Variability Of Soil Organic Carbon Research Articles

Tarım arazileri ve farklı yaşlardaki sarıçam meşçerelerinde (Pinus sylvestris L.) toprak organik karbon ve azot stoklarının değişimi

Bu çalışmada, Kastamonu ili İnebolu ilçesinde yayılış gösteren sarıçam (Pinus sylvestris L.) meşcereleri ve bitişiği tarım arazilerinde ölü örtü miktarı, toprak organik karbon (TOK) ve toplam azot (TA) stokları üzerinde farklı arazi kullanım şeklinin etkisi araştırılmıştır. Farklı yaşlardaki sarıçam meşcereleri (18- ve 30-yaş) ile en az 40 yıldır orman arazisinden tarım arazisine dönüştürülmüş alanlardan örnekleme yapılmıştır. Toprak örnekleri 5 farklı toprak derinlik kademesinden alınmış ve toprak örneklerinin pH’ı, hacim ağırlığı, alınabilir fosfor, yarayışlı potasyum, organik madde, toprak tekstürü, TOK ve TA miktarları belirlenmiştir. Toplam TOK ve TA stokları ise toprak derinliği, hacim ağırlığı, karbon ve azot miktarı çarpılarak hesaplanmıştır. Sonuçlar, farklı yaşlarda sarıçam meşcereleri ve tarım arazileri arasında ölü örtü miktarı, TOK ve TA stokları arasında önemli farklılıklar olduğunu göstermiştir. Genel olarak, 30 yaşındaki sarıçam meşceresinde TOK stoku en düşük belirlenirken (86.60 t C ha-1), bunu sırasıyla tarım alanı (93.70 t ha-1), ve 18 yaşındaki sarıçam meşceresi (115.0 t ha-1) takip etmiştir. Benzer şekilde, TA stoku ise en fazla 18 yaşındaki sarıçam meşceresinde (7.86 t ha-1), en düşük 30 yaşındaki sarıçam meşceresinde (5.74 t ha-1) tespit edilmiştir. Tarım arazilerinde ölü miktarı en düşük iken, sarıçam meşcerelerinde yaşa bağlı olarak orman katmanında biriken ölü örtü miktarı artış göstermiştir. Sonuç olarak, ölü örtü miktarı, TOK ve TA stokları üzerinde farklı arazi kullanım durumu ile toprak derinlik kademelerinin önemli derecede etkisi olduğu anlaşılmıştır. Bu nedenle, ölü örtü miktarı, TOK ve TA stokları ile ilgili gelecekteki araştırmalarda arazi kullanım türü de dikkate alınmalı ve daha detaylı çalışmalar yürütülmelidir.

Examining the Fluctuation of Soil Organic Carbon Levels: An Analysis of the Shuklaphanta National Park in Nepal

Soil organic carbon (SOC) is a crucial carbon reservoir that needs to be monitored for deforestation and forest degradation. The top one-meter layer of soil contains around 1500–1600 Pg of carbon. Assessing the SOC pool is essential for understanding the soil system’s carbon sequestration potential (CSP) as a mitigation strategy and determining whether it acts as a source or sink for atmospheric CO2, depending on the level of saturation. However, there are limited studies on SOC in Nepal’s forests. This research aims to assess SOC variation in the Shuklaphanta National Park in Nepal. It focuses on determining SOC according to depth and analyzing the variation of SOC among the core area of the national park, grasslands, and buffer zone community forests (CFs) and identifying the factors that contribute to the variation in soil carbon across different land uses. The study was conducted using a systematic sampling method with a sampling intensity of 6.59% on 180 soil samples taken from permanent plots set up by the Forest Resource Assessment (FRA) Nepal. The analysis was based on SOC estimated up to the depth of 0–10, 11–20, and 21–30 cm using a modified Walkley–Black wet oxidation method. The study also analyzed contributing factors affecting soil carbon such as vegetation, forest fire, rate of forest resource use, and different soil properties like pH and bulk density. The study found that the mean SOC% up to the depths of 0–10 cm, 11–20 cm, and 21–30 cm was 2.08, 0.98, and 0.68, respectively, in forest areas. Mean SOC% in grasslands was found to be 1.7, 1.68, and 1.87 in 0–10, 11–20, and 21–30 cm, respectively, and in community forests, it was found to be 1.3, 0.98, and 0.58 in 0–10, 11–20, and 21–30 cm, respectively. Similarly, the vertical mean SOC in tC⋅ha−1 (0–30 cm) was found to be 41.75 tC·ha−1 in the core area of the national park, 46.64 tC·ha−1 in grassland, and 37.50 tC·ha−1 in CFs. The study also found that there was variation in SOC with depth and that most of the SOC was concentrated in the topsoil in the core area of the national park and buffer zone community forests. Deep layers of SOC were found in grasslands, core area of the national park, and CF in decreasing order. The study implies that the national park has enormous potential to recapture atmospheric CO2 into the soil. Participating in the sustainable management of the national park can enhance the soil quality and help meet strategies to mitigate climate change. Factors such as vegetation cover, fire, bulk density, and vegetation type were found to be promising for SOC concentration.

