Effects of nitrogen fertilization on soil phosphorus speciation and soil microbial communities in different rotation cropping systems

Purpose Crop rotations (upland-upland and paddy-upland) are widely practiced in the Yangtze River Basin, where water and fertilization management strongly influence soil phosphorus (P) dynamics. However, the mechanisms by which nitrogen (N) fertilization strategies affect soil P speciation and microbial communities across these systems remain unclear. Methods Based on an 11-year field experiment, this study investigated soil P speciation, microbial communities, and soil physicochemical properties under different N regimes (single-season vs. dual-season) in the paddy-upland (rice-oilseed rape, RO) and upland-upland (corn-oilseed rape, CO) rotations. Using sequential P fractionation and high-throughput sequencing, we examined the responses of soil P speciation and microbial communities to varying N fertilization strategies. Results N fertilization significantly enhanced soil P availability. The balanced dual-season fertilization (N150-150) achieved the highest available P (AP) in RO (30.26 mg/kg), nearly double that in CO (14.79 mg/kg). N fertilization decreased soil pH in CO but maintained near-neutral pH in RO. Rotation mode was the primary driver of microbial community structure, with distinct taxa enriched in each system ( Actinobacteria in CO, Chloroflexi in RO). Soil pH droved microbial variation in CO, whereas soil organic matter and AP were key drivers in RO. Conclusion Water management serves as the overarching determinant of soil environment, within which N fertilization acts as a key regulatory factor. The balanced dual-season N application (N150-150) was associated with the highest P availability and microbial diversity in the rice-oilseed rape rotation system. N fertilization improves P availability by modulating soil properties and microbial communities, and the balanced dual-season strategy synergistically boosts P supply, promotes soil biodiversity, and supports sustainable cropping intensification in the Yangtze River Basin.