Soil Organic Matter Stabilization Research Articles

Impact of tillage and crop rotations on soil organic matter content in Northern Kazakhstan's chernozem soils: A 10-year study (2011-2021)

This extensive 10-year study conducted in Northern Kazakhstan investigates the intricate relationship between soil management techniques, crop rotations, and soil organic matter (SOM) content in Chernozem soils, an essential agricultural resource in the region. The experiments were established at the Karabalyk Agricultural Experimental Station, characterized by a arid continental climate. The study systematically examined the impact of two primary soil management techniques, conventional tillage (CT) and no-tillage (NT), in combination with various crop rotations. The crop rotations tested included grain-fallow rotations, fruit-exchange crop rotations, and an eight-field fruit-exchange crop rotation. The results provide valuable insights into the sustainable management of Chernozem soils in arid conditions, underscoring the role of crop rotation strategies in preserving SOM content. The findings reveal that among the crop rotations tested, the eight-field fruit-exchange crop rotation exhibited the most favorable outcomes for SOM preservation. This rotation helped maintain relatively stable SOM levels over the 10-year study period, contributing to soil health and fertility. In the context of the region's arid climate, the choice of soil management technique (CT or NT) had a limited impact on SOM content. The stability of SOM levels across diverse crop rotations and years highlights the dominant influence of crop management practices in this distinctive agricultural environment. This research serves as a valuable reference for tailored approaches to ensure soil health and organic matter preservation in the unique conditions of Northern Kazakhstan. It promotes the adoption of diversified crop rotations, with particular emphasis on the effectiveness of the eight-field fruit-exchange crop rotation, as a powerful strategy to mitigate organic matter loss, enhance soil quality, and optimize soil fertility in arid agricultural landscapes. The insights gained from this study are vital for sustainable land management in the region and underscore the importance of region-specific, holistic investigations to guide effective agricultural practices. The findings offer a solid foundation for the development of strategies that address soil health and safeguard the integrity of essential soil resources in these unique environments. The study conducted at the Karabalyk Agricultural Experimental Station in Northern Kazakhstan between 2011 and 2021 provides critical insights into the relationship between soil management techniques, crop rotations, and SOM content in Chernozem soils. The research suggests that diversified crop rotations, particularly the eight-field fruit-exchange crop rotation, represent a promising approach for mitigating organic matter loss and enhancing soil quality in arid regions.

Combining thermal analyses and wet-chemical extractions to assess the stability of mixed-nature soil organic matter

In this study, we combined Rock-Eval® analysis, analytical pyrolysis, and wet-chemical extractions, assisted by Fourier transform infrared spectroscopy (FTIR), and measurements of soil heterotrophic respiration. Our objective was to assess the biological and thermal stability of mixed-nature soil organic matter (SOM) derived from grass litter and kerogen. We studied a Technosol constructed with Ca, Mg and kerogen-rich waste rock (black shales), which has been under pasture cultivation for 20 years. We compared the results with those from black shales and an adjacent natural soil under long-term pasture cultivation, both used as endmembers representing kerogen and plant-derived SOM, respectively. Analytical pyrolysis and FTIR analyses revealed a mixed composition of SOM in the Technosol. Predominantly, polysaccharides, lignin, lipids and N-compounds were originated from plant-derived SOM, while (poly)aromatic and most aliphatic compounds were traced back to kerogen. Rock-Eval analysis showed that 58% of SOM in Technosol was kerogen-derived, which was poorly accessible to soil microbiota, as evidenced by heterotrophic respiration. In addition, an important portion of plant-derived SOM (>70%) was only released during the Rock-Eval oxidation stage. The impact of chemical recalcitrance of kerogen compounds on short-term biological stability was remarkable and demonstrated a strong correlation with the thermal indices derived from the Rock-Eval pyrolysis stage. Conversely, the parameters from Rock-Eval oxidation stage showed a positive correlation with the amount of SOM involved in mineral-organic associations, particularly with Ca2+ and Mg2+ (i.e., cation bridging). Thus, to disentangle the contribution of chemical recalcitrance and mineral-organic associations to SOM stability, we recommend the assessment of all Rock-Eval thermograms (from pyrolysis and oxidation stages) in combination with chemical and biological assessments, especially when studying soils containing mixed-nature SOM.

