Soil Organic Carbon In Subsoil Research Articles

Hydrology, vegetation, and soil properties as key drivers of soil organic carbon in coastal wetlands: A high-resolution study

Coastal wetlands are important blue carbon ecosystems that play a significant role in the global carbon cycle. However, there is insufficient understanding of the variations in soil organic carbon (SOC) stocks and the mechanisms driving these ecosystems. Here we analyze a comprehensive multi-source dataset of SOC in topsoil (0–20 cm) and subsoil (20–100 cm) across 31 coastal wetlands in China to identify the factors influencing their distribution. Structural equation models (SEMs) reveal that hydrology has the greatest overall effect on SOC in both soil layers, followed by vegetation, soil properties, and climate. Notably, the mechanisms driving SOC density differ between the two layers. In topsoil, vegetation type and productivity directly impact carbon density as primary sources of carbon input, while hydrology, primarily through seawater salinity, exerts the largest indirect influence. Conversely, in subsoil, hydrology has the strongest direct effect on SOC, with seawater salinity also influencing SOC indirectly through soil and vegetation mediation. Soil properties, particularly pH, negatively affect carbon accumulation, while climate influences SOC indirectly via its effects on vegetation and soil, with a diminishing impact at greater depths. Using Random Forest, we generate high-resolution maps (90 m × 90 m) of topsoil and subsoil carbon density (R2 of 0.53 and 0.62, respectively), providing the most detailed spatial distribution of SOC in Chinese coastal wetlands to date. Based on these maps, we estimate that SOC storage to a depth of 1 m in Chinese coastal wetlands totals 74.58 ± 3.85 Tg C, with subsoil carbon storage being 2.5 times greater than that in topsoil. These findings provide important insights into mechanism on driving spatial pattern of blue carbon and effective ways to assess carbon status on a national scale, thus contributing to the advancement of global blue carbon monitoring and management.

Winter wheat cover crop increased subsoil organic carbon in a long-term cotton cropping system in Tennessee

Long-term cover-cropping and no-tillage practices can facilitate soil organic carbon (SOC) accumulation in agroecosystems for soil health and climate mitigation benefits. However, the contribution of these conservation management practices to SOC gain from the subsoil layers is not extensively studied. To understand this knowledge gap, it is essential to determine the distribution of total SOC and SOC fractions in response to management practices across the soil profile. Therefore, this study was conducted by leveraging a 40-year replicated field experiment in a continuous cotton (Gossypium hirsutum) system. The management treatments examined included three cover crop treatments: 1) hairy vetch, HV (Vicia villosa); 2) winter wheat, WW (Triticum aestivum L.), and 3) no cover crop, NC; and two tillage treatments: 1) chisel-tillage, CT (10 cm deep) and 2) no-tillage, NT. Soil samples were collected from four depths (0–5, 5–10, 10–30, and 30–60 cm) and analyzed for total SOC and C fractions such as microbial biomass (MBC), particulate organic matter (POM-C), and mineral-associated organic matter (MAOM-C). Results showed that the profile-scale (0–60 cm) SOC stock was greater for WW (33.7 Mg ha −1) compared to HV (29.1 Mg ha −1) and NC (24.4 Mg ha −1), while the tillage effect was non-significant (28 Mg ha −1 for NT and 29.3 Mg ha −1 for CT). The NT increased SOC in the top 5-cm layer, which was reflected in the concentrations of POM-C and MBC. Below the tillage depth (10–30 cm), SOC accrual was greater under CT (10.4 Mg ha −1) than NT (8.9 Mg ha −1), despite similar profile-scale SOC stocks. Further analysis revealed that total SOC in topsoil and subsoil were driven by POM and MAOM-related parameters, respectively. We also observed a limited accumulation of C in POM fraction (0.2–6.1 g C kg−1 soil in tilled topsoil and 0.01–0.71 g C kg−1 soil in subsoil) with a weak relationship with total SOC concentration (topsoil slope = 0.29, subsoil slope = 0.13). Contrastingly, accumulation of C in MAOM fraction was greater (2.7–12.7 g C kg−1 soil in topsoil and 0.6–5.4 g C kg−1 soil in subsoil), and it was strongly related to total SOC concentration than POM-C, especially in the subsoil (topsoil slope = 0.43, subsoil slope = 0.71). Growing deep-rooted winter wheat cover crop increased SOC in the subsoil, indicating the need for climate, soil, and ecosystem-specific management practices for profile-scale net SOC accumulation and redistribution in agroecosystems.

