Organic Carbon Stability Research Articles

Exploring the Influence of Natural and Agricultural Land Use Systems on the Different Lability Organic Carbon Compounds in Eutric Endocalcaric Arenosol

It is important to ensure the ratio of stable and labile soil organic carbon (SOC) compounds in the soil as this influences ecosystem functions and the sustainability of soil management. The aim of this investigation was to determine the changes in SOC compounds and soil quality improvement in Arenosol soil after the conversion of arable land to natural and agricultural land use. The land use types included pine afforestation (PA), uncultivated abandoned land (UAL), unfertilised and fertilised cropland (CLunf, CLf), and unfertilised and fertilised grassland (GRunf, GRf). To assess the lability of organic carbon (OC) compounds, levels of mobile humic substances (MHSs), mobile humic acids (MHAs), mobile fulvic acids (MFAs), active C pool (POXC), and water-soluble C (WEOC) compounds were determined. It was found that faster OC accumulation occurs in PA soil than in CLf, and is somewhat slower in grassland uses (GRf and UAL). As the amount of SOC increased, more MHS formed. A significant increase in their quantity was found in PA (+92.2%) and CRf and UAL (+51.5–52.7%). The application of mineral fertilisers promoted the formation of MHSs in CLf and GRf. PA, GRunf, and GRf soils had more suitable conditions for MHA formation (MHA/MFA > 1.3), whereas CLunf soil contained more MFAs. The POXC pool was insensitive to land-use changes in the Arenosol. After land-use conversion, POXC amounts were significantly (p < 0.05) higher in natural ecosystems (UAL and PA) and fertiliser perennial grasses than in CL. The amount of WEOC increased the most in UAL, PA, and GRf (7.4–71.1%). The sequence of decrease in land use was GRf, UAL, and PA > CLunf, CLf, and GRunf. The decreasing order of the carbon management index (CMI) of different land uses (PA > UAL > GRf > GRunf > Clunf) confirms that faster OC accumulation in Arenosol soil occurred in PA and grassland land uses (GRf and UAL). The values of the carbon lability index (CLI) variation (CLunf > GRunf GRf > UAL > PA) show that in PA, UAL, and GRf land uses, mobile organic matter (OM) forms are relatively less formed, which stabilises OC accumulation in the soil. The CMI showed that UAL and GRf were the most suitable soil uses for Arenosol soils.

Phragmites australis straw and biochar additives regulate soil organic carbon fractions in a degraded coastal salt marsh

Severely degraded coastal salt marshes with impaired ecological functions urgently need to be restored to improve blue carbon sequestration. Although plant residues and biochar have been used to restore degraded wetlands, the dose-effects of these plant additives on soil organic carbon fractions in degraded coastal wetlands are still unclear. Here, we investigated the dynamic changes in DOC components and mineral-associated organic carbon, such as calcium-bound organic carbon (Ca-OC) and iron/aluminum-bound organic carbon (Fe/Al-OC), in a degraded coastal salt marsh in a 60-day incubation experiment with low (0.1%), medium (1%), and high (3%) doses of Phragmites australis straw (PS) and Phragmites australis-derived biochar (PB) addition treatments. The results showed that the medium and high doses of PB additives increased humus-like substances, with the maximum proportion occurring on day 60 of the incubation. The fluorescence index (FI) and biological index (BIX) were >1.9 and 0.8, respectively, and the humification index (HIX) was < 4 in most treatment groups. The mineral-associated organic carbon (Ca-OC and Fe/Al-OC) contents were higher in the 1% PS and 3% PS treatments (p < 0.05). The PS additives increased the soil Ca-OC/SOC and Fe/Al-OC/SOC ratios, whereas the PB additives decreased them. This study demonstrated that PS and PB are potential carbon-rich additives for enhancing soil organic carbon stability and carbon sequestration in coastal wetland restoration projects, contributing to regional carbon neutrality.

