Review on mercury transformations and trophic transfer in upland forest ecosystems.

Mercury (Hg), a global pollutant of widespread concern, poses significant threats to environmental and ecological health. Upland ecosystems are critical in the global Hg cycle, serving as major long-term sinks for atmospheric Hg, largely through vegetation mediated dry deposition. Despite this importance, substantial knowledge gaps remain on the global distribution patterns of Hg, particularly its more neurotoxic organic form, methylmercury (MeHg), in upland soils and the mechanisms governing its formation and fate. This review discusses current understanding of the global spatial pattern of Hg in upland soils and examines the controlling factors of Hg transformation under upland conditions. We propose that the pronounced spatial heterogeneity in soil Hg levels in upland ecosystems is primarily governed by macroscale drivers, including climate, topography, and vegetation type. Furthermore, the net MeHg concentration accumulated in surface soils is regulated by microscale biogeochemical and microbial processes, such as organic matter quantity and composition, redox dynamics and the microbial community structure. Here, we integrate nearly 50 existing published studies to investigate the distribution pattern of total Hg (THg) in upland soils and elucidates the drivers controlling the processes of methylation and demethylation, providing novel insight into Hg biogeochemical cycling in upland ecosystems and develops a conceptual foundation for future research aimed at assessing ecological risks and mitigating the impact of global Hg pollution.