Objectives This study aimed to investigate the effects of green manure (GM) species and phosphate rock powder (P) application rates on reducing soil available cadmium (Cd) content and cadmium uptake by rice, so as to provide a scientific basis for rice safe production.

Methods A pot experiment was conducted in a double-cropping rice area in Hunan Province from November 2021 to November 2022. Six winter treatments were established: winter fallow (CK), Chinese milk vetch (MV), hairy vetch (HV), common vetch (CV), perennial ryegrass (PR), and rape (ER), along with three P application levels: P0 (0 kg/hm²), P1 (1650 kg/hm²), and P2 (3300 kg/hm²), resulting in a total of 18 treatments. Soil and brown rice samples were collected at the maturity stage of early and late rice. Soil physicochemical properties, available cadmium (ACd) and brown rice cadmium (BCd) content were determined.

Results Green manure species, P application rate and their interaction significantly effected soil physicochemical properties, ACd and BCd content (P<0.05). All treatments increased soil pH, and available K in early-season rice soil, as well as available P, organic matter and cation exchange capacity in late-season rice soil, were significantly affected by the treatments (P<0.05). Compared with the CK-P0 treatment, the CK-P2 treatment reduced ACd in early-season rice soil by 42.15%. Under green manure treatments, changes in ACd ranged from −30.53% to 22.63% under P0, reductions ranged from 15.24% to 28.80% under P1, and from 40.82% to 60.79% under P2. The reductions under P2 were significantly greater than those under the corresponding P1 treatments, with the greatest decrease observed in HV-P2. In late-season rice soil, ACd under CK-P2 decreased by 38.16% compared with CK-P0. The reductions in ACd ranged from −4.26% to 97.43% under the green manure P0 treatments and from −16.51% to 14.14% under the P2 treatments, with the greatest reduction observed under CK-P2. For early rice, BCd concentrations under CK-P1 and CK-P2 were reduced by 58.70% and 59.93%, respectively, relative to CK-P0. Green manure alone reduced BCd by 63.09%–87.39%, while the combined application of green manure and phosphate rock powder reduced BCd by 76.16%–90.95%. The BCd concentrations in MV-P2, HV-P1, HV-P2, and all CV and PR treatments were below the safety threshold of 0.2 mg/kg. For late rice, BCd under CK-P2 decreased by 50.22% compared with CK-P0. The reductions under P1 and P2 treatments ranged from 34.85% to 69.93% and from 59.20% to 64.26%, respectively. BCd concentrations in all green manure P2 treatments were below 0.2 mg/kg. Random forest analysis showed that phosphate rock powder application rate and soil pH were the primary factors affecting ACd, whereas green manure species and soil pH were the major factors influencing BCd.

Conclusions Winter cultivation of green manure significantly and consistently reduced Cd accumulation in brown rice, although its effect on reducing soil available Cd varied among species. Application of phosphate rock powder alone effectively decreased soil available Cd but was less effective than green manure in suppressing Cd accumulation in brown rice. The combined application of green manure and phosphate rock powder further enhanced Cd immobilization in soil through their interactive effects, ultimately reducing BCd concentrations below the food safety threshold. Among the tested treatments, HV-P2 was the optimal combination for early rice, whereas PR-P2 was the most effective treatment for late rice.