Profound impact of water management on cadmium and arsenic accumulation in ratoon rice

There is limited information on the accumulation of cadmium (Cd) and arsenic (As) in grains of ratoon rice and how to regulate them through water management. This study aimed to determine the accumulation and transfer characteristics of Cd and As in main crop (MC) and ratoon crop (RC) under different water management practices, and to clarify the potential risk of Cd and As accumulation in the grains of MC and RC and related mechanisms. A two-year field experiment with two soil drying treatments (light drying and heavy drying, LD and HD) in the main season and two water treatments (flooded and rainfed) in the ratoon season was conducted in central China. Plants were sampled at the maturity of MC and RC, and the Cd and As concentrations in each organ were determined. In the main season of 2019, compared with LD, HD significantly increased grain Cd concentration by 9.2 times, but decreased grain As concentration by 22.1%. However, frequent rainfall in the main season of 2018 weakened the difference between soil drying treatments. Across years in the ratoon season, rainfed treatment significantly increased grain Cd concentration by 9.3 times, while decreased grain As concentration by 30.2%, in comparison with flooded irrigation. RC exhibited a higher Cd pollution risk in grain than MC (23.2–388.7 vs 8.2–152.5 µg kg−1), which was attributed to the difference in Cd accumulation capacity in shoot. However, grain As concentration of RC was lower than that of MC regardless of water treatments (87.2–158.2 vs 152.2–261.6 µg kg−1). The low accumulation in shoot and transfer factor from shoot to grain of As were responsible for the low grain As concentration of RC. Considering the climate conditions and the Cd and As accumulation capacity of MC and RC, it was recommended to heavily dry the soil in the main season and keep flooded in the ratoon season to produce safe rice with low grain Cd and As concentrations in ratoon rice. This study first clarified the accumulation and transfer characteristics of Cd and As in both seasons of ratoon rice, and provided recommendation of irrigation practices for meeting the national food safety standard of grain Cd and As concentrations for both seasons in the ratoon rice of central China.