening promotes accumulation of total, labile and stable SOC fractions. • Total SOC content increases asymptotically with the duration of cultivation. • SOC is continuously stabilized in micro-aggregates favored by Fe and Al oxides. • For SOC fractionation in Lixisols, sonication appears preferable to shaking. In sub-Saharan Africa, market gardening relies on the intensive use of organic amendments, but little is known about the impact of these practices on soil organic carbon (SOC) dynamics. Recent studies have demonstrated that a better understanding of carbon dynamics can be achieved by considering different carbon pools. Here we used a simple method of physical SOC fractionation to assess the impact of market gardening practices on the evolution of total SOC and its stable and labile sub-fractions over a period of several decades. The study was conducted at the Kuinima market gardening zone in Bobo-Dioulasso (Burkina Faso). Composite soil samples (0–15 cm depth) were collected from a chronosequence of 69 fields including control plots and plots farmed for more than 50 years. Samples were wet sieved at 20 µm after agitation or sonication to assess carbon content in the fine (i.e. stable) and coarse fractions. Our results show an asymptotic increase in total carbon content, from 9 g C kg−1 for uncultivated control plots to 28 g C kg−1 for plots cultivated for more than 50 years. This increase tends to stabilize after 30 years. A similar trend is observed for C content in the coarse fraction > 20 µm. In contrast, carbon content in the fraction < 20 µm increased linearly over time, and this increase was greater in sonicated than in agitated samples. Based on the comparison of the two fractionation methods, a strong contribution of micro-aggregation to the physical stabilization of SOC was hypothesized. This stabilization appears favored by short-range-order (SRO) iron and aluminum oxides and/or metal-humus complexes, as suggested by the strong correlation between stable C content and oxalate-extractable Fe and Al. Large and repeated additions of organic amendments in market gardening thus appear to lead to increased SOC content and promote SOC storage in the stable fraction, thereby contributing to soil quality improvement and the mitigation of climate change. [ABSTRACT FROM AUTHOR]

• Available Water Capacity does not represent the capacity of soils to supply water to plants. • Real Soil Moisture Supply Capacities can be obtained by dynamic modeling. • Increasing Soil Organic matter does not significantly increase MSC in clayey soils. • When modeling the impact of climate change, soil properties have a major impact. The Available Soil Water Capacity (AWC) is standard data in most soil databases and expresses soil water contents in the rootzone between field capacity (FC; –33 kPa) and permanent wilting point (WP; −1500 kPa). Literature suggests that increasing the content of soil organic matter (SOM) of a given soil does not significantly increase AWC and this has important implications when estimating soil moisture supply to crops and evaluating the potential for climate mitigation. For most crops, the real FC values vary between −10 and −50 kPa in different soils and WP values between −800 and −1500 kPa. Thus standard values for AWC of FC and WP do not represent field conditions in many soils. When exploring AWC for six Italian soil series, ranging from clay to sandy, AWC values at increasing %SOM were lower in clay soils and higher in sand as compared with actual conditions, which could be explained by considering the shape of the corresponding moisture retention curves. Rather than focus on static AWC values to define moisture supply to plants, real or actual soil moisture supply capacities (MSC) can be obtained by dynamic modeling of the soil-water-atmosphere-plant system, including a "sink-term" indicating a continuous relation between water uptake and negative pressure head of soil water and evaporative demand. Also, only models allow exploration of the effects of future severe IPCC climate scenario RCP 8.5. Thus, studying MSC for the six Italian soil series showed that MSC values were: (i) on average 30% higher than the corresponding AWC; (ii) distinctly different for the six soils; (iii) affected by declines of 1–9% as a result of the effects of future climate scenarios.; (iv) not significantly affected by increases of %SOM when considering climate change, except for the sand. Generalizations as to the effect of future climate scenarios and %SOM on MSC can only be realistic when modeling is performed for soil series in different climate zones. [ABSTRACT FROM AUTHOR]