Objectives To investigate the effects of winter oilseed rape cultivation, either for rapeseed production or as green manure, on soil phosphorus fractions and sorption–desorption characteristics in fallow fields of a double-rice system, and to provide a theoretical basis for efficient phosphorus utilization in rice–rice–oilseed rape systems.

Methods Two long-term field experiments were conducted in Qiyang, Hunan Province, and Jinxian, Jiangxi Province, namely, the “double-rice–winter green manure” experiment and the “multi-cropping system” experiment, respectively. The two experimental sites had similar soil types and climatic conditions. Based on the double-rice system, both experiments included three treatments: winter fallow, winter milk vetch (Astragalus sinicus), and winter oilseed rape. In the Qiyang experiment, oilseed rape was grown as green manure without fertilizer application, whereas in the Jinxian experiment, oilseed rape was grown for seed production with chemical fertilizer application, and the rapeseed straw was not returned to the field. Surface soil samples were collected after oilseed rape harvest in Qiyang and after late-rice harvest in Jinxian. Sequential chemical extraction and Langmuir model fitting were used to determine changes in soil phosphorus fractions and sorption–desorption characteristics and to explore their potential driving mechanisms.

Results In the Qiyang double-rice–winter green manure experiment, growing oilseed rape as green manure promoted the transformation of HCl-P into NaHCO₃-P. Compared with the winter fallow treatment, the contents of NaHCO₃-P and NaOH-P increased significantly by 11.2% and 7.59%, respectively, and were 28.5% and 20.8% higher than those under milk vetch treatment. Conversely, the residual-P fraction decreased by 50% compared with the milk vetch treatment. The green manure treatment increased the relative abundance of aromatic structures in soil dissolved organic matter while reducing the relative abundance of CHO components. Compared with winter fallow or milk vetch treatments, the green manure treatment increased the maximum phosphorus adsorption capacity (Qm), adsorption affinity constant (KL), and adsorption buffer capacity (ABC) by 13.0%–19.9%, 93.8%–144%, and 132%–176%, respectively, while decreasing the desorption rate by 4.0–6.8 percentage points. Correlation analysis showed that NaHCO₃-P and NaOH-P fractions were highly significantly positively correlated with phosphorus bound to weakly crystalline Fe/Al oxides. In the Jinxian experiment, the NaHCO₃-P content under the winter rapeseed treatment was 18.5% higher than that under milk vetch treatment. The NaOH-P and HCl-P fractions increased by 11.2%–12.0% and 14.1%–14.6%, respectively, compared to fallow or milk vetch treatments, although no significant effect was observed on the proportion of each fraction relative to total phosphorus. The Qm, KL, and ABC of soil under the rapeseed treatment increased by 4.35%–4.45%, 14.4%–22.4%, and 19.4%–27.6%, respectively, compared to the fallow and milk vetch treatments.

Conclusions Planting winter oilseed rape in fallow fields of a double-rice system contributes to improving phosphorus use efficiency. Using winter rapeseed as green manure promotes the transformation of HCl-P into NaHCO₃-P and increases the contents of NaHCO₃-P and NaOH-P. These effects may be associated with increases in the maximum phosphorus adsorption capacity, adsorption affinity constant, and adsorption buffer capacity following green manure incorporation, which may enhance phosphorus retention in the soil in adsorbed forms. These findings provide a basis for optimizing phosphorus input strategies by considering soil phosphorus transformation and retention processes, thereby promoting efficient phosphorus utilization in rice-based production systems.