Topsoil Soil Organic Carbon Stocks Research Articles

Projecting the urbanization effect on soil organic carbon stocks in polar and steppe areas of European Russia by remote sensing

This paper describes the effect of urbanization on soil organic carbon (SOC) stocks by projecting the main urban land cover classes over the initial pre-urban soil maps. Two cities different in climate and soil conditions as well as in availability of SOC data were chosen as the case studies. Rostov-on-Don is the center of Russian South, where croplands and natural steppes have been conventionally thoroughly studied by soil scientists. In contrast, soils of Murmansk located in Russian Arctics have always been overlooked due to low suitability for agriculture. Global, national and regional soil maps and databases were used to estimate pre-urban SOC stocks in the areas. The outcomes based on Harmonized World Soil Database were highly uncertain, underestimating 0-100 cm SOC stocks in the polar region and overestimating them in the steppe region, whereas the results based on Digital Soil Map of Russia and regional maps were comparable. Land cover structures of Rostov-on-Don and Murmansk were mapped based on the stepwise per-pixel and sub-pixel classification algorithms applied to Sentinel-2 and included the following classes: sealed soils, green lawns, trees and shrubs, bare soils and water. Murmansk was dominated by trees and shrubs (58.1%) with the proportion of area 17.5% covered by sealed soils. In Rostov-on-Don, less than 30% of the total area was covered by trees and shrubs which is also comparable with bare soils (19.6%)_and lawns (23.4%), whereas almost one third of the territory was sealed (27.6%). These land cover structures had a different impact on the topsoil SOC stocks: a 30-50% increase in Murmansk compared to the 18% decrease in Rostov-on-Don. An increase of the 0-100 cm SOC stocks was shown for both regions, however in the polar conditions it was two times higher compared to the steppe. In polar conditions, conversion of natural soils into urban non-sealed soils increased SOC stocks from 30% to more than 4 times in 0-10 cm layer and from 47% to almost 3 times in the 0-100 cm layer. The highest increase was reported for the lawns, whereas SOC under trees and shrublands were considerably lower. In Rostov-on-Don , sealed and bare soils stored less SOC compared to the initial natural soils. The conversion of natural areas into urban green infrastructure increased SOC up to 50-70%. Although the absolute SOC values based on the global and national legacy data are highly uncertain, especially for the polar areas, the research outcomes clearly reveal possible patterns in SOC changes induced by different urbanization pathways in contrast climatic conditions and highlight the complexity of the urbanization effect on soils

Topsoil organic carbon increases but its stability declines after five years of reduced throughfall

Climate change alters the regional precipitation pattern and causes seasonal droughts that affect forest growth and belowground carbon cycling. This is expected to create greater oscillations in soil moisture that affect the stability of the soil organic carbon (SOC) pool. This study examined the effects of the precipitation decline on the SOC dynamics by experimentally controlling the throughfall in a Pinus massoniana plantation in subtropical region of China. Soil physicochemical properties were analyzed in topsoil (0–5cm depth) during the dry-cool and the wet-warm season after five consecutive years of ~30% throughfall reduction. We also assessed the SOC stock and its stability, as indicated by the easily oxidizable carbon (EOC) pool, SOC mineralization rates, soil aggregate size distributions and fine root biomass in relation to soil microbial community structure. The throughfall reduction reduced the soil moisture by 15–20% between sampling dates, with an average 18% during the five-year experiment. With throughfall reduction, topsoil SOC stocks significantly increased by 0.18 kg C m-2 or 10%, but fine root biomass, EOC content, and the micro-aggregate fraction were significantly higher, suggesting less stable SOC. The abundance of bacterial r-strategists (e.g. Bacteroidetes, Betaproteobacteria, and Gammaproteobacteria) increased significantly while K-strategists (e.g. Planctomycetes) decreased. Both SOC and EOC mineralization were positively correlated with the abundance of r-strategists, but negatively correlated with the K-strategists abundance. We posit that micro-aggregates, fine root biomass, and altered microbial strategist ratios are associated with the decline in SOC stability in forests with less throughfall.

Mapping soil organic carbon stocks and trends with satellite-driven high resolution maps over South Africa

