Rainwater-Derived Reactive Oxygen Species Diminish Environmental Risk from Arsenic in Paddy Rice Systems

It has been previously observed that rainwater input into paddy rice soils reduced the level of grain-borne arsenic, and it is hypothesized that a Fenton-like reaction triggered by interaction between rainwater-borne hydrogen peroxide and ferrous iron in paddy soils is responsible for microbially mediated impediment of As uptake by rice plants. However, this hypothesis remains untested. This study tested the hypothesis through mesocosm experiments, confirming that rainwater-borne hydrogen peroxide triggered hydroxyl radical (^•OH) generation, elevating soil redox potential, and oxidizing arsenite to less phytoavailable arsenate in soil porewater, thereby reducing As uptake by rice and As accumulation in rice grain. Comparison between two crops of rice cultivation with different fluxes of rainwater-borne hydrogen peroxide confirms that seasonal rainfall variation has an impact on accumulation of rice grain-borne arsenic, with paddy soil receiving more rainfall having a lower arsenic concentration in the rice grain compared to that receiving less rainfall. Using China's major rice-producing region as an example, it is demonstrated that spatial variation in rainfall regime could impact the geographical distribution of rice grain-borne As at a national scale. The findings have implications for the assessment and management of the environmental risk from arsenic-contaminated rice grains.