Milk vetch activates low-grade phosphate rock for phosphorus supply and cadmium mitigation in paddy fields

Overuse of chemical phosphate fertilizers and cadmium (Cd) contamination in paddy soils threaten environmental health and rice safety in China. Low-grade phosphate rock (PR) could provide phosphorus (P) and immobilize Cd, but its low solubility severely restricts field application. Here, we tested whether milk vetch (MV) can biologically activate low-grade PR and support partial substitution of chemical P fertilizer. Two-year field experiments demonstrated that MV-PR increased PR recovery by 2.36-5.54 percentage points relative to winter fallow with PR (WF-PR), and reduced total P runoff loss by 14.8-49.2%, compared to winter fallow with chemical P fertilizer (WF-P). MV-PR-P_1/2, integrating MV planting and PR application with 50% less chemical P fertilizer, increased grain yield by 8.5-14.3% relative to WF-P and WF-PR and reduced brown rice Cd by 29.2-46.6% relative to WF-P. In three-year large-scale demonstrations, MV-PR-P_1/2 using pelletized PR increased net return by 25.5-59.1% compared with WF-P. Mechanistic evidence linked these benefits to enhanced PR dissolution and Cd immobilization. MV promoted Ca^2 + and PO₄^3- release from PR; together with DOM-mediated complexation and pH-regulated shifts in Cd and Ca fractions, these processes contributed to Cd immobilization through phosphate precipitation and ion exchange. A seven-day hydroponic experiment provided complementary seedling-level evidence that MV-derived DOM together with PR could alter Cd uptake and partitioning. Overall, MV-PR promoted the biological activation of low-grade PR, whereas MV-PR-P_1/2 achieved the most balanced agronomic, environmental, and economic performance under the tested conditions.