Soil organic carbon across scales

Microbial transformation of soil organic matter under varying agricultural management systems in Ukraine

Introduction

This paper presents comparative studies on the content and structure of organic matter (OM) and the activity of microbiological cellulose destruction in three types of Ukrainian soils intensively used in agricultural production.

Methods

The highest content of humus in the arable layer (4.9%), OM (410 t ha-1), and total carbon (30.9 mg C g-1 soil) was determined in chernic phaeozems, which is 2.2-2.5 times higher than in albic retisols. The soil of natural ecosystems is characterised by a high content of microbial carbon (Cmic) in the carbon fraction of organic soil compounds.

Results and discussion

In arable soils, the content and reserves of humus and soil organic matter (SOM) have decreased by an average of 1.5-2 times. The most considerable loss of humus reserves in the soil profile was identified in albic retisols (1.96-1.44 times) and the smallest in chernic phaeozems (1.27-1.81 times). During the long-term systematic application of mineral fertilisers, the Corg content decreased by 8-21% in chernic phaeozems, 12-33% in greyzemic phaeozems, and 6-38% in albic retisols. A significant difference of 2.1-8.0 times was determined regarding the number of aerobic cellulolytic microorganisms and 1.3-3.3 times in the potential cellulolytic activity of the studied soils. The high number of cellulose-destroying microorganisms is characteristic of chernic phaeozems with a high content of OM in the soil; the advantage over other types of studied soils was 1.4 times and 7.8 times for greyzemic phaeozems and albic retisols, respectively. Among the studied soil types, high values of CO2 emissions were identified in chernic phaeozems. Intensive agricultural practices in Ukrainian soils have significantly altered the content and composition of organic matter, leading to reduced humus and soil organic matter reserves. The study also underscores the importance of considering the abundance of cellulose-destroying microorganisms and their potential activity in assessing soil health and sustainability.

Afforestation increased the microbial necromass carbon accumulation in deep soil on the Loess Plateau

Microbial necromass carbon (MNC) is a stable part of SOC that makes up most of the C pool in land ecosystem. However, the contribution of MNC to SOC accumulation during afforestation is still unclear, particularly in the deep soil. Based on the collection and biomarker analysis of the forest succession sequence and soil profiles with significant depth on the Loess Plateau located China, we study the vertical distribution characteristics and control of MNC. The results found that MNC content increased with succession and decreased with soil depth. On average, the MNC content of a climax forest was 2.23 times higher than that of farmland. The FNC:BNC ratio increased with vegetation succession and decreased with soil depth. Although the MNC content decreased with soil depth, the necromass accumulation coefficient increased. The contribution of MNC to SOC in deep soil (60-100 cm) of pioneer forest was more than 10 % higher than that of farmland, suggesting that afforestation had a relatively positive effect on MNC stabilization and accumulation in deep soils. The microbial biomass and soil nutrient characteristics (i.e., TN, SOC, DOC, and DON) are important factors in mediating the accumulation of MNC in the succession of farmland to forest. These findings demonstrate the potential of MNC in deep soil and provide scientific guidance for sustainable reforestation management based on the carbon pump theory at regional scales.

Potential soil organic carbon sequestration vis-a-vis methane emission in lowland rice agroecosystem

Mitigating the atmospheric greenhouse effect while enhancing the inherent soil quality and productive capacity is possible through soil carbon (C) sequestration, which has a significant potential to counteract the adverse effects of agroecosystem level C emission through natural and anthropogenic means. Although rice is the most important food in India, feeding more than 60% of the country's population, it is commonly blamed for significant methane (CH4) emissions that accelerate climate change. Higher initial soil organic matter concentrations would create more CH4 under the flooded soil conditions, as reducible soil C is a prerequisite for CH4 generation. In India, rice is generally cultivated in lowlands under continuous flooding. Less extensive organic matter breakdown in lowland rice agroecosystems often significantly impacts the dynamics of soil active and passive C pools. Change from conventional to conservation agriculture might trap a significant quantity of SOC. The study aims to investigate the potential of rice-based soils to sequester C and reduce the accelerated greenhouse effects through modified farming practices, such as crop residue retention, crop rotation, organic farming, varietal selection, conservation agriculture, integrated nutrient management, and water management. Overall, lowland rice agroecosystems can sequester significant amounts of SOC, but this potential must be balanced against the potential for CH4 emissions. Management practices that reduce CH4 emissions while increasing soil C sequestration should be promoted and adopted to maximize the sustainability of rice agroecosystems. This review is important for understanding the effectiveness of the balance between SOC sequestration and CH4 emissions in lowland rice agroecosystems for adopting sustainable agricultural practices in the context of climate change.

Ecological health risk assessment of microplastics and heavy metals in sediments, water, hydrophytes (Alternanthera philoxeroides, Typha latifolia, and Ipomoea carnea), and fish (Labeo rohita) in Marala wetlands in Sialkot, Pakistan

For the ecological risk assessment of heavy metals and microplastics in Marala wetlands in Sialkot, Pakistan, samples of sediments, water, aquatic plants (Alternanthera philoxeroides, Typha latifolia, and Ipomoea carnea), and fish (Labeo rohita) were studied from five different locations. Pb, Cd, and Cr concentrations were above permissible limits devised by WHO in sediments and water at most of sites. High concentrations of Cd were recorded in water samples compared to sediments with maximum values recorded at Site-2 (52.08 ± 9.55 mg kg-1) and Site-5 (62.29 ± 10.12 mg kg-1). The maximum concentrations of Cr (7.23 ± 0.40 mg kg-1) and Pb (22.87 ± 0.83 mg kg-1) were found at Site-4 in water samples. The maximum abundance of microplastics (3047 pieces kg-1 of sediments) was at Site-1 with filaments in the highest proportion among the other types. Zn, Ni, and Cu remained generally low in concentrations in both sediments and waters. Plants showed accumulation of heavy metals, notably the amount of Cd (33.36 ± 0.26 mgkg-1) and Ni (163.3 ± 1.30 mgkg-1) absorbed by T. latifolia and A. philoxeroides, respectively were high. Also, photosynthetic pigments in plants seemed to be affected. However, estimated daily intake (EDI) and provisional tolerable weekly intake (PTWI) calculations for the human population consuming fish from this wetland remained below the FAO/WHO limits. PCA analysis revealed the anthropogenic origin of metals that might be causing adverse effects on the biota which depend on this wetland for their food.

