Article

Underestimated Sink of Atmospheric Mercury in a Deglaciated Forest Chronosequence

Authors:
  • Institute of Geochemistry, Chinese Academy of Sciences
  • Institute of geochemistry Chinese academy of science
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Abstract

Mercury (Hg) deposition through litterfall has been regarded as the main input of gaseous elemental mercury (Hg0) into forest ecosystems. We hypothesize that earlier studies largely underestimated this sink because the contribution of Hg0 uptake by moss and the downward transport to wood and throughfall is overlooked. To test the hypothesis, we investigated the Hg fluxes contributed via litterfall and throughfall, Hg pool sizes in moss covers and woody biomass as well as their isotopic signatures in a glacier-to-forest succession ecosystem of the Southeast Tibetan Plateau. Results show that Hg0 depositional uptake by and pool sizes stored in moss and woody biomass increase rapidly with the time after glacier retreat. Using the flux data as input to a Hg isotopic mixing model, Hg deposition through litterfall accounts for 27–85% of the total accumulation rate of Hg0 in organic soils of sites glacial retreat 20 to 90 years, revealing the presence of additional sources of Hg0 input. Atmospheric Hg0 accounts for 76±24% in ground moss, and 86±15% in tree moss, and 62–92% in above ground woody biomass (branch-bark-stem), and 44–83% in roots. The downward decreasing gradient of atmospheric Hg0 fractions from the aboveground woody biomass to roots suggests a foliage-to-root Hg transport in vegetation after uptake. Additionally, 34–82% of atmospheric Hg0 in throughfall further amplifies the accumulation of Hg0 from atmospheric sources. We conclude that woody biomass, moss and throughfall represent important Hg0 sinks in forest ecosystems. These previously unaccounted-for sink terms significantly increase the previously estimated atmospheric Hg0 sink via litterfall.

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... 3,4 Multiple lines of evidence show that dry deposition of gaseous elemental Hg (Hg(0)) via vegetation uptake, however, is the dominant Hg source to terrestrial environments whereby atmospheric Hg(0) is taken up by vegetation tissues and subsequently transferred and deposited to soils when tissues are shed (litterfall), plants die off (biomass turnover), and leaf surfaces are washed off (i.e., throughfall). 5,6 Global data analyses estimate global Hg deposition by litterfall in the range of 1020−1230 Mg yr −1 , and global throughfall deposition is considered in a similar range. 5,7−9 Global model simulations estimate total atmospheric deposition via vegetation in the range of 1310−1570 Mg yr −1 10 and 1400 Mg yr −1 , 11 constituting one of the largest global sinks of atmospheric Hg, e.g., exceeding global terrestrial wet deposition of 730−1070 Mg yr −1 . ...
... 10 Global estimates of Hg deposition associated with vegetation are commonly constrained using forest litterfall measurements, yet the use of litterfall alone to estimate vegetation Hg deposition may significantly underestimate deposition. 12 A study along a glacier-to-forest succession suggested that litterfall deposition underestimated total vegetation Hg deposition by 65% because it failed to account for assimilation of Hg(0) by mosses, inputs from woody tissues, and throughfall deposition, 6 and contributions by woody tissues and whole tree turnover have been estimated to enhance deposition by 60% compared to leaf litter deposition alone in forests. 13 A recent study showed that atmospheric Hg(0) deposition in a deciduous forest is three times greater than litterfall mercury deposition measurements. ...
... The term X% in eq 1 represents percentages of Hg derived from atmospheric Hg(0) and Hg(II) sources, as well as from geogenic uptake (i.e., derived from geologic substrates), based on stable Hg isotope studies by Wang et al. 6 and Zhou et al. 10 in forests and shrubland, by Mao et al. 30 in grassland, and by Yin et al. 31 in cropland tissues. Current source attributions to differentiate atmospheric Hg(0) and Hg(II) and geogenic sources in tissues rely on a large number of studies for foliage, which reveal consistent patterns of dominant atmospheric Hg(0) sources (summary by Zhou et al. 10 ). ...
... The fundamental assumption of the Hg dendrochemistry is that Hg accumulated in tree ring is derived from the atmospheric Hg sources, and that there is a linear correlation between variations of Hg in air and in tree ring. Earlier studies using the controlled dose-response experiments (Arnold et al., 2018) and Hg isotopes (Wang et al., 2020b) have identified that Hg in tree ring is mainly from foliage uptake of gaseous elemental mercury (Hg 0 ) and subsequent translocation by the phloem. Given the relatively well documented Hg concentration in tree ring, an examination of the relationship between tree-ring Hg concentration and atmospheric pollution level is beneficial to using available data for determining historical Hg pollution. ...
... Mercury undergoes both mass dependent fractionation (MDF, represented by δ 202 Hg) and mass independent fractionation (MIF,as Δ 199 Hg,Δ 200 Hg and Δ 201 Hg) during multiple processes occurring in forest ecosystems. Foliage uptake of atmospheric Hg 0 leads to a 2-3‰ MDF and lighter Hg isotopes accumulated in foliage (Wang et al., 2017(Wang et al., , 2020a(Wang et al., , 2020bYuan et al., 2020). The Hg odd-MIF signatures (Δ 199 Hg and Δ 201 Hg) in foliage is mainly inherited from atmospheric Hg 0 (Wang et al., 2017(Wang et al., , 2020a(Wang et al., , 2020bYuan et al., 2020) but slightly modified by the Hg re-emission after the reductive loss from foliage . ...
... Foliage uptake of atmospheric Hg 0 leads to a 2-3‰ MDF and lighter Hg isotopes accumulated in foliage (Wang et al., 2017(Wang et al., , 2020a(Wang et al., , 2020bYuan et al., 2020). The Hg odd-MIF signatures (Δ 199 Hg and Δ 201 Hg) in foliage is mainly inherited from atmospheric Hg 0 (Wang et al., 2017(Wang et al., , 2020a(Wang et al., , 2020bYuan et al., 2020) but slightly modified by the Hg re-emission after the reductive loss from foliage . Our recent work depicted that the MDF signatures in woody biomass vary among the tree components and species as a result of physiologically induced Hg accumulation and translocation, while the MIF does not occur because of little impact from physiological processes (Wang et al., 2020b). ...
Article
The accuracy of mercury (Hg) dendrochemistry has been questioned because significant knowledge gaps exist in understanding the Hg translocation and mobility in tree-ring. In this study, we evaluated Hg concentrations and isotopic profiles in the tree-ring at a Hg artisanal mining site and a control site with the documented local Hg production inventory. Results show that the Hg concentration accumulated in tree-ring fails to reconstruct the temporal trend of Hg production due to confounded tree physiological and environmental factors, specifically, the radial translocation and tree age effects occurring during the fast-growing period. The temporal profiles of δ²⁰²Hg exhibit pronounced tree-specific variabilities due to the complexity of Hg isotopic mass dependent fractionation during atmospheric Hg uptake and translocation in vegetation. The Hg odd-MIF (mass independent fractionation) profiles in tree-ring can reconstruct a decadal-scale temporal trend of the atmospheric Hg⁰ pollution level, and also be used as a tracer to distinguish the emission source shifts of atmospheric Hg⁰. However, the radial translocation would result in uncertainties at the higher resolution because of the mixing of odd-MIF signatures with active rings. Caution should be taken and additional supporting evidence collected from independent methods should be used for verifying the tree-ring records.
... Atmospheric Hg 0 uptake by vegetation then through litterfall deposition into the global terrestrial ecosystem is up to 1000-1200 Mg annually (Wang et al., 2016a). Recent studies suggested that using litterfall Hg deposition would underestimate the total atmospheric Hg 0 fluxes in the forests Wang et al., 2020b;Zhou and Obrist, 2021). Therefore, tracing Hg depositions and post-depositional sequestration in terrestrial ecosystems is critical for a complete understanding of global Hg cycling. ...
