Pioneer area construction reduces methylmercury production in mercury-contaminated paddy soils

Paddy fields in mercury (Hg) mining areas are identified as significant sites for Hg methylation, leading to an elevated accumulation of methylmercury (MeHg) in rice. The implementation of pilot zones for soil pollution prevention in China has significantly contributed to pollution control efforts. Its long-term effects on the Hg fate in surface paddy soil remains unclear. This study investigated the impacts of continuous rice cultivation and paddy-dryland conversion on Hg and MeHg in soil in Hg-mining areas over a span of ten years. The results demonstrated a gradual decline in the total Hg (THg) concentrations and a distinct fluctuation in the Hg methylation potential, gauged by the ratio of MeHg to THg, in soil subjected to two different planting patterns over the period from 2012 to 2023. Specifically, Hg methylation potential and MeHg concentrations in soil under continuous rice cultivation witnessed a substantial reduction (P < 0.05), falling by 48.6 % and 41.0 %, respectively. In contrast, paddy-to-vegetable conversion exhibited a trend of increase, although there was no statistical significance (P > 0.05). The decrease in continuous rice cultivation Hg methylation potential can be attributed not only to the pollution source blocking but also to the rise in molecular inertness of dissolved organic matter and a reduction in soil organic matter. Overall, with the control of pollution, the leaching of irrigation water and the humification of organic matter, Hg pollution in the soil is gradually decreasing, and the Hg methylation potential is also slowly weakening.