Influence of Land Use and Topographic Factors on Soil Organic Carbon Stocks and Their Spatial and Vertical Distribution

Soil organic carbon (SOC) plays a critical role in major ecosystem processes, agriculture, and climate mitigation. Accurate SOC predictions are challenging due to natural variation, as well as variation in data sources, sampling design, and modeling approaches. The goal of this study was to (i) understand SOC stock distribution due to land use (restored prairie grass—PG; lawn grass—LG; and forest—F), and local topography, and (ii) assess the scalability of SOC stock predictions from the study site in North Carolina (Lat: 36°7′ N; Longitude: 80°16′ W) to the geographic extension of the Fairview soil series based on the US Soil Survey Geographic (gSSURGO) database. Overall, LG had the highest SOC stock (82 Mg ha−1) followed by PG (79 Mg ha−1) and forest (73.1 Mg ha−1). SOC stock decreased with the depth for LG and PG, which had about 60% concentrated on the surface horizon (0–23 cm), while forest had only 40%. The differences between measured SOC stocks and those estimated by gSSURGO and modeled based on land use for the Fairview series extent were comparable. However, subtracting maps of the uncertainty predictions based on the 90% confidence interval (CI) derived from the measured values and estimated gSSURGO upper and lower values (an estimated CI) resulted in a range from −17 to 41 Mg ha−1 which, when valued monetarily, varied from USD 33 million to USD 824 million for the Fairview soil series extent. In addition, the spatial differences found by subtracting the gSSURGO estimations from measured uncertainties aligned with the county administrative boundaries. The distribution of SOC stock was found to be related to land use, topography, and soil depth, while accuracy predictions were also influenced by data source.

What Influence Does Conventional Tillage Have on the Ability of Soils to Sequester Carbon, Stabilise It and Become Saturated in the Medium Term? A Case Study in a Traditional Rainfed Olive Grove

Soils have the capacity to store three times more carbon (C) than the atmosphere. This fact has focused scientific and governmental attention because it is one way to mitigate climate change. However, there comes a time when the capacity of soils to store C reaches a limit, considering soil organic carbon (SOC) saturation. In the Mediterranean area, agricultural soils are traditionally exposed to conventional tillage (CT), causing soil properties and quality degradation. Therefore, this study aimed to determine whether CT modifies the carbon storage capacity (carbon saturation), linked to soil mineral fractions <20 µm in olive grove soil in a Mediterranean area over 15 years. The results showed losses of SOC and soil organic carbon stock (SOC-S) over the period studied. Moreover, CT significantly affected aggregate grain size, reducing the percentage of small macro-aggregates (2000–250 µm) by 51.1%, 32.9%, 46.6%, and 50.6% for the Ap, Bw, BC, and C horizons, respectively, and promoting an increase in fine fractions (large micro-aggregates (250–53 µm), silt + clay fraction (53–20 µm) and fine silt + clay (<20 µm)). After 15 years, SOC fractionation showed a decrease in SOC concentration within the large macro-aggregate fraction (>2000 µm) of 38.6% in the Bw horizon; however, in the small macro-aggregates (2000–250 µm), an increase in SOC concentration over time, of 33.5%, was observed in the Ap and Bw horizons. This increasing trend continued in the fine soil fractions. Concerning SOC bound to the fine mineral fraction (<20 µm), evolution over time with CT led to an increase in soil sequestration capacity in the first horizons of 44.7% (Ap horizon) and 42.9% (Bw horizon), and a decrease in depth (BC horizon) of 31.3%. Finally, the total saturated soil organic carbon stock (T-SOC-Ssat), after 15 years, experienced an increase of 30.5 Mg ha−1, and these results conditioned the soil organic carbon stock deficit (SOC-Sdef), causing a potential increase in the capacity of soils to sequester carbon, of 15.2 Mg ha−1 in 15 years. With these results, we can affirm that the effect of CT in the medium term has conditioned the degradation of these soils and the low SOC concentrations, and has therefore made it possible for these soils, with the application of sustainable management practices, to have a high carbon storage capacity and become carbon sinks.

Disturbance alters relationships between soil carbon pools and aboveground vegetation attributes in an anthropogenic peatland in Patagonia

Anthropogenic-based disturbances may alter peatland soil-plant causal associations and their ability to sequester carbon. Likewise, it is unclear how the vegetation attributes are linked with different soil C decomposition-based pools (i.e., live moss, debris, and poorly- to highly-decomposed peat) under grassing and harvesting conditions. Therefore, we aimed to assess the relationships between aboveground vegetation attributes and belowground C pools in a Northern Patagonian peatland of Sphagnum magellanicum with disturbed and undisturbed areas. We used ordination to depict the main C pool and floristic gradients and structural equation modeling (SEM) to explore the direct and indirect relationships among these variables. In addition, we evaluated whether attributes derived from plant functional types (PFTs) are better suited to predict soil C pools than attributes derived from species gradients. We found that the floristic composition of the peatland can be classified into three categories that follow the C pool gradient. These categories correspond to (1) woody species, such as Baccharis patagonica, (2) water-logged species like Juncus procerus, and (3) grasslands. We depicted that these classes are reliable indicators of soil C decomposition stages. However, the relationships change between management. We found a clear statistical trend showing a decrease of live moss, debris, and poorly-decomposed C pools in the disturbed area. We also depicted that plant diversity, plant height, and PFT composition were reliable indicators of C decomposition only under undisturbed conditions, while the species-based attributes consistently yielded better overall results predicting soil C pools than PFT-based attributes. Our results imply that managed peatlands of Northern Patagonia with active grassing and harvesting activities, even if small-scaled, will significantly alter their future C sequestration capacities by decreasing their live and poorly-decomposed components. Finally, aboveground vegetation attributes cannot be used as proxies of soil C decomposition in disturbed peatlands as they no longer relate to decomposition stages.