Contrasting effects of maize straw and its biochar on aggregation and soil organic matter stabilization

Contrasting effects of maize straw and its biochar on aggregation and soil organic matter stabilization

Soluble phenols in litter are reduced during passage through the soil macrofauna gut due to the formation of insoluble complexes with proteins: A case study with isopods and Diptera larvae

Plant litter represents a major source of soil organic matter and hence understanding the pathway of its decomposition and stabilisation is crucial for understanding the soil carbon dynamic. Importantly, more than half of annual litter production is consumed by soil fauna globally. Here we test the hypothesis that soluble phenols can be removed from the litter during gut passage and bound on nitrogen rich compounds, which results in formation of insoluble complexes in the faeces produced by soil fauna. Two invertebrate species, Diptera larvae Bibio marci and terrestrial isopod Armadilidium vulgare, and litter of two tree species, Quercus robur and Alnus glutinosa, were used. Both species of invertebrates were fed by both litter types in full factorial manner. The litter contained significantly more free phenols than the faeces. In opposite, the content of bound phenols was significantly higher in faeces than in the litter. The loss of total nitrogen associated with removal of bound phenolic compounds from faeces was higher than that of the litter. Thus, soil fauna contributes to the stabilisation of soil organic matter by the conversion of soluble phenols to insoluble ones together with nitrogen immobilization. In conclusion, this process of stabilisation likely plays an important role in mature soils where mineral surfaces have reached carbon saturation or in soils with higher organic horizons.

Deriving the Shannon Index from the soil molecular Humeome serves as a descriptor of soil organic matter stability under different cropping systems

Soil organic matter (SOM) is essential in maintaining soil structure, nutrient content, water retention, and biodiversity. It also represents an important C pool that, if efficiently stabilized, may play a role in the mitigation of global climate changes. However, knowledge is still limited on the dynamics of soil organic carbon (SOC) molecular composition occurring with changes in land use and management practices. We calculated the Shannon diversity index (SH) for the soil molecular characteristics as obtained by the humeomic sequential fractionation applied on two different soil systems including uncultivated control: (1) Soils under a short-term cultivation of maize for 1 and 3 years; (2) Soils under a 20-year long-term cultivation of either continuous maize (MAIZE) or maize-bean rotation (MIX). The SH values were derived for each of the nine molecular classes in which the soil humeomic results were grouped. When single class SH were summed, we found that the resulting total Shannon heterogeneity (SHtot) and its value weighted for the relative OC percent, were inversely proportional to a Stability Soil Organic Matter Ratio (SOMR) equation, that, in turn, was directly proportional to the normalized Chemical Protection Ratio (nCPR) equation, based on the quantity of highly hydrophobic (organosoluble) compounds present in soils. The physical–chemical stability of SOM, expressed as SOMR and nCPR, increased with the decrease of the heterogeneity of the total molecular system (SHtot) for both short- and long-term soil systems. In fact, in the short-term soil system the molecular humeome was least heterogeneous under uncropped conditions, while SH showed increased heterogeneity with Maize cultivation for 1 and 3 years, in the order. The greatest SH homogeneity was again shown by the uncultivated control soil for the 20-year long-term system, and SH heterogeneity increased passing from the soil under Maize-Broad bean rotation, to that under Maize monocultivation. The SH values of single molecular classes revealed that nitrogenated or oxygenated aromatic compounds determined the stability of soil humus, being the nitrogenated heterocyclic compounds responsible for the residual molecular stability of SOM in both soil systems. This work indicated that the molecular complexity of a soil humeome can be represented by a Shannon descriptor that provides a handy and direct information on the dynamics of humus in soils.Graphical

Sequential loss-on-ignition as a simple method for evaluating the stability of soil organic matter under actual environmental conditions