... The significantly negative odd-MIF signatures in our litterfall samples (Δ 199 Hg: − 0.23 ~ − 0.35‰) are comparable to the global average odd-MIF signatures in litterfall (− 0.30 ± 0.16‰, n = 123) and atmospheric Hg 0 (− 0.16 ± 0.11‰, n = 343) . This suggests that the tropical litterfall Hg is mainly derived from atmospheric Hg 0 uptake, consistent with that in other forest ecosystems (Jiskra et al., 2015;Wang et al., 2017Wang et al., , 2020aWang et al., , 2021Wang et al., , 2020bZheng et al., 2016). ...
... Fig. 6a clearly displays that the soil Hg odd-MIF signatures are close to values of litterfall, indicating the vegetation uptake of atmospheric Hg 0 dominating surface soil Hg sources. In addition, the throughfall Hg which also contains Hg derived from atmospheric Hg 0 sources (Wang et al., 2020b) can contribute to the tropical soil Hg accumulation. Finally, the previously deposited Hg re-emission is likely another important source for Hg accumulation in surface soil and litterfall. ...
Article
Tropical forest contributes to >50% of global litterfall mercury (Hg) inputs and surface soil Hg storage, while with limited understanding of Hg biogeochemical processes. In this study, we displayed the 5−m resolution of Hg spatial distribution in three 1-ha tropical forest plots across the latitudinal gradient in Southwest China, and determined Hg isotopic signatures to understand factors driving Hg spatial distribution and sequestration processes. Our results show that tropical forest at the lowest latitude has the highest litterfall Hg input (74.95 versus 34.14−56.59 μg m⁻² yr⁻¹ at higher latitude plots), but the smallest surface soil Hg concentration (2−3 times smaller than at higher latitude sites). Hg isotopic evidence indicates that the decreasing climate mediated microbial Hg reduction in forest floor leads to the increasing Hg accumulation along the latitudinal gradient in three tropical forests. The terrain induced indirect effects by influencing litterfall Hg inputs, soil organic matters distribution and interplays between surface and deep soils drive the heterogeneity of surface soil Hg distribution within each sampling plot. Our results highlight though the elevated litterfall Hg inputs, the distinct post-depositional reductions induced Hg loss would remarkedly decrease atmospheric Hg net sink in tropical forest.
... 22,23 Hence, the biodiversity and the complex 3D structure of canopies make it unfeasible to extrapolate net exchange processes based only on spatially confined branch-level observations in any representative manner to forest-atmosphere net exchange at an ecosystem scale, of which micrometeorological-based (MM) flux measurement methods covering a large footprint over the forest canopy are better suited. 24 It is also anticipated that further Hg 0 depositions occur via woody tissues, 25−27 cryptogamic covers, 9,28,29 and throughfall, 30,31 which is not accounted for by branch-level chamber measurements. 12,21,22 Using eddy covariance (EC, an MM technique), it has been established that the Mt. ...
... Article isotopes, it is becoming increasingly evident that atmospherically sourced Hg 0 participates not only in dry deposition/ uptake but also considerably in hydrological processes occurring in the canopy. 9,68 Interestingly, Hg in throughfall differs isotopically from that in wet deposition by significant negative shifts in δ 202 Hg, Δ 199 Hg, and Δ 200 Hg, which is consistent with mercury previously taken up as Hg 0 going into the solution when the substrate is wetted by precipitation rich in DOC. 9 Therefore, caution is warranted when scaling up Hg 0 fluxes measured with chambers to be representative of a large area. Interestingly, the common indirect method involving the sum of litterfall and throughfall minus bulk wet deposition as a proxy for total Hg dry deposition 69 and litterfall deposition as a proxy for Hg 0 dry deposition 70 also produces a substantial underestimation. ...
... 9,68 Interestingly, Hg in throughfall differs isotopically from that in wet deposition by significant negative shifts in δ 202 Hg, Δ 199 Hg, and Δ 200 Hg, which is consistent with mercury previously taken up as Hg 0 going into the solution when the substrate is wetted by precipitation rich in DOC. 9 Therefore, caution is warranted when scaling up Hg 0 fluxes measured with chambers to be representative of a large area. Interestingly, the common indirect method involving the sum of litterfall and throughfall minus bulk wet deposition as a proxy for total Hg dry deposition 69 and litterfall deposition as a proxy for Hg 0 dry deposition 70 also produces a substantial underestimation. ...
... An observed exception is that the root of wetland grass has a greater ability to take up soil Hg and transport to the aboveground parts (Meng et al., 2018). Recent observations using stable Hg isotope techniques (spike and natural stable isotopic compositions) support the translocation of Hg from foliage to stem via the phloem as the main pathway for Hg accumulation in aboveground woods (Graydon et al., 2009;Wang et al., 2020bWang et al., , 2021b. Due to the loose porous structure, bark has a strong absorption of atmospheric Hg, particularly for particle-bounded Hg, and therefore exhibit a higher Hg concentration compared to those found in wood (Hanson et al., 1997;Rodr ıguez Martin et al., 2018). ...
... Field observations have suggested that elevated Hg in throughfall is closely related to the pathway of rainfall scavenging deposited Hg 2þ and Hg P on canopy surface (Graydon et al., 2008;Hultberg et al., 1995;Rea et al., 1996Rea et al., , 2001St Louis et al., 2001). A recent study with the Hg isotopes has demonstrated that Hg can also be derived from the canopy epiphyte cover and tree detritus which formed by the degraded vegetation biomasses (Wang et al., 2020b). For stream/runoff Hg, the low Hg concentrations have been attributed to soil interception of particle-bounded Hg and selective absorption of dissolved Hg by organic matters when the water leaches through soil (Allan et al., 2001;Eklof et al., 2013;Lee et al., 2000;Lin et al., 2011;Schwesig & Matzner, 2000;Zhao et al., 2015). ...
... Though the Hg concentration in wood is one order of magnitude lower than that in foliage, the large biomass of aboveground wood leads to the Hg pool sizes 1 to 2 times greater than the foliage Hg pool sizes (Wang et al., 2020b;Yang et al., 2017). The soil Hg pool size in forests is much larger than the vegetation Hg pool size, with >90% Hg stored in soil (Navratil et al., 2009;Obrist et al., 2009Obrist et al., , 2011Wang et al., 2009Wang et al., , 2020b. ...
Article
Forest ecosystem accounts for 31% of global land areas and plays a key role in the global biogeochemical cycling of mercury (Hg). In this critical review, datasets of Hg flux measurements and Hg isotopic compositions in the environmental compartments of forests in the last three decades are synthesized to examine the budgets of Hg mass balance and storages. The primary goal of this synthesis is to provide insight into the source, transportation, translocation and fate of legacy Hg in forests. Existing data indicate that forests represent the largest atmospheric Hg sink in the terrestrial ecosystem, with atmospheric total Hg deposition of 2200–3400 Mg yr⁻¹ (i.e., relative to 40–65% atmospheric Hg pool size) and 500–1100 Gg of Hg stored in surface soils and vegetation. The climate and land cover changes, deforestation and wildfire re-volatilize several hundred tons of Hg into the atmosphere, thus increasing the ecological risk to the regional and global environments. Vegetative uptake of Hg⁰ vapor from air predominantly controls Hg accumulation and isotopic fractionation in the atmosphere and in global forests. With the ongoing Hg emission reduction from anthropogenic sources required by the Minamata Convention, an integrated assessment on the changing biogeochemical processes and isotopic fractionation in response to human and natural perturbations of emissions, climate, and land use is needed.
... For example, rice plants grown in contaminated soils showed root Hg with the same isotopic signature as the surrounding soil 110 , indicating root uptake. In contrast, substantial foliage-to-root Hg transport was observed in a forest, where atmospheric Hg(0) uptake via foliage accounted for 44-83% of Hg in tree roots 111 . In the latter study, large roots showed somewhat higher proportions of atmospheric Hg(0) compared with small roots (59% versus 64%) 111 , possibly related to lower surface areas and reduced absorptive potential of large roots 108,112 . ...