The soil organic carbon: Clay ratio in North Devon, UK: Implications for marketing soil carbon as an asset class

AbstractBuilding up stocks of agricultural soil organic carbon (SOC) can improve soil conditions as well as contribute to climate change mitigation. As a metric, the ratio of SOC to clay offers a better predictor of soil condition than SOC alone, potentially providing a benchmark for ecosystem service payments. We determined SOC:clay ratios for 50 fields in the North Devon UNESCO World Biosphere Reserve using 30 cm soil cores (divided into 0–10 cm and 10–30 cm depth samples), with soil bulk density, soil moisture and land‐use history recorded for each field. All the arable soils exceeded the minimum desirable SOC:clay ratio threshold, and the ley grassland soils generally exceeded it but were inconsistent at 10–30 cm. Land use was the primary factor driving SOC:clay ratios at 0–10 cm, with permanent pasture fields having the highest ratios followed by ley grass and then arable fields. Approximately half of the fields sampled had potential for building up SOC stock at 10–30 cm. However, at this depth, the effect of land use is significantly reduced. Within‐field variability in SOC and clay was low (coefficient of variation was ~10%) at both 0–10 cm and 10–30 cm, suggesting that SOC:clay ratios precisely characterized the fields. Due to the high SOC:clay ratios found, we conclude that there is limited opportunity to market additional carbon sequestration as an asset class in the North Devon Biosphere or similar areas. Instead, preserving existing SOC stocks would be a more suitable ecosystem service payment basis.

Historical and future variation of soil organic carbon in China

Revealing historical changes in soil organic carbon (SOC) and exploring its future status are important for safeguarding soil health and food security, giving full play to the service function of soil ecosystems, and coping with climate change. However, there is still a gap in our understanding of the SOC stocks in China and their spatial patterns in response to future climate change. Therefore, we attempted to fill this knowledge gap using a large amount of soil observation data, digital soil mapping technology, and global circulation models from Coupled Model Inter-comparison Project phase 6 (CMIP6). In this study, a random forest model was selected to construct the spatial relationship between SOC in the top 0–20 cm (SOC020) and 0–100 cm (SOC0100) of the soil and 21 environmental factors. Spatiotemporal changes in SOC from 1980 to 2100 were revealed at a resolution of 1 km and five-year interval for three climate scenarios in CMIP6. The cross-validation results indicated acceptable predictions for both depths of SOC; however, relatively large prediction uncertainties were observed for SOC0100 in the Tibetan Plateau and northeastern China. The mean values of SOC020 and SOC0100 over the last four decades were 35.77 and 84.62 Tg, respectively, and showed carbon sinks at the national scale, with accumulation rates of 0.05 Pg yr-1 and 0.036 Pg y-1 for the two depths. Compared with historical values (1980–2020), SOC will fluctuate significantly in the future under different climate scenarios. Among them, SOC showed a slow increasing trend at both depths under the SSP1-1.9 low emission scenario, while SOC presented a decreasing trend under the SSP2-4.5 and SSP5-8.5 medium–high emission scenarios. In particular, in most of China under the SSP2-4.5 scenario, SOC0100 presented a larger likelihood of being a carbon source. This study provides a reference for exploring the response of soil carbon pools to climate change and evaluating the effectiveness of land management and ecological protection.

