Soil organic carbon (SOC) constitutes the largest terrestrial carbon pool, playing a crucial role in the global carbon cycle. The transformation of plant-derived organic matter (OM) into stable SOC is primarily driven by microbial activity. Microbial processes, in turn, are significantly influenced by the soil environment, particularly the three-dimensional arrangement of soil particles (soil structure). Soil structure dictates the porous network, impacting gas and water movement, and ultimately, the bioavailability of carbon substrates for microorganisms. The size and connectivity of pores influence the contact between microorganisms and their energy sources like plant litter, thereby impacting microbial community composition and activity. Recent research highlights the importance of soil structure-driven mechanisms, such as aggregate formation and accessibility to reactive mineral surfaces, in regulating SOC persistence, surpassing the role of inherent chemical recalcitrance. SOC is largely stored in two pools: particulate organic matter (POM) and mineral-associated organic matter (MAOM). Understanding the physical mechanisms governing OM persistence, including OM adhesion to mineral surfaces and substrate accessibility, is key to comprehending SOC cycling. This study aims to understand how soil texture affects the transformation of litter-derived carbon into persistent SOC pools, considering the interactions between minerals, microorganisms, and OM at the microscale.

Existing literature emphasizes the importance of both the chemical recalcitrance of organic matter and the physical protection afforded by soil structure in determining the persistence of soil organic carbon. Studies have shown that soil carbon is predominantly stored in two major pools: particulate organic matter (POM) and mineral-associated organic matter (MAOM). The formation of organo-mineral complexes and the physical occlusion of organic matter within soil aggregates are identified as key mechanisms contributing to the stability of SOC. The role of soil texture and pore size distribution in influencing microbial activity and the accessibility of organic substrates has also been extensively discussed. Previous work has highlighted the importance of considering biogeochemical interfaces, where interactions between plant residues, microorganisms, and mineral surfaces occur, to fully understand SOC dynamics. However, a comprehensive understanding of how soil texture governs the pathway of litter-derived carbon into persistent SOM pools via microbial transformation in a relevant process scale (µm-mm) has been lacking. This gap highlights the need for a systematic approach that investigates these interactions.

This study employed a 95-day microcosm experiment to investigate the influence of soil texture on the fate of litter-derived carbon. Two differently textured soils (a fine-textured silty clay loam and a coarser-textured sandy clay loam, created by adding quartz sand) were used. 330 mg of ¹³C-labeled maize stalks were added to the top layer of half the microcosms to simulate litter input. Five replicates per treatment (two soil textures with or without added litter) were prepared. Soil respiration (CO₂ and ¹³CO₂) was monitored throughout the incubation period to assess litter decomposition and the priming effect on native soil carbon. After 95 days, each microcosm was divided into three depth increments (top, center, bottom). The soil samples underwent physical fractionation to separate free particulate organic matter (fPOM), occluded POM (oPOM), and mineral-associated OM (MAOM) fractions. The carbon (C), nitrogen (N), and ¹³C content of these fractions, as well as bulk soil, were determined using dry combustion coupled with isotope ratio mass spectrometry (EA-IRMS). Solid-state ¹³C nuclear magnetic resonance spectroscopy (NMR) was used to analyze the chemical composition of the POM fractions. Phospholipid fatty acid (PLFA) analysis was conducted to assess microbial community structure and the incorporation of ¹³C into microbial biomass. Finally, scanning electron microscopy (SEM) and nanoscale secondary ion mass spectrometry (NanoSIMS) were used to directly visualize and quantify ¹³C enrichment at the plant-soil interface, examining the spatial distribution of fungal hyphae, microorganisms, and soil minerals on litter surfaces. Statistical analyses, including t-tests and ANOVA, were employed to compare the different treatments and depths. The ¹³C content in CO2, soil, and OM fractions was calculated using specific formulas, taking into account isotope ratios and proportions of litter-derived carbon. PLFA analysis involved extraction, separation, and quantification of fatty acids, with the ¹³C labeling of fatty acids being accounted for via calculation to relate to fatty acid chain length. SEM and NanoSIMS measurements provided visual and isotopic data at the plant-soil interface to study organic matter transformation, providing information on elemental and isotopic distribution of fungal hyphae, microorganisms, and soil minerals. Data normality and homoscedasticity were checked, log-transformations were used where necessary, and various statistical tests were employed to analyze differences between treatments and depths.

The study revealed significant differences in microbial activity and carbon turnover between the two soil textures. The coarser-textured soil exhibited significantly higher total respiration (89.4 mg CO₂-C g^-1 C_bulk) and net litter-derived CO₂ (40.4 mg CO₂-C g^-1 C_bulk) compared to the finer-textured soil (55.3 mg CO₂-C g^-1 C_bulk and 24.6 mg CO₂-C g^-1 C_bulk, respectively; *P* < 0.001). The litter addition induced a higher priming effect in the coarse-textured soil (23.3 mg CO₂-C g^-1 C_bulk) than in the fine-textured soil (10.8 mg CO₂-C g^-1 C_bulk; *P* < 0.001). Analysis of OM fractions showed significantly higher litter-derived carbon content in the occluded POM (oPOM) fraction in the coarse-textured soil (71.1 mg C g^-1 C_bulk) compared to the fine-textured soil (36.8 mg C g^-1 C_bulk; *P* = 0.007). ¹³C NMR spectroscopy indicated that the chemical composition of the oPOM fraction resembled the original litter, rich in polysaccharides. PLFA analysis revealed a significantly stronger response of fungi to litter addition in the coarse-textured soil, with a five-fold increase in fungal abundance in the top layer (*P* = 0.01). A substantial portion (92%) of fungal biomass in the coarse-textured soil was derived from the added litter. NanoSIMS imaging directly visualized the close association between fungal hyphae, microorganisms, EPS, and soil minerals on POM surfaces, confirming the high ¹³C enrichment in these interfaces. Regardless of soil texture, fresh litter surfaces acted as hotspots for microbial activity and the formation of organo-mineral associations, leading to the simultaneous formation of occluded POM in aggregates and mineral-associated OM.

The findings demonstrate that soil texture strongly influences microbial activity and carbon turnover. The enhanced accessibility of litter in coarser soils leads to higher decomposition rates and a greater priming effect. Fungi, particularly in coarse-textured soils, play a crucial role in litter decomposition and the translocation of litter-derived carbon into deeper soil layers, contributing to the formation of more stable organic matter pools. The NanoSIMS results highlight the importance of biogeochemical interfaces at the microscale, where the close association of microorganisms with mineral surfaces and organic matter promotes the formation of organo-mineral complexes and contributes significantly to the stabilization of soil carbon. The observed simultaneous formation of occluded POM and MAOM across both soil textures underscores the crucial role of particulate organic matter as a nucleus for soil aggregate formation and the stabilization of SOC.

This study reveals the critical role of particulate organic matter as a functional component driving the persistence of soil organic carbon. Soil texture influences microbial activity and fungal growth, but the formation of stable SOC pools (occluded POM and MAOM) occurs through similar mechanisms across different soil structures, driven by interactions at plant-soil interfaces. Future research should focus on expanding this work to a broader range of soil types and land uses, investigating the long-term impacts of these mechanisms on SOC storage under different environmental conditions.

The study utilized a relatively short incubation period (95 days). Longer-term studies are necessary to fully assess the long-term stability of the formed SOC pools. The study focused on a specific type of litter (maize stalks) and a limited number of soil textures. Future studies should expand the scope to include diverse litter types and a wider range of soil conditions to increase the generalizability of the findings. The microcosm experiments may not fully capture the complexities of natural soil environments.