... In contrast, substantial foliage-to-root Hg transport was observed in a forest, where atmospheric Hg(0) uptake via foliage accounted for 44-83% of Hg in tree roots 111 . In the latter study, large roots showed somewhat higher proportions of atmospheric Hg(0) compared with small roots (59% versus 64%) 111 , possibly related to lower surface areas and reduced absorptive potential of large roots 108,112 . The role of atmospheric uptake in root Hg merits further detailed investigations, as this phenomenon would substantially increase estimates of plant Hg uptake from the atmosphere due to high turnover rates of roots, which could equal that of leaf litterfall 108 . ...
... Staple isotope analyses indicate that atmospheric Hg(0) accounts for 76% and 86% in ground and tree mosses, with the remaining 24% and 14% originating from Hg(II) contribution 111 . Hence, where lichens and mosses represent a significant component of plant communities, such as in the Arctic tundra, their high tissue concentrations are responsible for high atmospheric deposition loads via uptake of atmospheric Hg exceeding Hg deposition by vascular plants 2,58 . ...
Article
Mercury (Hg) is a global pollutant that emits in large quantities to the atmosphere (>6,000–8,000 Mg Hg per year) through anthropogenic activities, biomass burning, geogenic degassing and legacy emissions from land and oceans. Up to two-thirds of terrestrial Hg emissions are deposited back onto land, predominantly through vegetation uptake of Hg. In this Review, we assemble a global database of over 35,000 Hg measurements taken across 440 sites and synthesize the sources, distributions and sinks of Hg in foliage and vegetated ecosystems. Lichen and mosses show higher Hg concentrations than vascular plants, and, whereas Hg in above-ground biomass is largely from atmospheric uptake, root Hg is from combined soil and atmospheric uptake. Vegetation Hg uptake from the atmosphere and transfer to soils is the major Hg source in all biomes, globally accounting for 60–90% of terrestrial Hg deposition and decreasing the global atmospheric Hg pool by approximately 660 Mg. Moreover, it reduces the Hg deposition to global oceans, which, in the absence of vegetation, might receive an additional Hg deposition of 960 Mg per year. Vegetation uptake mechanisms need to be better constrained to understand vegetation cycling, and model representation of vegetation Hg cycling should be improved to quantify global vegetation impacts.
... Although data for shallow roots were reported in earlier studies, 3,8 the distribution of the Hg concentration in roots of various sizes has been largely unknown. 12,14 There are also mixed assessments regarding Hg sources in roots. The Hg concentration in global roots is in the range of 2−70 ng g −1 in the remote forest, up to an order of magnitude lower than the Hg concentration in surrounding soils. ...
... 26−28 Hg II in precipitation exhibits negative MDF and positive odd-MIF and even-MIF signals, 5,30−32 while Hg II from bedrock shows negative MDF and negligible odd-MIF and even-MIF signals. 33−36 Given the limited Hg mass translocated from foliage to other aboveground tissues, 14,37 we hypothesize that Hg in plant roots is predominantly derived from forest soil, rather than from foliage. In this study, the concentration of Hg, its vertical distribution, and the isotopic compositions of root Hg were investigated in a subtropical forest to understand the uptake, accumulation, and translocation of Hg in roots. ...
... Earlier studies showed that oven drying below 50°C did not lead to distinct Hg loss (<1%) from soil and woody biomass. 14 Hence, all soil and root samples were dried at 45°C in an oven for 3−4 days until there was no observable moisture weight loss. Finally, all soil samples were ground with an agate mortar and sieved with a 200-mesh (74 μm) nylon screen. ...
Article
Plant roots are responsible for transporting large quantities of nutrients in forest ecosystems and yet are frequently overlooked in global assessments of Hg cycling budgets. In this study, we systematically determined the distribution of total Hg mass and its stable isotopic signatures in a subtropical evergreen forest to elucidate sources of Hg in plant root tissues and the associated translocation mechanisms. Hg stored in roots and its isotopic signatures show significant correlations to those found in surrounding soil at various soil depths. The odd mass-independent fractionation (MIF) of root Hg at a shallow soil depth displays a −0.10‰ to −0.50‰ negative transition compared to the values in aboveground woody biomass. The evidence suggests that root Hg is predominantly derived from surrounding soil, rather than translocation of atmospheric uptake via aboveground tissues. The cortex has a more negative mass-dependent fractionation (MDF) of −0.10‰ to −1.20‰ compared to the soil samples, indicating a preferential uptake of lighter isotopes by roots. The similar MDF and odd-MIF signals found in root components imply limited Hg transport in roots. This work highlights that Hg stored in plant roots is not a significant sink of atmospheric Hg. The heterogeneous distribution of Hg mass in roots of various sizes represents a significant uncertainty of current estimates of Hg pool size in forest ecosystems.
... Using net daily CO 2 uptake to delineate periods of active vegetation growing periods (i.e., June 1 to October 1), the cumulative growing season dry deposition GEM sink amounted to 21.9 μg · m −2 (range of 20.9 to 23.1 μg · m −2 ) (Tables 1 and 2), which is 2.7-fold the value of litterfall mercury deposition, providing evidence that forest-level GEM uptake strongly exceeds litterfall deposition. This finding supports previous reports that large additional deposition fluxes in addition to foliar litterfall and wet deposition are needed to explain observed mercury accumulation in soils along a glacier retreat chronosequence (38). ...
... The forest GEM deposition sink at Harvard Forest may be enhanced by ongoing biomass growth and carbon sequestration driven by forest regrowth, climate warming, and increasing wetting and atmospheric CO 2 concentrations (16). Additional pathways of atmospheric GEM deposition include wash-off of GEM from plant surfaces and deposition via throughfall and stemflow deposition (38). A potentially larger and unaccounted for forest GEM sink will require major revision of other global pool sizes, atmospheric deposition and emission fluxes, and respective residence times in these environmental compartments. ...
Article
Full-text available
Significance Direct measurements of atmospheric deposition of gaseous elemental mercury (GEM) over a temperate forest showed a pronounced annual deposition of 25.1 µg ⋅ m ⁻² , which dominated as a source of mercury. GEM deposition was five times greater than wet deposition and three times greater than litterfall deposition, which has been used as a proxy for GEM deposition until now. Measured GEM deposition is driven by combined plant GEM uptake and underlying forest floor GEM uptake. Global forests may be a much larger global GEM sink than currently assumed, which may explain high mercury levels in soils across forests. Forest mercury mobilizes via watershed runoff and bioaccumulates in aquatic biota, ultimately leading to mercury exposures in wildlife and humans.
... In order to constrain future Hg levels in edible fish and to assess how Hg exposure responds to curbed anthropogenic Hg emissions under the policies implemented by the 2017 UN Minamata convention on mercury, it is essential to understand and quantify all major net deposition fluxes within the global Hg cycle. Wet deposition occurs when water-soluble oxidized Hg(II) is washed out from the atmosphere with rainwater (Driscoll et al., 2013;Sprovieri et al., 2017) or by cloud water interception (Weiss-Penzias et al., 2012). In a dry deposition process, gaseous elemental Hg(0) and Hg(II) directly bind to surfaces (Bishop et al., 2020), or Hg(0) is taken up by plants . ...
... Atmospheric Hg(0) taken up by vegetation is oxidized to Hg(II) within the plant tissue (Manceau et al., 2018) and transferred to soils via litterfall (Iverfeldt, 1991;Schwesig and Matzner, 2000;Rea et al., 2001;Graydon et al., 2008;Risch et al., 2012Risch et al., , 2017Jiskra et al., 2015;Wright et al., 2016;Wang et al., 2016). Moreover, in forests, Hg deposition to the ground may occur by wash-off of Hg(0) from plant surfaces via throughfall and by Hg(0) uptake into woody tissues, lichen, mosses, and soil litter (Wang et al., 2020;Obrist et al., 2021). Mercury sequestered by forest ecosystems accumulates in soil and may subsequently be transported from watersheds to streams, rivers, and the ocean, where it can bioaccumulate in fish (Drenner et al., 2013;Jiskra et al., 2017;Sonke et al., 2018). ...