Estimation and monitoring of soil organic carbon (SOC) stocks is important for maintaining soil productivity and meeting climate change mitigation targets. Current global SOC maps do not provide enough detail for landscape-scale decision making, and do not allow for tracking carbon sequestration or loss over time. Using an optical satellite-driven machine learning workflow, we mapped SOC stocks (topsoil; 0 to 30 cm) under natural vegetation (86% of land area) over South Africa at 30 m spatial resolution between 1984 and 2019. We estimate a total topsoil SOC stock of 5.6 Pg C with a median SOC density of 6 kg C m−2 (IQR: interquartile range 2.9 kg C m−2). Over 35 years, predicted SOC underwent a net increase of 0.3% (relative to long-term mean) with the greatest net increases (1.7%) and decreases (−0.6%) occurring in the Grassland and Nama Karoo biomes, respectively. At the landscape scale, SOC changes of up to 25% were evident in some locations, as evidenced from fence-line contrasts, and were likely due to local management effects (e.g. woody encroachment associated with increased SOC and overgrazing associated with decreased SOC). Our SOC mapping approach exhibited lower uncertainty (R2 = 0.64; RMSE = 2.5 kg C m−2) and less bias compared to previous low-resolution (250–1000 m) national SOC mapping efforts (average R2 = 0.24; RMSE = 3.7 kg C m−2). Our trend map remains an estimate, pending repeated measures of soil samples in the same location (time-series); a global priority for tracking SOC changes. While high resolution SOC maps can inform land management decisions aimed at climate mitigation (natural climate solutions), potential increases in SOC are likely limited by local climate and soils. It is also important that climate mitigation efforts such as planting trees balance trade-offs between carbon, biodiversity and overall ecosystem function.

Investigating the spatio-temporal variability of soil organic carbon stocks in different ecosystems of China

Soil organic carbon (SOC) significantly influences soil fertility, soil water holding capacity, and plant productivity. In this study, we applied two boosted regression tree (BRT) models to map SOC stocks across China in the 1980s and the 2010s. The models incorporated nine environmental variables (climate, topography, and biology) and 8897 (in the 1980s) and 4534 (in the 2010s) topsoil (0–20 cm) samples. During the two study periods, 20% of the soil samples were randomly selected for model testing, and the remaining samples were used as a training set to construct the models. The verification results showed that incorporating climate environment variables significantly improved the model prediction in both study periods. Mean annual temperature, mean annual precipitation, elevation, and the normalized difference vegetation index were the dominant environmental factors affecting the spatial distribution of SOC stocks. The full-variable model predicted similar spatial distributions of SOC stocks for the 1980s and the 2010s. SOC stocks in China showed an increasing trend over the past 30 years, from 3.9 kg m−2 in the 1980s to 4.6 kg m−2 in the 2010s. In both periods, topsoil SOC stocks were mainly stored in agroecosystems, forests, and grasslands in the 1980s, with values of 9.5, 12.0, and 11.4 Pg C, respectively. Our study provides reliable information on Chain's carbon distribution, which can be used by land managers and the national government to formulate relevant land use and carbon sequestration policies.

Changes in soil carbon and nitrogen stocks following degradation of alpine grasslands on the Qinghai‐Tibetan Plateau: A meta‐analysis

AbstractUntil now, nearly 90% of alpine grasslands on the Qinghai‐Tibetan Plateau (QTP) have been degraded. However, there is currently no consensus on how soil organic carbon (SOC) and soil total nitrogen (STN) stocks vary with the degradation succession of alpine grasslands in this region. Here, a meta‐analysis was conducted to quantify the dynamics of SOC and STN stocks in topsoil (0–30 cm) at different degradation stages of alpine grasslands on the QTP. The results showed that grassland degradation led to average losses of 48 and 39% for SOC and STN stocks, respectively. The changes in SOC and STN stocks following grassland degradation did not differ significantly between grassland types, but were significantly affected by grassland degradation stage. The reductions in both SOC and STN stocks increased with the degradation stage, and the highest reductions were all found at extreme degradation stage. The results indicated that the depletion of SOC and STN pools were aggravated with the degradation succession of grassland. The tightly coupled SOC and STN implied that the depletion of SOC stock was closely related to that of STN stock during the process of grassland degradation. Positive relationships were detected between the dynamics of vegetation coverage and that of SOC or STN stock, indicating that vegetation coverage could not only reflect grassland degradation, but might also be a potential indicator of SOC and STN status. The findings suggest that preventing the degradation succession of alpine grasslands is vital to maintain or promote SOC and STN levels on the QTP.

