Interactions between organic matter and Fe oxides at soil micro-interfaces: Quantification, associations, and influencing factors

Characteristics of nitrogen and phosphorus migration at sediment-water interface in seasonal frozen lakes and the mechanism of microbial driven cycling: a case study of Lake Daihai

Nitrogen and phosphorus play pivotal roles in determining the eutrophic conditions and nutrient provision in lakes. However, the mechanisms and processes of nutrient release at the sediment-water interface of shallow lakes in cold regions remain unclear, especially under the complex environmental conditions of freezing and open-water periods. Therefore, Diffusive Gradients in Thin-films (DGT) and High-resolution Peeper technologies (HR-Peeper) were used to investigate the nitrogen and phosphorus characteristics of the sediment water interface, and the process of bacteria affecting the nitrogen and phosphorus cycle was clarified by the high-throughput sequencing technology. The results indicated that sediment phosphorus (PO43-) flux ranged from -1.39 to 3.6 mg/m2·d, with the interstitial water-Soluble Reactive PO43- presenting notable fluidity and potential bioavailability. The ammonia nitrogen (NH4+-N) flux varied from -4.71 to 3.65 mg/m2·d. The nitrate nitrogen (NO3--N) flux varied from -11.64 to 1.18 mg/m2·d, exhibiting an opposite trend to NH4+-N, which was released into water bodies during the freezing period and migrated to the sediments in the open water period. Common metabolic pathways and functional genes for nitrogen and phosphorus were identified in Methylomicrobium, Marinobacter, and Psychrobacter. The dissimilatory nitrate reduction to ammonium (DNRA) facilitated the transformation of polyphosphates and the release of phosphorus. Water temperature indirectly regulated the fluxes of nitrogen and phosphorus at the sediment-water interface (SWI) by modulating the microbial abundance and dissolved oxygen (DO) content.

Microbial Carbon Limitation Mediates Soil Organic Carbon Sequestration in Sugarcane-Watermelon Intercropping System

Intercropping is an effective approach for enhancing soil organic carbon (SOC) sequestration. However, the effects of intercropping on SOC dynamics and the underlying factors in rhizosphere and bulk soils are still unclear. In this study, we examined the impacts of sugarcane monoculture and sugarcane-watermelon intercropping on soil properties, soil respiration, SOC fractions, and microbial C limitation with continuous two years in 2023-2024 years in the Nala area of Guangxi Province. Our results revealed that intercropping significantly decreased CO2/SOC by 25% and microbial C limitation by 21% in the rhizosphere, with more pronounced reductions observed in bulk soil by 33% and 25%, respectively. This means that the intercropping reduced soil respiration and this effect can be offset by the rhizosphere effects. Additionally, the sugarcane-watermelon intercropping increased the contents of mineral-associated organic carbon (MAOC) by 15~18% and particulate organic carbon (POC) by 34~46%. The random forest analysis indicated that enzyme activities (explaining 20~38% of variation) and soil properties (explaining 22% of variation) were the primary drivers of reduced CO2 emissions. The PLS-PM showed that intercropping decreased microbial C limitation by influencing soil pH and soil water content (SWC), and then increased MAOC, which finally led to a decline in CO2 emissions. Overall, these findings highlight the decreasing CO2 emissions during the use of the intercropping system and the importance of microbial C limitation in the soil C cycle via soil respiration and SOC fractions.

Seasonal Dynamics in Soil Properties Along a Roadway Corridor: A Network Analysis Approach

Understanding soil properties' spatial and temporal variability is essential for optimizing road construction and maintenance practices. This study investigates the seasonal variability of soil properties along a 4.8 km roadway in Maiduguri, Nigeria. Using a novel integration of network analysis and geotechnical testing, we analyzed nine soil parameters (e.g., particle size distribution (PSD), Atterberg limits, California bearing ratio) across wet (September 2024) and dry (January 2021) seasons from 25 test stations. Average Atterberg limits (LL: 22.8% wet vs. 17.5% dry; PL: 18.7% wet vs. 14.7% dry; PI: 4.2% wet vs. 2.8% dry; LS: 1.8% wet vs. 2.3% dry), average compaction characteristics (MDD: 1.8 Mg/m3 wet vs. 2.1 Mg/m3 dry; OMC: 12.3% wet vs. 10% dry), and average CBR (18.9% wet vs. 27.5% dry) were obtained. Network construction employed z-score standardization and similarity metrics, with multi-threshold analysis (θ = 0.05, 0.10, 0.15) revealing critical structural differences. During the wet season, soil networks exhibited a 5.0% reduction in edges (321 to 305) and density decline (1.07 to 1.02) as thresholds tightened, contrasting with dry-season networks retaining 99.38% connectivity (324 to 322 edges) and stable density (0.99). Seasonal shifts in soil classification (A-4(1)/ML wet vs. A-2(1)/SM dry) underscored moisture-driven plasticity changes. The findings highlight critical implications for adaptive road design, emphasizing moisture-resistant materials in wet seasons and optimized compaction in dry periods.