Topographic Position, Land Use and Soil Management Effects on Soil Organic Carbon (Vineyard Region of Niš, Serbia)

Spatial distribution of soil organic carbon (SOC) is the result of a combination of various factors related to both the natural environment and anthropogenic activities. The aim of this study was to examine (i) the state of SOC in topsoil and subsoil of vineyards compared to the nearest forest, (ii) the influence of soil management on SOC, (iii) the variation in SOC content with topographic position, (iv) the intensity of soil erosion in order to estimate the leaching of SOC from upper to lower topographic positions, and (v) the significance of SOC for the reduction of soil’s susceptibility to compaction. The study area was the vineyard region of Niš, which represents a medium-sized vineyard region in Serbia. About 32% of the total land area is affected, to some degree, by soil erosion. However, according to the mean annual soil loss rate, the total area is classified as having tolerable erosion risk. Land use was shown to be an important factor that controls SOC content. The vineyards contained less SOC than forest land. The SOC content was affected by topographic position. The interactive effect of topographic position and land use on SOC was significant. The SOC of forest land was significantly higher at the upper position than at the middle and lower positions. Spatial distribution of organic carbon in vineyards was not influenced by altitude, but occurred as a consequence of different soil management practices. The deep tillage at 60–80 cm, along with application of organic amendments, showed the potential to preserve SOC in the subsoil and prevent carbon loss from the surface layer. Penetrometric resistance values indicated optimum soil compaction in the surface layer of the soil, while low permeability was observed in deeper layers. Increases in SOC content reduce soil compaction and thus the risk of erosion and landslides. Knowledge of soil carbon distribution as a function of topographic position, land use and soil management is important for sustainable production and climate change mitigation.

Factors controlling sizes and stabilities of subsoil organic carbon pools in tropical volcanic soils

Soil organic carbon (SOC) in the subsoil may not be so resistant to decomposition as previously assumed, while the mechanisms controlling C dynamics in subsoils are not yet known. This study aimed to (1) identify the factors that control SOC pools in subsoil and (2) compare the differences in SOC pools and controlling factors between the subsoil and topsoil. Subsoils (20–40 cm) were sampled along elevational gradients from two volcanic regions with less-disturbed vegetation each from Tanzania (11 sites) and Indonesia (12 sites). The sizes and mean residence times of labile, intermediate, and stable SOC pools were estimated by fractionation and model fitting to CO2 release during long-term incubation. The controlling factors of each SOC pool were determined by accompanying partial correlation and path analyses. In subsoil, the intermediate SOC pool predominantly controlled the SOC stability within decades. Climatic, geochemical, and biotic factors controlled different SOC pools. Temperature negatively affected the sizes of all three pools. The nanocrystalline minerals contents predominantly and positively controlled the sizes of intermediate and stable SOC pools, and the mean residence time of intermediate SOC pool. Biotic and climatic factors (i.e., microbial biomass, available N for microbes, and excess precipitation) controlled the labile SOC pool. Compared with topsoil, stabilized organic matters were more in the intermediate rather than in the stable SOC pool, and the temperature had a more significant effect on the stable SOC pool in subsoil than in topsoil. Available N for microbes partially controlled the labile and intermediate SOC pools in subsoil (more limited available N for microbes), but not in topsoil. Thus, subsoil SOC would be more sensitive to climate change than topsoil SOC. This study helped to understand the SOC stabilization mechanism and emphasized the high climate- and mineral-dependence of SOC in subsoil of tropical volcanic regions.