Factor contribution to soil carbon and nitrogen accumulation after vegetation restoration on the Loess Plateau, China

Unraveling the distributions and controls of soil carbon (C) and nitrogen (N) accumulation is conducive to providing guidance for sustainable ecosystem management. Although the factors that affect soil C and N accumulation have been widely investigated, the relative importance of these factors and whether they confer direct or indirect effects remain unclear. Herein, we examined the distributions of soil organic carbon (SOC), soil inorganic carbon (SIC), and total nitrogen (TN) stocks across the 0–200 cm soil profiles in grasslands, forestlands, and shrublands on the Loess Plateau. Moreover, we determined the relative contributions of land use, topography, and soil properties (texture, water, phosphorus, and available nitrogen) to their variations. The results showed that with increasing soil depth, SOC and TN stocks decreased, while SIC stocks fluctuated. However, large amounts of SOC, SIC, and TN were stored in the deep soil layers (100–200 cm), with values of 3.06, 26.63, and 0.50 kg/m2, respectively. In the 0–200 cm soil layers, the total SIC stock was 53.40 kg/m2 and was 6.8 times higher than the total SOC stock. Among different plant communities, grasslands had the highest SIC stock (54.83 kg/m2), while Armeniaca sibirica and Robinia pseudoacacia had the highest SOC stock (10.06 kg/m2) and TN stock (1.49 kg/m2), respectively. Furthermore, approximately 85% of the variations in SOC, SIC, and TN stocks were jointly affected by land use, topography, and soil properties, in which soil properties acted as the most important driving factors, followed by topography and land use. In particular, topography affected SOC, SIC, and TN stocks mainly through its effects on soil texture and water. Overall, vegetation restoration scenarios incorporating grasslands and Robinia pseudoacacia are highly recommended and should be tailored according to specific topographic features.

Exploring the inter-decadal variability of soil organic carbon in China

Spatiotemporal variability in soil organic carbon (SOC) plays key roles in ecosystem stability and terrestrial carbon cycling. However, the inter-decadal characteristics of SOC variability and its climate drivers in China from the 1980s to 2010s remain unclear. In this study, a modeling framework for combining spatiotemporal prediction models was proposed. Within this model, soil samples from each of the two periods were combined and calibrated to obtain three models, and their prediction outputs were selectively combined to obtain SOC maps for the different inter-decadal periods. A total of 8473 soil samples from three periods (1980s, 2000s and 2010s) were used for model validation. The results suggested that the SOC prediction accuracy in the top 0–20 cm of soil (SOC020) was better than that of the first meter of soil (SOC0100), and the predicted spatial patterns shared characteristics with findings from previous studies. Predictions for the 1990s exhibited relatively high uncertainty, especially in the Tibetan Plateau, Tien Shan, and Northeast China. Both individual and coupled climatic factors exerted scale- and location-specific effects on the SOC. Coupled climatic factors accounted for more than 65% of the SOC variability in both soil layers. The regional SOC in North and Northeast China exhibited inter-decadal decline. Soil in China functioned as carbon sinks from the 1980s to 2010s at a national scale, with SOC020 increasing from 30.40 Pg to 33.97 Pg and SOC0100 from 79.29 Pg to 82.26 Pg. Overall, our results provide reliable baseline data for optimizing carbon cycle models of terrestrial ecosystems, and to guide future carbon sequestration policies.

Impact of rhizosphere priming on soil organic carbon dynamics: Insights from the perspective of carbon fractions

In the rhizosphere zone, soil microbes can be stimulated to increase the mineralization of soil organic matter (SOM), which is known as the rhizosphere priming effect (RPE). Soil organic carbon (SOC) can be separated into the coarse fraction (CF-SOC; fast turnover) and the fine fraction (FF-SOC; slow decomposition); however, the turnover of CF-SOC or FF-SOC during the rhizosphere processes remains unclear. Here, C4-plants (maize and sorghum) grew into C3-soil (paddy soil and lou soil), and the dynamic of the carbon fractions (CF-SOC and FF-SOC) and RPEs were monitored using the 13C natural abundance method during three plant growth stages. Positive RPEs were observed in most situations. There was a net SOC loss in both soils at the end of cultivation. The “new” plant-derived C can partially offset the SOC mineralization caused by rhizosphere priming. The formation of “new” FF-SOC was 1.51–60.20 times higher than that of “new” CF-SOC. Growing plants enhanced the sequestration of the “old” CF-SOC in comparison with the control. Moreover, the RPE strengthened the loss of “old” FF-SOC in the lou soil. The shift in microbial variables was found to strongly regulate SOC sequestration, which accounted for 38 % and 70 % of the variations in SOC in paddy and lou soil, respectively. The activated Gram-positive or Gram-negative bacteria were positively associated with soil CF-SOC and negatively correlated with soil “old” FF-SOC, indicating that stimulated rhizosphere bacteria enhanced the formation of “new” or “old” CF-SOC and the loss of “old” FF-SOC by co-metabolism and fast turnover during the priming process. Overall, our study provides insights into C fractions and microbial control of SOC turnover during the rhizosphere processes, which may have significant implications for better understanding the soil C cycle and increasing SOC sequestration.