Role of Fe and Mn in organo-mineral-microbe interactions: evidence of carbon stabilization and transformation of organic matter leading to carbon greenhouse gas emissions

Up to 90% of organic matter (OM) in soils and sediments are stabilized and protected against microbial decomposition through organo-mineral interactions, formation of soil aggregates, pH, and oxygen availability. In soils and sediment systems, OM is associated with mineral constituents promoting carbon persistence and sequestration of which iron (Fe) and manganese (Mn) are crucial components. Under anoxic condition, microbes couple the decomposition of OM to the oxidative/reductive transformation of Fe/Mn minerals leading to carbon greenhouse gas (C-GHG) emissions (i.e. CH4 and CO2). Although these organo-mineral-microbe interactions have been observed for decades, the bio-geochemical mechanisms governing the switch from OM stability toward OM degradation are not fully understood. Interest in this field have been growing steadily given the interest in global warming caused by OM decomposition leading to C-GHG emissions. This review emphasizes the dual role of Fe/Mn minerals in both OM stability and decomposition. Additionally, we synthesize the conceptual understanding of how Fe/Mn minerals govern OM dynamics and the resultant C-GHG emissions via microbial-mediated carbon transformation. We highlight the need for interdisciplinary research to better understand organo-Fe/Mn mineral-microbial interactions to develop management handles for climate mitigation strategies.

A new insight into the straw decomposition associated with minerals: Promoting straw humification and Cd immobilization

Organic matter (OM) derived from the decomposition of crop residues plays a key role as a sorbent for cadmium (Cd) immobilization. Few studies have explored the straw decomposition processes with the presence of minerals, and the effect of newly generated organo-mineral complexes on heavy metal adsorption. In this study, we investigated the variations in structure and composition during the rice straw decomposition with or without minerals (goethite and kaolinite), as well as the adsorption behavior and mechanisms by which straw decomposition affects Cd immobilization. The degree of humification of extracted straw organic matter was assessed using excitation-emission matrix (EEM) fluorescence and Ultraviolet-visible spectroscopy (UV-vis), while employing FTIR spectroscopy and XPS to characterize the adsorption mechanisms. The spectra analysis revealed the enrichment of highly aromatic and hydrophobic components, indicating that the degree of straw decomposition and humification were further intensified during incubation. Additionally, the existence of goethite (SG) accelerated the humification of OM. Sorption experiments revealed that the straw humification increased Cd adsorption capacity. Notably, SG exhibited significantly higher adsorption performance compared to the organic matter without minerals (RS) and the existence of kaolinite (SK). Further analysis using FT-IR spectroscopy and XPS verified that the primary mechanisms involved in Cd immobilization were complexion with -OH and -COOH, as well as the formation of Cd-π binds with aromatic C=C on the surface of solid OMs. These findings will facilitate understanding the interactions of the rice straw decomposing with soil minerals and its remediation effect on Cd-contaminated farmland.

Behavior of Silver Species in Soil: Ag Nanoparticles vs. Ionic Ag

Silver nanoparticles are one of the most commonly used forms of silver (Ag) in nanotechnology applications due to their antibacterial properties and electrical and thermal resistance. The increasing production and use of products containing nanoparticles has led to their release into and contamination of soil and water. This review summarizes the literature on the fate, behavior (adsorption/desorption, precipitation/oxidative dissolution, transformation), and transport/mobility of Ag forms in soils (Ag+ ions and Ag nanoparticles-AgNPs). The behavior of Ag+/AgNPs in soil is a complex process. It depends on many factors, including the characteristics of the Ag forms (ions, nanoparticle size, ligand type used for coating, surface charge, initial Ag concentration), the soil properties (organic matter and clay mineral content, textural properties, point of zero charge, cation exchange capacity, surface functional groups), and the solute properties (pH-Eh, ionic strength, cation type, oxygen content). The binding of Ag+ and AgNPs is significantly positively correlated with Al/Fe/Mn oxide and SOM content and depends on the surface charge of the minerals and CEC, which controls adsorption processes. Very important parameters to consider are the pH and Eh of the solution, which determine the durability of the ligands, the aggregation rate and the oxidation process of AgNPs, as well as the presence of sulfide and chloride and the Cl/Ag ratio, which determine the stability/mobility of Ag. Since AgNPs can be oxidized to Ag+ ions during their life cycle, it is necessary to consider the behavior of both forms of Ag in soils. Understanding the transport and behavior of Ag in soil is essential for the environmental risk assessment and management of wastes containing Ag.

