Coupling of Mercury Contamination and Carbon Emissions in Rice Paddies: Methylmercury Dynamics versus CO2 and CH4 Emissions

Abstract

Methylmercury (MeHg) accumulation in rice grains and greenhouse gas emissions are significant environmental concerns in rice paddy ecosystems. Dynamic of MeHg in paddy soils are likely interacted with the emissions of methane (CH₄) and carbon dioxide (CO₂), given the involvement of methanogenesis and organic matter mineralization in mercury (Hg) methylation, but poorly defined at present. Here, rice-growing pot experiments were performed with varying levels of paddy soil Hg to examine the interactions among CO₂ and CH₄ emissions and MeHg dynamics under variable Hg amendment scenarios. Mercury addition (20 mg kg^–1) significantly enhanced the cumulative emissions of CO₂ and CH₄ from a paddy system, and shifts in methanogen community explained the increased CH₄ emissions. In contrast, such enhancements were not observed at lower Hg addition levels. Under identical total Hg treatments, ecosystem-dependent negative correlations were observed between MeHg concentrations and C emissions during the rice growing period. The divergent kinetics associated with the production of MeHg, CO₂, and CH₄ likely explained the reason. In addition, methanogens mediated MeHg degradation as well as CH₄ production from oxidative demethylation may also contribute to the negative correlations. Overall, this study enhances our understanding of the complex interplay between C emissions and MeHg dynamics in rice paddy ecosystems.