Biochar affects methylmercury production and bioaccumulation in paddy soils: Insights from soil-derived dissolved organic matter

•Biochar significantly changes the nature of dissolved organic matter (DOM) in soil.•Biochar addition increases the aromaticity and molecular weight of soil DOM.•Modified biochar can effectively reduce the methylation rate in paddy soil.•Biochar significantly decreases MeHg bioaccumulation in rice plants, especially in grain.•Soil DOM modified by biochar is vital to the fate of mercury in rice field.