The danger posed by cadmium (Cd) pollution to rice production is continuously increasing. Co-utilizing milk vetch (Astragalus sinicus L.) and rice straw is a good practice for rice yield and soil fertility in south China. However, its effects on Cd availability in soil-rice systems remain unclear. A micro-plot trial of two typical paddy soils (alluvial sandy soil and reddish clayey soil) in south China was conducted to investigate the effects of milk vetch, rice straw, lime, and their combined application on Cd availability and the related mechanisms. Soil chemical properties, CaCl2-extractable Cd (CaCl2-Cd), total content of Cd (Total-Cd), Cd fractionation (BCR sequential-extraction method), and Cd accumulation in rice were measured. Results showed that the co-utilization of milk vetch, rice straw, and lime (GRFL) decreased the Cd content in rice grain by 91.43% and 15.63% in early rice of two soils, respectively. Cd was not detected in late rice grains. CaCl2-Cd decreased by 0.025 mg kg-1 in late rice of alluvial sandy soil, 0.057 and 0.044 mg kg-1 decreased in early and late rice of reddish clayey soil, and Total-Cd decreased by 19.4% and 9.1% for early rice of two soils, respectively. Co-utilizing milk vetch, rice straw, and lime changed the distribution of different chemical forms of Cd, decreased the content of bioavailable Cd in soil by reducing the Aci-Cd and RedCd, and benefited the formation of more stable residual fraction (ResCd). Redundancy analysis showed that the improvement in soil pH, dissolved organic matter (DOM), and other soil properties was the main cause of the transformation of Cd form. Among the soil properties, pH and DOM had the greatest impacts on Cd availability. In conclusion, co-utilizing milk vetch and rice straw can alleviate the danger of soil Cd in rice production, and this effect could be strengthened by applying lime.
Keywords: Cadmium bioavailability; Cadmium chemical speciation; Milk vetch; Rice cadmium; Rice straw.
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