Machine learning in space and time for modelling soil organic carbon change

AbstractSpatially resolved estimates of change in soil organic carbon (SOC) stocks are necessary for supporting national and international policies aimed at achieving land degradation neutrality and climate change mitigation. In this work we report on the development, implementation and application of a data‐driven, statistical method for mapping SOC stocks in space and time, using Argentina as a pilot. We used quantile regression forest machine learning to predict annual SOC stock at 0–30 cm depth at 250 m resolution for Argentina between 1982 and 2017. The model was calibrated using over 5,000 SOC stock values from the 36‐year time period and 35 environmental covariates. We preprocessed normalized difference vegetation index (NDVI) dynamic covariates using a temporal low‐pass filter to allow the SOC stock for a given year to depend on the NDVI of the current as well as preceding years. Predictions had modest temporal variation, with an average decrease for the entire country from 2.55 to 2.48 kg C m−2 over the 36‐year period (equivalent to a decline of 211 Gg C, 3.0% of the total 0–30 cm SOC stock in Argentina). The Pampa region had a larger estimated SOC stock decrease from 4.62 to 4.34 kg C m−2 (5.9%) during the same period. For the 2001–2015 period, predicted temporal variation was seven‐fold larger than that obtained using the Tier 1 approach of the Intergovernmental Panel on Climate Change and United Nations Convention to Combat Desertification. Prediction uncertainties turned out to be substantial, mainly due to the limited number and poor spatial and temporal distribution of the calibration data, and the limited explanatory power of the covariates. Cross‐validation confirmed that SOC stock prediction accuracy was limited, with a mean error of 0.03 kg C m−2 and a root mean squared error of 2.04 kg C m−2. In spite of the large uncertainties, this work showed that machine learning methods can be used for space–time SOC mapping and may yield valuable information to land managers and policymakers, provided that SOC observation density in space and time is sufficiently large.Highlights We tested the use of machine learning for space–time mapping of soil organic carbon (SOC) stock. Predictions for Argentina from 1982 to 2017 showed a 3% decrease of the topsoil SOC stock over time. The machine learning model predicted a greater temporal variation than the IPCC Tier 1 approach. Accurate machine learning SOC stock prediction requires dense soil sampling in space and time.

Soil parent material controls organic matter stocks and retention patterns in subtropical China

The influence of parent material on soil organic carbon (SOC) retention remains largely unstudied. Here, we aimed to reveal the role of soil parent material on SOC stocks and elucidate the underlying SOC retention patterns for soils derived from limestone, quaternary red earth, granite, basalt, and tertiary red sandstone in subtropical China. The study assessed 110 topsoils (0–20 cm) under two land uses (forest and cultivated), but with similar topography. We compared soil properties (pH, oxides, multivalent cations, texture, aggregates) and SOC stocks, total SOC concentration as well as three organic C fractions (active, slow, and passive C). Factors influencing SOC concentration were explored using a combination of stepwise multiple regression analysis and redundancy analysis. Topsoil SOC stocks and SOC concentration varied significantly among the five parent materials, with the greatest values all observed in limestone-derived soils and the lowest in tertiary red sandstone–derived soils. While parent material significantly influenced both quantity and quality of SOC, there was no land use effect on the proportion of organic C fractions. Our results have also shown that parent material mediates land use impacts on SOC. The SOC stocks and SOC concentration in limestone, quaternary red earth, and basalt-derived soils were significantly higher in forest than in arable soils, while no significant land use effect was found for soils derived on granite and tertiary red sandstone. Soil aggregates, texture, content of Fe oxides, and multivalent cations accounted for 67.8% of the variation in SOC concentration, and both SOC stocks and SOC concentration were more closely related to aggregation and soil texture. The major influencing factors for SOC retention differed with parent material, with Ca2+/Mg2+-mediated interactions between organic matter and mineral surfaces being the main regulating processes for limestone-derived soils and oxide-mediated mechanisms to form stable aggregates being the likely SOC protecting patters for quaternary red earth and basalt-derived soils. Due to the coarse texture and low aggregate stability, the soils derived from granite and tertiary red sandstone lacked the properties for SOC preservation, with pH the main predictor for SOC concentration. Macroaggregates (> 2 mm) played especially important role in shaping SOC stocks. Parent material exerts an influence on SOC stocks and retention patterns, and should be used in determining SOC storage potential for soils.

Terrestrial carbon stocks following 15 years of integrated watershed management intervention in semi-arid Ethiopia

Our study investigated the total terrestrial stock of organic carbon and its controlling factors in prevalent land‐use systems in semi-arid Ethiopia (610 mm of annual rainfall), as part of the impact assessment of the national Integrated Watershed Management (IWM) program. Above- and below-ground biomass and soil organic carbon (SOC) stocks of major land-use systems (i.e., exclosure, cropland, rangeland, and bare land) were quantified after field sampling along a topographic gradient. We found that aboveground carbon stocks peaked in the 15-year-old exclosures (9.08 ± 1.44 Mg ha−1) owing to intact woody and grass vegetation as well as substantial litter cover (>20% of the total biomass). Croplands cultivated with wheat and rangelands vegetated with perennial grasses showed average aboveground carbon stocks of 3.16 ± 0.24 and 1.45 ± 0.19 Mg ha−1, respectively. The belowground biomass carbon stock was particularly low in croplands (0.76 ± 0.09 Mg ha−1), exceeded by that in both exclosures and rangelands, where values averaged 3.67 ± 0.06 and 3.04 ± 0.42 Mg ha−1, respectively. The topsoil (0–30 cm) SOC stocks also varied with land-use systems but showed a different order, peaking in rangelands (53.9 ± 10.1 Mg ha−1) and exclosures (41.4 ± 8.1 Mg ha−1), followed by bare lands (29.0 ± 11.5 Mg ha−1) and croplands (26.4 ± 4.6 Mg ha−1). The sub-soils (30–100 cm) added 40% to this SOC storage. The greatest total SOC stock identified in exclosures that had been established primarily on degraded hillslopes may signify a successful restoration effort under the IWM program. However, croplands exhibited the lowest SOC stock, which implies the need for urgent interventions to improve the soil fertility.