Highly spatial variation of soil microbial respiration and temperature sensitivity in a subtropical forest

Quantifying the spatial variation and drivers of microbe-driven soil carbon (C) decomposition (also called soil microbial respiration, MR) and its temperature sensitivity (Q₁₀) is crucial for reducing the uncertainty in modelling the terrestrial C cycle under global warming. To this end, most previous studies sampled soils from multiple sites at regional scales and incubated them at the same temperature level in the laboratory. However, this unified incubation temperature is too warm to the cold sites, and too cold to the warm sites, thus causing a large bias in the MR and Q₁₀ estimations. Here, we conducted fine scale intensive sampling (194 soil samples) and measurements within a 4-ha subtropical forest plot to examine the underlying mechanisms driving the spatial pattern of MR and Q₁₀. Our results showed that both MR and Q₁₀ varied spatially within subtropical forests. The fine scale variation of MR was dominated by soil nitrogen concentration and slope position, and Q₁₀ was dominated by soil fungi abundance. Overall, the 35 investigated biotic and abiotic factors explained 38% of the spatial variation of MR and 9% of the spatial variation of Q₁₀ in the subtropical forest. This suggests that the fine scale variation of soil C dynamics is much more complex than that at the regional scale reported in previous studies, which should be considered in the assessments of terrestrial soil C cycles.

Global changes in soil organic carbon and implications for land degradation neutrality and climate stability

Soil organic carbon (SOC) is a critical indicator for healthy and fertile lands across the world. It is also the planet's largest terrestrial carbon pool, so any changes of this pool may have profound implications for both land productivity and climate stability. However, SOC changes have so far remained largely unexplored, although their understanding is essential for many international environmental policies. Here we investigate for the first time recent global SOC changes, based on some SOC stock interannual data that were processed for the 2001-2015 period on a planetary scale. We analysed the global SOC dynamics using the Mann-Kendall test and Sen's slope estimator, which are widely acknowledged to be reliable geostatistical tools for detecting various environmental trends from global to local scale. We explored SOC changes via three metrics (averages, quantities, areas) of negative and positive trends, but also of the balance between soil carbon trends, a key statistic for monitoring land quality stability and soil-atmosphere carbon fluxes in the global environmental policies. Globally, we estimated a net average decrease of -58.6 t C km² yr^-1, a total loss of ~3.1 Pg C, and an area affected by net SOC losses of ~1.9 million km². Using this triple statistic, we found that 79% of countries worldwide have been affected by net declines of SOC after 2001, which suggests that halting land degradation and mitigating climate change through the SOC pathway are still far from being achieved by international policies.

Tillage activates iron to prevent soil organic carbon loss following forest conversion to cornfields in tropical acidic red soils

Previous studies have shown that deforestation and planting of corn resulted in the loss of soil organic carbon (SOC). However, this is not inevitable in regions with acidic red soil. We selected six cornfields that have been planted for 34 years and adjacent forest plots in southwest China. Using a structural equation model, we identified the SOC contents and 42 soil environmental factors in 11 soil layers that are conducive to SOC storage, and evaluated their relative weights hierarchically (0-40, 40-100, and 100-140 cm). Our results surprisingly indicated that after forest had been converted into cornfield, the SOC density did not change in any layer. In acidic red soil, reactive iron (Fe_o), soil water content, nitrogen, and pH were the main soil environmental factors that affected the storage of SOC. In the 0-40 cm soil layer, compared to forests, the contribution of Fe_o in cornfields increased significantly (by 11.65%), due to farming promoting the activation of iron, while the contribution of nitrogen decreased significantly (by 9.65%). In the 100-140 cm soil layer, the contribution of soil environmental factors was similar to that in the forest system, but the pH in cornfields increasing significantly (by 21.5%) may result from the leaching of hydrogen ions. Although the cultivation of cornfields caused a loss of nitrogen in the 0-40 cm soil layer, the increase in Fe_o promoted combination of iron and soil organic carbon, avoiding the soil layer from SOC loss.

Regional environmental controllers influence continental scale soil carbon stocks and future carbon dynamics

Understanding the influence of environmental factors on soil organic carbon (SOC) is critical for quantifying and reducing the uncertainty in carbon climate feedback projections under changing environmental conditions. We explored the effect of climatic variables, land cover types, topographic attributes, soil types and bedrock geology on SOC stocks of top 1 m depth across conterminous United States (US) ecoregions. Using 4559 soil profile observations and high-resolution data of environmental factors, we identified dominant environmental controllers of SOC stocks in 21 US ecoregions using geographically weighted regression. We used projected climatic data of SSP126 and SSP585 scenarios from GFDL-ESM 4 Earth System Model of Coupled Model Intercomparison Project phase 6 to predict SOC stock changes across continental US between 2030 and 2100. Both baseline and predicted changes in SOC stocks were compared with SOC stocks represented in GFDL-ESM4 projections. Among 56 environmental predictors, we found 12 as dominant controllers across all ecoregions. The adjusted geospatial model with the 12 environmental controllers showed an R² of 0.48 in testing dataset. Higher precipitation and lower temperatures were associated with higher levels of SOC stocks in majority of ecoregions. Changes in land cover types (vegetation properties) was important in drier ecosystem as North American deserts, whereas soil types and topography were more important in American prairies. Wetlands of the Everglades was highly sensitive to projected temperature changes. The SOC stocks did not change under SSP126 until 2100, however SOC stocks decreased up to 21% under SSP585. Our results, based on environmental controllers of SOC stocks, help to predict impacts of changing environmental conditions on SOC stocks more reliably and may reduce uncertainties found in both, geospatial and Earth System Models. In addition, the description of different environmental controllers for US ecoregions can help to describe the scope and importance of global and local models.