Article
Despite the importance of vegetation uptake of atmospheric gaseous elemental mercury (Hg(0)) within the global Hg cycle, little knowledge exists on the physiological, climatic, and geographic factors controlling stomatal uptake of atmospheric Hg(0) by tree foliage. We investigate controls on foliar stomatal Hg(0) uptake by combining Hg measurements of 3569 foliage samples across Europe with data on tree species' traits and environmental conditions. To account for foliar Hg accumulation over time, we normalized foliar Hg concentration over the foliar life period from the simulated start of the growing season to sample harvest. The most relevant parameter impacting daily foliar stomatal Hg uptake was tree functional group (deciduous versus coniferous trees). On average, we measured 3.2 times higher daily foliar stomatal Hg uptake rates in deciduous leaves than in coniferous needles of the same age. Across tree species, for foliage of beech and fir, and at two out of three forest plots with more than 20 samples, we found a significant (p
... An elevated atmospheric Hg 0 concentration enhances Hg 0 deposition flux, potentially That leads us to examine the influence of tree physiological processes that control foliage uptake of atmospheric Hg 0 and its subsequent accumulation and translocation. The important physiological parameters include but are not limited to the inconsistent tree growth, rate of photosynthesis, stomata conductance, transpiration rate, foliage lifespan and canopy characteristics (Arnold et al., 2018;Ericksen et al., 2003;Laacouri et al., 2013;Luo et al., 2016;Stamenkovic and Gustin, 2009;X Wang et al., 2020b;L. P. Wright and Zhang, 2015), all of which are sensitive to environmental and climatic conditions. ...
... Yuan et al., 2020), >90% Hg in humus soil at ASSFERS is derived from the atmospheric Hg 0 deposition. Litterfall Hg deposition is the primary pathway of atmospheric Hg 0 deposition in forest ecosystems (Bishop et al., 2020;X Wang et al., 2016a;X Wang et al., 2020b), and the variation ...
Article
Full-text available
As climate change accelerates, extreme weather events become more severe and frequent. We analyzed the datasets of decade-long observation (2005-2020) of mercury (Hg) stored in two subtropical evergreen forests to understand the impacts of extreme weather on the sequestration of atmospheric Hg in forest ecosystems. Results show a weak correlation between litterfall Hg and atmospheric Hg0 concentration. Droughts and snowstorms significantly disturb Hg accumulation in litterfall and soils. Litterfall Hg concentration and deposition both display an increasing trend during the period of extended droughts in 2011–2014, but a decreasing trend after droughts. This is caused by the water stress that influences the change of tree physiology and processes of foliage Hg0 uptake. Snowstorm damages large areas of canopy, which leads to substantial canopy epiphyte cover mixed into the forest floor, thus considerably increasing soil Hg concentrations. Over a decadal timescale, soil Hg variabilities are shaped by the combined effects of atmospheric Hg inputs and processes of organic soil mineralization mediated climatic factors. Our study highlights that the accelerated climate change increases the unpredictability of Hg accumulation in terrestrial ecosystems. Future studies are needed for better understanding the response of Hg biogeochemical cycling to climate change among different terrestrial biomes.
... Recent work has provided evidence that dry deposition of GEM to vegetation via stomatal uptake and subsequent transfer via leaf/needle senescence, abscission, and litterfall is likely to be the dominant mechanism for Hg deposition from the atmosphere to terrestrial matrices (Obrist et al., 2017Jiskra et al., 2018). Similarly, there is strong evidence that GEM is also the major source of Hg in the bole wood of trees (Scanlon et al., 2020;Wang et al., 2020Wang et al., , 2021. Using Hg stable isotope measurements, stomatal assimilation of GEM has been estimated to supply 57 %-94 % of total Hg (THg) in vegetated terrestrial systems (Khan et al., 2019, and references therein). ...
Article
Full-text available
Trees predominantly take up mercury (Hg) from the atmosphere via stomatal assimilation of gaseous elemental Hg (GEM). Hg is oxidised in leaves/needles and transported to other tree anatomy including bole wood, where it can be stored long-term. Using Hg associated with growth rings facilitates archiving of historical GEM concentrations. Nonetheless, there are significant knowledge gaps on the cycling of Hg within trees. We investigate Hg archived in tree rings, internal tree Hg cycling, and differences in Hg uptake mechanisms in Norway spruce and European larch sampled within 1 km of a HgCl2-contaminated site using total Hg (THg) and Hg stable isotope analyses. Tree ring samples are indicative of significant increases in THg concentrations (up to 521 µg kg−1) from the background period (BGP; facility closed; 1992–present) to secondary industrial period (2ndIP; no HgCl2 wood treatment; 1962–1992) to primary industrial period (1stIP; active HgCl2 wood treatment; ≈ 1900–1962). Mass-dependent fractionation (MDF) Hg stable isotope data are shifted negative during industrial periods (δ202Hg of 1stIP: −4.32 ± 0.15 ‰, 2ndIP: −4.04 ± 0.32 ‰, BGP: −2.83 ± 0.74 ‰; 1 SD). Even accounting for a ≈ −2.6 ‰ MDF shift associated with stomatal uptake, these data are indicative of emissions derived from industrial activity being enriched in lighter isotopes associated with HgCl2 reduction and Hg0 volatilisation. Similar MDF (δ202Hg: −3.90 ± 0.30 ‰; 1 SD) in bark Hg (137 ± 105 µg kg−1) suggests that stomatal assimilation and downward transport is also the dominant uptake mechanism for bark Hg (reflective of negative stomatal-uptake MDF shift) rather than deposition to bark. THg was enriched in sapwood of all sampled trees across both tree species. This may indicate long-term storage of a fraction of Hg in sapwood or xylem solution. We also observed a small range of odd-isotope mass-independent fractionation (MIF). Differences in Δ199Hg between periods of different industrial activities were significant (Δ199Hg of 1stIP: 0.00 ± 0.03 ‰, 2ndIP: −0.06 ± 0.04 ‰, BGP: −0.13 ± 0.03 ‰; 1 SD), and we suggest MIF signatures are conserved during stomatal assimilation (reflect source MIF signatures). These data advance our understanding of the physiological processing of Hg within trees and provide critical direction to future research into the use of trees as archives for historical atmospheric Hg.
... 39,42 Recent experimental evidence using stable Hg isotopes excluded this pathway for cereal plants. 43,44 Vegetation foliage is denoted as a sink of Hg 0 in Wang et al. 40 (Fig. 3C), consistent with the observations in forest ecosystems. 34,45,46 The other advancement is the spatial quantication of soil Hg re-emissions with improved soil Hg data. ...
Article
Mercury (Hg) is a toxic metal released into the environment through human activities and natural processes. Human activities have profoundly increased the budget of Hg in the atmosphere and altered...
... Atmospheric Hg(0) taken up by vegetation is oxidized to Hg(II) within the plant tissue (Manceau et al., 2018) and transferred to soils via litterfall (Iverfeldt, 1991;Schwesig and Matzner, 2000;Rea et al., 2001;Graydon et al., 2008;Risch et al., 2012;Jiskra et al., 2015;Wright et al., 2016;Wang et al., 2016;Risch et al., 2017). Moreover, in forests, Hg deposition to the ground may occur by wash-off of Hg(0) from 80 plant surfaces via throughfall and by Hg(0) uptake into woody tissues, lichen, mosses and soil litter (Wang et al., 2020;Obrist et al., 2021). Mercury sequestered by forest ecosystems accumulates in soil and may subsequently be transported from watersheds to streams, rivers and the ocean, where it can bioaccumulate in fish (Drenner et al., 2013;Jiskra et al., 2017;Sonke et al., 2018). ...