Enhanced remediation of acetochlor-contaminated soils using phosphate-modified biochar: Impacts on environmental fate, microbial communities, and plant health

Given that acetochlor (ACT) persists in soil for extended periods, disrupting microbial community structure and causing phytotoxicity to sensitive crops, this study investigated the potential of phosphate-modified biochar (PBC-800) to remediate ACT-contaminated soil. Incorporating 0.5 % PBC-800 into fluvo-aquic, red, and black soils increased their adsorption capacities by 80.4 mg g-1, 76.6 mg g-1, and 76.0 mg g-1, respectively. Even after six months of aging, the Kf values remained 1.6 to 5.1 times higher than in untreated soils. PBC-800 also accelerated ACT degradation across all three soil types, reducing residual ACT levels by 34.3 % to 76.4 % after 60 days, and shortening the degradation half-life by 5 to 7 days. High-throughput sequencing revealed that ACT reduced soil microbial diversity and disrupted community structure, while 0.5 % PBC-800 amendments promoted the growth of degradation-capable genera such as Rhodococcus, Lysobacter, and Gemmatimonas, enhancing microbial ecosystem stability. Furthermore, the amendment of soil with 0.5 % PBC-800 reduced ACT residue concentrations in maize and soybeans by 76.5 % to 82.9 %, and restored plant biomass, leaf chlorophyll content, and mesophyll cell ultrastructure to levels comparable to the control. Therefore, amending ACT-contaminated soil with PBC-800 mitigates ecological and environmental risks, boosts microbial activity, and safeguards plant health.

Divergent Changes in Soil Iron-Bound Organic Carbon Between Distinct Determination Methods

Fe-OC is crucial for SOC preservation in the global ecosystem. However, there is still significant uncertainty in the determination methods of Fe-OC, and these methods are often not calibrated to each other, making the Fe-OC content by different methods impossible to compare. Here, Fe-OC is analyzed by the CBD method and the SD method from 45 soils from different land types (e.g., wetland, grassland, and forest) to compare and analyze the uncertainty and influencing factors between the two methods. Our results showed that the Fe-OC contributions to SOC (fFe-OC) measured by CBD and SD methods were significantly lower in the wetland ecosystem than in grassland and forest ecosystems. The Fe-OC content and fFe-OC in the grassland ecosystem was significantly higher using the CBD method compared to the SD method, with no significant difference between the methods in wetland and forest ecosystems. The random forest model revealed that Fe-OCCBD content was mainly affected by C/N, Clay%, and TC, whereas SOC, total nitrogen, and soil inorganic carbon were the main influences on Fe-OCSD. Taken together, our findings highlight the importance of incorporating ecosystem types and soil properties into soil carbon estimation models when predicting and estimating Fe-OC and its contribution to SOC.

Contribution of complexed Fe(Ⅱ) oxygenation to norfloxacin humification and stabilization: Producing and trapping of more humified products

Organic pollutants polymerization in advanced oxidation processes or environmental matrices has attracted increasing attention, but little is known about stabilization of the polymerization products. The results in this work revealed the contribution of Fe(Ⅱ) oxygenation to stabilization of the products from norfloxacin (NOR) humification. It was found that upon oxygenation of Fe(Ⅱ) complexed by catechol (CT), NOR polymerized into the products with larger molecular weight through nucleophilic addition. Around 83.9-89.7 % organic carbon (OC) can be retained in the reaction solution and the precipitates at different Fe(II)/CT molar ratio. In this system with humification potential, the produced hydroxyl radical (HO•) dominantly modified, instead of decomposed, the structure of transformation products (TPs). TPs with diversified side chains were formed through hydroxylation and ring-opening, leading to the more humified products. In the subsequent Fe(Ⅱ) oxidative precipitation, Fe-TPs composites were formed as spherical particle clusters, which could steadily incorporate OC species with molecular fractionation. Specifically, lignin-like, tannins-like, condensed aromatic and high-molecular-weight TPs were preferentially preserved in the precipitates, while the recalcitrant aliphatic products mainly retained in the solution. These findings shed light on the role of Fe(Ⅱ) oxygenation in stabilizing the products from pollutants humification, which could strengthen both decontamination and organics sequestration.