Does Grazing Exclusion Improve Soil Carbon and Nitrogen Stocks in Alpine Grasslands on the Qinghai-Tibetan Plateau? A Meta-Analysis

Grazing exclusion has been widely used to restore the degraded alpine grasslands on the Qinghai-Tibetan Plateau (QTP). However, the dynamics of soil organic carbon (SOC) and soil total nitrogen (STN) pools after grazing exclusion and their controlling factors are currently less understood in this region. Here, a meta-analysis was conducted to quantitatively assess the changes in SOC and STN stocks in topsoil (0–30 cm) following grazing exclusion in three major grassland types (alpine meadow, alpine steppe, and alpine desert steppe) on the QTP and to explore the potential factors controlling the effects of grazing exclusion on SOC and STN stocks. The results showed that overall, grazing exclusion significantly increased SOC stock by 16.5% and STN stock by 11.2%. Significant increases in both SOC and STN stocks were observed after grazing exclusion of alpine meadow. In contrast, grazing exclusion did not improve SOC and STN stocks in the other two grassland types. The difference in mean annual precipitation among grassland types was a likely reason for the different dynamics of SOC and STN stocks after grazing exclusion. The effect sizes of both SOC and STN stocks were positively related to the duration of grazing exclusion, and a positive relationship was detected between the effect size of SOC stock and that of STN stock, demonstrating that the dynamics of SOC and STN were closely coupled during the period of grazing exclusion. However, grazing exclusion had no impact on soil C:N ratio for all grassland types, indicating that soil C:N ratio was generally stable after grazing exclusion. Therefore, it is suggested that the increase in STN can support continuous SOC accumulation following grazing exclusion. In conclusion, the findings suggest that the effects of grazing exclusion on SOC and STN stocks differ among grassland types on the QTP, and grazing exclusion of alpine meadows may provide substantial opportunities for improving SOC and STN stocks in this region.

Effect of high-intensity rotational grazing on the growth of cattle grazing buffel pasture in the Northern Territory and on soil carbon sequestration

Context Scientific and anecdotal reports conflict on the effects of intensive rotational grazing (IRG) on cattle growth, and very limited objective data are available from cattle grazing in northern Australia that producers can use to decide whether to adopt IRG. Aims This study aimed to compare liveweight gain and sequestration of soil organic carbon when cattle grazed buffel grass (Cenchrus ciliaris L.) under either continuous grazing (CG) or IRG. Methods In each year of this 9-year study, a cohort of Brahman and Brahman-cross weaners was randomly allocated to IRG and CG treatments. They grazed predominantly buffel pasture at Douglas Daly Research Farm from shortly after weaning for about a year, at which time they were replaced by the next year’s group, and the average liveweight gains of the treatments over the post-weaning year were compared each year for 9 years. Soil organic carbon was measured in the topsoil (0–30 cm) twice each year for 5 years (2009–14) and changes in carbon stocks over time were compared between treatments. Key results In each year of this study, the growth of cattle grazing buffel pasture was lower under IRG than CG. In each year, liveweight gain was lower (P < 0.05) per head and per hectare under IRG. Topsoil soil organic carbon stocks did not increase in the IRG treatment over the 5 years of this study. Conclusions This study found that cattle growth, both per head and per hectare, was lower under IRG than CG, and that IRG did not result in any increase in soil organic carbon over time. Implications The lower per head and per area production from the IRG system, combined with the extra infrastructure and operating costs for IRG systems, make it unlikely that adoption of IRG would improve the profitability of cattle-grazing operations on similar pasture systems in northern Australia. However, the findings of this study may not apply to other pasture systems and environments.

Short-range-order minerals as powerful factors explaining deep soil organic carbon stock distribution: the case of a coffee agroforestry plantation on Andosols in Costa Rica

Abstract. Soil organic carbon (SOC) constitutes the largest terrestrial C stock, particularly in the Andosols of volcanic areas. Quantitative information on distribution of SOC stocks is needed to construct a baseline for studying temporal changes in SOC. The spatial variation of soil short-range-order minerals such as allophane usually explains the variability of topsoil SOC contents, but SOC data for deeper soil layers are needed. We found that within a 1 km2 Costa Rican basin covered by coffee agroforestry, SOC stocks in the upper 200 cm of soil were highly variable (24 to 72 kg C m−2). Topsoil SOC stocks were not correlated with SOC stocks present in deeper layers. Diffuse-reflectance mid-infrared (MIR) spectroscopy made possible the analysis of a large number of samples (69 soil profiles, i.e. 598 soil samples) for ammonium-oxalate and sodium-pyrophosphate-extractable forms of Al, Fe, and Si, as well as SOC content and bulk density. Using the MIR spectra, we identified two different soil materials, which were identified as allophanic and halloysitic soil material. Allophanic soil occurred on top of the halloysitic soil. The thickness of the allophanic soil material, rich in SRO minerals and related to a young andic A horizon, explained the variability of SOC. This study illustrates that knowledge of topography and pedogenesis is needed to understand and extrapolate the distribution of SOC stocks at landscape scales.

