Cadmium accumulation and bioavailability in paddy soil under different water regimes for different growth stages of rice (Oryza sativa L.)

Cadmium (Cd) is a ubiquitous environmental pollutant, and rice (Oryza sativa L.) is the main staple food in China. Water regimes are promising, controllable, and environment-friendly agricultural measures for remediation Cd contaminated soil. The optimum water regime for ensuring high rice yield with low Cd contents in brown rice was investigated. A pot experiment was conducted to study the effects of Cd migration and accumulation from paddy soil to rice plants under four water regimes with three growth stages of two rice cultivars, ‘Xiangwanxian 12’ (X12) and ‘Weiyou 46’ (W46) grown in a polluted soil with 5 mg kg−1 of Cd. The relationship between Cd accumulation amounts in rice plant and soil Eh was also investigated. The four regimes were moisture throughout growth (M), moisture before the filling stage and flooding after filling (M-F), flooding before filling and moisture after filling (F-M), and continual flooding (F). Water regimes effectively reduced Cd accumulation in rice at the three growth stages, whereas the filling stage was suggested to be as the key stage for interventions to prevent Cd transport and accumulation. Compared with M, the M-F, F-M, and F regimes significantly reduced Cd contents of rice tissues (root, stem, husk, and brown rice) at the maturity stage. Under the M, F-M, M-F, and F regimes, Cd contents were 2.17, 2.10, 0.21, 0.06 mg kg−1 in X12 brown rice and 3.27, 0.47, 0.19, 0.10 mg kg−1 in W46 brown rice, respectively. Importantly, Cd contents of brown rice of X12 (0.06 mg kg−1 under regime F) and W46 (0.19 mg kg−1 under regime M-F and 0.10 mg kg−1 under regime F) did not exceed China’s food safety standard of 0.20 mg kg−1 (GB 2762–2017). Moreover, there were significant positive exponential relationships between soil Eh and Cd accumulation amounts in belowground and aboveground parts of rice. The M-F regime (W46) and F regime (X12) could ensure high rice yield with low Cd contents in brown rice and could be recommended as the irrigation management in rice production. Simultaneously, keeping soil Eh under 50 mV is also necessary after the rice filling stage.