Spatial variation in iodine content with relation to soil physicochemical properties in lower Himalayan region

Topography of a place has a significant impact on soil characteristics that ultimately influence soil iodine levels. Lower Himalayan region (LHR) in Pakistan has a wide range of climatic and geological variations. Hence, an investigation was conducted to analyze the iodine concentration and other physicochemical properties of soils in two LHR districts, Haripur and Mansehra. Spatial analysis indicated a decrease in iodine levels in the mountainous regions in comparison to the flat portions of LHR. Soil samples obtained from different locations across Haripur had a stronger affinity for iodine due to variations in solubility and adsorption of iodine to soil clay components, which can be attributed to lower pH, higher organic matter, and a higher cation exchange capacity (CEC). In contrast to the plains of Haripur, elevated locations in the Mansehra district had decreased levels of iodine, along with a higher soil pH and reduced soil organic matter. The soil erosion and depletion of soil micronutrients in the hilly region of Mansehra may be attributed to the unfavorable soil conditions and excessive precipitation. Presence of clay, iron (Fe), and aluminum (Al) in the soil led to a rise in iodine levels. Iodine concentrations exhibited an inverse relationship with soil acidity. Study revealed a direct correlation between soil iodine levels and their cation exchange capacity (CEC) and clay content. This study aims to gather fundamental data for the chosen regions of LHR to address illnesses caused by iodine deficiency.

Marble Powder as a Soil Stabilizer: An Experimental Investigation of the Geotechnical Properties and Unconfined Compressive Strength Analysis

Fine-grained soils present engineering challenges. Stabilization with marble powder has shown promise for improving engineering properties. Understanding the temporal evolution of Unconfined Compressive Strength (UCS) and geotechnical properties in stabilized soils could aid strength assessment. This study investigates the stabilization of fine-grained clayey soils using waste marble powder as an alternative binder. Laboratory experiments were conducted to evaluate the geotechnical properties of soil-marble powder mixtures, including Atterberg's limits, compaction characteristics, California Bearing Ratio (CBR), Indirect Tensile Strength (ITS), and Unconfined Compressive Strength (UCS). The effects of various factors, such as curing time, molding water content, and composition ratios, on UCS, were analyzed using Exploratory Data Analysis (EDA) techniques, including histograms, box plots, and statistical modeling. The results show that the CBR increased from 10.43 to 22.94% for unsoaked and 4.68 to 12.46% for soaked conditions with 60% marble powder, ITS rose from 100 to 208 kN/m2 with 60-75% marble powder, and UCS rose from 170 to 661 kN/m2 after 28 days of curing, molding water content (optimum at 22.5%), and composition ratios (optimum at 60% marble powder). Complex modeling yielded R2 (0.954) and RMSE (29.82 kN/m2) between predicted and experimental values. This study demonstrates the potential of utilizing waste marble powder as a sustainable and cost-effective binder for soil stabilization, transforming weak soils into viable construction materials.

Composition of DOM along the depth gradients in the paddy field treated with crop straw for 10 years

Crop straw return is a widely used agricultural management practice. The addition of crop straw significantly alters the pool of dissolved organic matter (DOM) in agricultural soils and plays a pivotal role in the global carbon (C) cycle, which is sensitive to climate change. The DOM concentration and composition at different soil depths could regulate the turnover and further storage of organic C in terrestrial systems. However, it is still unclear how crop straw return influences the change in DOM composition in rice paddy soils. Therefore, a field experiment was conducted in which paddy soil was amended with crop straw for 10 years. Two crop straw-addition treatments [NPK with 50% crop straw (NPK+1/2S) and NPK with 100% crop straw (NPK + S)], a conventional mineral fertilization control (NPK) and a non-fertilized control were included. Topsoil (0-20 cm) and subsoil (20-40 cm) samples were collected to investigate the soil DOM concentration and compositional structure of the profile. Soil nutrients, iron (Fe) fraction, microbial biomass carbon (MBC), and concentration and optical properties (UV-Vis and fluorescence spectra) of soil DOM were determined. Here, we found that the DOM in the topsoil was more humified than that in the subsoil. The addition of crop straw further decreased the humidification degree of DOM in the subsoil. In crop straw-amended topsoil, microbial decomposition controlled the composition of DOM and induced the formation of aromatic DOM. In the straw-treated subsoil, selective adsorption by poorly crystalline Fe(oxyhydr)oxides and microbial decomposition controlled the composition of DOM. In particular, the formation of protein-like compounds could have played a significant role in the microbial degradation of DOM in the subsoil. Overall, this work conducted a case study within long-term agricultural management to understand the changes in DOM composition along the soil profile, which would be further helpful for evaluating C cycling in agricultural ecosystems.

