Long-Term Straw Returning Combined With Mineral Fertilization Enhances Soil Quality but Elevates P Loss Risks in a Subtropical Rice–Wheat Rotation System, Central China

Abstract

Long-term straw returning is widely promoted to improve soil quality in intensive cropping systems, yet its combined effects with mineral fertilization on soil quality and phosphorus (P) loss risks remain unclear. Using a 16-year field experiment in a rice–wheat rotation system, we evaluated the impacts of straw returning (S), NPK fertilization (NPK), their combination (NPK + S), and an unfertilized control (CK) on soil properties, soil quality index (SQI), and P sorption indicators. Straw-return treatments improved soil chemical and biological properties, including soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon, and available potassium, but had little effect on physical properties. NPK + S markedly increased the degrees of P saturation (DPS) and equilibrium P concentrations (ECP₀), indicating elevated P loss risk. A minimum data set (MDS) identified silt, SOC, N:P ratio, and total P as key indicators. SQI derived from the MDS (SQI_MDS) was highest in NPK + S and strongly correlated with both SQI from the total dataset and rice yield (r > 0.92, p < 0.01). Silt was the dominant soil quality factor, contributing 56.7%–64.8% to SQI_MDS. Both NPK and NPK + S increased SQI_MDS and P-related risk indicators relative to CK, with NPK + S showing the greatest improvement in soil quality but also the greatest P loss risk. Overall, long-term straw returning with NPK fertilization enhances soil fertility and productivity but increases soil P saturation and leaching potential. Sustainable residue management should therefore integrate straw returning with reduced or controlled P inputs to balance agronomic benefits and environmental protection.