Using Mercury Isotopes To Understand Mercury Accumulation in the Montane Forest Floor of the Eastern Tibetan Plateau

Abstract

Mercury accumulation in montane forested areas plays an important role in global Hg cycling. In this study, we measured stable Hg isotopes in soil and litter samples to understand Hg accumulation on the forest floor along the eastern fringe of the Tibetan Plateau (TP). The low atmospheric Hg inputs lead to the small Hg pool size (23 ± 9 mg m^-2 in 0-60 cm soil horizon), up to 1 order of magnitude lower than those found at sites in Southwest China, North America, and Europe. The slightly negative Δ¹⁹⁹Hg (-0.12 to -0.05‰) in the litter at low elevations (3100 to 3600 m) suggests an influence of local anthropogenic emissions, whereas the more significant negative Δ¹⁹⁹Hg (-0.38 to -0.15‰) at high elevations (3700 to 4300 m) indicates impact from long-range transport. Hg input from litter is more important than wet deposition to Hg accumulation on the forest floor, as evidenced by the negative Δ¹⁹⁹Hg found in the surface soil samples. Correlation analyses of Δ¹⁹⁹Hg versus total carbon and leaf area index suggest that litter biomass production is a predominant factor in atmospheric Hg inputs to the forest floor. Precipitation and temperature show indirect effects on Hg accumulation by influencing litter biomass production in the eastern TP.