Abstract: To explore the effects of straw return, either alone or in combination with animal manure, and of replacing part of the chemical nitrogen (N) fertilizer on paddy soil phosphorus (P) availability and rice yield, we conducted a field experiment with four treatments in a hilly paddy field of central Sichuan. The treatments included unfertilized control (CK), chemical N, P, and potassium (K) fertilizer (NPK), pig manure in combination with straw return and replacing part of chemical N fertilizer (MSNPK), and straw return alone and replacing part of chemical N fertilizer (SNPK). The results showed that the SNPK treatment significantly increased labile organic P (NaHCO₃-Po) and moderately labile organic P (NaOH Ⅰ-Po) contents, and decreased labile inorganic P (NaHCO₃-Pi) and highly stable organic P (NaOH Ⅰ-Po) contents compared to NPK treatment. The MSNPK treatment led to a significant increase in water-soluble P (NH₄-Cl-Pi) and moderately labile inorganic P (NaOH Ⅰ-Pi) contents, and a significant decrease in NaOH Ⅰ-Po and NaOH Ⅰ-Po contents compared to NPK and SNPK treatments. Moreover, the SNPK treatment significantly increased soil organic carbon (SOC) content, alkaline phosphatase activity, rice P uptake, yield and aboveground biomass compared to NPK and MSNPK treatments. Results of correlation analysis revealed that SOC, alkaline phosphatase, and P uptake were significantly positively correlated with rice yield and aboveground biomass. In addition, NH₄-Cl-Pi and NaHCO₃-Po showed significantly positive correlation with rice P uptake, yield and aboveground biomass. Results of partial least squares path modeling (PLS-PM) further indicated that NaHCO₃-Po could directly affect NH₄-Cl-Pi or indirectly affect NH₄-Cl-Pi by influencing NaHCO₃-Pi under the catalytic hydrolysis of alkaline phosphatase. Overall, the straw return along and replacing part of chemical N fertilizer can enhance the activity of soil alkaline phosphatase and its mediated organic P fraction mineralization process. This approach can increase the paddy soil P availability and rice P uptake, yield, and aboveground biomass, thus can be used as a suitable fertilizer application method for the hilly paddy fields of central Sichuan.

Key words: phosphatase, soil phosphorus fraction, straw return, paddy field, phosphorus uptake