Deposition of Mercury in Forests along a Montane Elevation Gradient

Blackwell, Bradley D.; Driscoll, Charles T.

doi:10.1021/es505928w

Cited by 85 publications

(84 citation statements)

References 48 publications

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“…Although all our litter depth profiles are located in coniferous forests, similar enrichments of Hg concentrations in older litter has been observed in both coniferous and deciduous forests [ Hall and St . Louis , ; Demers et al ., ; Obrist et al ., , ; Blackwell et al ., ; Blackwell and Driscoll , ].…”

Section: Resultsmentioning

confidence: 99%

Mercury isotope compositions across North American forests

Zheng

Obrist

Weis

et al. 2016

Global Biogeochemical Cycles

184

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Forest biomass and soils represent some of the largest reservoirs of actively cycling mercury (Hg) on Earth, but many uncertainties exist regarding the source and fate of Hg in forest ecosystems. We systematically characterized stable isotope compositions of Hg in foliage, litter, and mineral soil horizons across 10 forest sites in the contiguous United States. The mass‐independent isotope signatures in all forest depth profiles are more consistent with those of atmospheric Hg(0) than those of atmospheric Hg(II), indicating that atmospheric Hg(0) is the larger source of Hg to forest ecosystems. Within litter horizons, we observed significant enrichment in Hg concentration and heavier isotopes along the depth, which we hypothesize to result from additional deposition of atmospheric Hg(0) during litter decomposition. Furthermore, Hg isotope signatures in mineral soils closely resemble those of the overlying litter horizons suggesting incorporation of Hg from litter as a key source of soil Hg. The spatial distribution of Hg isotope compositions in mineral soils across all sites is modeled by isotopic mixing assuming atmospheric Hg(II), atmospheric Hg(0), and geogenic Hg as major sources. This model shows that northern sites with higher precipitation tend to have higher atmospheric Hg(0) deposition than other sites, whereas drier sites in the western U.S. tend to have higher atmospheric Hg(II) deposition than the rest. We attribute these differences primarily to the higher litterfall Hg input at northern wetter sites due to increased plant productivity by precipitation. These results allow for a better understanding of Hg cycling across the atmosphere‐forest‐soil interface.

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Section: Resultsmentioning

confidence: 99%

Mercury isotope compositions across North American forests

Zheng

Obrist

Weis

et al. 2016

Global Biogeochemical Cycles

184

195

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“…At Whiteface, the summit has been estimated to be covered by clouds for 40–45% of the year, with significant cloud coverage also found in the coniferous zone [ Mohnen , ]. Consequently, previous studies have found cloudwater contribution to THg deposition at Whiteface to be the most important input of Hg to the alpine zone and a comparable input to throughfall Hg deposition in the coniferous zone [ Blackwell and Driscoll , ]. We also find cloudwater to be an important contributor in alpine and coniferous zones.…”

Section: Discussionmentioning

confidence: 99%

“…Additionally, conifers are more efficient at filtering atmospheric Hg particles than deciduous trees due to greater surface roughness, high leaf area index, and a canopy structure that decreases air flow and thereby enhances particle adsorption [ Kolka et al, ; Rea et al, ; Johnson et al, ; Witt et al, ]. As a result, THg inputs via throughfall are typically the predominant source of THg to coniferous forests [ Demers et al, ], which is consistent with our results and previous work at Whiteface [ Blackwell and Driscoll , ]. Throughfall inputs of THg to alpine zones, however, are reduced due to lower total leaf area associated with sparse tree density and lower canopy height.…”

Section: Discussionmentioning

confidence: 99%

“…Forest communities on Whiteface Mountain consist of three major zones: deciduous, coniferous, and alpine [ Miller et al, ; Blackwell and Driscoll , ]. The deciduous forest zone is located at low elevations of 400 to 900 m, has a mean canopy height ranging from 8.3 to 11.8 m, and is dominated by sugar maple ( Acer saccharum ), yellow birch ( Betula alleghaniensis ), red maple ( Acer rubrum ), and American beech ( Fagus grandifolia ).…”

