Isotopic Characterization of Mercury Atmosphere-Foliage and Atmosphere-Soil Exchange in a Swiss Subalpine Coniferous Forest.

To understand the role of vegetation and soil in regulating atmospheric Hg, exchange fluxes and isotope signatures of Hg were characterized using a dynamic flux bag/chamber at the atmosphere-foliage/soil interfaces at the Davos-Seehornwald forest, Switzerland. The foliage was a net Hg sink and took up preferentially the light Hg isotopes, consequently resulting in large shifts (-3.27‰) in δHg values. The soil served mostly as net sources of atmospheric Hg with higher Hg emission from the moss-covered soils than from bare soils. The negative shift of δHg and ΔHg values of the efflux air relative to ambient air and the ΔHg/ΔHg ratio among ambient air, efflux air, and soil pore gas highlight that Hg re-emission was strongly constrained by soil pore gas evasion together with microbial reduction. The isotopic mass balance model indicates 8.4 times higher Hg emission caused by pore gas evasion than surface soil photoreduction. Deposition of atmospheric Hg to soil was noticeably 3.2 times higher than that to foliage, reflecting the high significance of the soil to influence atmospheric Hg isotope signatures. This study improves our understanding of Hg atmosphere-foliage/soil exchange in subalpine coniferous forests, which is indispensable in the model assessment of forest Hg biogeochemical cycling.