Afforestation and agroforestry enhance soil nutrient status and carbon sequestration capacity in eastern China

AbstractAfforestation and agroforestry both sequester atmospheric carbon (C) and store C in the soil. However, the response of soil nutrients and soil organic carbon (SOC) to these practices has not been fully examined, especially in deep soil layers. We therefore investigated the effects of afforestation and agroforestry on the nutrient contents and organic carbon (OC) stocks within a 100‐cm depth in eastern China. The soil nutrient contents and stoichiometry as well as the SOC contents and stocks exhibited different vertical patterns and varied among the five planting systems. The total nitrogen (TN), total phosphorus (TP), and SOC contents decreased dramatically at a 20‐cm depth, and the total potassium (TK) content exhibited a different pattern. Compared with adjacent croplands and nurseries, the forest plantation and agroforestry systems had higher SOC stocks, ranging from 42.9 to 111.6 t·hm−2 in the whole soil profile. The SOC distribution among the depth gradient varied among planting systems, and the topsoil SOC stocks played crucial roles in total SOC storage within a depth of 100 cm in the cropland and nursery systems. The soil pH and bulk density (BD) were negatively correlated with the SOC content, and a significant positive linear relationship occurred between the TN and SOC contents and between the TP and SOC contents. We recommend that combinations of nitrogen (N) and phosphorus (P) fertilizers would benefit SOC storage and improve stabilization in agroforestry and plantation systems.

Carbon accrual in the Atacama Desert

Abstract The Atacama Desert is the oldest and driest desert on Earth, nevertheless traces of life have been observed in several places, accumulating residues of organic matter (OM) in the desert soil. We evaluated to which degree the distribution of soil organic carbon (SOC) stocks depends on aridity. We questioned that these OM traces of life preferentially accumulate in topsoil and investigated whether there was also an enrichment of OM in deeper subsoil. We sampled four west-east directed transects with increasing distance to the coast, spanning the Atacama Desert from north to south, plus a hyperarid site at Yungay in the centre of the desert. With a nested sampling design we addressed topsoil heterogeneity at each sampling site (n ≤ 18). For 12 of these sites soil profiles were dug to 0.6–2.0 m depth. The SOC concentrations were determined for each sample by temperature-dependent differentiation of total carbon. We found that with increasing distance to the coast (2.3 to 86.8 km) and thus aridity, SOC stocks decreased from 53.91 ± 53.90 to 4.11 ± 1.46 t SOC ha−1 in the topsoil. Hence, traces of life were ubiquitously preserved in both, arid and hyperarid desert areas, whereas a local hyperarid minimum of 1.02 ± 0.18 t SOC ha−1 at 49.2 km distance, right in between the Andes and the coast, was found. Intriguingly, relatively large amounts of OM were found at depth below 15 cm, adding 74.0 to 94.3 % of subsoil SOC to the topsoil SOC stock. Furthermore, the subsoil SOC concentrations peaked between 40 and 150 cm depth in the soil, irrespectively of the presence or absence of visible vegetation. We conclude that carbon accrual in this arid to hyperarid system shifts from preferential carbon enrichment in topsoil to subsoil, therewith providing the potential for deep(er) biosphere food-webs and demonstrating the future need to dig into soil for discovering traces of life in comparable environments, including other planets such as Mars.

Relationship between carbon stocks and tree species diversity in a humid Guinean savanna landscape in northern Sierra Leone

Global sustainable development goals include reducing greenhouse gas emissions from land-use change and maintaining biodiversity. Many studies have examined carbon stocks and tree species diversity, but few have studied the humid Guinean savanna ecosystem. This study focuses on a humid savanna landscape in northern Sierra Leone, aiming to assess carbon stocks and tree species diversity and compare their relationships in different vegetation types. We surveyed 160 sample plots (0.1 ha) in the field for tree species, aboveground carbon (AGC) and soil organic carbon (SOC). In total, 90 tree species were identified in the field. Gmelina arborea, an exotic tree species common in the foothills of the Kuru Hills Forest Reserve, and Combretum glutinosum, Pterocarpus erinaceous and Terminaria glaucescens, which are typical savanna trees, were the most common species. At landscape level, the mean AGC stock was 29.4 Mg C ha−1 (SD 21.3) and mean topsoil (0–20 cm depth) SOC stock was 42.2 Mg C ha−1 (SD 20.6). Mean tree species richness and Shannon index per plot were 7 (SD 4) and 1.6 (SD 0.6), respectively. Forests and woodlands had significantly higher mean AGC and tree species richness than bushland, wooded grassland or cropland (p < 0.05). In the forest and bushland, a small number of large diameter trees covered a large portion of the total AGC stocks. Furthermore, a moderate linear correlation was observed between AGC and tree species richness (r = 0.475, p < 0.001) and AGC and Shannon index (r = 0.375, p < 0.05). The correlation between AGC and SOC was weak (r = 0.17, p < 0.05). The results emphasise the role of forests and woodlands and large diameter trees in retaining AGC stocks and tree species diversity in the savanna ecosystem.

