Fate of mercury in tree litter during decomposition

Abstract

We performed a controlled laboratory litter incubation study to assess changes in dry mass, carbon (C) mass and concentration, mercury (Hg) mass and concentration, and stoichiometric relations between elements during decomposition. Twenty-five surface litter samples each, collected from four forest stands, were placed in incubation jars open to the atmosphere, and were harvested sequentially at 0, 3, 6, 12, and 18 months. Using a mass balance approach, we observed significant mass losses of Hg during decomposition (5 to 23% of initial mass after 18 months), which we attribute to gaseous losses of Hg to the atmosphere through a gas-permeable filter covering incubation jars. Percentage mass losses of Hg generally were less than observed dry mass and C mass losses (48% to 63% Hg loss per unit dry mass loss), although one species showed similar losses. A field control study using the same litter types exposed at the original collection locations for one year showed that field litter samples were enriched in Hg concentrations by 8 to 64% compared to samples incubated for the same time period in the laboratory, indicating strong additional sorption of Hg in the field. Solubility of Hg, assessed by exposure of Hg to water upon harvest, was very low (< 0.22 ng Hg g<sup>−1</sup> dry mass) and decreased with increasing stage of decomposition for all litter types. Our results indicate large gaseous emissions, or re-emissions, of Hg originally associated with plant litter upon decomposition. Results also suggest that Hg accumulation in litter and surface layers in the field is driven mainly by sorption of Hg – such as from atmospheric deposition – with minor contributions from "internal" accumulation due to preferential loss of C over Hg. Litter types showed highly species-specific differences in Hg levels during decomposition – suggesting that emissions, retention, and sorption of Hg are dependent on litter type

Effects of vegetation type on mercury concentrations and pools in two adjacent coniferous and deciduous forests

“…Increasing Hg concentrations through decomposing litter are in agreement with other studies: in litterdecomposition studies, Hall and St. Louis (2004) and Demers et al (2007) observed pronounced Hg-concentration increases in forest litter decomposing in the field. Pokharel and Obrist (2011) observed similar Hg-concentration increases during litter decomposition in the field and attributed Hg-concentration increases in the field mainly to sorption of additional atmospheric deposition. Three possible reasons may explain more pronounced Hg-concentration increases in Douglas fir litter compared to red alder.…”

Effects of Precipitation on Mercury Accumulation on Subtropical Montane Forest Floor: Implications on Climate Forcing

“…Two possible reason can explain the Hg loss during litter decomposition at lower altitudes. One is Hg 0 evasion due to the C and N microbial mineralization during litter decomposition (D. Obrist et al, ; Pokharel & Obrist, ; Strauss et al, ). The other is Hg leaching with the formed dissolved organic carbons (Pokharel & Obrist, ; X. Wang, Lin, et al, ).…”

“…A more plausible explanation is that the Hg source of uptake by decomposing litter is from throughfall (Demers et al, ; Pokharel & Obrist, ; X. Wang, Lin, et al, ). However, Hg isotopic signatures in throughfall samples have not been reported.…”

Mercury isotope compositions across North American forests

“…There are several possible explanations for these increases including "internal enrichment" (i.e., preferential loss of litter mass over the loss of Hg Pokharel and Obrist, 2011] and fungal translocation [Demers et al, 2007]) and/or external (additional) Hg inputs from atmospheric deposition [Hall and St. Louis, 2004;Demers et al, 2007;Obrist et al, 2011;Pokharel and Obrist, 2011;Blackwell and Driscoll, 2015a]. It has been argued that preferential mass loss of litter compared to Hg alone cannot fully account for the increase of THg and Hg/C based on litter decomposition experiments [Pokharel and Obrist, 2011]. Therefore, additional Hg sources besides litterfall are required to explain the increase of THg in decomposing litter layers.…”