Soil development following glacier retreat shapes metagenomic and metabolomic functioning associated with asynchronous C and N accumulation

Glacier retreat caused by global warming may result in the variation of soil organic carbon and nutrient cycling. Yet, the dynamic change of soil microbial functional profiles, especially C metabolism-related, with soil development following glacier retreat are still unclear. In the present study, we investigated the soil microbial communities, metagenomic functioning, and metabolomic profiles along the Hailuogou Glacier forefield representing a 120-year chronosequence. The alpha diversity indices of soil bacteria, protozoa and nifH genes showed an upward trend with increased soil ages, and the beta diversity of soil archaea, bacteria, fungi, protozoa, nifH and nirS genes were significantly correlated with soil ages, in which increasing soil C and P while decreased C/N and pH significantly contributed to the differences of soil microbial communities among the analyzed environmental variables. The metagenomic functional genes related to the metabolisms of Glycogen and Cellulosome, Iron Acquisition and Metabolism were significantly decreased with chronosequence, while the utilization of Xylose and Lactate, Potassium Metabolism, Sulfur Metabolism showing an upward trend with soil ages, in which soil C/N ratios and pH were the most influential factors. In addition, soil C and C/N ratios were also significantly correlated to metabolomic compositions, in which the complexity of the metabolite structure increased with soil ages. Our results indicate that glacier retreat may lead to the asynchronous C and N accumulation along the chronosequence, thereby affecting the metagenomic and metabolomic functioning of soil microbial communities related to C metabolisms during soil development following glacier retreat.

Assessing the carbon sequestration potential and identifying influential factors of cultivated soils in Northeast China

Cultivated lands play a crucial role in terrestrial carbon cycle, and enhancing soil organic carbon (SOC) sequestration in these areas can help effectively mitigate the rise of atmospheric CO2 concentration. In this study, topsoil (0–20 cm) saturated SOC and its density of cultivated soil in Northeast China were mapped using a boosted regression trees (BRT) model. The distribution of the SOC sequestration potential was also calculated based on the difference between saturated SOC and SOC density in ArcGIS. Nine environmental factors including climate, topography and lengths of cultivation data (LCD) and 197 soil samples are used. A 10-fold cross-validation technique is applied to derive four statistical indices - mean absolute prediction error (MAE), root mean square error (RMSE), coefficient of determination (R2) and Lin's consistent correlation coefficient (LCCC) to verify the model performance. The model explains 81% and 85% of the spatial variation of saturated SOC and SOC density, respectively. Mean annual temperature and mean annual precipitation are key factors controlling SOC density and saturated SOC distribution. In addition, LCD showed a similar spatial pattern to SOC sequestration potential, influencing the distribution of SOC density and saturated SOC in the study area. We recommend LCD as an important factor to consider in saturated SOC and SOC density predictions, especially in the farmland ecosystem with a long reclamation history. Accurate mapping SOC sequestration potential and identifying environmental factors will help manage land use and promote soil quality evaluation and improve soil carbon sequestration in this region.

Response of soil organic carbon and its fractions to natural vegetation restoration in a tropical karst area, southwest China