Effects of coupled biochar and snow cover on soil carbon components and CO2 emissions in seasonally frozen soil areas under climate change conditions

Global warming is profoundly altering soil freeze–thaw cycle (FTC) patterns, and the formation of different thicknesses and durations of snow cover by snowfall results in heterogeneity of environmental and biological factors, which can have complex effects on soil water and carbon cycle processes. In order to better develop rational regulation strategies to increase the potential of soil carbon sequestration and emission reductions under climate change conditions, a three-year in situ control trial of field snow was set up to simulate climate scenarios using two treatments: snow removal and natural snow. The effects of FTCs and biochar on soil CO2 emission flux (CO2 Flux) were analyzed by constructing a driven coupling model between soil hydrothermal environmental factors, unstable organic carbon components and stable organic carbon components. The results showed that CO2 Flux decreased by 9.36% to 11.34% for 1% biochar treatment, while CO2 Flux increased by 15.41% to 18.32% for 2% biochar treatment. Moreover, the snow removal treatment increased CO2 Flux by 9.86% to 13.99% compared to the natural snow treatment. The snow during freezing and thawing has a dual effect on soil hydrothermal dynamics, with snow removal making the freeze–thaw action more intense in perturbing the soil carbon matrix, while the interfacial behavior of biochar with soil minerals protects the stability of the soil structure. Biochar reduces soil carbon emissions thanks to its highly stabilized components and unique surface structure, which enhances the carbon sequestration and emission reduction effect by increasing the proportion of inert organic carbon, promoting the formation of organic–inorganic complexes, and encapsulating and adsorbing soil organic matter. The results of the study can provide important theoretical support and practical models for the assessment of the environmental effects of biochar and the reduction of carbon sequestration in agriculture under climate change conditions.

Effects of ant mounds construction on the carbon cycle in a drained peatland, Northeast China

Acting as ecosystem engineers, ant activities are significantly crucial in altering carbon (C) cycles by modifying the energy flows and nutrient cycles in soils. However, how ant mound constructions reshape soil organic carbon (SOC) pool heterogeneity and stability remains elusive. In this study, we compared differences in soil properties, vegetation biomass, greenhouse gas (GHG) emission, and molecular composition of SOC between ant mounds and nearby soils from a drained peatland, intending to decipher factors that govern carbon dynamics during ant excavates. Results indicated that ant mound construction simultaneously caused a decrease in SOC contents and an increase in nutrient level (total nitrogen and phosphorus), dissolved organic carbon (DOC), and its proportion to SOC (DOC/SOC) dramatically. The proportions of pyrolytic products derived from lignin, phenol, and aromatics decreased significantly from 31.70 % in nearby soils to 27.43 % in ant mounds. However, no significant difference in iron-bound organic carbon (OC-Fe) was found between ant mounds and nearby soils (p > 0.05). Ant mound building greatly accelerated SOC mineralization, with average CO2 emission fluxes 1–2 times higher than that in nearby soils. Notably, CH4 emissions from ant mounds was negative, meaning ant mounds acted as CH4 sinks. Meanwhile, net CH4 flux emission into the atmosphere from nearby soils were positive. Both plant biomass and SOC density (SOCd) in ant mounds were considerably lower than those observed in nearby soils. The structural equation model indicated that soil nutrient enrichment in ant mounds indirectly stimulated SOC decomposition by stimulating microbial abundance. Still, ant mounds construction had direct positive effects on labile carbon components accumulation and posed negative effects on resistant SOC compounds. Moreover, redundancy analysis and variation partitioning analysis confirmed that SOCd decline in ant mounds was primarily attributed to reduced soil moisture and elevated microbial abundance. The conceptual model implied that ants could effectively strengthen SOC content heterogeneity at the microscales, characterized by lower SOC storage and stability, and higher CO2 emission than those in nearby soils.

13C evidence for the selective loss of active and inert organic carbon in soil aggregates under rain-induced overland flow erosion

