Combined Utilization of Chinese Milk Vetch, Rice Straw, and Lime Reduces Soil Available Cd and Cd Accumulation in Rice Grains

Cadmium (Cd) pollution poses a growing threat to rice production in acidic paddies. In south China, a common agricultural practice involves the combined utilization of Chinese milk vetch (M) and rice straw (R). However, it is unclear how the addition of lime to these amendments affects Cd bioavailability and accumulation in soil. Control (CK), chemical fertilizer (F), Chinese milk vetch + rice straw + chemical fertilizer (MRF), and Chinese milk vetch + rice straw + chemical fertilizer + lime (MRFL) treatments were applied to develop a kind of green, efficient, and practical amendment for acidic paddies. We conducted a microplot experiment to explore Cd immobilization in paddy soil and the Cd content in rice grains with these treatments. The results showed that compared with F, the rice Cd in the MRF and MRFL treatments were significantly decreased by 51.7% and 65.2% in early rice and 23.0% and 43.3% in late rice, respectively. Both the MRF and MRFL treatments significantly reduced soil available Cd and weak acid-extractable cadmium (Aci-Cd) concentrations and increased soil organic matter (SOM), exchangeable cation concentrations, and pH, which converted Cd into a stable form in soil. In addition, the MRF and MRFL treatments increased soil pH value by reducing soil exchangeable hydrogen ion concentration (E-H). Additionally, recombination of Cd forms was the primary factor in the reduction in available Cd concentration according to partial least squares path modeling (PLS-PM) analysis. The Cd concentration of rice grains was primarily associated with soil available Cd, soil pH value, and SOM. Overall, these results provide useful data and novel insights into reducing rice grain Cd in south China.