The dynamics of soil organic carbon (SOC) and its fractions are important for evaluating the vegetation restoration effect and carbon cycle of the ecosystem. Here, SOC fractions, including light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), and labile organic carbon (LOC) fractions (including water-soluble organic carbon, WSOC, readily oxidizable organic carbon, ROC, particulate organic carbon, POC, and microbial biomass carbon, MBC), were investigated at four soil depths under five restoration stages in a tropical karst area in southwest China. This study showed that the content of SOC and its fractions significantly increased with vegetation restoration and decreased with increasing soil depth at each restoration stage (p < 0.05). Additionally, LFOC was more sensitive to vegetation restoration, whereas HFOC was the main storage form of SOC. The LOC fractions in the surface soil layer were significantly higher than those in the lower, and the percentages of some LOC fractions (POC/SOC and MBC/SOC) significantly decreased with increasing soil depth, indicating that SOC was more stable in the lower layer than in the surface layer. Correlation analysis showed that SOC was significantly and positively correlated with its fractions. Moreover, SOC and its fraction were positively correlated with soil chemical factors (TN, TP, AP, AK, ECa, EMg, NH4+-N, and NO3–-N) and negatively correlated with bulk density (BD) at a significant level (p < 0.05). Moreover, redundancy analysis showed that the 12 soil physicochemical factors explained 70.99% of the variation in SOC and its fractions, with AK, NH4+-N, and BD being the main factors, explaining 19.38, 17.24, and 10.52% of the variation, respectively. The structural equation mode analysis showed that soil properties, above-ground biomass, and litterfall explained most of the variation in SOC (59%), LFOC (79%), HFOC (81%), and LOC (61%). Soil properties and above-ground biomass significantly affected SOC, LFOC, and HFOC content mainly through indirect effects, while the total phosphorus content of the litterfall could directly and significantly affect SOC, LFOC, and HFOC content. NH4+-N and AK of soil factors had direct effects on LFOC and LOC accumulation, respectively. This study provides a valuable perspective for estimating carbon sink potential and constructing carbon sink models in karst areas.

Mapping soil organic carbon stocks in Nepal’s forests

Comprehensive forest carbon accounting requires reliable estimation of soil organic carbon (SOC) stocks. Despite being an important carbon pool, limited information is available on SOC stocks in global forests, particularly for forests in mountainous regions, such as the Central Himalayas. The availability of consistently measured new field data enabled us to accurately estimate forest soil organic carbon (SOC) stocks in Nepal, addressing a previously existing knowledge gap. Our method involved modelling plot-based estimates of forest SOC using covariates related to climate, soil, and topographic position. Our quantile random forest model resulted in the high spatial resolution prediction of Nepal’s national forest SOC stock together with prediction uncertainties. Our spatially explicit forest SOC map showed the high SOC levels in high-elevation forests and a significant underrepresentation of these stocks in global-scale assessments. Our results offer an improved baseline on the distribution of total carbon in the forests of the Central Himalayas. The benchmark maps of predicted forest SOC and associated errors, along with our estimate of 494 million tonnes (SE = 16) of total SOC in the topsoil (0–30 cm) of forested areas in Nepal, carry important implications for understanding the spatial variability of forest SOC in mountainous regions with complex terrains.

Topography and Soil Organic Carbon in Subtropical Forests of China

Soil organic carbon (SOC) strongly contributes to the operation of the global carbon cycling, and topographical factors largely influence its spatial distribution. However, SOC distribution and its leading topographical impact factors in subtropical forest ecosystems (e.g., the Zhejiang Province in China) have received relatively limited attention from researchers. In this study, 255 forest soil samples were collected from the Zhejiang Province to quantify the spatial variation in SOC and impact factors in subtropical forests. The SOC contents over soil profiles were 35.95 ± 22.58 g/kg, 20.98 ± 15.26 g/kg, and 13.77 ± 11.28 g/kg at depths of 0–10 cm, 10–30 cm, and 30–60 cm, respectively. The coefficient variations at different depths were 62.81% (0–10 cm), 72.74% (10–30 cm), and 81.92% (30–60 cm), respectively. SOC content shows a moderate intensity variation in the Zhejiang Province. The nugget coefficients of the SOC content for the three depths were 0.809 (0–10 cm), 0.846 (10–30 cm), and 0.977 (30–60 cm), respectively. Structural factors mainly influence SOC content. SOC content is positively correlated with elevation and slope, and negatively correlated with slope position (p < 0.05). However, the SOC content was negatively correlated with slope in mixed coniferous and broad-leaved forest. The distribution of the SOC content was relatively balanced between different slope positions. However, the differences became obvious when forest types were distinguished. Topographical factors affected the SOC content differently: elevation > slope > slope position. Slope becomes the main influencing factor in 30–60 cm soil. Forest type significantly influenced the SOC content but with a low statistical explanation compared to topographical factors. Topography has different effects on SOC of different forest types in subtropical forests. This reminds us that in future research, we should consider the combination of topography and forest types.

