Mosses and lichens enhance atmospheric elemental mercury deposition in a subtropical montane forest†

Abstract

Environmental context Mosses and lichens are widely distributed in montane forests and their important role in global biogeochemical cycles has been increasingly recognised. This study of mercury accumulation and sources in mosses and lichens, using mercury isotopic data, provides evidence that they promote atmospheric mercury deposition in these forests, which is an important function that should be incorporated into current mercury mass balance budgets for forests. Rationale Mosses and lichens, which are widely distributed in montane forests, are often used to monitor the atmospheric mercury (Hg) depositions. In this study we hypothesised that atmospheric Hg uptake by mosses and lichens could greatly promote Hg depositions in montane forests. Methodology We comprehensively determined the Hg concentration and isotopic signatures of various species of mosses and lichens in a subtropical montane forest, to quantify the Hg accumulation, influencing factors and potential Hg depositions induced by mosses and lichens. Results Our results show that the higher Hg concentrations in mosses than in lichens are mainly due to their species-specific, morphological and physiological differences. Hg isotopic mixing model results display that uptake of atmospheric elemental Hg (Hg0) contributes 89.2 ± 22.8% of Hg in mosses and 88.4 ± 24.4% in lichens. The lichens growing on trees have a lower atmospheric Hg0 source contribution than on the ground (61.3 ± 42.5% versus 93.6 ± 10.0%) because of the elevated rainfall Hg uptake on trees. The Hg storage in live moss and lichen is 28.0 ± 16.5 and 0.9 ± 1.0 μg m−2, respectively. Given the 1–2-year lifespan of moss, the moss induced atmospheric Hg deposition is almost equivalent to litterfall Hg deposition which was previously used as a proxy for atmospheric Hg0 deposition in forests. Discussion Overall, we suggest mosses and lichens play an important role in atmospheric Hg depositions and recommend more research in montane forests.