Dynamics of water extractable organic carbon at a subtropical catchment using fluorescence excitation‐emission matrix spectroscopy coupled with parallel factor analysis

AbstractThe vertical and horizontal movement of water extractable organic carbon (WEOC) has a significant influence on soil organic carbon (SOC) storage and water quality. However, how the amount, components and chemical qualities of WEOC change at horizontal and vertical levels, as well as desorption in the cascade model in subtropical red soils, are not fully understood. We sampled soils from four types of land (Elaeocarpus, Masson Pine, Crape‐Myrtle and a deposition site) in a subtropical catchment as replicated cores (n = 96) and investigated changes in the molecular and structural constituents of WEOC using excitation‐emission matrix fluorescence and UV–visible absorbance spectroscopy. Our results showed that WEOC concentration changed remarkably in all four plots, and that the concentration coupled with the relative abundance of aromatic compounds decreased with increasing soil depth, whereas low‐molecular‐weight materials increased with increasing soil depth. Five fluorescent components were identified by parallel factor analysis (i.e., tryptophan‐like, tyrosine‐like and three humic‐substance‐like components). The relative abundance of these components suggested that WEOC composition shifted along vertical soil layers. Dynamic variations in fluorescent indices indicated that WEOC from the subsoil has strong biological characteristics. Additionally, there was no significant correlation between WEOC and SOC in subsoil, indicating that much of the WEOC within mineral soils is of local microbial origin rather than from local soil organic matter. Sorption processes alone or stepwise exchange processes of WEOC on the soil minerals do not readily occur, as the red soil in a subtropical area is typically unsaturated and potentially has a large sorption capacity. Once soil carbon is adsorbed, it is difficult to release it into soil water by microbial processing. Data from the deposition site suggest that components of hydrophilic WEOC with low molecular weight are preferentially transported.

Residue incorporation enhances the effect of subsoiling on soil structure and increases SOC accumulation

Subsoil has great potential for increasing soil organic carbon (SOC) stock. Soil compaction decreases the content of SOC in the topsoil layer and leads to the formation of a plow pan in subsoil. However, measures for improving poor soil structure mainly concentrate on topsoil. This study aims to understand the effect of subsoiling management practices on soil aggregation and accumulation of SOC in both topsoil (0–20 cm) and subsoil (20–35 cm). A field experiment was carried out in a Mollisol in northeastern China. The treatments included conventional tillage (CT), subsoiling tillage (ST), and subsoiling with residue incorporation tillage (SST). The results showed that in comparison to CT, ST and SST not only had lower subsoil bulk densities (reduced by 9.42% and 13.61%, respectively) but also promoted the formation of macroaggregates (> 250 μm) in both soil layers; thus, soil structure stability increased. In the topsoil layer, the content of the > 53 μm aggregate-associated carbon (C) under ST and SST was greater than that under CT. In the subsoil layer, compared with CT, SST increased the organic C content (increased by 0.73–12.32%) in all the aggregate classes, but ST decreased the content of the < 2000 μm aggregate-associated C (6.09–18.96%) on day 150. The soil structure stability decreased by 13.06% and 22.21% under ST and SST after day 150, respectively, which was probably due to the clayey characteristics of the subsoil. We concluded that subsoiling would only improve the soil structure while increasing the loss of organic C. However, the effect of subsoiling on structure improvement was enhanced by residue incorporation, not only promoting soil aggregation but also increasing SOC accumulation.

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

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

Land-use change affects stocks and stoichiometric ratios of soil carbon, nitrogen, and phosphorus in a typical agro-pastoral region of northwest China

