Contamination severity shifts the multi-mode protective functions of edaphic phosphorus against cadmium in rice

Phosphorus (P) amendments are increasingly recognized as a critical strategy to mitigate cadmium (Cd) contamination in paddy soils. However, how varying Cd contamination intensity shapes rhizosphere geochemistry and determines the dominant pathway of P-mediated Cd mitigation remains unclear. This study aimed to elucidate the distinct mechanisms by which exogenous P modulates Cd bioavailability under different Cd stress levels.

We integrated a rhizo-bag pot system with diffusive gradients in thin-films (DGT) and time-resolved porewater monitoring. This approach allowed us to resolve high-resolution spatiotemporal patterns of Cd, P, Fe, and pH dynamics in both rhizosphere (RH) and non-rhizosphere (NRH) zones throughout the rice cultivation period.

Under low Cd concentration level, P acted as a nutrient, stimulating biomass growth (2.2- to 3.1-fold). The resulting biomass increase generated an apparent biomass-dilution effect on a concentration basis, with enhanced iron plaque potentially serving as a Cd sink. Under high Cd concentration level, P raised rhizosphere pH (from 7.08 to 7.45-7.69) and likely promoted Cd/Fe phosphate precipitation, reducing labile Cd and Cd accumulation in rice. Correlation analyses supported the dilution effect under low Cd and precipitation-driven immobilization under high Cd.

The mechanism of P-mediated Cd remediation shifts from biological dilution to chemical immobilization with increasing Cd stress. P-based management should be tailored to pollution severity: growth promotion in low-risk soils and chemical stabilization in heavily polluted fields.