Our incubation experiment shows that flooding of polluted floodplain soils may induce pulses of both mercury (Hg) and methylmercury to the soil solution and threaten downstream ecosystems. We demonstrate that mobilization of Hg bound to manganese oxides is a relevant process in organic-matter-poor soils. Addition of organic amendments accelerates this mobilization but also facilitates the formation of nanoparticulate Hg and the subsequent fixation of Hg from soil solution to the soil.