The impacts of land-use change on dynamics of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in the subsoil (> 30 cm) are poorly understood. This study aims to investigate whether the effects of land-use change on stocks and stoichiometric ratios (RCN, RCP, and RNP) of SOC, TN, and TP can be different between topsoil (0–30 cm) and subsoil (30–60 cm) in the Ili River Valley, northwest China. Soil samples (0–10, 10–20, 20–30, 30–40, 40–50, and 50–60 cm) were collected from a pasture (PT), a 27-year-old cropland (CL) converted from PT, and a 13-year-old poplar (Populus tomentosa Carr.) plantation (PP) converted from CL. SOC, TN, and TP concentrations and soil bulk density were determined to calculate stocks and stoichiometric ratios (molar ratios) of SOC, TN, and TP. Conversion from PT to CL led to substantial losses in SOC, TN, and TP pools in both topsoil and subsoil, and the reduction rates in subsoil (13.8–24.7%) were higher than those in topsoil (8.5–17.3%), indicating that C, N, and P pools in subsoil could also be depleted by cultivation. Similar to topsoil, significant increases in SOC, TN, and TP stocks were detected after afforestation on CL in subsoil, although the increase rates (31.2–56.2%) were lower than those in topsoil (47.8–69.1%). Soil pH and electrical conductivity (EC), which generally increased after conversion from PT to CL while decreased after CL afforestation, showed significant negative correlations with SOC, TN, and TP, suggesting that cultivation might lead to soil degradation, whereas afforestation contributed to soil restoration in this area. Significant changes in C:N:P ratios in topsoil were only detected for RNP after conversion from CL to PP. By contrast, land-use change significantly altered both RCN and RNP in the subsoil, demonstrating that the impacts of land-use change on RCN and RNP were different between topsoil and subsoil. The significant relationship between soil EC and RNP suggested that RNP might be a useful indicator of soil salinization. Stocks of SOC, TN, and TP as well as RCN and RNP in subsoil showed different responses to land-use change compared to those in topsoil in this typical agro-pastoral region. Therefore, it is suggested that the effects of land-use change on dynamics of SOC, TN, and TP in subsoil should also be evaluated to better understand the role of land-use change in global biogeochemical cycles.

Cropland has higher soil carbon residence time than grassland in the subsurface layer on the Loess Plateau, China

Carbon (C) residence time is one of the key factors that determine the capacity of C storage and the potential C loss in soil, but it has not been well quantified. Assessing C residence time is crucial for an improved understanding of terrestrial C dynamics. To investigate the responses of C residence time to land use, soil samples were collected from millet cropland (MC) and enclosed grassland (EG) on the Loess Platea, China. The soil samples were incubated at 25°C for 182days. A Bayesian inverse analysis was applied to evaluate the residence times of different C fractions based on the information contained in the time-series data from laboratory incubation. Our results showed that soil organic carbon (SOC) mineralization rates and the amount of SOC mineralized significantly was increased after the conversion of cropland into grassland due to the increase in labile C fraction. At the end of the incubation study, 2.1%–6.8% of the initial SOC was released. Our modeling analysis indicated that the sizes and pool of active C in EG were larger than those in MC. The residence times of slow C pool at the 10–30cm layers were longer in MC (11.8–15.7 years) than those in EG (14.7–21.9 years). Our inverse analysis also directly reflected that the active C pool dominated C mineralization in the first 92days, while the slow C pool was the largest contributor after 92days. Conversion of cropland to grassland increases SOC decomposition, probably through increased active C pool resulted from higher litter input and no ploughing activities. The declined SOC in subsoil may be attributed to the short carbon residence times in grassland.

Statistically estimated storage potential of organic carbon by its association with clay content for Korean upland subsoil

Soil organic carbon (SOC) retention has gradually gotten attention due to the need for mitigation of increased atmospheric carbon dioxide and the simultaneous increase in crop productivity. We estimated the statistical maximum value of soil organic carbon (SOC) fixed by clay content using the Korean detailed soil map database. Clay content is a major factor determining SOC of subsoil because it influences the vertical mobility and adsorption capacity of dissolved organic matter. We selected 1,912 soil data of B and C horizons from 13 soil series, Sangju, Jigog, Jungdong, Bonryang, Anryong, Banho, Baegsan, Daegog, Yeongog, Bugog, Weongog, Gopyeong, and Bancheon, mainly distributed in Korean upland. The ranges of SOC and clay content were <TEX>$0-40g\;kg^{-1}$</TEX> and 0 - 60%, respectively. Soils having more than 25% clay content had much lower SOC in subsoil than topsoil, probably due to low vertical mobility of dissolved organic carbon. The statistical analysis of SOC storage potential of upland subsoil, performed using 90%, 95%, and 99% maximum values in cumulative SOC frequency distribution in a range of clay content, revealed that these results could be applicable to soils with 1% - 25% of clay content. The 90% SOC maximum values, closest to the inflection point, at 5%, 10%, 15%, and 25% of clay contents were <TEX>$7g\;kg^{-1}$</TEX>, <TEX>$10g\;kg^{-1}$</TEX>, <TEX>$12g\;kg^{-1}$</TEX>, and <TEX>$13g\;kg^{-1}$</TEX>, respectively. We expect that the statistical analysis of SOC maximum values for different clay contents could contribute to quantifying the soil carbon sink capacity of Korean upland soils.

