title: Mercury deposition and re-emission pathways in boreal forest soils investigated with Hg isotope signatures.
abstract: Soils comprise the largest terrestrial mercury (Hg) pool in exchange with the atmosphere. To predict how anthropogenic emissions affect global Hg cycling and eventually human Hg exposure, it is crucial to understand Hg deposition and re-emission of legacy Hg from soils. However, assessing Hg deposition and re-emission pathways remains difficult because of an insufficient understanding of the governing processes. We measured Hg stable isotope signatures of radiocarbon-dated boreal forest soils and modeled atmospheric Hg deposition and re-emission pathways and fluxes using a combined source and process tracing approach. Our results suggest that Hg in the soils was dominantly derived from deposition of litter (∼90% on average). The remaining fraction was attributed to precipitation-derived Hg, which showed increasing contributions in older, deeper soil horizons (up to 27%) indicative of an accumulation over decades. We provide evidence for significant Hg re-emission from organic soil horizons most likely caused by nonphotochemical abiotic reduction by natural organic matter, a process previously not observed unambiguously in nature. Our data suggest that Histosols (peat soils), which exhibit at least seasonally water-saturated conditions, have re-emitted up to one-third of previously deposited Hg back to the atmosphere. Re-emission of legacy Hg following reduction by natural organic matter may therefore be an important pathway to be considered in global models, further supporting the need for a process-based assessment of land/atmosphere Hg exchange.
authors.affiliation: †Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland.|‡Isotope Geochemistry Group, Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Jiskra
authors.affiliation: †Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland.|‡Isotope Geochemistry Group, Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland.
authors.forename: Jan G
authors.initials: JG
authors.lastname: Wiederhold
authors.affiliation: §Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Skogmarksgränd, 90183 Umeå, Sweden.
authors.forename: Ulf
authors.initials: U
authors.lastname: Skyllberg
authors.affiliation: §Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Skogmarksgränd, 90183 Umeå, Sweden.
authors.forename: Rose-Marie
authors.initials: RM
authors.lastname: Kronberg
authors.affiliation: ∥Laboratory of Ion Beam Physics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland.
authors.forename: Irka
authors.initials: I
authors.lastname: Hajdas
authors.affiliation: †Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland.
authors.forename: Ruben
authors.initials: R
authors.lastname: Kretzschmar
mesh_terms: D005591:Chemical Fractionation; D008628:Mercury / Q000032:analysis*; D008629:Mercury Isotopes; D008962:Models, Theoretical; D008970:Molecular Weight; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis; D066188:Taiga; D013997:Time Factors
chemical_list: D008629:Mercury Isotopes; D012987:Soil; D012989:Soil Pollutants; D008628:Mercury