Trade-off of microbial community involved in decomposition of labile and recalcitrant substrate enhances soil carbon sequestration with diverse fertility

Soil bacteria and fungi, coexist with each other or fierce compete for different carbon(C) resources. However, their critical roles in decomposition of labile and recalcitrant substrates, and its C sequestration mechanism under diverse soil fertility remains unexplored. An incubation experiment was conducted following the amendments of crop straw(ST) and woody peat(WP) in hilly red soil and yellow paddy, and the degradation characteristics of glucose/lignin and their dominant microbes were determined using 13 C-PLFA technique. Results demonstrated that ST in red soil and WP in yellow paddy increased soil organic C, aligning with an opposite trends of glucose and lignin accumulation, and diverse C components and activities of C-/N-metabolic enzymes. In hilly red soil, ST accelerated glucose decomposition (lower t 1/2 ) and its microbial C efficiency(CUE), but suppressed lignin decomposition. Conversely, WP in yellow paddy enhanced the degradation rates and microbial CUE of glucose/lignin. Although amendments altered the abundance of key glucose- and lignin-utilizers (e.g., Proteobacteria , Bacteroidota , Ascomycota , Chloroflexi ), their microbial structure (Gram-positive:Gram-negative bacteria, Fungi:Bacteria) showed no divergence differences between ST and WP. It indicates that bacteria-fungi cross-feeding interaction may stabilize microbial functional complementarity in soil C decomposition. Soil dissolved organic C, microbial biomass C, and Glucose-CUE were primary predictors of SOC in hilly red soil. Whereas soil oxidative degradation function, particulate organic C, and lignin-t 1/2 exhibited higher predictive importance in yellow paddy. Collectively, these findings highlight that bacterial-fungal interactions through exploitative competition and cross-feeding jointly regulate exogenous C decomposition and sequestration, and provide mechanistic insights for developing targeted C farming practices across contrasting agricultural soils. The mechanism model of exogenous organic residues for improving soil organic carbon. LOC, DOC, POC, and MAOC, represent the labile organic C, dissolved organic C, particulate organic C, and mineral-associated organic C, respectively. CF, NF, and OF, represent soil C-cycle, N-cycle, and oxidation functional enzymes, respectively. CUE represents soil microbial C use efficiency. CK, ST, and WP, represent the control without exogenous organic residue application, rice straw application, and woody peat application treatments, respectively. Red and purple arrows represent the increasing and decreasing trends, respectively. ns represents no significant difference. • ST and WP enhance soil C sequestration varying with soil fertility. • ST and WP boost the decomposition rates of glucose and lignin, respectively. • Recalcitrant C (lignin) exhibits unexpectedly high turnover rates (half-life: 3.01–3.98 h). • Exploitative competition and cross-feeding strategy of microbes enhance soil C sequestration.