Chinese milk vetch (Astragalus sinicus L.) and rice straw are high-quality organic resources in southern rice-growing regions. Their combined application plays a significant role in enhancing rice yield and improving soil fertility. However, the specific processes and mechanisms by which this practice activates insoluble phosphorus (P) in the soil remain unclear. This study systematically investigated the effects of combined incorporation of Chinese milk vetch and rice straw on soil P fraction composition, P activation coefficient, and rice P accumulation in paddy fields, aiming to provide the theoretical support for promoting this practice and managing P nutrients.

Long-term field positioning experiments were conducted in Hunan, Jiangxi (initiated in 2016), and Guangxi (initiated in 2019). Four treatments were established: (1) Winter fallow+Conventional fertilization (F); (2) Chinese milk vetch incorporation+Conventional fertilization (FM); (3) Winter fallow+Rice straw return+Conventional fertilization (FS); (4) Chinese milk vetch incorporation+Rice straw return+Conventional fertilization (FMS). After early rice harvest, soil samples (0-20 cm plough layer) were collected. Soil P fractions and phosphatase activities were measured. The soil P activation coefficient was calculated. Actual rice yield was recorded, and rice P accumulation was calculated.

Compared with the F treatment, the FMS treatment increased grain yield by 9.7%-29.5% and rice P accumulation by 20.6%-51.4% across the three experimental sites. The FMS treatment significantly increased the proportion of the active P pool in the total soil P pool by 13.5%-30.1%, while decreasing the proportion of the stable P pool by 2.3%-6.0%. Specifically, compared with the F treatment, the FMS treatment primarily increased the content of inorganic P fractions: NaHCO₃-Pi and NaOH-Pi increased by 13.5%-33.3% and 10.7%-17.4%, respectively. Furthermore, the soil P activation coefficient under FMS increased by 5.4%-18.2%. Soil phosphatase activities were also significantly enhanced, compared with the F treatment, acid phosphatase activity and alkaline phosphatase activity increased by 2.1%-21.5% and 2.2%-26.3%, respectively, thereby promoting organic P mineralization.

Long-term combined incorporation of Chinese milk vetch and rice straw enhanced soil P availability by increasing the content of active P forms and boosting phosphatase activity. Compared with their individual application, the combined use of Chinese milk vetch and rice straw further improved soil P fertility, promoted rice P uptake, and consequently achieved synergistic growth in rice yield.