Due to the hierarchical structure of aggregates, raindrops cause changes in the composition, distribution and state of aggregate-wrapped organic carbon (OC) fractions during erosion, greatly affecting soil carbon turnover and sequestration. To explore these regulatory mechanisms, the selective transport of light (LFoc) and heavy (HFoc) fraction OC within aggregates of varying sizes was traced via the 13C/12C carbon isotope ratio (δ13C) under splash and sheet erosion conditions. A “three-zone” variable-slope soil pan was filled with loess soil with a high aggregate concentration to monitor rainfall erosion on a slope. The OC aggregate composition, aggregate stripping and δ13C values of the sediment aggregates of various particle sizes were measured. When the erosion intensity was low, the splash-eroded sediment was mainly enriched in 13C-rich HFoc, and as the rainfall intensity increased, the concentrations of 12C-rich LFoc and HFoc gradually increased. That is, under heavy rainfall, aggregate fragmentation exposed more large fragments rich in younger HFoc and LFoc, and these fragments underwent saltation. There was no obvious correlation between the trends in splash erosion and OC, so runoff transport was an important factor influencing the correlation between δ13C and OC (P < 0.05). Raindrop impact exposed HFoc-rich aggregate fragments of different sizes, e.g., stable mineral-associated OC. Runoff promoted the obvious preferential transport of LFoc and the redistribution of particulate OC (POC) among different sediment particles. Mineral-associated OC and 2–0.05 mm easily decomposable LFoc or POC were preferentially transported, causing OC with the highest and median δ13C values to be preferentially transported. Overall, the transport order of aggregate-exposed OC particles was clay + silt particle-bonded OC, POC, POC-bonded aggregate fragments and silt-bonded aggregate fragments. These results verified the selective loss of aggregate-detached OC fragments during erosion, which led to a change in the OC sequestration and reaggregation potential of OC in the eroded and deposited soil.

Aridity affects soil organic carbon concentration and chemical stability by different forest types and soil processes across Chinese natural forests

Forest soils play a critical role in carbon (C) reservoirs and climate change mitigation globally. Exploring the driving factors of soil organic carbon (SOC) concentration and stability in forests on a large spatial scale can help us evaluate the role of forest soils in regulating C sequestration. Based on SOC quantification and solid-state 13C nuclear magnetic resonance spectroscopy, we investigated the SOC concentration and SOC chemical stability (indicated by alkyl-to-O-alkyl ratio and hydrophobic-to-hydrophilic ratio) in top 0–5 and 5–10 cm soils from 65 Chinese natural forest sites and explored their driving factors. Results showed that SOC concentration in 0–5 cm soils were highest in mixed forests but SOC chemical stability in 0–5 cm soils were highest in coniferous forests, while SOC concentration and chemical stability in 5–10 cm soil layers did not differ across forest types. SOC concentration in 0–5 cm was directly related to soil pH and soil bacterial diversity. Structural equation models showed that aridity indirectly affected SOC concentration in 0–5 cm by directly affecting soil pH. While SOC chemical stability in 0–5 cm soils was higher with increased aridity. According to the correlations, the potential mechanisms could be attributed to higher proportion of coniferous forests in more arid forest sites, lower relative abundance of O-alkyl C, higher MgO and CaO contents, and higher bacterial diversity in soils from more arid forest sites. Our study reveals the important role of aridity in mediating SOC concentration and chemical stability in top 0–5 cm soils in Chinese natural forests on a large-scale field investigation. These results will help us better understand the different mechanisms underlying SOC concentration and stability in forests and assess the feedback of forest SOC to future climate change.

The limited effect of deforestation on stabilized subsoil organic carbon in a subtropical catchment

Abstract. Predicting the quantity of soil organic carbon (SOC) requires understanding how different factors control the amount of SOC. Land use has a major influence on the function of the soil as a carbon sink, as shown by substantial organic carbon (OC) losses from the soil upon deforestation. However, predicting the degree to which land use change affects the OC content in soils and the depth down to which this occurs requires context-specific information related to, for example, climate, geochemistry, and land use history. In this study, 266 samples from forests and agricultural fields were collected from 94 soil profiles down to 300 cm depth in a subtropical region (Arvorezinha, southern Brazil) to study the impact of land use on the amount of stabilized OC along the soil profile. We found that the stabilized OC content was not affected by land use below a depth of 90 cm. Along the soil profile, the amount of stabilized OC was predominantly controlled by land use and depth in addition to the silt and clay content and aluminium ion concentrations. Below 100 cm, none of the soil profiles reached a concentration of stabilized OC above 50 % of the stabilized OC saturation point (i.e. the maximum OC concentration that can physically be stabilized in these soils). Based on these results, we argue that it is unlikely that deeper soil layers can serve as an OC sink over a timescale relevant to global climate change due to the limited OC input in these deeper layers. Furthermore, we found that the soil weathering degree was not a relevant control on the amount of stabilized OC in our profiles because of the high weathering degree of the studied soils. It is therefore suggested that, while the soil weathering degree might be an effective controlling factor of OC stabilization over a large spatial scale, it is not an informative measure for this process at regional and local scales (with similar climate, bedrock, and weathering history) in highly weathered soils.