Field scale estimates of soil carbon stocks on ten heavy textured farms across Ireland

The world's soils store vast amounts (≈2,500 GT) of Carbon which acts as a vital sink to counterbalance the effects of increasing atmospheric carbon dioxide. There have been fruitful efforts to quantify soil Carbon stocks at national scales, which are required for policy level decisions but lack the high resolution required to support farm specific decisions. It is hypothesised that farm scale evaluations of soils can provide insight that is masked in national scale studies and can allow for spatially explicit management approaches to optimise soil Carbon storage and sequestration, such that it can be prioritized within profitable production systems. The objective of the present study was to estimate Carbon stocks on a range of heavy textured soils at field and farm scale and to quantify Carbon storage relative to national scale estimates. Ten grassland dairy farms (mean area of 52.2 Ha) were surveyed, sampled and classified to determine soil types and quantify soil Carbon stores. The level of Carbon present (mean: 346.0 T/Ha) at these sites was greater than previous averages on such soils quantified at national scale (by a factor of 1.1-3.9 depending on soil type). Furthermore, if Carbon saturation potential was realised, the amount of Carbon stored could be increased by an average of 792.1 T/Ha in each profile (from 346.0 to 1138.1 T/Ha). Current management has fostered the retention of large stores of soil Carbon on such soils/farms which co-exist within highly productive farm systems. As there is a societal demand to retain and enhance soil carbon stores to mitigate climate change, high Carbon soils should be identified and, under appropriate policies, commodified to offer a direct incentive to retain soil Carbon. The value of this resource should be recognised and polices to ensure a spatially explicit approach for soil Carbon management should be adopted.

Investigating spatially varying relationships between total organic carbon contents and pH values in European agricultural soil using geographically weighted regression

Total organic carbon (TOC) has received increased attention in recent years, not only as an important indicator in soil fertility, but also due to its close relationship with the atmosphere. Generally, soil TOC and pH values follow a negative correlation, which was revealed by traditional statistical methods. However, the conventional global models lack the ability to capture the spatial variation locally. In this study, spatially varying local relationships between TOC and pH values are studied by geographically weighted regression (GWR) on continental-scale data of European agricultural soil from the project 'Geochemical Mapping of Agricultural and Grazing land Soil' (GEMAS). In this study, TOC is the dependent and pH the independent variable. Both negative and positive local correlation coefficients are observed, showing the existence of 'special' spatially varying relationships between TOC and pH values. Original negative relationships change to positive values in more than 50% of the study area. Novel finding of significant positive correlations is observed in central-eastern Europe, while negative correlations are found mainly in northern Europe. Mixed relationships occur in southern Europe. These special patterns are strongly associated with specific natural factors, especially the extensive occurrence of quartz-rich soil in the central-eastern part of Europe. Anthropogenic inputs may have also played a role in the mixed southern European areas. The GWR technique is powerful and effective for revealing spatially varying relationships at the local level. Thus, it provides a new way to further explore the related influencing factors on the TOC and pH spatial distribution.

Factors controlling the spatial distribution of soil organic carbon in Daxing'anling Mountain

Daxing'anling Mountain, in the northeastern part of China, contains a large amount of soil organic carbon (SOC). Using data including topography, climate, and vegetation, the spatial distribution of SOC content was modeled using classical and geography-based statistics, as well as a geographically weighted kriging model. The study findings include: (1) SOC content generally ranges 40-70 g/kg, with high SOC content in the southwest and low SOC content in the southeast; (2) Results of principal component analysis suggested the normalized difference vegetation index is the best predictor of patterns in SOC; and (3) The geo-weighted regression Kriging model well reflects factors influencing spatial distribution of SOC content. This study provides important baseline information for environmental protection in the Daxing'anling Mountain area, as well as general information as to important factors that mediate this important reservoir of soil carbon.

Interaction effects of the main drivers of global climate change on spatiotemporal dynamics of high altitude ecosystem behaviors: process-based modeling

Soil organic carbon and nitrogen (SOC-N) dynamics are indicative of the human-induced disturbances of the terrestrial ecosystems the quantification of which provides insights into interactions among drivers, pressures, states, impacts, and responses in a changing environment. In this study, a process-based model was developed to simulate the eight monthly outputs of net primary productivity (NPP), SOC-N pools, soil C:N ratio, soil respiration, total N emission, and sediment C-N transport effluxes for cropland, grassland, and forest on a hectare basis. The interaction effect of the climate change drivers of aridity, CO₂ fertilization, land-use and land-cover change, and best management practices was simulated on high altitude ecosystems from 2018 to 2070. The best management practices were developed into a spatiotemporally composite index based on SOC-N stock saturation, 4/1000 initiative, and RUCLE-C factor. Our model predictions differed from the remotely sensed data in the range of - 64% (underestimation) for the cropland NPP to 142% (overestimation) for the grassland SOC pool as well as from the global mean values in the range of - 97% for the sediment C and N effluxes to 60% for the total N emission from the grassland. The interaction exerted the greatest negative impact on the monthly sediment N efflux, total N emission, and soil respiration from forest by - 90.5, - 82.7, and - 80.3% and the greatest positive impact on the monthly sediment C effluxes from cropland, grassland, and forest by 139.3, 137.1, and 133.3%, respectively, relative to the currently prevailing conditions.

