Partial substitution of rice straw with biochar alters soil microbial biomass phosphorus and turnover by regulating carbon:phosphorus stoichiometry

Microbial biomass phosphorus (MBP) and its turnover play a crucial role in crop phosphorus (P) nutrition. However, the response of MBP to organic amendments, especially under the combined application of biochar and straw in Oxisols remains poorly understood. This study investigated the effects of substituting rice straw with different biochar proportions (0 %, 10 %, 30 %, and 50 %) on soil CO₂ emissions, microbial biomass carbon (MBC) and MBP content, their turnover parameters and carbon:phosphorus (C:P) stoichiometry during a 60-day incubation. The results showed that 10 % biochar substitution did not alter MBP content but decreased MBP flux and turnover rate, consequently prolonging turnover time. In contrast, higher biochar substitution rates (30 % and 50 %) increased MBP content by 7.37 % and 5.71 %, respectively, and gradually recovered MBP turnover. Notably, these higher biochar ratios also significantly decreased dissolved C:P and microbial C:P ratios, exacerbating C:P stoichiometric imbalance. The partial least squares path model (PLS-PM) indicated the MBC:MBP ratio as the primary driver of microbial P immobilization and turnover, demonstrating a positive correlation with CO₂ emission rates. These results imply that strategic substitution of straw with biochar can increase potential soil P supply by enlarging MBP pools and accelerating MBP turnover, thereby improving crop P utilization efficiency.