Trade-off between Pore-Throat Structure and Mineral Composition in Modulating the Stability of Soil Organic Carbon.

The preservation of soil organic carbon (OC) is an effective way to decelerate the emission of CO2 emission. However, the coregulation of pore structure and mineral composition in OC stabilization remains elusive. We employed the in situ nondestructive oxidation of OC by low-temperature ashing (LTA) combined with near edge X-ray absorption fine structure (NEXAFS), high-resolution microtomography (μ-CT), field emission electron probe microanalysis (FE-EPMA) with C-free embedding, and novel Cosine similarity measurement to investigate the C retention in different aggregate fractions of contrasting soils. Pore structure and minerals contributed equally (ca. 50%) to OC accumulation in macroaggregates, while chemical protection played a leading role in C retention with 53.4%-59.2% of residual C associated with minerals in microaggregates. Phyllosilicates were discovered to be more prominent than Fe (hydr)oxides in C stabilization. The proportion of phyllosilicates-associated C (52.0%-61.9%) was higher than that bound with Fe (hydr)oxides (45.6%-55.3%) in all aggregate fractions tested. This study disentangled quantitatively for the first time a trade-off between physical and chemical protection of OC varying with aggregate size and the different contributions of minerals to OC preservation. Incorporating pore structure and mineral composition into C modeling would optimize the C models and improve the soil C content prediction.

Variation of root traits and its influences on soil organic carbon stability in response to altered precipitation in an alpine meadow

Soil organic carbon (SOC) dynamics are strongly controlled by plant roots. Yet, how variation of root traits under precipitation change influences SOC stability remains unclear. As part of a 5-year field experiment manipulating precipitation including 90 % (0.1P), 50 % (0.5P), 30 % (0.7P) decrease, and 50 % increase (1.5P), this study was designed to assess the effects of changing precipitation on root traits and production dynamics by minirhizotron and examine how such influences regulate SOC stability in an alpine meadow on the Qinghai-Tibetan Plateau. We found that root length density (RLD), specific root length (SRL), root branching intensity (RBI), and root residue carbon input (RC input) exhibited no significant response, whereas root turnover (RT), root carbon (C), nitrogen (N) concentrations and C/N ratio were altered by precipitation change with nonlinear trends. Absorptive root RT positively correlated to manipulated precipitation within the interannual precipitation range in topsoil, but it showed no significant change under extreme drought treatment. Alpine meadows can maintain the SOC content and density under varied precipitation. However, it showed significant variation in aggregate stability and organic carbon (OC) distribution in aggregates in topsoil, which were mainly due to the strong direct effects of soil moisture and partly related to RLD and RC input of transport roots. Although subsurface soil aggregate stability and OC associated with aggregates were not modified, our results indicated a risk of SOC stability variation in subsurface soil if absorptive root RT and SRL changed. These findings provide vital information to predict responses of SOC dynamics of alpine meadow to future climate change.

Drivers of organic carbon distribution and accumulation in the northern Barents Sea

Sedimentary properties and accumulation rates on the continental shelf and in the deep sea reflect temporal oceanographic, biological and chemical processes occurring in the water column and the sediment surface. We used the radionuclides 210Pb, 226Ra, and 137Cs activities to estimate sedimentation rates during the last century at nine stations in the northern Barents Sea region. Elemental (C, N) and stable isotopic composition (δ13C, δ15N) were also analysed from the nine stations sampled in August 2018, and, for five other stations sampled in August and December 2019, and in March and May 2021. Sediment accumulation rates varied between 130 and 1 410 g m−2 y−1. The < 63 μm normalized total organic carbon (TOC63) and the total nitrogen from the sediment surface varied between 0.90–2.56 % and 0.13–0.33 %, respectively. Ice-free shelf stations had higher TOC63 and possibly fresher organic matter (high δ13C, low δ15N) than ice-covered more northern stations. The opposite trend was observed for total inorganic carbon. We found that these trends in biogeochemical parameters were spatially structured by the winter sea ice concentration and biological production differences, and exhibited a south-north separation of the Polar Front region. The low and stable organic carbon accumulation rate (1.7–13.4 g Corg m-2 y−1; ARtoc) is a function of slow sedimentation rates, and high degradation and residence time in the water column and at the sediment–water interface. Overall, the ARtoc has been stable for the past 100 years, with a slight increase from the early 1970s to the present at the shelf and slope stations. Our results highlight that spatial scales of variability of the studied sedimentary parameters are linked to spatial patterns of important environmental variables (e.g., chlorophyll-a, sea ice concentration) in the region. In contrast, no seasonal differences were observed in the sediment parameters of revisited stations, and the dated sediment geochemical profiles did not exhibit substantial longer-term variation. This means that climate-induced changes in variables that modify the sedimentary geochemistry of the environment may affect benthic community activity and structure before leaving a record in ARtoc.

