Abstract

Methylmercury (MeHg) accumulation in rice is a major source of human MeHg exposure in some inland areas, yet the mechanism controlling mercury (Hg) accumulation in high geological background (HGB) regions remains poorly understood. Here, a field-scale remediation experiment was conducted in a karst HGB region of Guizhou, China, using a synergistic strategy combining selenium foliar spraying and calcium oxide-based soil conditioner. The combined treatment reduced total Hg and MeHg concentrations in rice grains by 63.0% and 80.0%, respectively. The root uptake from the soil–water system was found to be the primary pathway controlling Hg transfer into rice grain. Mechanistically, the soil conditioner increased soil pH and reduced bioavailable Hg, porewater Hg, and soil MeHg by 53.6%, 59.8%, and 62.6%, respectively, whereas selenium foliar spraying promoted Hg–Se complexation and reduced Hg mobility in the paddy system. In parallel, the combined treatment suppressed Hg-methylating microorganism (e.g., Geobacter decreased by 65%) and hgcAB gene (hgcA: −65%; hgcB: −58%), while enriching Hg-resistant taxa and merA-mediated detoxification pathway. These coupled physicochemical and microbial processes substantially reduced Hg bioavailability, MeHg production, and Hg bioaccumulation in rice grain of the paddy ecosystem, providing an effective strategy for mitigating Hg-related food safety risk in the karst HGB region.

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