Zones of influence for soil organic matter dynamics: A conceptual framework for data and models

Soil organic matter (SOM) is an indicator of sustainable land management as stated in the global indicator framework of the United Nations Sustainable Development Goals (SDG Indicator 15.3.1). Improved forecasting of future changes in SOM is needed to support the development of more sustainable land management under a changing climate. Current models fail to reproduce historical trends in SOM both within and during transition between ecosystems. More realistic spatio-temporal SOM dynamics require inclusion of the recent paradigm shift from SOM recalcitrance as an 'intrinsic property' to SOM persistence as an 'ecosystem interaction'. We present a soil profile, or pedon-explicit, ecosystem-scale framework for data and models of SOM distribution and dynamics which can better represent land use transitions. Ecosystem-scale drivers are integrated with pedon-scale processes in two zones of influence. In the upper vegetation zone, SOM is affected primarily by plant inputs (above- and belowground), climate, microbial activity and physical aggregation and is prone to destabilization. In the lower mineral matrix zone, SOM inputs from the vegetation zone are controlled primarily by mineral phase and chemical interactions, resulting in more favourable conditions for SOM persistence. Vegetation zone boundary conditions vary spatially at landscape scales (vegetation cover) and temporally at decadal scales (climate). Mineral matrix zone boundary conditions vary spatially at landscape scales (geology, topography) but change only slowly. The thicknesses of the two zones and their transport connectivity are dynamic and affected by plant cover, land use practices, climate and feedbacks from current SOM stock in each layer. Using this framework, we identify several areas where greater knowledge is needed to advance the emerging paradigm of SOM dynamics-improved representation of plant-derived carbon inputs, contributions of soil biota to SOM storage and effect of dynamic soil structure on SOM storage-and how this can be combined with robust and efficient soil monitoring.

Impacts of Climate and Land Cover on Soil Organic Carbon in the Eastern Qilian Mountains, China

Soil, as the largest organic carbon pool of terrestrial ecosystem, plays a significant role in regulating the global carbon cycle, atmospheric carbon dioxide (CO2) levels, and global climate change. It is of great significance to scientifically understand the change rule and influence mechanism of soil organic carbon (SOC) to further understand the "source–sink" transformation of SOC and its influence on climate change. In this paper, the spatiotemporal distribution characteristics and influencing mechanism of SOC were analyzed by means of field investigation and laboratory analysis and the measured data in the Eastern Qilian Mountains. The results showed that the average SOC content of 0–50 cm was 35.74 ± 4.15 g/kg and the range of coefficients of variation (CV) between 48.84% and 75.84%, which suggested that the SOC content exhibited moderate heterogeneity at each soil layer of the Eastern Qilian Mountains. In four land cover types, the SOC content of forestland was the highest, followed by alpine meadow, grassland, and wilderness, which presented surface enrichment, and there was a decreasing trend with the soil depth. From the perspective of seasonal dynamics, there was a uniform pattern of SOC content in different land cover types, shown to be the highest in winter, followed by autumn, spring, and summer, and with the biggest difference between winter and summer appearing in the surface layer. At the same time, our study suggested that the SOC content of different land cover types was closely related to aboveground biomass and negatively related to both the mean monthly temperature and the mean monthly precipitation. Therefore, the distribution and variation of SOC was the result of a combination of climate, vegetation, and other factors.

Phytoremediation: Climate change resilience and sustainability assessment at a coastal brownfield redevelopment

Phytoremediation offers a nature based solution (NBS) for contaminated soil remediation; however, its application under a brownfield redevelopment context has not been well studied. Moreover, climate change could impact large numbers of contaminated sites, yet there remains little research on the potential impacts for remediation. This study examined phytoremediation at a brownfield redevelopment in the San Francisco Bay area, where thousands of cleanup sites are vulnerable to rising sea levels. Life cycle assessment (LCA) was used to determine both primary and secondary impacts and the system's resilience to various sea level scenarios and hydroclimatic conditions was investigated. It was found that the phytoremediation project rendered only a small environmental footprint, and was associated with low cost and substantial socioeconomic benefits. For instance, it fitted well with the site redevelopment setting by offering attractive landscape features. Moreover, under a modeled moderate sea level rise scenario, the groundwater hydraulic gradient at the site decreased, which was coupled with greater natural biodegradation and reduced plume migration, and, therefore, lower life cycle impact. There was also minimal increase in the vapor intrusion risk with increased sea level. Overall, phytoremediation at the site was found to be resilient to a moderate sea level rise and other hydroclimatic effects induced by climate change. However, the system performance responded to increasing sea level rise in a non-linear manner. Under a high sea level rise scenario, the system is predicted to perform abruptly worse.

Identification of the co-existence of low total organic carbon contents and low pH values in agricultural soil in north-central Europe using hot spot analysis based on GEMAS project data

Total organic carbon (TOC) contents in agricultural soil are presently receiving increased attention, not only because of their relationship to soil fertility, but also due to the sequestration of organic carbon in soil to reduce carbon dioxide emissions. In this research, the spatial patterns of TOC and its relationship with pH at the European scale were studied using hot spot analysis based on the agricultural soil results of the Geochemical Mapping of Agricultural Soil (GEMAS) project. The hot and cold spot maps revealed the overall spatial patterns showing a negative correlation between TOC contents and pH values in European agricultural soil. High TOC contents accompanying low pH values in the north-eastern part of Europe (e.g., Fennoscandia), and low TOC with high pH values in the southern part (e.g., Spain, Italy, Balkan countries). A special feature of co-existence of comparatively low TOC contents and low pH values in north-central Europe was also identified on hot and cold spot analysis maps. It has been found that these patterns are strongly related to the high concentration of SiO₂ (quartz) in the coarse-textured glacial sediments in north-central Europe. The hot spot analysis was effective, therefore, in highlighting the spatial patterns of TOC in European agricultural soil and helpful to identify hidden patterns.