Preprint
Full-text available
Despite the importance of vegetation uptake of atmospheric gaseous elemental mercury (Hg(0)) within the global Hg cycle, little knowledge exists on the physiological, climatic and geographic factors controlling stomatal uptake of atmospheric Hg(0) by tree foliage. We investigate controls on foliar stomatal Hg(0) uptake by combining Hg measurements of 3,569 foliage samples across Europe with data on tree species traits and environmental conditions. To account for foliar Hg accumulation over time, we normalized foliar Hg concentration over the foliar life period from the simulated start of the growing season to sample harvest. The most relevant parameter impacting daily foliar stomatal Hg uptake was tree functional group (deciduous versus coniferous trees). On average, we measured 3.2 times higher daily foliar stomatal Hg uptake rates in deciduous leaves than in coniferous needles of the same age. Across tree species, for foliage of beech and fir, and at two out of three forest plots with more than 20 samples, we found a significant (p
... Anthropogenic warming led changes in AMDEs related oxidation chemistry 86 , air-surface exchanges 2 and precipitation 55 , with consequences to Hg deposition in the Arctic, are suggested to be responsible for the changing atmos pheric Hg cycling. Vegetation Hg deposition is pro jected to increase with increasing vegetation cover and density 114 , but Hg evasion from soils is projected to increase in response to accelerating wildfires and permafrost thaw led microbial reduction and release of stored Hg in soils 15 . ...
Article
Full-text available
Anthropogenic mercury (Hg) emissions have driven marked increases in Arctic Hg levels, which are now being impacted by regional warming, with uncertain ecological consequences. This Review presents a comprehensive assessment of the present-day total Hg mass balance in the Arctic. Over 98% of atmospheric Hg is emitted outside the region and is transported to the Arctic via long-range air and ocean transport. Around two thirds of this Hg is deposited in terrestrial ecosystems, where it predominantly accumulates in soils via vegetation uptake. Rivers and coastal erosion transfer about 80 Mg year⁻¹ of terrestrial Hg to the Arctic Ocean, in approximate balance with modelled net terrestrial Hg deposition in the region. The revised Arctic Ocean Hg mass balance suggests net atmospheric Hg deposition to the ocean and that Hg burial in inner-shelf sediments is underestimated (up to >100%), needing seasonal observations of sediment-ocean Hg exchange. Terrestrial Hg mobilization pathways from soils and the cryosphere (permafrost, ice, snow and glaciers) remain uncertain. Improved soil, snowpack and glacial Hg inventories, transfer mechanisms of riverine Hg releases under accelerated glacier and soil thaw, coupled atmosphere–terrestrial modelling and monitoring of Hg in sensitive ecosystems such as fjords can help to anticipate impacts on downstream Arctic ecosystems.
... The higher atmospheric Hg °concentration at lower altitude also can promote the greater Hg concentration in broadleaf foliage (Manceau et al., 2018;Zhou et al., 2021 ). The Hg isotopes have identified that Hg in bole wood and branch is mainly derived from the foliage uptake of atmospheric Hg 0 and subsequent translocation by the phloem ( Wang et al., 2020 ;Zhou et al., 2021 ). Hence, the Hg concentrations in aboveground woods are lower than foliage. ...
Article
Understanding atmospheric mercury (Hg) accumulation in remote montane forests is critical to assess the Hg ecological risk to wildlife and human health. To quantify impacts of vegetation, climatic and topographic factors on Hg accumulation in montane forests, we assessed the Hg distribution and stoichiometric relations among Hg, carbon (C), and nitrogen (N) in four forest types along the elevation of Mt. Gongga. Our results show that Hg concentration in plant tissues follows the descending order of litter > leaf, bark > root > branch > bole wood, indicating the importance of atmospheric Hg uptake by foliage for Hg accumulation in plants. The foliar Hg/C (from 237.0 ± 171.4 to 56.8 ± 27.7 µg/kg) and Hg/N (from 7.5 ± 3.9 to 2.5 ± 1.2 mg/kg) both decrease along the elevation. These elevation gradients are caused by the heterogeneity of vegetation uptake of atmospheric Hg and the variation of atmospheric Hg° concentrations at different altitudes. Organic soil Hg accumulation is controlled by forest types, topographic and climatic factors, with the highest concentration in the mixed forest (244.9 ± 55.7 µg/kg) and the lowest value in the alpine forest (151.9 ± 44.5 µg/kg). Further analysis suggests that soil Hg is positively correlated to C (r² = 0.66) and N (r² = 0.57), and Hg/C and Hg/N both increase with the soil depth. These stoichiometric relations highlight the combined effects from environmental and climatic factors which mediating legacy Hg accumulation and selective Hg absorption during processes of organic soil mineralization.
... The important physiological parameters related to the process of atmospheric Hg 0 uptake by foliage, include but are not limited to the inconsistent tree growth, rate of photosynthesis, stomata conductance, transpiration rate, foliage lifespan and canopy characteristics (Arnold et al., 2018;Ericksen et al., 2003;Laacouri et al., 2013;Luo et al., 2016;Stamenkovic and Gustin, 2009;Wang et al., 2020;Wright and Zhang, 2015), all of which are sensitive to environmental and climatic conditions. We suggest the vegetation types, climate and terrain along the elevation induced variations of the tree physiological factors are the important cause to control the elevation gradient of litterfall Hg concentration. ...
Article
Litterfall mercury (Hg) input has been regarded as the dominant Hg source in montane forest floor. To depict combining effects of vegetation, climate and topography on accumulation of Hg in montane forests, we comprehensively quantified litterfall Hg deposition and decomposition in a serial of subtropical forests along an elevation gradient on both leeward and windward slopes of Mt. Ailao, Southwest China. Results showed that the average litterfall Hg deposition increased from 12.0 ± 4.2 μg m⁻² yr⁻¹ in dry-hot valley shrub at 850–1000 m, 14.9 ± 6.8 μg m⁻² yr⁻¹ in mixed conifer-broadleaf forest at 1250–2400 m, to 23.1 ± 8.3 μg m⁻² yr⁻¹ in evergreen broadleaf forest at 2500–2650 m. Additionally, the windward slope forests had a significantly higher litterfall Hg depositions at the same altitude because the larger precipitation promoted the greater litterfall biomass production. The one-year litter Hg decomposition showed that the Hg mass of litter in dry-hot valley shrub decreased by 29%, while in mixed conifer-broadleaf and evergreen broadleaf forests increased by 22–48%. The dynamics of Hg in decomposing litter was controlled by the temperature mediated litter decomposition rate and the additional adsorption of environmental Hg during decomposition. Overall, our study highlights the litterfall mediated atmospheric mercury inputs and sequestration increase with the montane elevation, thus driving a Hg enhanced accumulation in the high montane forest.
... Origin and dynamics of this fraction are not understood, but large aerosol reservoirs sit in long-lived tissues of the forest canopy (lichen, moss, bark, etc., collectively "phyllosphere"), 77 for example, storing a ∼5 year equivalent of total Hg atmospheric deposition on an areal basis. 81 FRN chronometry provides a means for estimating residence times and fluxes, across such biogeochemical boundaries that otherwise remain challenging to quantify. 82 For geomorphic tracer applications, the irreversibility of FRN sorption reinforces our confidence that FRNs maintain particle fidelity through erosion and redistribution. ...
