A synthesis of terrestrial mercury in the western United States: Spatial distribution defined by land cover and plant productivity

Abstract

A synthesis of published vegetation mercury (Hg) data across 11 contiguous states in the western United States showed that aboveground biomass concentrations followed the order: leaves (26μgkg(-1))~branches (26μgkg(-1))>bark (16μgkg(-1))>bole wood (1μgkg(-1)). No spatial trends of Hg in aboveground biomass distribution were detected, which likely is due to very sparse data coverage and different sampling protocols. Vegetation data are largely lacking for important functional vegetation types such as shrubs, herbaceous species, and grasses. Soil concentrations collected from the published literature were high in the western United States, with 12% of observations exceeding 100μgkg(-1), reflecting a bias toward investigations in Hg-enriched sites. In contrast, soil Hg concentrations from a randomly distributed data set (1911 sampling points; Smith et al., 2013a) averaged 24μgkg(-1) (A-horizon) and 22μgkg(-1) (C-horizon), and only 2.6% of data exceeded 100μgkg(-1). Soil Hg concentrations significantly differed among land covers, following the order: forested upland>planted/cultivated>herbaceous upland/shrubland>barren soils. Concentrations in forests were on average 2.5 times higher than in barren locations. Principal component analyses showed that soil Hg concentrations were not or weakly related to modeled dry and wet Hg deposition and proximity to mining, geothermal areas, and coal-fired power plants. Soil Hg distribution also was not closely related to other trace metals, but strongly associated with organic carbon, precipitation, canopy greenness, and foliar Hg pools of overlying vegetation. These patterns indicate that soil Hg concentrations are related to atmospheric deposition and reflect an overwhelming influence of plant productivity - driven by water availability - with productive landscapes showing high soil Hg accumulation and unproductive barren soils and shrublands showing low soil Hg values. Large expanses of low-productivity, arid ecosystems across the western U.S. result in some of the lowest soil Hg concentrations observed worldwide.

Mercury isotope compositions across North American forests

“…While this observation seems to be opposite to the expectation that higher precipitation would deliver more Hg(ox) via wet deposition, precipitation has been shown to increase plant productivity (including litterfall) in forests [ Lonsdale , ; Benfield , ; Kay et al ., ]. A recent study on soil Hg distribution patterns across the western U.S. showed that soil Hg concentrations strongly follow land cover types and argued that this is due to an overwhelming effect of plant productivity on soil Hg accumulation [ Obrist et al ., ]. Our observed spatial distribution in Hg isotopes is also consistent with wetter sites receiving higher proportions of atmospheric Hg(0) deposition due to higher plant productivity and litterfall.…”

“…Our isotope data further suggest that incorporation of litter Hg is the major source of Hg to upper mineral soil. These conclusions are consistent with the current understanding on the importance of litterfall flux to forest floor and soils [ Obrist et al ., ; Wang et al ., ]. In addition, we observed strong enrichment of Hg concentration accompanied by positive shifts of δ 202 Hg and invariant or decreasing MIF with increasing depth in litter horizons, which cannot be fully explained by litterfall, atmospheric Hg(ox) deposition, or reductive Hg losses based on our current understanding on Hg isotope fractionation and source isotope signatures.…”

Methylmercury Dynamics in Upper Sacramento Valley Rice Fields with Low Background Soil Mercury Levels

“…Butte and Yolo soil THg content (25 ± 7 and 57 ± 4 ng g −1 , respectively) were similar to background soil THg reported for the United States and China (Shacklette and Boerngen, 1984;Mingcai and Qinghua, 1997;Obrist et al, 2016), and five to ten times lower than reported for fields in the Delta . Prior studies suggest that MeHg production can be limited when soil THg is <1000 ng g −1 (Rudd et al, 1983;Krabbenhoft et al, 1999), as is the case in Butte and Yolo.…”

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

“…Recent studies suggested a Hg distribution defined by land cover and water‐limited plant productivity via the analysis of correlations among soil Hg concentration, leaf area index, and Hg/C ratios (D. Obrist et al, ; X. Wang, Luo, et al, ; Zheng et al, ). We combined comprehensive field observations and Hg isotopic signatures along with windward and leeward slopes of Mt.…”