Hydrological processes significantly increase mercury-related ecological risk in karst paddy ecosystem: Evidence from mercury isotope.

Mercury (Hg) transported by runoff poses potential ecological risks to aquatic ecosystem. However, its mechanism remains unclear. This study investigated a typical karst paddy ecosystem in southwestern China. By analyzing isotopic compositions of inorganic Hg (IHg) and methylmercury (MeHg) across different environmental matrices, we clarified the significant impact of hydrological processes on Hg migration and accumulation in paddy ecosystem. Rainfall-driven runoff transported Hg from hillslope soils into irrigation water, with hillslope soils-derived Hg accounting for 72.6% of the total Hg in irrigation water. Irrigation water subsequently contributed 47.6% of the IHg to pore water in paddy soil. Importantly, pore water served as the primary source of IHg in rice grain and also supplied the dominant source of bioavailable Hg for methylation in paddy soil, thereby facilitating MeHg accumulation in rice grain. These coupled Hg migration and transformation processes resulted in elevated Hg accumulation in rice grain. Therefore, the distinct geological and hydrological processes in karst regions can accelerate Hg migration and enhance Hg bioavailability in paddy ecosystem, thereby significantly increasing Hg loading and risk in these systems. This study provided new insights into Hg-related risks driven by natural processes in agricultural ecosystems and offered guidance for developing targeted mitigation strategies.