Soil organic matter (SOM) is one of the largest carbon (C) reservoirs on Earth, and therefore its stability attracts a great deal of interest from the perspective of the global C cycle. This study examined the applicability of loss-on-ignition with a stepwise increase in temperature (SIT-LOI) of soil to evaluate the stability of SOM using soil samples having different organic matter (OM) and mineral contents and different mean residence times (MRTs) for SOM. The responses of SOM to the SIT-LOI varied depending on the samples but were all successfully approximated by a liner regression model as a function of the temperature of LOI. The slope value in the liner model that determines the residual potential of carbon during the SIT-LOI highly correlated with MRT of SOM, suggesting that this value reflects the overall stability of SOM over a range of soil properties. This hypothesis was consistent with the observation that Δ14C values of SOM decreased with increasing LOI temperature and thus, older, slower-cycling SOM was preferentially left in the soil samples by SIT-LOI. Additionally, the hypothesis was also supported by the significant correlations (p < 0.01) between the slope value and OM and mineral contents in the samples because these components are considered to regulate SOM stability. In addition to the regression analysis of the SIT-LOI data, changes in carbon to nitrogen (C/N) and carbon to hydrogen (C/H) ratios and stable carbon isotope signatures (δ13C) of the samples were investigated. The results suggest that the mineral association of SOM is an important factor characterizing the response of SOM to LOI. Hence, it was concluded that SIT-LOI is a simple and useful method for evaluating the stability of SOM under actual environmental conditions.

Drivers of microbially and plant-derived carbon in topsoil and subsoil.

Plant- and microbially derived carbon (C) are the two major sources of soil organic matter (SOM), and their ratio impacts SOM composition, accumulation, stability, and turnover. The contributions of and the key factors defining the plant and microbial C in SOM along the soil profile are not well known. By leveraging nuclear magnetic resonance spectroscopy and biomarker analysis, we analyzed the plant and microbial C in three soil types using regional-scale sampling and combined these results with a meta-analysis. Topsoil (0-40 cm) was rich in carbohydrates and lignin (38%-50%), whereas subsoil (40-100 cm) contained more proteins and lipids (26%-60%). The proportion of plant C increases, while microbial C decreases with SOM content. The decrease rate of the ratio of the microbially derived C to plant-derived C (CM:P ) with SOM content was 23%-30% faster in the topsoil than in the subsoil in the regional study and meta-analysis. The topsoil had high potential to stabilize plant-derived C through intensive microbial transformations and microbial necromass formation. Plant C input and mean annual soil temperature were the main factors defining CM:P in topsoil, whereas the fungi-to-bacteria ratio and clay content were the main factors influencing subsoil CM:P . Combining a regional study and meta-analysis, we highlighted the contribution of plant litter to microbial necromass to organic matter up to 1-m soil depth and elucidated the main factors regulating their long-term preservation.

Effect of soil microbial community structure on the chemical compositions of different soil organic matter fractions in land uses of the Pearl River Estuary

Understanding the interactions between the dynamics of soil organic matter (SOM) and soil microbial communities is important for predicting the stability and fate of estuarine SOM following anthropogenic disturbance. However, current knowledge on the influence of microbial communities on the chemical compositions of different SOM fractions in estuarine land uses still remains incomplete. In this study, we used high-throughput sequencing, Fourier transform infrared spectroscopy, UV–Vis absorption, and fluorescence spectroscopy to investigate soil microbial communities and characterize the chemical compositions of bulk SOM and water-extractable organic matter (WEOM) across different land uses (artificial forests (AF), farmland (FL), natural wetland (NW), construction land (CL), and abandoned land (AL)) in the Pearl River Estuary. The results showed that the bulk SOM in FL and AL exhibited comparable or higher levels of aromatic functional groups (aromatic CH and aromatic CC) compared to other land uses. The HIX and SUV254 indexes further confirmed that AF and FL had more aromatic and hydrophobic WEOM. Apart from FL, where anthropogenic humus constituted the largest proportion, the WEOM fraction predominantly consist of three components in the following order across all land uses, ordered as anthropogenic humus < microbial humus < protein humus. The chemical compositions of the different SOM fractions showed a stronger correlation with the dominant bacterial phylum composition, especially for the WEOM fraction, compared to the fungal composition. Meanwhile, our findings demonstrate that bacterial diversity showed a stronger relationship with the chemical composition of WEOM compared to abundance. Overall, this study sheds light on the impact of land use on the quantity and quality of SOM fractions, underscoring the varying influence of microorganisms on the chemical compositions of different SOM fractions within estuarine environments.

