Impacts of Spectral Characteristics of Dissolved Organic Matter on Methyl Mercury Contents in Permafrost Wetlands, Northeast China

Dissolved organic matter (DOM) plays an important role in promoting or suppressing methylmercury (MeHg) production in wetlands. However, in the context of climate warming, the regulatory mechanism of DOM composition and molecular structure changes in permafrost wetland soil on mercury methylation remains unclear. In this study, we analyzed the distribution characteristics of mercury and methylmercury in permafrost wetland soils in the Greater Khingan Mountains (GKM), and elucidated the driving mechanism of mercury methylation by basic physical and chemical properties and DOM spectral characteristics of soils. The results showed that the mean value of total mercury in the permafrost wetlands of the GKM was 111 ng·g^-1, the mean value of methylmercury was 5.69 ng·g^-1, and the mean percentage of methylmercury was 6.16%. Hg and MeHg concentrations showed different vertical distribution patterns in the four wetlands, with Hg and MeHg concentrations in both scrub and moss wetlands showing a decreasing trend with soil deepening, but herb and forest wetlands did not satisfy this pattern. Soil warming, associated with the decomposition and mineralization of Soil Organic Matter (SOM), induces an elevation in Dissolved Organic Matter (DOM) content, subsequently contributing to increased levels of mercury methylation and an upswing in methylmercury output. Throughout this process, the spectral properties of DOM play a pivotal role in regulating Hg methylation, with the input of plant sources following closely behind. The content of MeHg in the soil is minimally influenced by the mercury content in the soil.