The effect of green manure combined with phosphate rock powder on reducing cadmium pollution in paddy fields

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

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

Results The green manure species, P application level and their interaction had significant effects on soil physiochemical properties, ACd content and BCd (P<0.05). All treatments increased soil pH, and the available K in early rice soil, available P, organic matter and cation exchange in late rice soil were significantly affected by the treatment effect (P<0.05). Compared with the CK-P0 treatment, the ACd in early rice soil under the CK-P1 and CK-P2 treatments decreased by 22.91% and 42.15%, respectively; the ACd redection in the five green manure P0 treatments ranged from −30.53% to 22.63%, in the five green manure P1 treatments were from 15.24% to 28.80%, in the five P2 treatments ranged from 40.82% to 60.79%, the reduction in manure P2 treatment was higher than in the same manure P1 treatment, with the greatest reduction observed in the HV-P2 treatment. Compared with CK-P0 in late-season rice soil, the CK-P1 and CK-P2 treatment reduced ACd in late-season rice soil by 9.28% and 38.16%, respectively; the ACd of five manure P0 treatments in late-season rice reduced from −4.26% to 97.43%, whilst the ACd reduction in the five manure P1 and P2 treatments ranged from −16.51% to 14.14%, with the greatest reduction observed in the CK-P2 treatment. The BCd of the early rice in CK-P1 and CK-P2 treatments were reduced by 58.70% and 59.93%, respectively, compared with that of the CK-P0 treatment. The five green manure P0 treatments reduced BCd by 63.09%−87.39%, whilist the five green manure P1 and P2 treatments reduced it by 76.16%−90.95%, and the BCd in early rice of the MV-P2, HV-P1, HV-P2, CV-P0 and PR-P0 treatments were below the safety threshold of 0.2 mg/kg. The CK-P1 and CK-P2 treatments reduced the BCd in the late rice by 15.15% and 50.22% respectively, compared to the CK-P0 treatment, whilst the MV-P0, HV-P0, CV-P0 and ER-P0 treatments reduced BCd by 16.91%, 27.10%, 22.21% and 14.24%, respectively. The P1 and P2 treatments reduced BCd by a range of 34.85% to 69.93%, with the BCd content in all the P2 treatments below 0.2 mg/kg. Random forest analysis indicated that the primary factors influencing soil ACd were the application rate of phosphate rock and pH, whilst the primary factors influencing BCd were the green manure variety and pH.

Conclusions The cultivation of green manure during winter season demenstrates significant and stable effect on reducing Cd content in brown rice, but varied effect on soil available cadmium reduction. The application of phosphate rock powder demonstrate significantly higher effect on reducing soil available cadmium content, and weaker effect on inhibiting Ca accumulation in brown rice than green manures The combined application of green manure and phosphate rock powder further strengthened the effect of phosphorous rock powder on reducing soli available Ca content, and the Ca content of brown rice as result. The optimal combination is high phosphorous rock powder application rate with hairy vetch for early rice, and perennial ryegrass for late rice.