Linking clay mineral surfaces, organic matter stabilization and microbial communities

Clay minerals play a central role in the soil organic matter (OM) stabilization, yet the limits of mineral-associated organic carbon and their functional consequences remain debated. This study investigated how clay mineral type influences OM stabilization, microbial communities and saturation limits.

A six-month laboratory incubation was conducted using soil microcosms composed of sand amended with clay minerals differing in specific surface area (SSA): kaolinite (8 m² g⁻¹), montmorillonite (265 m² g⁻¹), and sepiolite (386 m² g⁻¹). Increasing proportions of compost (1-50%, m/m) were added to determine saturation thresholds. Carbon mineralization was monitored through CO₂ measurements (micro-GC). Post-incubation analyses included microbial biomass (qPCR), OM quality (Rock-Eval® pyrolysis), and scanning electron microscopy (SEM). Temperature sensitivity of respiration (Q10 index) was assessed by re-incubating microcosms at different temperatures.

CO₂ emissions decreased with increasing clay surface area, indicating reduced microbial accessibility to OM. With only 1% compost addition, mineralization rates were reduced by 85% with kaolinite, 90% with montmorillonite, and 97% with sepiolite compared to the control. A saturation threshold was observed for kaolinite at 10% compost input, whereas montmorillonite and sepiolite remained unsaturated even at 50% compost input. SEM observations confirmed a higher proportion of mineral-associated OM in montmorillonite and sepiolite compared to kaolinite. Enhanced thermal and biological OM stability was observed in high-surface-area clays, accompanied by three order magnitude reductions in microbial abundance, particularly with sepiolite. These results demonstrate that OM stabilization is constrained by mineral surface saturation and strongly depends on clay mineralogy, with implications for carbon persistence and soil functioning.