Section: Methodsmentioning

confidence: 99%

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Deposition of mercury in forests across a montane elevation gradient: Elevational and seasonal patterns in methylmercury inputs and production

et al. 2017

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Global mercury contamination largely results from direct primary atmospheric and secondary legacy emissions, which can be deposited to ecosystems, converted to methylmercury, and bioaccumulated along food chains. We examined organic horizon soil samples collected across an elevational gradient on Whiteface Mountain in the Adirondack region of New York State, USA to determine spatial patterns in methylmercury concentrations across a forested montane landscape. We found that soil methylmercury concentrations were highest in the midelevation coniferous zone (0.39 ± 0.07 ng/g) compared to the higher elevation alpine zone (0.28 ± 0.04 ng/g) and particularly the lower elevation deciduous zone (0.17 ± 0.02 ng/g), while the percent of total mercury as methylmercury in soils decreased with elevation. We also found a seasonal pattern in soil methylmercury concentrations, with peak methylmercury values occurring in July. Given elevational patterns in temperature and bioavailable total mercury (derived from mineralization of soil organic matter), soil methylmercury concentrations appear to be driven by soil processing of ionic Hg, as opposed to atmospheric deposition of methylmercury. These methylmercury results are consistent with spatial patterns of mercury concentrations in songbird species observed from other studies, suggesting that future declines in mercury emissions could be important for reducing exposure of mercury to montane avian species.

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“…Laboratory studies have suggested that plants are a net sink for atmospheric Hg through leaf assimilation (Millhollen et al, 2006a;Stamenkovic and Gustin, 2009;Rutter et al, 2011b;Cui et al, 2014). Using Hg concentrations in plant tissues and net primary productivity as a proxy for atmospheric Hg deposition, Obrist (2007) (Blackwell and Driscoll, 2015a, b;Obrist et al, 2011); and ∼ 90 % boreal forest soil Hg was believed to be originated from litterfall input (Jiskra et al, 2015). These studies suggest that the forest ecosystem is likely a large atmospheric Hg sink, although these bulk proxy methods are not sufficiently sophisticated to resolve the global Hg mass balances.…”

Section: Fluxes Over Vegetated Surfaces Likely a Sink But Large Uncermentioning

confidence: 99%

Global observations and modeling of atmosphere–surface exchange of elemental mercury: a critical review

et al. 2016

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Abstract. Reliable quantification of air-surface fluxes of elemental Hg vapor (Hg 0 ) is crucial for understanding mercury (Hg) global biogeochemical cycles. There have been extensive measurements and modeling efforts devoted to estimating the exchange fluxes between the atmosphere and various surfaces (e.g., soil, canopies, water, snow, etc.) in the past three decades. However, large uncertainties remain due to the complexity of Hg 0 bidirectional exchange, limitations of flux quantification techniques and challenges in model parameterization. In this study, we provide a critical review on the state of science in the atmosphere-surface exchange of Hg 0 . Specifically, the advancement of flux quantification techniques, mechanisms in driving the air-surface Hg exchange and modeling efforts are presented. Due to the semi-volatile nature of Hg 0 and redox transformation of Hg in environmental media, Hg deposition and evasion are influenced by multiple environmental variables including seasonality, vegetative coverage and its life cycle, temperature, light, moisture, atmospheric turbulence and the presence of reactants (e.g., O 3 , radicals, etc.). However, the effects of these processes on flux have not been fundamentally and quantitatively determined, which limits the accuracy of flux modeling.We compile an up-to-date global observational flux database and discuss the implication of flux data on the global Hg budget. Mean Hg 0 fluxes obtained by micrometeorological measurements do not appear to be significantly greater than the fluxes measured by dynamic flux chamber methods over unpolluted surfaces (p = 0.16, one-tailed, Mann-Whitney U test). The spatiotemporal coverage of existing Hg 0 flux measurements is highly heterogeneous with large data gaps existing in multiple continents (Africa, South Asia, Middle East, South America and Australia). The magnitude of the evasion flux is strongly enhanced by human activities, particularly at contaminated sites. Hg 0 flux observations in East Asia are comparatively larger in magnitude than the rest of the world, suggesting substantial re-emission of previously deposited mercury from anthropogenic sources. The Hg 0 exchange over pristine surfaces (e.g., background soil and water) and vegetation needs better constraints for global analyses of the atmospheric Hg budget. The existing knowledge gap and the associated research needs for future measurements and modeling efforts for the air-surface exchange of Hg 0 are discussed.

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Deposition of Mercury in Forests along a Montane Elevation Gradient

Cited by 85 publications

References 48 publications

Mercury isotope compositions across North American forests

Mercury isotope compositions across North American forests

Deposition of mercury in forests across a montane elevation gradient: Elevational and seasonal patterns in methylmercury inputs and production

Global observations and modeling of atmosphere–surface exchange of elemental mercury: a critical review

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