Agricultural soil organic carbon stocks in the north-eastern Iberian Peninsula: Drivers and spatial variability

Estimating soil organic carbon (SOC) stocks under agriculture, assessing the importance of their drivers and understanding the spatial distribution of SOC stocks are crucial to predicting possible future SOC stocks scenarios under climate change conditions and to designing appropriate mitigation and adaptation strategies. This study characterized and modelled SOC stocks at two soil depth intervals, topsoil (0–30 cm) and subsoil (30–100 cm), based on both legacy and recent data from 7245 agricultural soil profiles and using environmental drivers (climate, agricultural practices and soil properties) for agricultural soils in Catalonia (NE Spain). Generalized Least Square (GLS) and Geographical Weighted Regression (GWR) were used as modelling approaches to: (i) assess the main SOC stock drivers and their effects on SOC stocks; (ii) analyse spatial variability of SOC stocks and their relationships with the main drivers; and (iii) predict and map SOC stocks at the regional scale. While topsoil variation of SOC stocks depended mainly on climate, soil texture and agricultural variables, subsoil SOC stocks changes depended mainly on soil attributes such us soil texture, clay content, soil type or depth to bedrock. The GWR model revealed that the relationship between SOC stocks and drivers varied spatially. Finally, the study was only able to predict and map topsoil SOC stocks at the regional scale, because controlling factors of SOC stocks at the subsoil level were largely unavailable for digital mapping. According to the resulting map, the mean SOC stock value for Catalan agriculture at the topsoil level was 4.88 ± 0.89 kg/m2 and the total magnitude of the carbon pool in agricultural soils of Catalonia up to 30 cm reached 47.9 Tg. The present study findings are useful for defining carbon sequestration strategies at the regional scale related with agricultural land use changes and agricultural management practices in a context of climate change.

Spatial Distribution of Soil Organic Matter and Soil Organic Carbon Stocks in Semi-Arid Area of Northeastern Syria

Although soil organic matter (SOM) forms a small portion of the soil body. Nevertheless, it is the most important component of the soil ecosystem, as well as of the carbon global cycle. In the semi-arid environment, there has been little research on the spatial distribution of SOM and soil organic carbon (SOC) stock. In this study, stratified random samples of total 30 soils were collected from two different soil depth (topsoil, subsoil) of Al Balikh plain and used for mapping the spatial variability of SOC and to estimating the SOC stock. The result showed that the values were relatively homogenate, with the normal decreasing trend with increasing the depth. The standard deviation (Std. D) for both SOC and SOC stock indicates homogeneous and absence of outliers values, whereas the coefficient of variation (C.V) indicates non-dispersion and clustering of values around the average. SOC was 0.38%, 0.17% in topsoil and subsoil respectively; the corresponding averages of SOC stock were 1.23 kg·m-2 and 1.14 kg·m-2 respectively, these values reflecting typical characteristics of poor SOC semi-arid soil. The correlation between SOC and SOC stock was (R2 = 0.996, p < 0.001) in topsoil and it was (R2 = 0.941, p < 0.001) for subsoil. The semivariograms were indicated that both SOC and SOC stock were best fitted to the exponential model. Nugget, range, and sill were equal to 0.002, 0.036, and 0.044, respectively for SOC in topsoil, and 0.014, 0.071, and 0.081, for SOC in the subsoil. For SOC stock, it was 0.0, 0.036, and 0.0508, respectively in topsoil. In the subsoil, the values were 0.1899, 0.086, and 4.159, respectively. SOC and SCO stock in both two layers are shown a strong spatial dependence, for which were 4.3, 17.2 for SOC in topsoil and subsoil respectively, and 0.0, 4.5 for SOC stock in topsoil and subsoil respectively, thus, which can be attributed to intrinsic factors.