Seagrass decline weakens sediment organic carbon stability

Seagrass meadows are globally recognized as critical natural carbon sinks, commonly known as ‘blue carbon’. However, seagrass decline attributed to escalating human activities and climate change, significantly influences their carbon sequestration capacity. A key aspect in comprehending the impact of seagrass decline on carbon sequestration is understanding how degradation affects the stored blue carbon, primarily consisting of sediment organic carbon (SOC). While it is widely acknowledged that seagrass decline affects the input of organic carbon, little is known about its impact on SOC pool stability. To address this knowledge, we examined variations in total SOC and recalcitrant SOC (RSOC) at a depth of 15 cm in nine seagrass meadows located on the coast of Southern China. Our findings revealed that the ratio of RSOC to SOC (RSOC/SOC) ranged from 27 % to 91 % in the seagrass meadows, and the RSOC/SOC increased slightly with depth. Comparing different seagrass species, we observed that SOC and RSOC stocks were 1.94 and 3.19-fold higher under Halophila beccarii and Halophila ovalis meadows compared to Thalassia hemprichii and Enhalus acoroides meadows. Redundancy and correlation analyses indicated that SOC and RSOC content and stock, as well as the RSOC/SOC ratio, decreased with declining seagrass shoot density, biomass, and coverage. This implies that the loss of seagrass, caused by human activities and climate change, results in a reduction in carbon sequestration stability. Further, the RSOC decreased by 15 %, 29 %, and 40 % under unvegetated areas compared to adjacent Halophila spp., T. hemprichii and E. acoroides meadows, respectively. Given the anticipated acceleration of seagrass decline due to climate change and increasing coastal development, our study provides timely information for developing coastal carbon protection strategies. These strategies should focus on preserving seagrass and restoring damaged seagrass meadows, to maximize their carbon sequestration capacity.

Development of unique soil organic carbon stability index under influence of integrated nutrient management in four major soil orders of India

Stability of soil organic carbon (SOC) is pre-requisite for stabilization of C leading to long-term C sequestration. However, development of a comprehensive metric of SOC stability is a major challenge. The objectives for the study were to develop novel SOC stability indices by encompassing physical, chemical, and biochemical SOC stability parameters and identifying the most important indicators from a Mollisol, an Inceptisol, a Vertisol, and an Alfisol under long-term manuring and fertilization. The treatments were control, 100%NPK, 50% NPK+ 50% N through either farmyard manure, cereal residue, or green manure. SOC stability indicators were selected, transformed and integrated into unique SOC stability indices via conceptual framework and principal component analysis. Principal component analysis identified Al-macroaggregate, humic acid C-microaggregate, microaggregate-C, particulate organic matter-C-macroaggregate and polyphenol-microaggregate as the important SOC stability indicators. The principal component analysis -based SOC stability index varied from 0.2 to 0.9, 0.1 to 0.5, 0.2 to 0.6, 0.1 to 0.5 for Mollisol, Inceptisol, Vertisol and Alfisol, respectively. The SOC-stability index derived from conceptual framework and principal component analysis significantly lined up well with one another, although NaOCl-Res-C showed a high correlation with both conceptual framework (r = 0.8) and principal component analysis-based (r = 0.7) SOC stability indexes, suggesting that both methods might be used to quickly assess SOC stability in four soil orders. Overall, 50%NPK+50%N by farmyard manure or green manure emerged as the most effective management practices for enhancing stability of SOC in Mollisol, Inceptisol, Vertisol, and Alfisol of India which might act as major C sink in rice-wheat and maize-wheat cropping systems. The other aspect of C sequestration is to enhance agricultural productivity without depending much on expensive chemical fertilizers. The model yardstick thus developed for assessing SOC stability might be useful to other systems as well.

