Mineral-Imposed Accessibility and Microbial Processing Drive Contrasting Mineralization Regimes and Carbon Balance of MAOC

Wheat straw is a plant-derived substrate rich in cellulose, hemicellulose, and lignin and represents a major carbon input to agricultural soils. Mineral-associated organic carbon (MAOC) is the most stable soil carbon pool, yet how mineral structure regulates the stability of straw-derived MAOC through microbial processing remains unclear. Here, straw-derived MAOC was formed in artificial soils containing five clay minerals (halloysite, kaolinite, illite, vermiculite, and montmorillonite) during a two-year incubation, followed by a 45-day incubation with a standardized microbial community to quantify CO2 emission and net carbon balance. Mineral type regulated MAOC mineralization (38.54–54.48 mg C g−1 MAOC). Vermiculite produced the highest CO2 emission but maintained a positive net carbon balance, whereas illite showed net carbon loss (−0.53 g kg−1). Kaolinite, halloysite, and montmorillonite exhibited lower mineralization and retained net carbon. The 2:1 clay minerals enhanced interlayer interactions and favored accumulation of C=O and aromatic compounds, reflecting stronger microbial transformation and residue retention. In contrast, 1:1 minerals stabilized carbon via edge hydroxyl bonding, which restricted substrate accessibility and slowed decomposition. Cumulative mineralization decreased with initial MAOC carbon but increased with dissolved organic carbon and bacterial abundance. Net carbon retention increased with N-acetylglucosaminidase activity and fungal abundance, indicating joint microbial control via nutrient acquisition and fungal processing. Two contrasting stabilization regimes were observed: high turnover driven by vermiculite and halloysite, and strong protection dominated by montmorillonite and kaolinite. These differences indicate that MAOC stability is jointly constrained by mineral-regulated accessibility and microbial transformation processes.