Insights into soil carbon metabolism and carbon sequestration capacity under organic fertilizer substitution model

Organic fertilizer substitution is important for improving soil carbon stocks and soil quality. However, the mechanisms underlying changes in soil microbe-mediated carbon sequestration capacity under organic fertilizer substitution remain unclear. In this study, we investigated the associations between soil properties, enzyme activities, soil carbon fixation genes and C-degradation genes including chemical fertilizer (CF), low-ratio organic fertilizer substitution (MF), high-ratio organic fertilizer substitution (AF), organic fertilizer (WF) and no fertilizer control (CK). The results showed that the application of chemical fertilizer significantly increased the degradation of plant-derived C; the exogenous organic carbon from organic fertilizer was mainly stored in easily degradable particulate organic carbon (POC), alleviating microbial carbon metabolic limitation; and the balanced nutrient structure in the low-ratio organic fertilizer substitution stimulated the microbial degradation of chitin in the bulk soil, thus improving the efficiency of soil carbon source utilization. The combined application of organic and chemical fertilizers stimulated the growth of Pseudomonadota in the rhizosphere soil, which played an important role in organic matter sequestration and carbon fixation. From the results of the structural equation model, the overall total effect of C-degradation gene (0.61) and carbon metabolic limitation (−0.67) on microbial carbon use efficiency were higher compared to soil properties (0.09), highlighting the key role of microorganisms in regulating soil carbon sequestration capacity. This study provides a scientific basis to guide the use of organic fertilizer substitution techniques to enhance soil carbon storage potential and soil quality.