Anaerobically mineralized nitrogen as a potential indicator of the activity and abundance of mycorrhizal fungi in Mollisols

Anaerobically mineralized nitrogen (AN) is a suitable soil health indicator. The AN is sensitive to soil use changes and is related to soil and particulate organic carbon and aggregate stability. This work aims to evaluate the relationship between AN and 1) easily extractable glomalin-related soil proteins; 2) abundance of arbuscular mycorrhizal fungi measured by the number of arbuscular mycorrhizal fungi spores, and 3) arbuscular mycorrhizal fungi activity (root colonization). Soil samples were taken at depths of 0-5 and 5-20 cm from cultivated and uncultivated plots throughout the southeastern province of Buenos Aires. Anaerobically mineralized nitrogen, soil organic carbon, particulate organic carbon, aggregate stability, easily extracted glomalin-related soil proteins and the logarithm of the number of arbuscular mycorrhizal fungi spores (log spores) at 0-5, 5-20 and 0-20 cm depths were determined. In wheat roots, the percentages of total infection and arbuscules at 0-20 cm were measured. At all depths, AN was positively correlated to easily extractable glomalin-related soil proteins (r=0.34-0.65), which is an indicator of arbuscular mycorrhizal fungi activity and abundance. Likewise, AN was positively related to log-spores (r=0.58-0.78), which is an indicator of arbuscular mycorrhizal fungi abundance. However, AN was not related to root colonization (the percentages of total infection and arbuscules) that manifests the activity of arbuscular mycorrhizal fungi at a specific moment. Thus, anaerobically mineralized nitrogen would be an indicator of mid- to long-term changes in arbuscular mycorrhizal fungi abundance and activity (easily extractable glomalin-related soil proteins and log-spores) resulting from soil use. Consequently, the AN would allow monitoring an important aspect of soil microbiological health associated with arbuscular mycorrhizal fungi. However, it is necessary to evaluate the relationships studied in this work in a wider range of soil situations.

Soil erosion‐induced decline in aggregate stability and soil organic carbon reduces aggregate‐associated microbial diversity and multifunctionality of agricultural slope in the Mollisol region

AbstractSoil erosion has become a serious ecological problem hindering the agricultural and economic development in Northeast China. Soil aggregates are basic units that regulate soil fertility and microbial activity. However, there have been few reports on the role of soil erosion in regulating the microbial diversity, multiple ecosystem services, and functions (multifunctionality) of soil aggregates. In this study, we investigated the effects of soil erosion on microbial diversity and the activities of various extracellular enzymes associated with soil aggregates by using Mollisol soil in Northeastern China at different positions of a slope, and how soil erosion affects soil multifunctionality indexes obtained by normalized calculation of soil enzyme activities. The results showed that soil erosion could reduce the physicochemical properties, soil enzyme activity, and microbial diversity of the aggregates, resulting in accumulation of aggregate nutrients and increase in microbial diversity and enzyme activity in the sedimentation area. Compared with smaller soil aggregates, larger soil aggregates had higher nutrient content, enzyme activity, and microbial diversity. However, with the intensification of soil erosion, the differences in microbial diversity and soil enzyme activities among different sizes of aggregates were reduced. Soil erosion also significantly reduced soil multifunctionality (p &lt; 0.001). The multifunctionality of 2000–250 μm, 250–53 μm, and &lt;53 μm aggregates respectively declined from 0.52, 0.20, and 0.08 (no erosion sites) to −0.17, −0.66, and −0.77 (heavy erosion sites). Additionally, microbial diversity was found to have a linear positive correlation with soil multifunctionality (R2 = 0.23, p &lt; 0.01). Structural equation modeling and random forest analysis revealed that soil organic carbon and aggregate water stability are main predictors of soil multifunctionality. Furthermore, soil physicochemical properties had a greater influence on soil multifunctionality than microbial diversity. Collectively, our results demonstrate that soil erosion negatively affects the structure, resource effectiveness, and microbial activity of soil aggregates, thereby reducing to differential soil multifunctionality.

Upper limit of mineral-associated organic carbon in temperate and sub-tropical soils: How far is it?