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Residential coal combustion (RCC) is a major source of atmospheric mercury (Hg) in rural areas of China, but little is known about the isotope signatures of Hg from this source. In the present study, the isotope compositions of speciated Hg (Hg⁰, Hg²⁺, and Hgp) in flue gas emitted from RCC were investigated in two important coal-producing areas (Xingren (XR) and Jinsha (JS)) of Guizhou Province, Southwest China. The total Hg concentration in discharged flue gas is in the range of 1.80-9.65 μg/m³ at the two sites, and the emission ratio reaches 99.87%. Isotope signatures of total Hg in flue gas are similar to those of Hg in feed coal, with near-zero Δ¹⁹⁹Hg and negative δ²⁰²Hg (-1.47‰ for XR and -3.00‰ for JS) in discharged flue gas. Such isotope signatures are very different from those of Hg emissions from modern coal-fired power plants with much higher δ²⁰²Hg signals. Negative shifts from Hg⁰ to Hg²⁺ in flue gas for δ²⁰²Hg (-0.94‰) and Δ¹⁹⁹Hg (-0.51‰) were observed, suggesting that both mass-dependent fractionation and mass-independent fractionation occurred during RCC. The nuclear volume effect produced by chlorine oxidation of Hg⁰ to Hg²⁺ may play an important role in the observed mass-independent fractionation in Hg²⁺. Without any air pollution control devices, RCC potentially increases the atmospheric Hg levels and has a negative-shifting impact on the δ²⁰²Hg of atmospheric Hg.
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The heavy metal accumulation in the Tibet Plateau (TP) poses a serious ecologic risk to the health of human and the other biota. Given the TP far away from the large anthropogenic emission sources, the rapid development of traffic activities during last several decades possibly leads to the elevated heavy metal concentration in the roadside soils. Therefore, we comprehensively assessed the heavy metal distribution in the 0–5 cm and 15–20 cm depth soils located at 5 m, 50 m, and 100 m distance to the edge of two major roads among the different vegetation covers and climatic conditions in the TP to verify this hypothesis. Results show that most of heavy metal concentrations in soils of different distance to the major road display an insignificant difference. The Nemero Synthesis indexes which represent the risk of pollution for these regions almost range 1 to 2 (low pollution risk), except 12.7 (extreme pollution risk) at one site. These indicate the limited impacts from the traffic activities for the whole region, but at some specific sites with the elevated traffic pollution. The forest cover at the altitude of 3700–4100 m has the highest mercury accumulation due to the vegetation and climatic factors induced the higher atmospheric depositions and stronger complexation with the organic matters. The statistical analysis finally suggests the geogenic weathering processes, climate, terrain and vegetation play an important role in shaping heavy metal distribution along the roadside of the TP.
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Previous research has found total mercury (THg) and methylmercury (MeHg) levels increase with litterfall decay, thus suggesting litterfall decomposition plays an essential role in the biogeochemical transformation of mercury (Hg). However, it remains unclear how Hg accumulates in the decaying litter, how bacterial taxa networks vary and what roles various microorganisms play during litterfall decomposition, especially nitrogen (N)-fixing, MeHg-degrading and Hg-methylating microbes. Here, we demonstrated as degradation proceeded, a gradually-complex network evolved for litterfall bacteria for the subtropical mixed broadleaf-conifer (MBC) forest, whereas a relatively static network existed for the evergreen broadleaf (EB) forest. N-fixing and MeHg-degrading bacteria dominated throughout litterfall decomposition process, with relative abundances of N-fixing genera and nifH copies maximum and relative abundances of MeHg-degrading bacteria and merAB copies minimum in summer. Hence, N-fixing bacteria likely mediate THg increase in the decomposing litterfall, while MeHg enhancement may be regulated by aerobic MeHg-degrading microbes which can transform MeHg to inorganic divalent Hg (Hg²⁺) or further to elemental Hg (Hg⁰). Together, this work elucidates variations of N-fixing and MeHg-degrading microbes in decaying litterfall and their relationships with Hg accumulation, providing novel insights into understanding the biogeochemical cycle of Hg in the forest ecosystem.
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Mercury (Hg) accumulation in rice is an emerging health concern worldwide. However, sources and interactions responsible for Hg species accumulation in different rice tissues are still uncertain. Four experimental plots were carefully designed at an artisanal Hg mining site and a control site to evaluate the effect of atmospheric and soil Hg contents on Hg accumulation in rice. We showed that inorganic Hg (IHg) contents in rice tissues grown either in contaminated or control site soil (non-contaminated soil) were higher at Hg artisanal mining site than those at the control site. Elevated total gaseous mercury (TGM) levels in ambient air were the predominant source of IHg to rice at the Hg mining area. Methylmercury (MeHg) concentrations in rice plant tissues increased in proportionality with MeHg contents in paddy soil. Our results suggest that both atmosphere and soil Hg sources have been impacted the IHg accumulation in rice. Above ground rice tissues, grains, leaves, and stalk accumulated IHg from both atmosphere and soil to varying degrees. Nonetheless, the study also provides the first direct evidence that atmospheric Hg accumulated by above-ground rice tissues could be translocated to below-ground tissues (roots). However, the extent to which atmosphere or soil Hg contributes to IHg in rice tissues may vary with each source's concentration gradient at the given site. No evidence of in planta Hg methylation was found during the current study. Hence, paddy fields are potential MeHg production sites, whereas paddy soil is a unique MeHg accumulation source in rice plants. This study expands and clarifies the contribution of various sources involved in Hg accumulation in the soil rice system. The findings here provide the basis for future research strategies to deal with the global issue of Hg contaminated rice.
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Foliar accumulation of atmospheric mercury (Hg), particularly gaseous elemental mercury (Hg(0)), accounts for a significant amount of atmospheric Hg deposition and influences the Hg geochemical cycle. Foliar Hg accumulation occurs as the net result of uptake, adsorption, fixation, re-emission, and other processes. The combination of these processes influences foliar Hg speciation, translocation, toxicity, transport, and bioavailability, which eventually affects the environmental risk of Hg. Therefore, a systematic understanding of the various processes involved in foliar Hg accumulation is urgently needed. This review summarizes the current knowledge, research progress, and remaining knowledge gaps regarding foliar Hg accumulation processes and the biochemical mechanisms involved. Atmospheric Hg is biochemically fixed within foliage after stomatal and/or nonstomatal uptake, with foliar Hg then volatilized after reduction, washed off via throughfall, deposited on soil surfaces via litterfall or stored within vegetation over time. Mechanisms underlying these processes and future perspectives on foliar accumulation are hereby discussed with the aim of reducing knowledge gaps on foliar Hg accumulation. The combination of different methods, such as Hg speciation analysis, isotope tracer techniques, element imaging, biological technologies, and ecologically microcosmic methods, will help clarify Hg speciation, transformation, and subcellular distribution in foliage, and quantitate different Hg fluxes during foliar Hg accumulation and litterfall decomposition. Measurements and modeling on the foliar accumulation process of atmospheric Hg under natural and human perturbations are also essential. These future investigations will help comprehensively understand the processes influencing atmospheric Hg interactions with foliage and evaluate the environmental fates and risks of foliar Hg.
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Bio-monitoring of mercury (Hg) in air using transplanted and in-situ lichens was conducted at three locations in Slovenia: (I) the town of Idrija in the area of the former Hg mine, where Hg contamination is well known; (II) Anhovo, a settlement with a cement production plant, which is a source of Hg pollution, and (III) Pokljuka, a part of a national park. Lichens from Pokljuka were transplanted to different sites and sampled four times—once per season, from January 2020 to February 2021. Lichens were set on tree branches, fences, and under cover, allowing them to be exposed to different environmental conditions (e.g., light and rain). The in-situ lichens were sampled at the beginning and the end of the sampling period. The highest concentrations were in the Idrija area, which was consistent with previous research. Significant mass-dependent fractionation has been observed in transplanted lichens during summer period. The δ²⁰²Hg changed from −3.0‰ in winter to −1.0‰ in summer and dropped again to the same value in winter the following year. This trend was observed in all samples, except those from the most polluted Idrija sampling site, which was in the vicinity of the former Hg ore-smelting plant. This was likely due to large amounts of Hg originating from polluted soil close to the former smelting plant with a distinct isotopic fingerprint in this local area. The Δ¹⁹⁹Hg in transplanted lichens ranged from −0.5‰ to −0.1‰ and showed no seasonal trends. These findings imply that seasonality, particularly in summer months, may affect the isotopic fractionation of Hg and should be considered in the sampling design and data interpretation. This trend was thus described in lichens for the first time. The mechanism behind such change is not yet fully understood.