Soil organic carbon stocks and their determining factors in the Dano catchment (Southwest Burkina Faso)

Although the evaluation of soil organic carbon (SOC) stocks across different types of land use and major reference soil groups is essential for mitigating climate change, there remains, to date, limited comprehensive understanding of whole tropical soil profiles. Therefore, this study aimed to explain the amount of SOC stocks in different land-use systems and across various soil groups, as well as its spatial pattern in the topsoil (0–30 cm) and subsoil (30–100 cm) within the savannah zone of Burkina Faso. Roughly 70 soil profiles were considered along with additional auger sampling to account for spatial variation in both cropland (CR) and savannah (SA). The machine learning technique random forest regression (RFR) and multiple linear regression (MLR) were used for modeling the surface and subsurface SOC stocks. For model calibration, covariates including land use, topographic, texture, and climatic data were considered as surrogate for soil forming factors. The prediction maps produced by the calibrated models were validated by an independent dataset. The results indicated that about 53% of the SOC stock over 1 m depth was held in the upper 30 cm. Only a marginal difference was recorded between the topsoil SOC stock in SA (41.4 t C ha−1) and CR (39.1 t C ha−1) soils. For the subsoil, a significant difference (p < 0.05) was observed between the SOC stock of CR soils recording about 40.2 t C ha−1 and SA soils with 26.3 t C ha−1. Among the reference soil groups, the Gleysols located at lower elevation positions revealed the highest SOC stocks over 0–30 cm (44 t C ha−1) and 100 cm depth (86.6 t C ha−1). The Stagnosols (45.2 t C ha−1) followed by the Gleysols (42.7 t C ha−1) recorded the highest SOC stocks over 30–100 cm. The variability of SOC stock in the topsoil was primarily related to site-specific elements, such as particle-size fraction and wetness index, while its distribution in the subsoil was mainly associated with the topographical orientation represented by the slope aspect. Compared to the MLR, RFR estimated mean top- and subsoil SOC stocks of the catchment fairly well, along with lower statistical error metrics, though extreme values were not covered. Nevertheless, the findings on SOC stocks reinforce the view that the semi-arid ecosystems of West Africa still offer a significant opportunity for carbon sequestration for both topsoil and subsoil, and these results represent a baseline for future modeling of SOC dynamics in the region.

Soil organic carbon stocks in topsoil and subsoil controlled by parent material, carbon input in the rhizosphere, and microbial-derived compounds

Despite a large body of studies investigating soil organic carbon (SOC) stocks and potential influencing factors, the impact of contrasting parent material, particularly in the subsoil, has received little attention. To reveal potential effects varying parent materials exert on SOC stocks, we investigated chemical (14C content and overall chemical composition via 13C NMR spectroscopy) and plant/microbial related parameters (root mass, amino sugars) of bulk soil and soil organic matter fractions from topsoil, subsoil, and rhizosphere soil at three European beech stands (Fagus sylvatica L.) only differing in parent material (Tertiary sand, Quaternary loess, and Tertiary basalt).The results suggest that the clay fraction, its amount being largely dependent on the respective parent material, took a central role in shaping differences in SOC stocks among the investigated sites by affecting soil organic matter stabilization via organo-mineral association and aggregation. This fraction was particularly relevant in the subsoil, where it accounted for up to 80% of the bulk soil SOC stocks that decreased with decreasing amounts of the clay fraction (basalt > loess > sand site). Determining the soil's nutrient composition, parent material likely also indirectly affected SOC stocks by changing rhizosphere traits (such as fine root density or mortality) and by attracting root growth (and thus organic matter inputs) to subsoil with higher nutrient contents, where in situ root inputs in the form of rhizodeposits were likely the prime source of plant-derived SOC. However, root inputs also contributed in large part to topsoil SOC stocks and were associated with higher abundance of microbial compounds (amino sugars), whose relative importance increased with increasing soil depth.Independent of soil depth and site, amino sugars and the amount of the clay fraction, combined with parameters related to the input of organic matter (root mass and amount of the particulate organic matter fraction) explained more than 90% of the variability in SOC stocks, indicating a key role of these measures in impacting SOC stocks. Because parent material directly or indirectly influenced these parameters, we demonstrate the necessity to consider differences in parent material when estimating and predicting SOC stocks.