Soil organic carbon (SOC) is the main terrestrial C pool and is roughly divided into stable mineral-associated organic carbon (MAOC) and particulate organic matter carbon (POC). However, the sequestration capacity of MAOC remains unquantified. Currently, soils are considered to have a limited capacity to accumulate MAOC, primarily due to the presence of fine fractions, i.e. silt and clay particles. However, it is unlikely that saturation will occur in all soils due to land use. Therefore it is necessary to investigate whether soils from different latitudes and their silt and clay fractions that comprise MAOC are saturated. We selected 71 soil samples that were physically fractionated from a Mexican inventory of vegetation with a wide range of SOC (4–186 g kg−1 soil) and clay (126–777 g kg−1 soil) contents. Soils were grouped into non-volcanic soils and allophanic Andosols. In the first group, we found a positive and highly significant linear relationship between SOC and MAOC contents, but no upper limit was detected. We found a common slope of 0.86 (R2 = 0.97, p < 0.01), i.e. 86% of the SOC was contained in MAOC irrespective of soil types and management, a value very similar to that in other studies. Conversely, for allophanic Andosols, which had SOC content twice that of non-volcanic soils, a second-order polynomial function (R2 = 0.98, p < 0.01) was established, the upper limit of which exceeded 186 g SOC kg−1 for a MAOC of 134 g kg−1. This was also supported by an increasing polynomial function of particulate organic carbon (POC) as the SOC increased. In both types of soils, maximum MAOCs were found between 700 and 800 g kg−1 of silt and clay in medium-textured soils. The amount of silt and clay particles was limited by SOC accumulation in the silt fraction as clay became saturated. These results are influenced by the mass proportion of silt and clay that follows a unique pattern in allophanic Andosols and non-volcanic soils. An investigation of a comprehensive database comprising 21,315 soil profiles sampled 0–30 cm across Mexico showed that most soils are below 186 g SOC kg−1 the bending point above which the upper limit does exist.

Labile and Stable Organic Carbon Fractions in Water Stable Aggregates and Their Contribution to Aggregate Stability in Paddy Soils

Labile and Stable Organic Carbon Fractions in Water Stable Aggregates and Their Contribution to Aggregate Stability in Paddy Soils

Residual effect of legumes on maize yield, nitrogen balance and soil organic carbon stabilization under legume – maize cropping systems

Inclusion of legumes in cereal based cropping system restores soil fertility and improves carbon storage and soil quality. In this background a field experiment was conducted in a strip-split plot design using three legume species viz., green gram, groundnut and cluster bean as vertical strip, two residue management practices i.e. residue removal (-R) and residue retention (+R) as horizontal strip, and three nitrogen levels i.e. 100%, 75% and 50% recommended rate of nitrogen (N) application in intersectional sub plots with four replications during summer 2017 and 2018. The grain and stover yield of maize, N uptake and soil properties were monitored after each crop harvest; soil nitrogen balance and soil organic carbon were estimated. Under 100% N fertilization, cluster bean – maize cropping system with residue (+R) recorded 12.78 and 9.78% higher maize grain and stover yield, respectively than without residue (–R) treatment. Highest available N (189.9 kg ha−1) and available P (54.5 kg ha−1) were registered under cluster bean- maize cropping system. Legume residue incorporation resulted 9.65, 8.46, 7.09 and 8.86% higher (p = 0.05) WBC content, WBC stock, available N and P contents over residue removal scenario. Annually, 1.71% added carbon converted into soil organic carbon. Legume residue retention and 100% recommended rate of nitrogen application enhanced nitrogen balance. Soil N balance was highest under groundnut – maize system (165 kg ha−1). Overall, for better soil health and optimum crop production, cluster bean – maize cropping system with legume residue retention and 100% N fertilization may be promoted in semi-arid Western India.