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As the “lungs of the city”, urban forests can improve air quality by absorbing air pollutants, becoming hotspots for mercury (Hg) pollution from anthropogenic activities. However, the bioaccumulation and transfer of Hg in the urban forest food web are unclear. In this study, total mercury (THg) and methylmercury (MeHg) concentrations, as well as the stable isotopes of carbon (δ13C) and nitrogen (δ15N) in organisms with different trophic levels (TLs) were investigated in a mid-subtropical urban forest of the Changpoling Forest Park (CFP) in Guiyang City, Guizhou Province, southwestern China. The results showed that THg and MeHg among all taxa ranged from 5.6 to 1267 ng·g-1 and 0.046 to 692 ng·g-1, respectively. MeHg% (% of Hg present as MeHg) at different TLs exhibited a wide range of 5.0-69% on average. Both THg and MeHg increased with the TLs from plants to nestling birds, indicating distinct biomagnification through the food web of grasses/pine needles - grasshoppers/caterpillars/katydids/mantis - spiders/songbird nestlings. The trophic magnification slope (TMS) of THg and MeHg were 0.18±0.05 and 0.37±0.08, respectively, suggesting both of them significantly increase along food webs. These findings improve the understanding of biogeochemical Hg cycles in terrestrial food webs and highlight the impacts of terrestrial MeHg on nestling birds.
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As a major entry point of mercury (Hg) to aquatic food webs, algae play an important role in taking up and transforming Hg species in aquatic ecosystems. However, little is known how and to what extent Hg reduction, uptake, and species transformations are mediated by algal cells and their exudates, algal organic matter (AOM), under either sunlit or dark conditions. Here, using Chlorella vulgaris (CV) as one of the most prevalent freshwater model algal species, we show that solar irradiation could enhance the reduction of mercuric Hg(II) to elemental Hg(0) by both CV cells and AOM. AOM reduced more Hg(II) than algal cells themselves due to cell surface adsorption and uptake of Hg(II) inside the cells under solar irradiation. Synchrotron radiation X-ray absorption near-edge spectroscopy (SR-XANES) analyses indicate that sunlight facilitated the transformation of Hg to less bioavailable species, such as β-HgS and Hg-phytochelatins, compared to Hg(Cysteine)2-like species formed in algal cells in the dark. These findings highlight important functional roles and potential mechanisms of algae in Hg reduction and immobilization under varying lighting conditions and how these processes may modulate Hg cycling and bioavailability in the aquatic environment.
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Mercury (Hg), a neurotoxic heavy metal, is transferred to marine and terrestrial ecosystems through atmospheric transport. Recent studies have highlighted the role of vegetation uptake as a sink for atmospheric elemental mercury (Hg0) and a source of Hg to soils. However, the global magnitude of the Hg0 vegetation uptake flux is highly uncertain, with estimates ranging 1000-4000 Mg per year. To constrain this sink, we compare simulations in the chemical transport model GEOS-Chem with a compiled database of litterfall, throughfall, and flux tower measurements from 93 forested sites. The prior version of GEOS-Chem predicts median Hg0 dry deposition velocities similar to litterfall measurements from Northern hemisphere temperate and boreal forests (∼0.03 cm s-1), yet it underestimates measurements from a flux tower study (0.04 cm s-1vs. 0.07 cm s-1) and Amazon litterfall (0.05 cm s-1vs. 0.17 cm s-1). After revising the Hg0 reactivity within the dry deposition parametrization to match flux tower and Amazon measurements, GEOS-Chem displays improved agreement with the seasonality of atmospheric Hg0 observations in the Northern midlatitudes. Additionally, the modelled bias in Hg0 concentrations in South America decreases from +0.21 ng m-3 to +0.05 ng m-3. We calculate a global flux of Hg0 dry deposition to land of 2276 Mg per year, approximately double previous model estimates. The Amazon rainforest contributes 29% of the total Hg0 land sink, yet continued deforestation and climate change threatens the rainforest's stability and thus its role as an important Hg sink. In an illustrative worst-case scenario where the Amazon is completely converted to savannah, GEOS-Chem predicts that an additional 283 Mg Hg per year would deposit to the ocean, where it can bioaccumulate in the marine food chain. Biosphere-atmosphere interactions thus play a crucial role in global Hg cycling and should be considered in assessments of future Hg pollution.
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The biogeochemical cycling of trace elements (TEs) in forest ecosystems is important because it plays a role in providing essential nutrients to plants and soils and because it can potentially have toxic effects. In this study, we investigated the concentration of TEs in atmospheric wet deposition, vegetation and soil in Qinghai spruce (QS) and Qilian juniper (QJ) forests of the Qilian Mountains. Our results show that the average concentrations of Cu in rainwater in QS and QJ forests were 10.30 and 5.35 μg L⁻¹, respectively, the highest concentrations of all TEs in these environments. We suggest that the particulate matter present in the air was the main contributor of TEs in atmospheric wet deposition, which is affected by element specificity, regional factors, and the scavenging process during rainfall events. Most vegetation and tissues had high concentrations of Zn, Ni, Pb, and Cu, suggesting that these elements have accumulated in plants. The Zn, Pb, and Ni levels in forest plants may be correlated with those in forest soils. Our study highlights the role that atmospheric wet deposition can play in affecting TEs cycling across forest ecosystems. Managers need to further reduce TEs levels in emissions from surrounding sources and improve long-term observation of TEs in forest ecosystems.
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Significant knowledge gaps in mercury (Hg) cycling challenge our ability to assess the effectiveness of the Minamata Convention on Mercury in reducing human and wildlife Hg exposure due to the complicated impacts of climate change. In this study, we comprehensively quantified the Hg spatial distribution and determined the Hg stable isotopes to understand the Hg sources and accumulation pathways in forests along the timberline ecotone, an ecosystem that provides an early bellwether of global warming effects. Our results show that although there were subtle variations in meteorological and environmental conditions among the < 50 m elevation variation, a spruce to shrub vegetation structure shift led to a distinct decrease in vegetation and soil Hg pool sizes by 36 − 56% along the timberline ecotone. Further Hg isotopic evidence verifies that vegetation-induced atmospheric Hg0 deposition is the main source for Hg accumulation in soils, and the source contribution of such input decreases from 73% to 45% along the timberline ecotone. Our results indicate that if global warming induced a spruce treeline upward shift would significantly promote the atmospheric Hg burden, thus resulting in a conspicuous “Hg hotspot”. We recommend further studies to assess the Hg ecological risk promoted by global warming in alpine ecosystems.
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Litterfall mercury (Hg) deposition is the dominant source of soil Hg in forests. Identifying reduction processes and tracking the fate of legacy Hg on forest floor are challenging tasks. Interplays between isotopes of carbon (C) and nitrogen (N) may shed some lights on Hg biogeochemical processes because their biogeochemical cycling closely links with organic matters. Isotope measurements at the evergreen broadleaf forest floor at Mt. Ailao (Mountain Ailao) display that δ202Hg and Δ199Hg both significantly correlate with δ13C and δ15N in soil profiles. Data analysis results show that microbial reduction is the dominant process for the distinct δ202Hg shift (up to ∼1.0‰) between Oi and 0-10 cm surface mineral soil, and dark abiotic organic matter reduction is the main cause for the Δ199Hg shift (∼-0.18‰). Higher N in foliage leads to greater Hg concentration, and Hg0 re-emission via microbial reduction on forest floor is likely linked to N release and immobilization on forest floor. We thus suggest that the enhanced N deposition in global forest ecosystems can potentially influence Hg uptake by vegetation and litter Hg sequestration on forest floor.