Functional groups and mineralization kinetics of soil organic matter under contrasting hydro‐thermal regimes under conservation agriculture‐based rice–wheat system in eastern Indo‐Gangetic Plains

AbstractSoil organic carbon (SOC) sequestration is important to counteract anthropogenic climate change at the global level. Studying the effect of conservation agriculture (CA) on SOC dynamics in the presence of two distinctively different hydro‐thermal regimes across the year in rice–wheat (RW) systems in eastern Indo‐Gangetic Plains (E‐IGP) is of topical interest. The stabilization mechanism of soil organic matter (SOM), and its effect on C mineralization kinetics in these conditions is not well understood. We collected soil samples from six combinations of CA and conventional farming in an ongoing experiment at CIMMYT‐Borlaug Institute for South Asia, Bihar, India. CA enhanced SOC, specifically labile C, while decreasing mineral N, as a result of encapsulation/assimilation in aggregates/microbial biomass. Humic acids registered characteristic Fourier transform infrared (FTIR) peaks at 3200–3600 cm−1, 2920–2930 cm−1, 1645–1655 cm−1 and 1220–1240 cm−1, and displayed lesser degree of humification, aromaticity and redox status under CA. SOC and N mineralization were studied in two different hydro‐thermal regimes pertaining to rice (submergence, 35°C; SM35) and wheat (field capacity, 25°C; FC25) growing periods of E‐IGP. SM35 displayed temperature‐mediated higher decay of SOC. Decay constants of C mineralization were lesser under CA compared with CT. CA promoted higher SOM stability, evidenced by lower decay rates of SOC and N, attributed to (1) better protection of SOM in well‐aggregated soil structure and (2) an excess supply of fresh crop residues ensuring higher rate of SOM formation than its decay. Practising CA in E‐IGP is imperative towards C‐neutral agriculture, especially in the impending global warming scenario.

Changes in Molecular Structure of Humic Substances in Cambisols under Agricultural Use

Soil organic matter stability and transformation affected by agriculture is a global problem of great concern. This study aimed to reveal structural changes in humic molecules under conventional soil tillage in Cambisols of the Transbaikal area. Humic acids were isolated from the humus horizons of native and arable variants of Eutric Cambisol Cryic (gleyic, humic, loamic) from the forest steppe, and Haplic Cambisol (arenic, humic, protocalcic) from the steppe zone. The changes in the humic substances’ quality were evaluated by means of an elemental analysis and 13C nuclear magnetic resonance spectroscopy. The results indicate that different agronomic management affected the changes in the composition and molecular structure of humic acids. Soil tillage induced an increase in the carbon content, the proportion of aromatic compounds and carboxyl groups and the depletion in nitrogen. As measured by 13C NMR spectroscopy, the intensity of these changes was determined by soil properties and the hydrothermal regime. Organic matter from Eutric Cambisol Cryic was suggested as providing significant environmental resistance to the agricultural impact due to the optimization of the water regime and a loamy texture. Prolonged ploughing of sandy Haplic Cambisol under the arid conditions promoted deep changes in the humic substances’ composition and chemical structure.

Soil organic carbon priming co-regulated by labile carbon input level and long-term fertilization history

Labile carbon (C) input and fertilization have important consequences for soil organic matter (SOM) decomposition via the priming effect (PE), thereby impacting soil fertility and C sequestration. However, it remains largely uncertain on how the labile C input levels interact with long-term fertilization history to control PE intensity. To clarify this question, soil samples were collected from a 38-year fertilization field experiment (including five treatments: chemical nitrogen fertilizer, N; chemical fertilizer, NPK; manure, M1; 200 % manure, M2; NPK plus M2, NPKM2), with strongly altered soil physiochemical properties (i.e., soil aggregation, organic C and nutrient availability). These soil samples were incubated with three input levels of 13C-glucose (without glucose, control; low, 0.4 % SOC; high, 2.0 % SOC) to clarify the underlying mechanisms of PE. Results showed that the PE significantly increased with glucose input levels, with values increasing from negative or weak (−2.21 to 3.55 mg C g−1 SOC) at low input level to strongly positive (5.62 to 8.57 mg C g−1 SOC) at high input level across fertilization treatments. The increased PE intensity occurred along with decreased dissolved total nitrogen (DTN) contents and increased ratios of dissolved organic C to DTN, implying that the decline in N availability largely increased PE via enhanced microbial N mining from SOM. Compared to N and NPK treatments, the PE was significantly lower in the manure-amendment treatments, especially for low input level, due to more stable SOM by aggregate protection and higher N and phosphorus availability. These results suggested that manure application could alleviate SOM priming via increased soil C stability and nutrient availability. Collectively, our findings emphasize the importance of long-term fertilization-driven changes in labile C inputs, SOM stability, and nutrient availability in regulating PE and soil C dynamics. This knowledge advances our understanding of the long-term fertilization management for soil C sequestration.

