How bacterial traits scale to soil organic matter pool buildup revealed by long-term maize straw mulching experiment

No-till with maize straw mulching can enhance soil organic matter (SOM) accumulation. Soil bacterial communities, which are sensitive to habitat changes, can influence carbon (C) and nitrogen (N) cycling during SOM pool buildup, yet their temporal mediation under long-term conservation tillage remains unclear. We used 16S rRNA high-throughput sequencing to analyze bacterial traits across four maize straw mulching durations, 0 (M0), 6 (M6), 10 (M10), and 14 (M14) years in Northeast China. Compared with aboveground removal, bacteria at M6 exhibited traits of active growth and metabolism, such as higher phylogenetic diversity, increased copiotroph to oligotroph ratio (Copio/Oligo), enhanced species cross-feeding, and greater potential for biomass and carbohydrate biosynthesis efficiency. Similar trends persisted at M10, accompanied by stronger competition between Actinobacteria and other species. Till M14, bacteria further shifted toward reduced efficiency in carbohydrate biosynthesis. Correspondingly, SOM increased by 42.7 % at M10 and by 53.6 % at M14, accompanied by an increased C to N ratio, with M14 also showing higher labile C content. Such findings indicated that SOM pool buildup evolved from initial stability via bacterial turnover, to accelerated accumulation driven by bacterial transformation, and ultimately to enhanced both quantity and C availability. Structural equation modeling highlighted that temporally coupled bacterial succession, shaped by Copio/Oligo and growth strategies in response to shifting C availability, drove changes in SOM quality. Our findings suggest that bacterial traits can be effectively scaled up to understand SOM buildup under long-term maize straw mulching.