Soil conditioners promote the formation of Fe-bound organic carbon and its stability

The close association of soil organic carbon (SOC) with Fe oxides is an important stabilization mechanism for soil organic matter (SOM) against biodegradation. Soil conditioners are of great importance in improving soil quality and soil health. Yet it remains unclear how different conditioners would affect the fractionation of SOC, particularly the Fe-bound organic carbon (Fe-OC). Field-based experiments were conducted in farmland to explore the fractionation of organic carbon (OC) and Fe oxides under the effects of three different soil conditioners (mineral, organic, and microbial conditioners). The results showed that all soil conditioners increased the total OC and Fe-OC contents, with the contribution of Fe-OC to total OC increasing from 1.57% to 2.99%. The low OC/Fe molar ratio indicated that surface adsorption played a crucial role in soil Fe-OC accumulation. Nuclear magnetic resonance (NMR) results suggested that soil conditioner altered the composition of SOM, accelerating O-alkyl C degradation and increasing recalcitrant alkyl C and aromatic C sequestration. Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) analysis indicated that all conditioners promoted the association of OC and Fe oxides. Furthermore, comprehensive analysis of 13C isotope and synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy showed that the mineral conditioner enhanced the association of microbial-derived OC and Fe oxides, whereas the organic conditioner increased the association of plant-derived OC with Fe oxides. These findings provide important insights into the potential mechanisms through which soil conditioners regulate the stability of OC and guide agricultural management.

Research progress of nano-scale secondary ion mass spectrometry (NanoSIMS) in soil science: Evolution, applications, and challenges

Nano-scale secondary ion mass spectrometry (NanoSIMS) has emerged as a powerful analytical tool for investigating various aspects of soils. In recent decades, the widespread adoption of advanced instrumentation and methods has contributed significantly to our understanding of organic-mineral assemblages. However, few literature reviews have comprehensively summarized NanoSIMS and its evolution, applications, limitations, and integration with other analytical techniques. In this review, we addressed this gap by comprehensively overviewing the development of NanoSIMS as an analytical tool in soils. This review covers studies on soil organic matter (SOM) cycling, soil-root interactions, and the behavior of metals, discussing the capability and limitations related to the distribution, composition, and interactions of various soil components that occur at mineral-organic interfaces. Furthermore, we examine recent advancements in high-resolution imaging and mass spectrometry technologies and their impact on the utilization of NanoSIMS in soils, along with potential new applications such as utilizing multiple ion beams and integrating them with other analytical techniques. The review emphasizes the importance of employing advanced techniques and methods to explore micro-interfaces and provide in situ descriptions of organic-mineral assemblages in future research. The ongoing development and refinement of NanoSIMS may yield new insights and breakthroughs in soil science, deepening our understanding of the intricate relationships between soil components and the processes that govern soil health and fertility.

Pyrolysis temperature and biochar redox activity on arsenic availability and speciation in a sediment

Biochar is widely used for water and soil remediation in part because of its local availability and low production cost. However, its effectiveness depends on physicochemical properties related to its feedstock and pyrolysis temperature, as well as the environmental conditions of its use site. Furthermore, biochar is susceptible to natural aging caused by changes in soil or sediment moisture, which can alter its redox properties and interactions with contaminants such as arsenic (As). In this study, we investigated the effect of pyrolysis temperature and biochar application on the release and transformations of As in contaminated sediments subjected to redox fluctuations. Biochar application and pyrolysis temperature played an important role in As species availability, As methylation, and dissolved organic carbon concentration. Furthermore, successive flooding cycles that induced reductive conditions in sediments increased the As content in the solution by up to seven times. In the solid phase, the application of biochar and the flooding cycle altered the spatial distribution and speciation of carbon, iron (Fe) and As. In general, the application of biochar decreased the reduction of Fe(III) and As(V) after the first cycle of flooding. Our results demonstrate that the flooding cycle plays an important role in the reoxidation of biochar to the point of enhancing the immobilization of As.