Chicken manure as an organic fertilizer: composting technologies and impact on soil properties (a review)

Chicken manure and composts derived from it are valuable organic fertilizers that have a positive impact on soil fertility and significantly enhance crop yields. However, the application of fresh manure can lead to excessive concentrations of nitrogen and other nutrients in the soil, as well as contamination of the soil with microorganisms that are pathogenic to humans and animals. Therefore, it is recommended to compost chicken manure prior to its use in agriculture. This review examines the composition and properties of chicken manure, the techniques involved in its composting, and its influence on the physical, chemical, and biological properties of the soil. The utilization of composted manure results in increased levels of organic carbon (on average by 1–1.5%), nitrogen (by 0.1–0.2%), and other biophilic elements. Moreover, it improves various physical properties of the soil, such as porosity, aeration, and density. Levels of pH increase (by 0.2–0.4), as well as electrical conductivity (by 2–2.5 times), and the quantity of water-resistant aggregates (by 5–10%). The application of these fertilizers also promotes biological activity, microbial diversity, microbial biomass carbon (by 25–75%), and soil enzymatic activity (phosphatase and catalase increase by 5–7 times, and urease by 3–5 times). When chicken manure is incorporated into the soil, a significant (2–4 times) increase in the population of bacteria from the phyla Firmicutes, Bacteroidetes, and Actinobacteria is observed. The necessity of investigating the effects of compost derived from chicken manure on the chemical composition and storage of soil organic matter, pore space, and the ecotrophic structure of the soil microbiome is also discussed.

Soil organic matter dynamics mediated by arbuscular mycorrhizal fungi - an updated conceptual framework.

Arbuscular mycorrhizal (AM) fungi play an important role in soil organic matter (SOM) formation and stabilization. Previous studies have emphasized organic compounds produced by AM fungi as persistent binding agents for aggregate formation and SOM storage. This concept overlooks the multiple biogeochemical processes mediated by AM fungal activities, which drive SOM generation, reprocessing, reorganization, and stabilization. Here, we propose an updated conceptual framework to facilitate a mechanistic understanding of the role of AM fungi in SOM dynamics. In this framework, four pathways for AM fungi-mediated SOM dynamics are included: 'Generating', AM fungal exudates and biomass serve as key sources of SOM chemodiversity; 'Reprocessing', hyphosphere microorganisms drive SOM decomposition and resynthesis; 'Reorganizing', AM fungi mediate soil physical changes and influence SOM transport, redistribution, transformation, and storage; and 'Stabilizing', AM fungi drive mineral weathering and organo-mineral interactions for SOM stabilization. Moreover, we discuss the AM fungal role in SOM dynamics at different scales, especially when translating results from small scales to complex larger scales. We believe that working with this conceptual framework can allow a better understanding of AM fungal role in SOM dynamics, therefore facilitating the development of mycorrhiza-based technologies toward soil health and global change mitigation.

Silicon Incorporation Reduces the Reactivity of Short-range Ordered Aluminosilicates Toward Organic Acids

The structure and composition of short-range ordered aluminosilicates (SROAS) may control their affinity for organic acids with potential effects on soil organic matter stabilization. Adsorption mechanisms of model organic acids were studied to resolve the effect of Si incorporation. Adsorption of oxalic, salicylic, and octanoic acid on Al-rich (Al:Si = 3.7) and Si-rich (Al:Si = 1.4) SROAS was quantified by analyses of dissolved organic carbon using catalytic high-temperature combustion. The initial pH of 5 and 6.5 increased to 6.3–8.2 during adsorption of oxalic and salicylic acid, demonstrating hydroxyl release by ligand exchange. Minor changes in pH indicated weak interactions of octanoic acid with both SROAS. Adsorbates were characterized by Fourier-transform infrared spectroscopy. Asymmetric stretching of carboxylate groups at 1720 and 1700 cm–1, and symmetric stretching at 1430 cm–1 evinced the formation of chelate complexes for oxalic acid. An absorption band centered at 1545 cm–1 indicated partial inner-sphere binding of salicylic acid on both SROAS. Silicon-rich SROAS adsorbed 80–90% less than Al-rich SROAS, suggesting that adsorption of oxalic and salicylic acid was controlled by surface aluminol groups. Fast kinetics of oxalate adsorption on Al sites was studied by a conductivity-based stopped-flow technique. Ligand exchange proceeded at a rate constant of 3.5 s–1 (25°C), similar to solute Al complexation, with an activation energy of up to 34.1 kJ mol–1. A slow process with a rate constant of 0.13 s–1 (25°C) was attributed to diffusion of oxalate at the surface or into SROAS particles. As supported by structural characterization of Si-rich SROAS, the much lower susceptibility of Si-rich SROAS to ligand exchange relates to Al speciation. The formation of tetrahedral Al precludes its complexation by carboxyl groups.