Black soils in the southwest of the Brazilian Pantanal: Organic carbon and secondary carbonates accumulation in Phaeozems–Gleysol-Chernozem

The southwest of the Brazilian Pantanal contains areas formed by calcareous sediments, which give this hydromorphic environment special characteristics in the formation of soils such as Phaeozems, Chernozems and Gleysols. These soils have a dark surface horizon, enriched with organic matter, high natural fertility in the sorption complex and high clay activity. These soils are important because they store large amounts of organic carbon and can be used in carbon sequestration and greenhouse gases studies. However, despite their abundance in the Pantanal region and importance, these soils are few studied. Thus, our study aimed to characterize the morphology, physical, and chemical attributes of black soils in this region, identifying the main mechanisms of organic carbon and secondary carbonate accumulation. Three profiles located in the municipality of Corumbá-MS were studied, namely Phaeozem (profiles P1), Gleysol (profile P2) and Chernozem (profiles P3). In all profiles, high concentrations of Ca2+, Mg2+ and K+ elements were observed, mainly derived from the sediments of the Serra da Bodoquena and the Urucum-Amolar residual plateau, ion transport fluxes and secondary carbonate dissolution. The preservation of these elements and 2:1 clays is closely linked to the rainfall dynamics of the region, alternating between floods and droughts. The high 2:1 clays and exchangeable Ca2+ benefits the stabilization of organic carbon by means of organic–mineral interactions and promote the increase of humin content. In Phaeozem and Chernozem, the highest levels of secondary carbonates were observed at the top of the B horizons of these soils, where the pressure of CO2 is lower. Being a lowland area under the direct influence of flooding, the development of secondary carbonates is limited in the B horizons of the Gleysol, where access to the water table and root activity at depth increases the CO2 pressure and hinders the re-precipitation of carbonates in this soil, precipitating in the capillary bangs and in the upper zone (surface horizons).

High levels of soil calcium and clay facilitate the recovery and stability of organic carbon: Insights from different land uses in the karst of China.

Soil organic carbon (SOC) is a crucial medium of the global carbon cycle and is profoundly affected by multiple factors, such as climate and management practices. However, interactions between different SOC fractions and land-use change have remained largely unexplored in karst ecosystems with widespread rock outcrops. Owing to the inherent heterogeneity and divergent response of SOC to land-use change, soil samples with close depth were collected from four typical land-use types (cropland, grassland, shrubland, and forestland) in the karst rocky desertification area of China. The aim of this study was to explore the responses of SOC dynamics to land-use types and underlying mechanism. The results showed that land-use type significantly affected SOC contents and its fractions. Compared with cropland, the other three land uses increased the total organic carbon (TOC), microbial biomass carbon (MBC), and non-labile organic carbon (NLOC) contents by 6.11-129.44%, 32.58-173.73%, and 90.98-347.00%, respectively; this demonstrated that a decrease in both labile and recalcitrant carbon resulted in SOC depletion under agricultural land use. Readily oxidized organic carbon (ROC) ranged from 42 to 69%, accounting for almost half of the TOC in the 0-40-cm soil layer. Cropland soil showed significantly higher ROC:TOC ratios than other land-use types. These results indicated that long-term vegetation restoration decreased SOC activity and improved SOC stability. Greater levels of soil exchangeable calcium (ECa) and clay contents were likely responsible for higher stabilization and then accumulation of SOC after vegetation restoration. The carbon pool index (CPI) rather than the carbon pool management index (CPMI) exhibited consistent variation trend with soil TOC contents among land-use types. Thus, further study is needed to validate the CPMI in evaluating land use effects on soil quality in karst ecosystems. Our findings suggest that land-use patterns characterized by grass or forest could be an effective approach for SOC-sequestration potential and ensure the sustainable use of soil resources in the karst area.

Soil's Hidden Power: The Stable Soil Organic Carbon Pool Controls the Burden of Persistent Organic Pollutants in Background Soils.

Persistent organic pollutants (POPs) tend to accumulate in cold regions by cold condensation and global distillation. Soil organic matter is the main storage compartment for POPs in terrestrial ecosystems due to deposition and repeated air-surface exchange processes. Here, physicochemical properties and environmental factors were investigated for their role in influencing POPs accumulation in soils of the Tibetan Plateau and Antarctic and Arctic regions. The results showed that the soil burden of most POPs was closely coupled to stable mineral-associated organic carbon (MAOC). Combining the proportion of MAOC and physicochemical properties can explain much of the soil distribution characteristics of the POPs. The background levels of POPs were estimated in conjunction with the global soil database. It led to the proposition that the stable soil carbon pools are key controlling factors affecting the ultimate global distribution of POPs, so that the dynamic cycling of soil carbon acts to counteract the cold-trapping effects. In the future, soil carbon pool composition should be fully considered in a multimedia environmental model of POPs, and the risk of secondary release of POPs in soils under conditions such as climate change can be further assessed with soil organic carbon models.