Mercury isotopes in a forested ecosystem: Implications for air-surface exchange dynamics and the global mercury cycle

Forests mediate the biogeochemical cycling of mercury (Hg) between the atmosphere and terrestrial ecosystems; however, there remain many gaps in our understanding of these processes. Our objectives in this study were to characterize Hg isotopic composition within forests, and use natural abundance stable Hg isotopes to track sources and reveal mechanisms underlying the cycling of Hg. We quantified the stable Hg isotopic composition of foliage, forest floor, mineral soil, precipitation, and total gaseous mercury (THg(g)) in the atmosphere and in evasion from soil, in 10-year-old aspen forests at the Rhinelander FACE experiment in northeastern Wisconsin, USA. The effect of increased atmospheric CO2 and O-3 concentrations on Hg isotopic composition was small relative to differences among forest ecosystem components. Precipitation samples had delta Hg-202 values of -0.74 to 0.06 parts per thousand and Delta Hg-199 values of 0.16 to 0.82 parts per thousand. Atmospheric THg(g) had delta Hg-202 values of 0.48 to 0.93 parts per thousand and Delta Hg-199 values of -0.21 to -0.15 parts per thousand. Uptake of THg(g) by foliage resulted in a large (-2.89 parts per thousand) shift in delta Hg-202 values; foliage displayed delta Hg-202 values of -2.53 to -1.89 parts per thousand and Delta Hg-199 values of -0.37 to -0.23 parts per thousand. Forest floor samples had delta Hg-202 values of -1.88 to -1.22% and Delta Hg-199 values of -0.22 to -0.14%. Mercury isotopes distinguished geogenic sources of Hg and atmospheric derived sources of Hg in soil, and showed that precipitation Hg only accounted for similar to 16% of atmospheric Hg inputs. The isotopic composition of Hg evasion from the forest floor was similar to atmospheric THg(g); however, there were systematic differences in delta Hg-202 values and MIF of even isotopes (Delta Hg-200 and Delta Hg-204). Mercury evasion from the forest floor may have arisen from air-surface exchange of atmospheric THg(g), but was not the emission of legacy Hg from soils, nor re-emission of wet-deposition. This implies that there was net atmospheric THg(g) deposition to the forest soils. Furthermore, MDF of Hg isotopes during foliar uptake and air-surface exchange of atmospheric THg(g) resulted in the release of Hg with very positive delta Hg-202 values to the atmosphere, which is key information for modeling the isotopic balance of the global mercury cycle, and may indicate a shorter residence time than previously recognized for the atmospheric mercury pool.