Rainwater input reduces greenhouse gas emission and arsenic uptake in paddy rice systems

This work aimed to test the hypothesis that rainwater-borne hydrogen peroxide (H₂O₂) can affect arsenic uptake by rice plants and emission of greenhouse gases in paddy rice systems. A mesocosm rice plant growth experiment, in conjunction with rainwater monitoring, was conducted to examine the effects of rainwater input on functional groups of soil microorganisms related to transformation of arsenic, carbon and nitrogen as well as various arsenic species in the soil and plant systems. The fluxes of methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂) were measured during selected rainfall events. The results showed that rainwater-borne H₂O₂ effectively reacted with Fe²⁺ present in paddy soil to trigger a Fenton-like reaction to produce ^•OH. Both H₂O₂ and ^•OH inhibited As(V)-reducing microbes but promoted As(III)-oxidizing microbes, leading to a net increase in arsenate-As that is less phytoavailable compared to arsenite-As. This impeded uptake of soil-borne As by the rice plant roots, and consequently reduced the accumulation of As in the rice grains. The input of H₂O₂ into the soil caused more inhibition to methanogens than to methane-oxidizing microbes, resulting in a reduction in CH₄ flux. The microbes mediating the transformation of inorganic nitrogen were also under oxidative stresses upon exposure to the rainwater-derived H₂O₂. And the limited conversion of NO₃^- to NO played a crucial role in reducing N₂O emission from the paddy soils. The results also indicated that the rainwater-borne H₂O₂ could significantly affect other biogeochemical processes that shape the wider ecosystems, which is worth further investigations.