Responses of soil organic carbon cycle to land degradation by isotopically tracing in a typical karst area, southwest China.

The loss of soil organic carbon (SOC) under land degradation threatens crop production and reduces soil fertility and stability, which is more reflected in eco-sensitive environments. However, fewer studies simultaneously compared SOC variations and δ13CSOC compositions under diverse land uses, especially in karst areas. Soil profiles from two agricultural lands and a secondary forest land were selected to analyze SOC contents and their stable isotope composition (δ13CSOC) in a typical karst area located in southwest China to understand the response of the SOC cycle to land degradation. Moreover, the relationships between SOC contents and mean weight diameter (MWD) and soil erodibility (K) factor were comprehensively analyzed for assessing the response of SOC to soil degradation risk. The mean SOC content was found to be the lowest in abandoned cropland (6.91 g/kg), followed by secondary forest land (9.31 g/kg) and grazing shrubland (34.80 g/kg), respectively. Meanwhile, the δ13CSOC values exhibited the following trend: secondary forest land (mean: -23.79‰) ≈abandoned cropland (mean: -23.76‰) >shrubland (mean: -25.33‰). The isotopic tracing results suggested that plant litter was the main contributor to SOC in the secondary forest land. Whereas abundant nitrogen from goat feces enhanced plant productivity and resulted in additional accumulation of SOC in the grazing shrubland. Conversely, long-term cultivation led to the depletion of SOC sequestration by the loss of calcium. In surface soils, the fractionation of δ13CSOC were considerably affected by the decomposition of SOC by soil microorganisms and covered vegetation rather than agricultural influences. The findings indicate that the cycling of SOC and soil stability in the calcareous soil of southwest China are largely regulated by different land uses and the presence of vegetation cover. The depletion of SOC and soil physical degradation pose significant challenges for abandoned cropland, particularly in the karst area, where land degradation is inevitable. Nevertheless, moderate grazing enhances SOC levels, which is beneficial to the land fertility maintenance in the karst region. Therefore, more emphasis should be placed on the cultivation methods and management strategies for abandoned cropland in the karst area.

Spatial Variability of Soil Organic Carbon, pH and Electrical Conductivity and Its Influencing Factors in a Watershed of Coastal Region of Odisha, India

ABSTRACT Soil organic carbon (SOC), as a main component of the carbon cycle, contributes to biological and physico-chemical processes in the soil that influence plant growth and soil health. The pH of soil determines how acidic and basic the soil is, while the electrical conductivity (EC) of soil is used to determine the extent of soil salinity. Therefore, the quantification and spatial distribution of these soil properties are important for agricultural crop planning. In this study, the spatial variability of SOC, pH, and EC in soils of a watershed in Nayagarh (Odisha, India) was analyzed using geostatistical (ordinary kriging) techniques. The study revealed that the experimental semi-variogram of SOC and EC fits well with the spherical model; however, the Gaussian model was found to be the best fit for soil pH. The coefficients of variation for SOC, pH, and EC were 15%, 7.63%, and 40.3%, respectively, which suggests SOC may have moderate variability. SOC had moderate spatial variability, as shown by the nugget to sill ratio. The geostatistical method was able to accurately assess the spatial variability of SOC content and its influencing factors at the study watershed. The findings could help decision-makers and farmers in developing more effective land resource management.

Soil carbon stabilization and potential stabilizing mechanisms along elevational gradients in alpine forest and grassland ecosystems of Southwest China

