Cycling of elemental mercury vapor controls the sink and isotopic fractionation of atmospheric mercury in forest ecosystems

Significant knowledge gaps exist in the fate of mercury (Hg) and its isotopic fractionation in forests and limit the understanding of the global Hg mass budget. This study, for the first time, conducted a whole-ecosystem Hg isotope study to depict the Hg biogeochemical processes in a subtropical evergreen forest. Results show that atmospheric Hg⁰ is the primary source of Hg in foliage, woody biomass, throughfall water, runoff water and the food chains of birds. The studied subtropical evergreen forest is an atmospheric Hg⁰ sink of 57.6±43.9 μg m^-2 year^-1, and an atmospheric Hg²⁺ sink of 11.5± 6.2 μg m^-2 year^-1. The Hg mass-dependent fractionation Hg⁰ driven by the biogeochemical processes leads to a -0.69±0.58‰ in δ²⁰²Hg of atmospheric Hg⁰ but an insignificant shift for Δ¹⁹⁹Hg. This study provides a protocol for quantifying the atmospheric Hg⁰ and Hg²⁺ sink over the whole forest ecosystem and the impact of vegetation on Hg⁰ isotopic shift; and demonstrates the use of stable Hg isotopes in tracing atmospheric Hg cycle in terrestrial ecosystems.