Soil organic carbon stabilization by organic amendments through iron gate and enzyme latch mechanisms

Abstract The stabilization of soil organic carbon in agricultural lands is crucial for mitigating climate change and enhancing soil fertility. While organic amendments like manure and biochar enhance carbon sequestration, their micro-scale mechanisms at the soil aggregate level remain unclear. Here we integrated analyses of enzyme activities, iron-bound organic carbon, iron oxides, and microbial communities across soil aggregates to clarify these mechanisms. Compared with conventional fertilization, manure increased the activities of cellobiohydrolase, β-1,4-glucosidase, β-1,4-N-acetylglucosaminidase, and phosphatase by 92%, 147%, 202%, and 89%, respectively, while increasing iron-bound organic carbon content (288%) and ferrihydrite-like iron oxide content (4%). Manure also boosted the abundance of iron-oxidizing bacteria and the expression of iron-oxidation related genes ( coxA, coxB ). In contrast, biochar inhibited polyphenol activities (33–54%) and reduced carbon mineralization (2.6–17.3%). These findings indicate that pig manure stabilizes carbon via a microbial-mediated iron gate, whereas biochar stabilizes carbon mainly through a physical enzyme latch driven by enzyme suppression.