Soil organic matter stabilization during early stages of Technosol development from Ca, Mg and pyrite-rich parent material

The mechanisms underlying soil organic matter (SOM) accrual in Technosols are still poorly understood. Here we studied a 20-year chronosequence of three Technosols constructed with dolomitic limestone mining waste: two cultivated with sugarcane (for 3 and 7 years) and one under pasture (for ∼20 years). Through a combination of SOM physical fractionation and wet-chemical extractions, we aimed to unravel the processes driving SOM accumulation in these soils and developed a conceptual model for embryonic soils developed from Ca, Mg and pyrite-rich parent material. Our results revealed that the weathering of comminuted waste rocks (e.g., shale, siltstone) provided 2:1 minerals (especially smectite) to participate in mineral-organic associations. The weathering of primary minerals also promoted the formation of secondary minerals, which interacted with SOM and increased the amount of mineral associated organic matter (MAOM) up to 20 g SOC kg−1. Pyrite oxidation produced highly reactive secondary Fe phases, while the dolomite dissolution increased the concentrations of Ca2+ and Mg2+ and maintained near-neutral pH conditions, favoring the formation of cation-bridges. The progressive increase in the different operational Fe fractions (Fe in organomineral complex, poorly crystalline, and crystalline Fe oxides) along the Technosol chronosequence was strongly correlated with MAOM, highlighting the significant role of Fe in SOM accumulation. Furthermore, we observed substantial increase in particulate organic matter (POM), up to 22 g SOC kg−1, which was associated with increased aggregate formation and stability. These processes collectively contribute to the protection of SOM during early stages of soil development from in Ca, Mg, and pyrite-rich parent material, and thus indicate a high potential for carbon sequestration.

Evidence for the formation of fused aromatic ring structures in an organic soil profile in the early diagenesis

The presence of fused aromatic ring (FAR) structures in soil define the stability of the recalcitrant soil organic matter (RSOM). FAR are important skeletal features in RSOM that contribute to its extended residence time. During the early diagenesis, FAR structures are formed through condensation and polymerization of biomolecules produced during plant residue and microbial product decay. Molecular level characterization of the RSOM extracted from an organic soil profile gives important insights into the formation of FAR. Advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, including recoupled long-range C–H dipolar dephasing experiments on extracted humic acids (HA) showed that they contain diagenetically formed FAR different from charcoal and lignin. Peaks characteristic of FAR are observed at all depths in the soil profile, with a greater prevalence observed in the HA extracts from the clay soil layer at the bottom. In the clay soil layer, 78% of the aromatic carbon was non-protonated, and this was 2.2-fold higher than the topsoil. These data further strengthen our understanding of the humification process that could occur in early diagenesis and help explain the importance of incorporating diagenesis as an important phenomenon for long-term carbon sequestration in soil.

The Role of Bedrock Geochemistry and Climate in Soil Organic Matter Stability in Subtropical Karst Forests of Southwest China

The stability of soil organic matter (SOM) plays a critical role in soil carbon (C) dynamics under global warming. However, the factors influencing SOM stability, particularly the significance of bedrock geochemistry and its hierarchical relationship with climate and soil properties, remain poorly understood. To address this gap, we conducted a study along a large climatic gradient (Δtemperature &gt; 9 °C) in the subtropical karst forests of southwest China, quantifying SOM stability using thermal analysis and investigating the contributions of bedrock geochemistry, climate, and soil properties. Our results showed that SOM stability was positively correlated with mineral-associated organic C (MAOC) rather than particulate organic C. Hierarchical partitioning analysis further demonstrated that bedrock geochemistry was the predominant contributor to SOM stability variance, accounting for 23.7%. Following this, soil minerals contributed to 21.1%–22.6% of the variance, the mean annual temperature to 20.3%, and microbial biomass C to 17.2%. In particular, bedrock geochemistry—specifically the presence of calcium-rich bedrock—was found to enhance SOM stability by promoting the accumulation of exchangeable calcium and calcium carbonate in soils. Additionally, high temperature improved SOM stability by increasing the content and proportion of MAOC and soil pH. These results highlight the fundamental role of bedrock geochemistry in controlling SOM stability and emphasize the importance of considering hierarchical relationships among bedrock–soil–climate interactions for evaluating soil C dynamics.