Soil C, N, P and Its Stratification Ratio Affected by Artificial Vegetation in Subsoil, Loess Plateau China.

Artificial vegetation restoration can induce variations in accumulation and distribution of soil carbon (C), nitrogen (N) and phosphorus (P). However, little is known about variations in soil C, N and P nutrient fraction stratification following artificial vegetation in Loess Plateau China. Based on the hypothesis that re-vegetated can improve soil quality and stratification ratios (SR) can be used as an indicator to evaluate soil quality. This study measured contents and storages of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) and their SRs in topsoil (0–20 cm) and subsoil (20–60 cm) in three 30-year re-vegetated lands that had been converted from arable land (Robinia pseudoacacia L., Caragana Korshinskii Kom. and abandoned cropland with low interferences and few management measures) and one slope cropland (SC) as a control for three soil profiles(0–20 cm, 20–40 cm and 40–60 cm) from June 2009 to June 2013. The results showed that the contents and storages of SOC, TN and TP in re-vegetated land were significantly higher than those in the SC in both topsoil and subsoil. The storages of SOC, TN and TP in the topsoil (0–20 cm) of the re-vegetated lands increased by 16.2%-26.4%, 12.7%-28.4% and 16.5%-20.9%, respectively, and increased by smaller but significant amounts in subsoil from 2009 to 2013. The SRs for SOC, TN and TP in the re-vegetated lands were mostly >2 (either for 0–20:20–40 cm or 0–20:40–60 cm) and greater than that in the SC. The SRs showed an increasing trend with increasing restoration age. The results also showed that the land use type and soil depth were the most influential factors for the SRs and storages, and the SRs of SOC and TN had significantly positive correlations with their storages. The SRs were concluded to be a good indicator for evaluating the soil quality, which can be significantly enhanced through vegetation restoration. Moreover, vegetation restoration can significantly enhance SOC, TN and TP accumulation in both topsoil and subsoil.

Integrating physical and chemical methods for isolating stable soil organic carbon

A better understanding of the effects of climate and management on long-term soil organic carbon (SOC) sequestration necessitates separation of the stable from the labile SOC fraction. An array of physical and chemical methods is currently used for the separation of SOC fractions, but these methods have not been subjected to a comparative evaluation to understand their effectiveness. Therefore, the present study was conducted with the objective to evaluate the pool size and 13C isotopic signatures of stable SOC fractions derived from physical and chemical approaches applied separately and in combination on a Typic Fragiudalf of Ohio, USA. Five methods were evaluated: recovery of silt-associated SOC by physical method (method 1a), clay-associated SOC by physical method (method 1b), bulk soil-associated SOC by chemical method (method 2), chemically stable silt-associated SOC by combined physical and chemical methods (method 3a), and chemically stable clay-associated SOC by combined physical and chemical methods (method 3b). The corn-derived fraction of the stable SOC pool was calculated using 13C natural abundance technique because there was a clear vegetation shift from C 3 (forest) to C 4 species (corn, Zea mays L.) at the study site. Across land use and soil depth increments, the pool sizes of stable SOC isolated by chemical treatment, individually and in combination with physical method (methods 2, 3a, and 3b), were substantially lower (1.5% to 29.4% of total SOC) than that of standalone physical method (methods 1a and 1b) (17.3% to 64.8% of total SOC). However, among the chemical methods (method 2, 3a and 3b), the pool size of chemically resistant SOC was generally comparable. In the surface layer, the proportion of young, corn-derived SOC was lower after chemical treatment than after individual physical method. But, the chemically stabilized SOC in subsoil showed an enrichment of 13C signatures and increased proportion of C 4–C than that of surface soil, and it is mostly similar to that derived from physical methods. The data obtained from this study indicate that integrating physical and chemical methods is effective in isolating the SOC fraction with greatest stability in the surface soil layer, but its effectiveness to separating the stable SOC fractions in the subsoil was comparable to individual physical and chemical methods.