干旱区沙漠化逆转过程植被–土壤碳储量的恢复演变规律研究

碳汇作用是陆地生态系统的重要生态功能之一，也是生态修复关注的重要方面。以地带性植被区为对照，应用空间代替时间的方法，在石羊河中游选择流动沙丘及造林恢复5年、15年和25年沙化土地样地，研究了沙漠化逆转过程沙地生态系统植被–土壤有机碳储量的恢复演变过程、规律及其影响因素。结果表明：固沙植被碳储量随沙漠化逆转过程先增加后降低，造林恢复25年平均固碳速率仅达到0.05 Mg/hm2•a。尽管沙漠化逆转过程的不同恢复阶段地上植被碳储量大，但是造林恢复5年、15年和25年沙化土地地下植被总碳储量也仅占到地带性植被的22.6%、54.9%和45.3%。0~1 m土壤和生态系统有机碳储量均随沙漠化逆转过程持续增加，造林恢复25年平均固碳速率分别达到0.48 Mg/hm2•a和0.53 Mg/hm2•a，远远高于固沙植被的固碳速率，而且0~5 cm表层土壤是有机碳储量为增长最快的层次。土壤有机碳储量所占比例均在87.5%以上，是沙漠化逆转不同恢复阶段沙化土地和地带性植被区的主要碳库。上述结果证明，固沙造林加速了沙漠化逆转过程，快速提高了沙化土地植被和表层土壤的碳储量，是固定大气CO2于沙地植被和土壤中的有效途径。但是，造林恢复25年沙化土地碳储量与地带性植被间依然存在明显差距，特别是地下植被和深层土壤有机碳储量还存在较大的增长潜力。 Carbon sequestration is one of the important ecological functions of land ecosystem, and it is also an important aspect of ecological restoration. An unrestored shifting sand dune and three formerly shifting sand dunes that had been afforested for 5, 15 and 25 years were selected in the middle reaches of Shiyang River to study restoration changes in biomass carbon (BC) and soil organic carbon (SOC) stocks of the desertified lands in the reversion process of desertification and their influence factors. Results showed that BC stocks of the desertified lands was increased and then reduced in the reversion process of desertification, and the average vegetation carbon se-questration rate reached only 0.05 Mg/hm2•a in 25 years. Total BC stocks of the desertified lands afforested for 5 years, 15 years and 25 years only accounted for 22.6%, 54.9% and 45.3% of native zonal vegetation, respectively. SOC in 0 - 1 m layer and ecosystem stocks increased gradually in the reversion process of desertification, and the average carbon sequestration rate reached 0.48 Mg/hm2•a and 0.53 Mg/hm2•a in 25 years, respectively. 0 - 5 cm topsoil was the fastest layer for restoration of SOC stocks. SOC stocks accounted for more than 87.5% of the ecosystem organic C stocks, and soil was major C stocks of the different restoration stages in the reversion process of desertification and native zonal vegetation. Therefore, sand-fixation afforestation not only promoted the reversal of desertification, but also increased BC and topsoil SOC stocks rapidly, which proved that sand-fixation afforestation was the effective way to fix atmospheric CO2 in the desertified lands. However, C stocks of the desertified lands at the different restoration stages were lower than native zonal vegetation obviously and there was still a growth potential for BC of underground vegetation and SOC stocks of deep soil of the desertified lands afforested for 25 years.

Matching policy and science: Rationale for the ‘4 per 1000 - soils for food security and climate’ initiative

At the 21st session of the United Nations Framework Convention on Climate Change (UNFCCC, COP21), a voluntary action plan, the ‘4 per 1000 Initiative: Soils for Food Security and Climate’ was proposed under the Agenda for Action. The Initiative underlines the role of soil organic matter (SOM) in addressing the three-fold challenge of food and nutritional security, adaptation to climate change and mitigation of human-induced greenhouse gases (GHGs) emissions. It sets an ambitious aspirational target of a 4 per 1000 (i.e. 0.4%) rate of annual increase in global soil organic carbon (SOC) stocks, with a focus on agricultural lands where farmers would ensure the carbon stewardship of soils, like they manage day-to-day multipurpose production systems in a changing environment. In this paper, the opportunities and challenges for the 4 per 1000 initiative are discussed. We show that the 4 per 1000 target, calculated relative to global top soil SOC stocks, is consistent with literature estimates of the technical potential for SOC sequestration, though the achievable potential is likely to be substantially lower given socio-economic constraints. We calculate that land-based negative emissions from additional SOC sequestration could significantly contribute to reducing the anthropogenic CO2 equivalent emission gap identified from Nationally Determined Contributions pledged by countries to stabilize global warming levels below 2 °C or even 1.5 °C under the Paris agreement on climate. The 4 per 1000 target could be implemented by taking into account differentiated SOC stock baselines, reversing the current trend of huge soil CO2 losses, e.g. from agriculture encroaching peatland soils. We further discuss the potential benefits of SOC stewardship for both degraded and healthy soils along contrasting spatial scales (field, farm, landscape and country) and temporal (year to century) horizons. Last, we present some of the implications relative to non-CO2 GHGs emissions, water and nutrients use as well as co-benefits for crop yields and climate change adaptation. We underline the considerable challenges associated with the non-permanence of SOC stocks and show how the rates of adoption and the duration of improved soil management practices could alter the global impacts of practices under the 4 per 1000 initiative. We conclude that the 4 per 1000 initiative has potential to support multiple sustainable development goals (SDGs) of the 2030 Agenda. It can be regarded as no-regret since increasing SOC in agricultural soils will contribute to food security benefits that will enhance resilience to climate change. However, social, economic and environmental safeguards will be needed to ensure an equitable and sustainable implementation of the 4 per 1000 target.