Conservation tillage and wheat straw managements improve soil organic carbon sequestration via calcium-mediated...

Conservation tillage improves soil organic carbon (SOC) management by balancing microbial decomposition and physico-chemical protection. Minerals stabilize SOC, but how tillage practices affect calcium (Ca) speciation and its role in microbial-mineral-organic matter interactions in calcareous soils remains unclear. Thus, a 22-year tillage experiment and soil incubation were conducted to investigate Ca-mediated SOC sequestration and mineralization. Conservation tillage, including no-tillage with straw mulch (NTS) and subsoil tillage with straw mulch (STS), increased SOC storage by 9.9-14.3 % at 0-20 cm soil depth by promoting organo-Ca associations, compared to conventional tillage without straw return (CTN) and reduced tillage without straw return (RTN). Moreover, NTS and STS increased aggregate stability and macroaggregate proportion, reducing total SOC mineralization, as macroaggregates had lower SOC mineralization than microaggregates. In aggregates, increased exchangeable Ca was linked to higher particle- and mineral-associated SOC and lower SOC mineralization, showing positive (p < 0.01) and negative (p < 0.001) relationships, respectively. Compared to CTN and RTN, long-term NTS and STS not only promoted the transformation of CaCO₃ to exchangeable Ca, but also increased microbial biomass, especially the proportion of Gram-negative and arbuscular mycorrhizal fungi. Mechanistically, higher Ca²⁺ reshaped bacterial communities and promoted the microbial by-product binding to the minerals to form stable organo-Ca complexes, which was supported by the positive correlations between Ca²⁺, microbial composition and SOC content (PLS-PM, p < 0.01). Overall, conservation tillage increased Ca availability for carbon binding by mediating microbial structures, thereby promoting aggregate protection and SOC stability in Calcaric Cambisols.