The role of wetland microorganisms in plant-litter decomposition and soil organic matter formation: a critical review

New soil organic matter (SOM) models highlight the role of microorganisms in plant litter decomposition and storage of microbial-derived carbon (C) molecules. Wetlands store more C per unit area than any other ecosystem, but SOM storage mechanisms such as aggregation and metal complexes are mostly untested in wetlands. This review discusses what is currently known about the role of microorganisms in SOM formation and C sequestrations, as well as, measures of microbial communities as they relate to wetland C cycling. Studies within the last decade have yielded new insights about microbial communities. For example, microbial communities appear to be adapted to short-term fluctuations in saturation and redox and researchers have observed synergistic pairings that in some cases run counter to thermodynamic theory. Significant knowledge gaps yet to be filled include: (i) What controls microbial access to and decomposition of plant litter and SOM? (ii) How does microbial community structure shape C fate, across different wetland types? (iii) What types of plant and microbial molecules contribute to SOM accumulation? Studies examining the active microbial community directly or that utilize multi-pronged approaches are shedding new light on microbial functional potential, however, and promise to improve wetland C models in the near future.

Climate and litter C/N ratio constrain soil organic carbon accumulation

Soil organic carbon (SOC) plays critical roles in stabilizing atmospheric CO2 concentration, but the mechanistic controls on the amount and distribution of SOC on global scales are not well understood. In turn, this has hampered the ability to model global C budgets and to find measures to mitigate climate change. Here, based on the data from a large field survey campaign with 2600 plots across China's forest ecosystems and a global collection of published data from forested land, we find that a low litter carbon-to-nitrogen ratio (C/N) and high wetness index (P/PET, precipitation-to-potential-evapotranspiration ratio) are the two factors that promote SOC accumulation, with only minor contributions of litter quantity and soil texture. The field survey data demonstrated that high plant diversity decreased litter C/N and thus indirectly promoted SOC accumulation by increasing the litter quality. We conclude that any changes in plant-community composition, plant-species richness and environmental factors that can reduce the litter C/N ratio, or climatic changes that increase wetness index, may promote SOC accumulation. The study provides a guideline for modeling the carbon cycle of various ecosystem scales and formulates the principle for land-based actions for mitigating the rising atmospheric CO2 concentration.

Current and emerging methodologies for estimating carbon sequestration in agricultural soils: A review

This review covers the current and emerging analytical methods used in laboratory, field, landscape and regional contexts for measuring soil organic carbon (SOC) sequestration in agricultural soil. Soil depth plays an important role in estimating SOC sequestration. Selecting appropriate sampling design, depth of soil, use of proper analytical methods and base line selection are prerequisites for estimating accurately the soil carbon stocks. Traditional methods of wet digestion and dry combustion (DC) are extensively used for routine laboratory analysis; the latter is considered to be the "gold standard" and superior to the former for routine laboratory analysis. Recent spectroscopic techniques can measure SOC stocks in laboratory and in-situ even up to a deeper depth. Aerial spectroscopy using multispectral and/or hyperspectral sensors located on aircraft, unmanned aerial vehicles (UAVs) or satellite platforms can measure surface soil organic carbon. Although these techniques' current precision is low, the next generation hyperspectral sensor with improved signal noise ratio will further improve the accuracy of prediction. At the ecosystem level, carbon balance can be estimated directly using the eddy-covariance approach and indirectly by employing agricultural life cycle analysis (LCA). These methods have tremendous potential for estimating SOC. Irrespective of old or new approaches, depending on the resources and research needed, they occupy a unique place in soil carbon and climate research. This paper highlights the overview, potential limitations of various scale-dependent techniques for measuring SOC sequestration in agricultural soil.

Assessment of Blue Carbon Storage Loss in Coastal Wetlands under Rapid Reclamation

Highly productive coastal wetlands play an essential role in storing blue carbon as one of their ecosystem services, but they are increasingly jeopardized by intensive reclamation activities to facilitate rapid population growth and urbanization. Coastal reclamation causes the destruction and severe degradation of wetland ecosystems, which may affect their abilities to store blue carbon. To assist with international accords on blue carbon, we evaluated the dynamics of blue carbon storage in coastal wetlands under coastal reclamation in China. By integrating carbon density data collected from field measurement experiments and from the literature, an InVEST model, Carbon Storage and Sequestration was used to estimate carbon storage across the reclamation area between 1990 and 2015. The result is the first map capable of informing about blue carbon storage in coastal reclamation areas on a national scale. We found that more than 380,000 hectares of coastal wetlands were affected by reclamation, which resulted in the release of ca. 20.7 Tg of blue carbon. The carbon loss from natural wetlands to artificial wetlands accounted for 72.5% of total carbon loss, which highlights the major task in managing coastal sustainability. In addition, the top 20% of coastal wetlands in carbon storage loss covered 4.2% of the total reclamation area, which can be applied as critical information for coastal redline planning. We conclude that the release of blue carbon due to the conversion of natural wetlands exceeded the total carbon emission from energy consumption within the reclamation area. Implementing the Redline policy could guide the management of coastal areas resulting in greater resiliency regarding carbon emission and sustained ecosystem services.