Fate and stabilization of labile carbon in a sandy boreal forest soil – A question of nitrogen availability?

Labile carbon (C) fractions, such as sugars, may persist in soil due to their incorporation into microbial biomass and are ultimately stabilized as microbial necromass as part of stable soil organic matter (SOM). However, the underlying factors and mechanisms are currently highly debated. To address this knowledge gap, we conducted a 1-year greenhouse experiment including four treatments: (1) bare soil, (2) bare soil and nitrogen (N) fertilization, (3) soil planted with a tree, and (4) tree and N. The boreal forest soil was a sandy and nutrient-poor Podzol taken from 0 to 20 cm depth and trees were Pinus sylvestris. We hypothesized that: (1) originally labile C does not accumulate under N-deficient conditions, as microbial residues may be intensely recycled for N acquisition and (2) differences in N supply and demand change the functionality and composition of the microbial community, which will be reflected in the stabilization of microbial C. We added 13C glucose to the soil and measured 13C recovery to trace the fate of added C in soil, microbial biomass (MBC), dissolved organic C (DOC), phospholipid fatty acids (PLFA), and amino sugars as biomarker for microbial necromass. We also analyzed microbial community structure and enzyme activities. Around 40 % of the added C was mineralized after one day. Mineralization of the added C continued for 6 months, but stabilized thereafter. After 1 year, the treatment with both tree and N fertilization had the highest amount of added 13C (34 %) remaining in soil compared to the other treatments (18 %). The recovery of 13C in DOC was <1 % from the 3rd day onwards, but remained higher in MBC (2 %) and microbial necromass (1.5 %) after 1 year. N fertilization increased bacterial growth on 13C-glucose and abundance of gram-positive bacteria, while trees increased the abundance of symbiotrophic fungi. The formation of more stable C in the treatment with both tree and N indicates that under those conditions, recycling of microbial necromass for N acquisition is lower and the changed microbial composition leaves behind more stable residues.

Stable C and N isotope abundances in water-extractable organic matter from air-dried soils as potential indices of microbially utilized organic matter

Stable carbon (C) and nitrogen (N) isotopes (13C and 15N) in water-extractable organic matter (WEOM) derived from air-dried soils may be applicable to elucidate the microbial decomposition of soil organic matter (SOM), which is crucial in terrestrial C cycles. A total of 40 soil samples were collected from a depth of 0–6 cm from a temperate broadleaved forest in Japan with vegetation succession from grassland approximately 150 years ago. Those soil samples were air-dried before the water extraction process and organic matter analysis. The C and N concentrations of WEOM were &amp;lt;3.6% of those of the bulk soil and were positively correlated with those of the bulk soil at a p-value of &amp;lt; 0.01. A positive correlation between the two fractions (i.e., WEOM and bulk soils) was also found for natural 13C and 15N abundances (δ13C and δ15N; p &amp;lt; 0.01). However, the C/N ratio of WEOM was slightly correlated with that of bulk soils, exhibiting a narrow range of values of ~10. Thus, those features of the WEOM were similar to the well-known features of microbial biomass. The δ13C and δ15N enrichments in WEOM relative to bulk soil, the difference in stable isotope abundances between bulk SOM and WEOM were negatively and positively correlated, respectively, with the concentrations of organo-mineral complexes and short-range order minerals (non-crystalline oxyhydroxides of aluminum and iron, allophane, imogolite, and allophane-like constituents), which play significant roles in SOM stabilization in soils. These relationships suggest that the stable isotopic enrichments in WEOM can be a good indicator of the microbial utilization of soil C and N under different substrate availabilities, which are crucial to SOM decomposition and decomposability substantially varying from local to global scales.

A large nitrogen supply from the stable mineral-associated soil organic matter fraction

A large nitrogen supply from the stable mineral-associated soil organic matter fraction