Objectives Investigating the effects of winter oilseed rape cultivation (for rapeseed or green manure) on soil phosphorus fractions and adsorption–desorption characteristics in fallow fields of a double-rice system can provide a theoretical basis for the efficient utilization of phosphorus nutrients in the “Rice–Rice–Rapeseed” system.

Methods This study relied on two long-term positioning experiments: the “Double-Rice–Winter Green Manure” experiment in Qiyang, Hunan, and the “Multi-cropping System” experiment in Jinxian, Jiangxi. The two sites share similar soil types and climatic conditions. Both experiments included three treatments based on the double-rice system: winter fallow, winter milk vetch (Astragalus sinicus), and winter oilseed rape. In the Qiyang trial, oilseed rape was grown as green manure without fertilization; in the Jinxian trial, oilseed rape was cultivated for seed production with chemical fertilizer application, and the straw was not returned to the field. Surface soil samples were collected after the harvest of rapeseed and late rice at both sites. Sequential chemical extraction and Langmuir model fitting were applied to analyze changes in soil phosphorus fractions and adsorption–desorption characteristics, and to explore potential driving mechanisms.

Results In the Qiyang double-rice–winter green manure positioning experiment, growing rapeseed 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 to 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 4–6.8 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 (P<0.05). 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 fractions. This enhancement is likely attributed to the improved maximum phosphorus adsorption capacity, adsorption affinity constant, and adsorption buffer capacity induced by green manure incorporation, which facilitates the retention of phosphorus in the soil in an adsorbed state. These findings suggest that optimizing phosphorus input strategies by fully considering phosphorus transformation processes can promote the efficient utilization of phosphorus in production systems.