Alpine ecosystems potentially store a large amount of soil organic carbon (SOC), but they are highly sensitive to climate change. The assessment of SOC stabilization mechanisms in these ecosystems is therefore vital to understanding the C dynamics in terrestrial ecosystems. Here, we investigated the soil aggregate distribution, aggregate stability, and associated SOC contents within alpine forest and grassland soils along an elevation gradient (2600–3900 m a.s.l) in the Yulong Mountains of Southwest China. Our results showed that the SOC contents in bulk soils and aggregates were higher within grassland soils than in forestland soils, with higher levels in the topsoil (0–10 cm) than in the subsurface soil (10–20 cm) layers at each elevation. The large macroaggregates (>2000 µm) accounted for the largest proportions of aggregate fractions (50.54% and 49.11%) and contributed the greatest proportions of C (55.44% and 52.12%) to the whole soil C within forest and grassland soils, respectively. Generally, the proportions of large macroaggregates, microaggregates (53–250 µm), mean weight diameter, and geometric mean diameter increased significantly with increasing elevation across different soil depths and land types, suggesting that soil C stability was improved with increasing elevation. The SOC contents in bulk soils and aggregates also showed an increasing trend with increasing elevation, with the highest level observed at 3900 m within forestland and 3200 and 3900 m within grassland soils at both soil depths. Climate factors, soil factors, Al/Fe oxides, and aggregate stability indices interactively explained 86% and 52% of the variation in soil C within forest and grassland soils, respectively. Among them, the incorporation of the Al/Fe oxides increased the total explained SOC variations by approximately 10%, exceeding the SOC variations explained by climate or soil factors. Overall, our results provide useful insights into the patterns and mechanisms of soil C stabilization along elevation gradients in alpine ecosystems.

A Continental‐Scale Estimate of Soil Organic Carbon Change at NEON Sites and Their Environmental and Edaphic Controls

AbstractCurrent carbon cycle models focus on the effects of climate and land‐use change on primary productivity and microbial‐mineral dependent carbon turnover in the topsoil, while less attention has been paid to vertical soil processes and soil‐dependent response to land‐use change along the profile. In this study, a spatial‐temporal analysis was used to estimate soil organic carbon (SOC) change in topsoil/A horizon and subsoil/B horizon at National Ecological Observatory Network (NEON) sites, USA over 30 years. To separate the effects of land‐use, environmental, and edaphic factors on SOC change, space‐for‐time substitution was used in combination with the Continuous Change Detection and Classification algorithm and Structural Equation Modeling. Results showed that (a) under natural vegetation, Spodosols and Inceptisols found in the eastern NEON sites had substantial topsoil SOC accumulation (+0.4 to +1.2 Mg C ha−1 year−1), while Inceptisols and Andisols in the west had a comparable magnitude of topsoil SOC loss (−0.5 to −1.8 Mg C ha−1 year−1); (b) Mollisols and Alfisols in the Central Plains sites were susceptible to significant SOC loss under farming and grazing; (c) Runoff/erosion and leaching potential, vertical translocation, and mineral sorption were the most important factors controlling SOC variation across the NEON sites. Our work could be used to parameterize ecosystem models simulating SOC change.

Storage and controlling factors of soil organic carbon in alpine wetlands and meadow across the Tibetan Plateau

AbstractAlpine wetlands and meadows across the Three Rivers Source Region (TRSR) store high soil organic carbon (SOC). However, information on factors affecting SOC storage is scanty. Herein, we investigated SOC storage and explored factors affecting SOC storage, including climate, soil properties and above‐ and belowground biomass, using 50 soil profiles across the TRSR on the Tibetan Plateau. The SOC storage was 491.9 ± 158.5 Tg C and 545.2 ± 160.8 Tg C in the TRSR alpine wetlands and meadow, respectively. The SOC stock was positively correlated with the mean annual precipitation. However, no significant correlation between SOC stock and mean annual temperature was observed, as opposed to the global trend. In addition, SOC stock was positively correlated with both the aboveground biomass (AGB) and belowground biomass (BGB). Soil pH indirectly affected SOC stock, while SOC stock positively correlated with Al and Fe oxyhydroxides. Compared with vegetation biomass and climatic factors, soil properties, including soil pH and Al and Fe oxyhydroxides (Alo and Feo), affected not only SOC stock variation but also affected the impact of vegetation and climatic factors on SOC stock. Climate factors, AGB, BGB, soil pH, Alo and Feo jointly accounted for 59% of SOC stock variation in alpine wetlands and 64% of SOC stock variation in alpine meadow. This study suggests that soil properties are the dominant factors affecting SOC variation in alpine wetlands and meadow on the Tibetan Plateau.