Fine resolution map of top- and subsoil carbon sequestration potential in France

Although soils have a high potential to offset CO₂ emissions through its conversion into soil organic carbon (SOC) with long turnover time, it is widely accepted that there is an upper limit of soil stable C storage, which is referred to SOC saturation. In this study we estimate SOC saturation in French topsoil (0-30cm) and subsoil (30-50cm), using the Hassink equation and calculate the additional SOC sequestration potential (SOC_sp) by the difference between SOC saturation and fine fraction C on an unbiased sampling set of sites covering whole mainland France. We then map with fine resolution the geographical distribution of SOC_sp over the French territory using a regression Kriging approach with environmental covariates. Results show that the controlling factors of SOC_sp differ from topsoil and subsoil. The main controlling factor of SOCsp in topsoils is land use. Nearly half of forest topsoils are over-saturated with a SOC_sp close to 0 (mean and standard error at 0.19±0.12) whereas cropland, vineyard and orchard soils are largely unsaturated with degrees of C saturation deficit at 36.45±0.68% and 57.10±1.64%, respectively. The determinant of C sequestration potential in subsoils is related to parent material. There is a large additional SOC_sp in subsoil for all land uses with degrees of C saturation deficit between 48.52±4.83% and 68.68±0.42%. Overall the SOCsp for French soils appears to be very large (1008Mt C for topsoil and 1360Mt C for subsoil) when compared to previous total SOC stocks estimates of about 3.5Gt in French topsoil. Our results also show that overall, 176Mt C exceed C saturation in French topsoil and might thus be very sensitive to land use change.

The location- and scale- specific correlation between temperature and soil carbon sequestration across the globe

Much research has been conducted to understand the spatial distribution of soil carbon stock and its temporal dynamics. However, an agreement has not been reached on whether increasing global temperature has a positive or negative feedback on soil carbon stocks. By analysing global maps of soil organic carbon (SOC) using a spherical wavelet analysis, it was found that the correlation between SOC and soil temperature at the regional scale was negative between 52° N and 40° S parallels and positive beyond this region. This was consistent with a few previous studies and it was assumed that the effect was most likely due to the temperature-dependent SOC formation (photosynthesis) and decomposition (microbial activities and substrate decomposability) processes. The results also suggested that the large SOC stocks distributed in the low-temperature areas might increase under global warming while the small SOC stocks found in the high-temperature areas might decrease accordingly. Although it remains unknown whether the potential increasing soil carbon stocks in the low-temperature areas can offset the loss of carbon stocks in the high-temperature areas, the location- and scale- specific correlations between SOC and temperature should be taken into account for modeling SOC dynamics and SOC sequestration management.

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

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

Understanding the spatial distribution of factors controlling topsoil organic carbon content in European soils

Soil Organic Carbon (SOC) constitutes the largest terrestrial carbon pool. The understanding of its dynamics and the environmental factors that influence its behaviour as sink or source of atmospheric CO₂ is crucial to quantify the carbon budget at the global scale. At the European scale, most of the existing studies to account for SOC stocks are centred in the fitting of predictive model to ascertain the distribution of SOC. However, the development of methodologies for monitoring and identifying the environmental factors that control SOC storage in Europe remains a key research challenge. Here we present a modelling procedure for mapping and monitoring SOC contents that uses Visible-Near Infrared (VNIR) spectroscopic measurements and a series of environmental covariates to ascertain the key environmental processes that have a major contribution into SOC sequestration processes. Our results show that it follows a geographically non-stationary process in which the influencing environmental factors have different weights depending on the spatial location. This implies that SOC stock modelling should not rely on a single model but on a combination of different statistical models depending on the environmental characteristics of each area. A cluster classification of European soils in relation to those factors resulted in the determination of four groups for which specific models have been obtained. Differences in climate, soil pH, content of coarse fragments or land cover type are the main factors explaining the differences in SOC in topsoil from Europe. We found that climatic conditions are the main driver of SOC storage at the continental scale, but we also found that parameters like land cover type influence SOC content found at the local scales in certain areas. Our methodology developed at continental scale could be used in future research aimed to improve the predictive performance of SOC assessments at European scale.

Multi-decadal time series of remotely sensed vegetation improves prediction of soil carbon in a subtropical grassland

Soil carbon sequestration in agroecosystems could play a key role in climate change mitigation but will require accurate predictions of soil organic carbon (SOC) stocks over spatial scales relevant to land management. Spatial variation in underlying drivers of SOC, such as plant productivity and soil mineralogy, complicates these predictions. Recent advances in the availability of remotely sensed data make it practical to generate multidecadal time series of vegetation indices with high spatial resolution and coverage. However, the utility of such data largely is unknown, only having been tested with shorter (e.g., 1-2 yr) data summaries. Across a 2,000 ha subtropical grassland, we found that a long time series (28 yr) of a vegetation index (Enhanced Vegetation Index; EVI) derived from the Landsat 5 satellite significantly enhanced prediction of spatially varying SOC pools, while a short summary (2 yr) was an ineffective predictor. EVI was the best predictor for surface SOC (0-5 cm depth) and total measured SOC stocks (0-15 cm). The optimum models for SOC in the upper soil layer combined EVI records with elevation and calcium concentration, while deeper SOC was more strongly associated with calcium availability. We demonstrate how data from the open access Landsat archive can predict SOC stocks, a key ecosystem metric, and illustrate the rich variety of analytical approaches that can be applied to long time series of remotely sensed greenness. Overall, our results showed that SOC pools were closely coupled to EVI in this ecosystem, demonstrating that maintenance of higher average green leaf area is correlated with higher SOC. The strong associations of vegetation greenness and calcium concentration with SOC suggest that the ability to sequester additional SOC likely will rely on strategic management of pasture vegetation and soil fertility.