New mechanisms of biochar-assisted vermicomposting by recognizing different active di-(2-ethylhexyl) phthalate (DEHP) degraders across pedosphere, charosphere and intestinal sphere

Biochar-assisted vermicomposting can significantly accelerate soil DEHP degradation, but little information is known about the underlying mechanisms as different microspheres exist in soil ecosystem. In this study, we identified the active DEHP degraders in biochar-assisted vermicomposting by DNA stable isotope probing (DNA-SIP) and surprisingly found their different compositions in pedosphere, charosphere and intestinal sphere. Thirteen bacterial lineages (Laceyella, Microvirga, Sphingomonas, Ensifer, Skermanella, Lysobacter, Archangium, Intrasporangiaceae, Pseudarthrobacter, Blastococcus, Streptomyces, Nocardioides and Gemmatimonadetes) were responsible for in situ DEHP degradation in pedosphere, whereas their abundance significantly changed in biochar or earthworm treatments. Instead, some other active DEHP degraders were identified in charosphere (Serratia marcescens and Micromonospora) and intestinal sphere (Clostridiaceae, Oceanobacillus, Acidobacteria, Serratia marcescens and Acinetobacter) with high abundance. In biochar-assisted vermicomposting, the majority of active DEHP degraders were found in charosphere, followed by intestinal sphere and pedosphere. Our findings for the first time unraveled the spatial distribution of active DEHP degraders in different microspheres in soil matrices, explained by DEHP dynamic adsorption on biochar and desorption in earthworm gut. Our work highlighted that charosphere and intestinal sphere exhibited more contribution to the accelerated DEHP biodegradation than pedosphere, providing novel insight into the mechanisms of biochar and earthworm in improving contaminant degradation.

Variability of blue carbon storage in arid evaporitic environment of two coastal Sabkhas or mudflats

Coastal Sabkhas are mudflats found in arid coastal regions that are located within the supratidal zone when high rates of evaporation lead to high salinity. While evaporitic minerals often accumulate underneath the surface, the microbial mats are present on the surface of Sabkhas. Coastal Sabkha, an under-studied ecosystem in Qatar, has the potential to store blue carbon. In the present study, we investigated the carbon storage capacity of two Sabkhas from contrasting geological backgrounds. The spatial and temporal variabilities of the carbon stocks were examined. The results showed that both studied Sabkhas exhibit a considerable potential for soil carbon storage with carbon stocks of 109.11 ± 7.07 Mg C ha-1 and 67.77 ± 18.10 Mg C ha-1 in Dohat Faishakh and Khor al Adaid Sabkha respectively. These values fall within the reported range for carbon stocks in coastal Sabkhas in the region (51-194 Mg C ha-1). Interestingly, the carbon stocks in the sediments of the Sabkhas were higher than those in the sediments of Qatari mangroves (50.17 ± 6.27 Mg C ha-1). These finding suggest that coastal Sabkhas can serve as blue carbon ecosystems in arid environments.

Carbon sequestration in paddy soils: Contribution and mechanisms of mineral-associated SOC formation

In this work, comparative study of paddy and upland soils were carried out to unravel mechanisms of enhanced soil organic carbon (SOC) sequestration in paddy soils using fractionation methods, ¹³C NMR and Nano-SIMS analysis, as well as organic layer thickness calculations (Core-Shell model). The results showed that although there is a strong increase in particulate SOC in paddy soils compared to that in the upland soils, the increase in mineral-associated SOC is more important, explaining 60-75% of SOC increase in the paddy soils. In the wet and dry alternate cycles of paddy soil, iron (hydr)oxides adsorb relatively small and soluble organic molecules (fulvic acid-like), promote catalytic oxidation and polymerization, thus accelerating formation of larger organic molecules. Upon reductive iron dissolution, these molecules are released and incorporated into existing less soluble organic compounds (humic acid or humin-like), which are coagulated and associated with clay minerals, becoming part of the mineral-associated SOC. The functioning of this "iron wheel" process stimulates accumulation of relatively young SOC into mineral-associated organic carbon pool, and reduces the difference in chemical structure between oxides-bound and clay-bound SOC. Further, the faster turnover of oxides and soil aggregates in paddy soil also facilities interaction between SOC and minerals. The formation of mineral-associated SOC may delay degradation of organic matter during both wet and dry period in the paddy field, therefore enhancing carbon sequestration in paddy soils.