Effects of Climate on Variation of Soil Organic Carbon and Alkali-Hydrolyzed Nitrogen in Subtropical Forests: A Case Study of Zhejiang Province, China

Subtropical forests play an important role in the global carbon cycle and climate change mitigation. In order to understand the effects of climate factors on soil carbon in subtropical forest ecosystems, it is necessary to make full use of carbon sequestration potential. Soil organic carbon (SOC) and soil alkali-hydrolyzed nitrogen (SAN) were tested in 255 plots of subtropical forests in Zhejiang Province, and their forest reserves from 2020 in Zhejiang Province were compared with those from 2010. The results showed that SOC content significantly increased, but SAN content decreased over those ten years. Combined with random forest (RF) and correlation analysis, the contribution of different climate factors (temperature, precipitation, etc.) to soil carbon storage was analyzed, and the main driving factors were evaluated. The RF model explained that winter (December to February) and spring (March to May) were the most dominant drivers to the 0–10 cm and 10–30 cm increases in SOC. There was a significant positive correlation between precipitation and SOC accumulation (0–30 cm) during winter and spring. The minimum temperatures in summer (June to August) and autumn (September to November) were negatively correlated with SOC accumulation (0–30 cm). Increasing the precipitation or irrigation (cloud seeding) in winter could improve the carbon sequestration capacity of subtropical forest soils. This study provides a new perspective on the sensitivity and potential response of the carbon cycle to climate change in subtropical forest ecosystems.

Spatial Distribution of Soil Organic Carbon and Total Nitrogen in a Micro-Catchment of Northeast China and Their Influencing Factors

This study aimed to analyze the spatial distribution of soil organic carbon (SOC) and total nitrogen (STN) in a micro-catchment area comprising farmland and a gully with vegetation restoration (artificial forest and grassland) and their influencing factors. We surveyed a total of 52 topsoil sampling sites to measure the SOC and STN content, as well as topographical factors (elevation, curvature, slope gradient, and aspect), soil properties (bulk density, aggregate distribution, soil texture, and soil moisture), and land use and management. We used traditional statistical and geostatistical methods to analyze the spatial variability of SOC and STN. The results of this study indicate that SOC and STN content varied moderately across the entire micro-catchment area, with greater values in the west (gully head area) than in the east (gully mouth area). Additionally, SOC and STN were significantly positively correlated with soil water content, >2 mm size aggregate fraction, and elevation, but negatively correlated with <0.053 mm size fraction. Land use types also influenced the spatial distribution of SOC and STN contents, with the highest averages of SOC and STN content found at the edge of farmland road and grassland, respectively, and the lowest values in bare land. This study provides a valuable supplement to the understanding of SOC and STN in micro-catchment areas, and the research results also support the evaluation of the impact of gully erosion control on SOC and STN dynamics after vegetation restoration. Overall, vegetation restoration should be adopted for gully erosion control and sustainable agricultural development in the Mollisols region of Northeast China.

Shifting relationships between SOC and molecular diversity in soils of varied carbon concentrations: Evidence from drained wetlands

The molecular diversity of soil organic matter (SOM) is considered to be a critical factor influencing soil organic carbon (SOC) persistence, and is found to show a positive relationship with SOC in many upland soils with relatively low SOC concentrations. However, the SOM diversity-SOC relationship remains under-investigated in high-SOC soils such as wetlands, which exhibit divergent SOC accumulation mechanisms compared to upland soils and may induce contrasting shifts in SOM composition during drainage-induced SOC changes. Here we utilized three contrasting wetlands (i.e., two fens and a bog) that encompassed a wide SOC range and experienced decades of artificial drainage to investigate shifts in SOM diversity after drainage. The molecular diversity of SOM was assessed based on SOM pyrolysis products using pyrolysis–gas chromatography-mass spectrometry, and its influencing factors (including plant inputs, soil minerals, and extracellular enzyme activity, etc.) were evaluated. We observed divergent responses of SOM diversity to long-term drainage in the investigated wetlands. While SOM diversity increased with SOC concentration in the low-SOC (i.e., ≤ 60 mg g−1 soil) fen after drainage, it did not change in high-SOC (i.e., > 60 mg g−1 soil) wetlands after drainage regardless of SOC variations. While plant inputs (e.g., root mass) drove SOC accumulation in these wetlands, microbial processing rather than plant inputs dominated increases in SOM diversity, thereby resulting in the decoupling of SOC concentration and SOM diversity in high-SOC soils. These findings suggest that SOC and SOM diversity had a non-linear relationship in divergent soils. In organic-rich soils such as peatlands, SOM diversity did not increase with SOC accumulation after drainage, in contrast to low-SOC soils. Our study provides a novel, molecular diversity-based perspective on understanding wetland SOC stability in the context of drainage caused by increasing human activity and/or climate changes.