Assessing top- and subsoil organic carbon stocks of Low-Input High-Diversity systems using soil and vegetation characteristics

The soil organic carbon (SOC) stock is an important indicator in ecosystem service assessments. Even though a considerable fraction of the total stock is stored in the subsoil, current assessments often consider the topsoil only. Furthermore, mapping efforts are hampered by the limited spatial density of these topsoil measurements. The aim of this study was to assess the SOC stock in the upper 100cm of soil in 30,556ha of Low-Input High-Diversity systems, such as nature reserves, in Flanders (Belgium) and compare this estimate with the stock found in the topsoil (upper 15cm). To this end, we combined depth extrapolation of 139 measurements limited to the topsoil with four digital soil mapping techniques: multiple linear regression, boosted regression trees, artificial neural networks and least-squares support vector machines. Particular attention was given to vegetation characteristics as predictors. For both the stock in the upper 15cm and 100cm, a boosted regression trees approach was most informative as it resulted in the lowest cross-validation errors and provided insights in the relative importance of predictors. The predictors of the stock in the upper 100cm were soil type, groundwater level, clay fraction and community weighted mean (CWM) and variance (CWV) of plant height. These predictors, together with the CWM of specific leaf area, aboveground biomass production, CWV and CWM of rooting depth, terrain slope, CWM of mycorrhizal associations and species diversity also explained the topsoil stock. Our total stock estimates show that focusing on the topsoil (1.63Tg OC) only considers 36% of the stock in the upper 100cm (4.53Tg OC). Given the magnitude of subsoil OC and its dependency on typical ecosystem characteristics, it should not be neglected in regional ecosystem service assessments.

Spatial distribution of soil organic carbon stock in Moso bamboo forests in subtropical China

Moso bamboo (Phyllostachys heterocycla (Carr.) Mitford cv. Pubescens) is an important timber substitute in China. Site specific stand management requires an accurate estimate of soil organic carbon (SOC) stock for maintaining stand productivity and understanding global carbon cycling. This study compared ordinary kriging (OK) and inverse distance weighting (IDW) approaches to study the spatial distribution of SOC stock within 0–60 cm using 111 soil samples in Moso bamboo forests in subtropical China. Similar spatial patterns but different spatial distribution ranges of SOC stock from OK and IDW highlighted the necessity to apply different approaches to obtain accurate and consistent results of SOC stock distribution. Different spatial patterns of SOC stock suggested the use of different fertilization treatments in Moso bamboo forests across the study area. SOC pool within 0–60 cm was 6.46 and 6.22 Tg for OK and IDW; results which were lower than that of conventional approach (CA, 7.41 Tg). CA is not recommended unless coordinates of the sampling locations are missing and the spatial patterns of SOC stock are not required. OK is recommended for the uneven distribution of sampling locations. Our results can improve methodology selection for investigating spatial distribution of SOC stock in Moso bamboo forests.

Future C loss in mid-latitude mineral soils: climate change exceeds land use mitigation potential in France

Many studies have highlighted significant interactions between soil C reservoir dynamics and global climate and environmental change. However, in order to estimate the future soil organic carbon sequestration potential and related ecosystem services well, more spatially detailed predictions are needed. The present study made detailed predictions of future spatial evolution (at 250 m resolution) of topsoil SOC driven by climate change and land use change for France up to the year 2100 by taking interactions between climate, land use and soil type into account. We conclude that climate change will have a much bigger influence on future SOC losses in mid-latitude mineral soils than land use change dynamics. Hence, reducing CO2 emissions will be crucial to prevent further loss of carbon from our soils.

Variation of Soil Organic Carbon and Its Major Constraints in East Central Asia

Variation of soil organic carbon (SOC) and its major constraints in large spatial scale are critical for estimating global SOC inventory and projecting its future at environmental changes. By analyzing SOC and its environment at 210 sites in uncultivated land along a 3020km latitudinal transect in East Central Asia, we examined the effect of environmental factors on the dynamics of SOC. We found that SOC changes dramatically with the difference as high as 5 times in north China and 17 times in Mongolia. Regardless, C:N remains consistent about 12. Path analysis indicated that temperature is the dominant factor in the variation of SOC with a direct effect much higher than the indirect one, the former breaks SOC down the year round while the latter results in its growth mainly via precipitation in the winter half year. Precipitation helps accumulate SOC, a large part of the effect, however, is taken via temperature. NH₄⁺-N and topography also affect SOC, their roles are played primarily via climatic factors. pH correlates significantly with SOC, the effect, however, is taken only in the winter months, contributing to the decay of SOC primarily via temperature. These factors explained as much as 79% of SOC variations, especially in the summer months, representing the major constraints on the SOC stock. Soil texture gets increasingly fine southward, it does not, however, constitute an apparent factor. Our results suggested that recent global warming should have been adversely affecting SOC stock in the mid-latitude as temperature dominates other factors as the constraint.

Scale-dependent variation in nitrogen cycling and soil fungal communities along gradients of forest composition and age in regenerating tropical dry forests

Rates of ecosystem nitrogen (N) cycling may be mediated by the presence of ectomycorrhizal fungi, which compete directly with free-living microbes for N. In the regenerating tropical dry forests of Central America, the distribution of ectomycorrhizal trees is affected by succession and soil parent material, both of which may exert independent influence over soil N fluxes. In order to quantify these interacting controls, we used a scale-explicit sampling strategy to examine soil N cycling at scales ranging from the microsite to ecosystem level. We measured fungal community composition, total and inorganic N pools, gross proteolytic rate, net N mineralization and microbial extracellular enzyme activity at multiple locations within 18 permanent plots that span dramatic gradients of soil N concentration, stand age and forest composition. The ratio of inorganic to organic N cycling was correlated with variation in fungal community structure, consistent with a strong influence of ectomycorrhiza on ecosystem-scale N cycling. However, on average, > 61% of the variation in soil biogeochemistry occurred within plots, and the effects of forest composition were mediated by this local-scale heterogeneity in total soil N concentrations. These cross-scale interactions demonstrate the importance of a spatially explicit approach towards an understanding of controls on element cycling.