Reduction of chemical phosphate fertilizer application in a rice–rapeseed cropping system through continuous straw return

Straw return has the potential to significantly contribute to nutrient recycling and utilization, thus reducing the reliance on chemical fertilizers. Extensive research has focused on nitrogen and potassium fertilizer management and the associated potential for reduction through straw return. However, the potential effect of straw return on reducing phosphorus (P) fertilizer application has been relatively overlooked due to the inherently low P content in straw. Objective or research question This study aimed to determine the potential benefits and underlying mechanism of reducing chemical P fertilizer application through straw return in a rice–rapeseed cropping system. We conducted a five-year field experiment in a rice–rapeseed cropping system to investigate the effects of chemical P fertilizer reduction through straw return. Six fertilization treatments were investigated: no chemical P fertilization, 30, 60, 90, 120, and 60 kg P2O5 ha−1 with 6000 kg ha−1 straw return. The result showed that continuous straw return increased yield, P uptake and P utilization during rice and rapeseed seasons. According to the P uptake level, the benefits from straw return were tantamount to applying 69.8–88.0 kg ha−1 of P fertilizer, which exceeded the actual amount of P input (64.5–70.3 kg ha−1) from fertilizer and straw. Straw C input mainly increased the content of soil microbial biomass C and P (MBC and MBP), which enhanced the supply capacity of available P, thereby increasing crop P uptake and reducing P fertilizer. The rapeseed season showed a higher chemical P fertilizer reduction rate (31.8%) than the rice season (14.0%). Moreover, rapeseed P uptake was more sensitive to soil Olsen-P, MBP and MBC than rice. The substitution efficiency of P fertilizer through the priming effect of straw C was higher in the rapeseed season (7.5 kg P2O5/1000 kg C) than in the rice season (2.1 kg P2O5/1000 kg C). Overall, under the rice–rapeseed cropping system, straw return increased soil MBC and MBP, thereby enhancing the supply capacity of available P. The C priming effect induced by straw return can partially reduce P fertilizer application. Compared with the rice season, straw return significantly substituted P fertilizer in the rapeseed season. This study articulately delineates the role of straw return in enhancing soil P availability and crop P uptake and further highlights the potential of straw return as a strategy to reduce dependence on chemical P fertilizers in the rice–rapeseed cropping system.