Converting straw into biochar shifts soil carbon stabilization from plant-derived to microbial-derived pathways in a rice–wheat system

Sustainable utilization of agricultural residues is essential for reducing environmental pollution while enhancing long-term soil carbon sequestration. Converting crop straw into biochar has emerged as a promising environmental technology for agricultural waste utilization, however, whether this conversion fundamentally alters soil organic carbon (SOC) stabilization pathways remains poorly understood. A three-year field experiment in an intensive rice–wheat cropping system was conducted to compare the effects of direct straw return and straw-derived biochar application on SOC fractions, microbial responses, and microbial necromass formation. Results showed that straw-derived biochar combined with N fertilization (NBC treatment) significantly increased total SOC (TSOC) content and stock by17-22% and 12-15% compared to the CK, CS, and NPK treatments ( P < 0.05), respectively, while it exhibited a 48% increase ( P < 0.05) in mineral-associated organic C (MAOC) relative to the CK treatment. In contrast, direct straw return (NS treatment) promoted the accumulation of plant-derived lignin phenols. Biochar amendment also increased fungal α-diversity, strengthened bacterial–fungal network connectivity, and enhanced microbial necromass C accumulation. Microbial necromass C, especially bacterial necromass, contributed up to 16.7% of TSOC and was identified as the strongest predictor of SOC accumulation in partial least squares-path modeling. Our results suggest that converting crop residues into biochar prior to soil return, combined with N fertilization, promotes a transition in SOC formation from plant residue accumulation toward microbial-mediated stabilization pathways. Our results highlight that integrating straw pyrolysis with nutrient management represents an effective strategy to optimize residue management and support climate change mitigation efforts in intensive rice-wheat system, though evaluating its long-term persistence requires continued multi-site field validation.