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 Hg0, 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-0 sink and took up preferentially the light Hg isotopes, consequently resulting in large shifts (-3.27 parts per thousand ) in delta Hg-202 values. The soil served mostly as net sources of atmospheric Hg0 with higher Hg0 emission from the moss-covered soils than from bare soils. The negative shift of delta Hg-202 and Delta Hg-199 values of the efflux air relative to ambient air and the Delta Hg-199/Delta Hg-201 ratio among ambient air, efflux air, and soil pore gas highlight that Hg-0 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-0 emission caused by pore gas evasion than surface soil photoreduction. Deposition of atmospheric Hg-0 to soil was noticeably 3.2 times higher than that to foliage, reflecting the high significance of the soil to influence atmospheric Hg-0 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.