Objectives We assessed the effects of milk vetch on rice yield, mitigation of cadmium (Cd) toxicity in rice, and the physiological mechanism underlying Cd mitigation.
Methods A 5-year field micro-compartment experiment was conducted. The experimental treatments were fertilizer–free (CK), milk vetch (GM), chemical fertilizer (F), and milk vetch with chemical fertilizer (F+GM). The milk vetch was returned to the field as winter planting green manure. Rice plant material was sampled at tillering, filling and maturity stage of double-cropping rice. Cd content in root, stem and leaf, and grain were analyzed. The pH, total Cd and available Cd content in 0–20 cm soil layer were analyzed after rice harvest.
Results 1) Compared with CK, the rice yield of F+GM and F treatment increased significantly, GM treatment increased significantly from 2017. Compared with F, the rice yield of F+GM treatment increased, especially in 2016 and 2020. 2) The Cd reduction effects of returning milk vetch on the same part of rice at different stages were different. The Cd content of early rice root of GM treatment was significantly lower than CK in tillering, filling and maturity stage, but there was no significant difference in late rice. The Cd content in stem and leaf of early rice GM treatment was significantly lower than CK at grain filling stage, but there was no significant difference in late rice. The content of Cd in grain of early rice, GM treatment was significantly lower than that of CK at grain filling and maturity stage (decreased by 85.7% and 57.6% respectively), and there was no significant difference in late rice. The Cd content of root in early rice, F+GM treatment was significantly lower than F at tillering, filling and maturity stage, but there was no significant difference in late rice. The Cd content in stem and leaf of early rice F+GM treatment was significantly lower than that of F at tillering stage, but there was no significant difference in late rice. 3) There were differences in Cd transport in rice at different stages under the return milk vetch. In early rice, the root-stem leaf Cd transport coefficient of F+GM treatment was significantly higher than F and CK at tillering stage. The stem and leaf-grain Cd transport coefficient of GM treatment was significantly lower than CK at maturity, with a decrease of 52.2%. There was no significant difference in late rice. 4) The accumulation of Cd in the roots of F+GM and GM treatments was significantly lower than that of F; the Cd accumulation of early rice grains in GM treatment was significantly lower than that of F; and the Cd accumulation of stems and leaves of F+GM late rice was significantly higher than that of CK. The distribution ratio of Cd in the grains of F+GM and GM is lower than that of CK and F, the distribution ratio of Cd in stems and leaves of F+GM is higher than that of F, and the distribution ratio of Cd in stems and leaves of GM is higher than that of CK. 5) There was no significant difference in soil total Cd content among early rice, while GM treatment was significantly lower than CK in late rice. Compared with CK, the soil available Cd of early rice of GM and F+GM was decreased significantly, and no significant difference in late rice.
Conclusions Returning milk vetch to soil proved to be effective in increasing rice yield and decreasing Cd content in rice. The two reasons for the positive effects of the milk vetch were: (1) the decreased availability of soil Cd reduced Cd absorption by rice; and (2) the reduced transport coefficient of Cd from stems and leaves to grains at maturing stage made the rice Cd content lower than the safety limit recommended by the National standard.
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