Mitigating methylmercury in rice through ridge tillage: A sustainable solution for mercury-polluted paddy fields in populated regions

Mercury (Hg) contamination of rice poses a health risk to consumers. Sustainable use of Hg-contaminated farmland is necessary to meet increasing demand for safe rice caused by rapid population growth. Ridge tillage is a water-saving tillage method that rice is planted on raised ridges, typically with furrows between them. This study aims to elucidate the application of ridge tillage on the immobilization of Hg in paddy fields and its transfer to rice plants, as well as the underlying mechanisms. The results showed that ridge tillage treatment increased the biomass of aboveground tissues of rice by up to 30 % compared to the control method (flooding). The concentration of total Hg (THg) in pore water was 58 % lower in the ridge tillage treatments than the control, which was attributed to limited reductive dissolution of Fe/Mn (hydr)oxides as a consequence of increased redox potential (control: −111 to −85 mV; ridge tillage: +48 to +280 mV) caused by ridge tillage. Moreover, the concentration of methylmercury (MeHg) in the pore water significantly decreased under ridge tillage. The concentrations of THg and MeHg in the roots, stalks, and leaves decreased under ridge tillage. In particular, the concentrations of THg and MeHg in the rice grains from ridge tillage treatments were 3–27 % and 22–47 % lower than those in the control, respectively, with significant reductions observed in the 15-cm ridge height treatments. A ridge height of 15-cm is recommended as the optimal parameter. Overall, the application of ridge tillage can inhibit Hg mobilization in paddy soils and its accumulation in rice plants, serving as a promising approach to managing the health risks of Hg in paddy fields within populated regions worldwide.