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Recent studies have shown that mercury (Hg) concentrations in tree rings have the potential to archive historical Hg exposure from local, regional, and global sources. The southeastern United States has received elevated Hg deposition, yet no studies have evaluated tree ring Hg in this region. Here, we quantify Hg accumulation and isotopic composition in tree rings collected in Shenandoah National Park, Virginia. Cores were collected from three individuals of three tree species—white oak (Quercus alba), northern red oak (Quercus rubra), and pitch pine (Pinus rigida)—within the northern, central, and southern areas of the Park (n = 27 cores). The cores were analyzed for Hg content in 10‐year increments, with some cores dating back to the early 1800s. Overall, tree ring Hg concentrations (ranging from below detection to 4.4 ng/g) were similar to other studies and varied between species, with pitch pine having higher concentrations than the deciduous species. The most notable feature of the tree ring Hg time series was a peak that occurred during the 1930s through 1950s, coinciding with the use of Hg at a local industrial facility. Atmospheric modeling indicates that potential emissions from the plant likely had a stronger impact on the southern region of the Park, consistent with the latitudinal gradient in tree ring Hg concentrations. Mass‐dependent and mass‐independent fractionation of Hg isotopes suggests contributions from both regional anthropogenic and local industrial sources during this period. This study demonstrates the potential usefulness of tree ring dendrochemistry for identifying historical sources of atmospheric Hg exposure.
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Processes facilitated by precipitation play an important role on mercury (Hg) accumulation on forest floor and therefore key to Hg cycling in forest ecosystems. Sites along the windward slope of 1,250 to 2,400 m at Mt. Ailao, Southwestern China, have higher precipitation than the leeward slope sites. In this study, measurements of Hg concentration and associated stable isotope composition for soil, fresh, and degraded litterfall samples were made at sites along two slopes of Mt. Ailao to quantify the direct and indirect effects of precipitation on Hg accumulation on forest floor. Higher soil Hg concentrations, larger litterfall Hg depositions, and faster litter decomposition rates were observed on the windward slope (1,250–2,400 m). Data of Hg isotopic signatures suggest that Hg in surface soils is mainly derived from litterfall Hg input. Precipitation enhances litterfall Hg deposition by increasing litter biomass production, reduces litter decomposition rate, facilitates short‐term Hg uptake to decomposing litter, and potentially increases microbial activity that increases Hg loss via microbial reduction or runoff. Structural equation modeling results support that the indirect effect of precipitation on increased biomass production merge as the most important factor controlling soil Hg variation. Given the climate forcing on global precipitation pattern and vegetation growth cycle, Hg biogeochemical cycling is likely to continue to evolve under the changing climate.
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Isotopic composition of atmospheric particulate bound mercury (PBM) was obtained at four remote sites in different geographical regions of China for three to 12 month periods. Mean (±1σ) ∆199HgPBM was the highest at the site in southwestern China (0.66 ± 0.32‰), followed by the site in northeastern China (0.36 ± 0.34‰), the site in the marine boundary layer of East China Sea (0.35 ± 0.33‰), and was the lowest at the site in northwestern China (0.27 ± 0.22‰). ∆199HgPBM was relatively higher in cold than warm season at the sites in northwestern and southwestern China, whereas the opposite was found at the site in northeastern China. We propose that the seasonal variations of ∆199HgPBM were influenced by the exposure of air masses to regional (e.g., anthropogenic and dust related ) and long-range transport (e.g., anthropogenic and oceanic) sources in the preceding several days, with the former characterized by lower ∆199HgPBM and the latter characterized by more positive ∆199HgPBM due to sufficient atmospheric transformations. Modeling results from Potential Source Contribution Function suggested that domestic anthropogenic emission was the major contributor to PBM pollution at the sites in northeastern and eastern China, whereas long-range transboundary transport from South Asia played a more important role at the sites in southwestern and northwestern China.
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We review recent progress in our understanding of the global cycling of mercury (Hg), including best estimates of Hg concentrations and pool sizes in major environmental compartments and exchange processes within and between these reservoirs. Recent advances include the availability of new global datasets covering areas of the world where environmental Hg data were previously lacking; integration of these data into global and regional models is continually improving estimates of global Hg cycling. New analytical techniques, such as Hg stable isotope characterization, provide novel constraints of sources and transformation processes. The major global Hg reservoirs that are, and continue to be, affected by anthropogenic activities include the atmosphere (4.4–5.3 Gt), terrestrial environments (particularly soils: 250–1000 Gg), and aquatic ecosystems (e.g., oceans: 270–450 Gg). Declines in anthropogenic Hg emissions between 1990 and 2010 have led to declines in atmospheric Hg⁰ concentrations and HgII wet deposition in Europe and the US (− 1.5 to − 2.2% per year). Smaller atmospheric Hg⁰ declines (− 0.2% per year) have been reported in high northern latitudes, but not in the southern hemisphere, while increasing atmospheric Hg loads are still reported in East Asia. New observations and updated models now suggest high concentrations of oxidized HgII in the tropical and subtropical free troposphere where deep convection can scavenge these HgII reservoirs. As a result, up to 50% of total global wet HgII deposition has been predicted to occur to tropical oceans. Ocean Hg⁰ evasion is a large source of present-day atmospheric Hg (approximately 2900 Mg/year; range 1900–4200 Mg/year). Enhanced seawater Hg⁰ levels suggest enhanced Hg⁰ ocean evasion in the intertropical convergence zone, which may be linked to high HgII deposition. Estimates of gaseous Hg⁰ emissions to the atmosphere over land, long considered a critical Hg source, have been revised downward, and most terrestrial environments now are considered net sinks of atmospheric Hg due to substantial Hg uptake by plants. Litterfall deposition by plants is now estimated at 1020–1230 Mg/year globally. Stable isotope analysis and direct flux measurements provide evidence that in many ecosystems Hg⁰ deposition via plant inputs dominates, accounting for 57–94% of Hg in soils. Of global aquatic Hg releases, around 50% are estimated to occur in China and India, where Hg drains into the West Pacific and North Indian Oceans. A first inventory of global freshwater Hg suggests that inland freshwater Hg releases may be dominated by artisanal and small-scale gold mining (ASGM; approximately 880 Mg/year), industrial and wastewater releases (220 Mg/year), and terrestrial mobilization (170–300 Mg/year). For pelagic ocean regions, the dominant source of Hg is atmospheric deposition; an exception is the Arctic Ocean, where riverine and coastal erosion is likely the dominant source. Ocean water Hg concentrations in the North Atlantic appear to have declined during the last several decades but have increased since the mid-1980s in the Pacific due to enhanced atmospheric deposition from the Asian continent. Finally, we provide examples of ongoing and anticipated changes in Hg cycling due to emission, climate, and land use changes. It is anticipated that future emissions changes will be strongly dependent on ASGM, as well as energy use scenarios and technology requirements implemented under the Minamata Convention. We predict that land use and climate change impacts on Hg cycling will be large and inherently linked to changes in ecosystem function and global atmospheric and ocean circulations. Our ability to predict multiple and simultaneous changes in future Hg global cycling and human exposure is rapidly developing but requires further enhancement.
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Anthropogenic emissions of the toxic heavy metal mercury (Hg) have substantially increased atmospheric Hg levels during the 20th century compared to pre-industrial times. However, on a regional scale, atmospheric Hg concentration or deposition trends vary to such an extent during the industrial period that the consequences of recent Asian emissions on atmospheric Hg levels are still unclear. Here we present a 320-year Hg deposition history for Central Asia, based on a continuous high-resolution ice-core Hg record from the Belukha glacier in the Siberian Altai, covering the time period 1680-2001. Hg concentrations and deposition fluxes start rising above background levels at the beginning of the 19th century due to emissions from gold/silver mining and Hg production. A steep increase occurs after the 1940s culminating during the 1970s, at the same time as the maximum Hg use in consumer products in Europe and North America. After a distinct decrease in the 1980s, Hg levels in the 1990s and beginning of the 2000s return to their maximum values, which we attribute to increased Hg emissions from Asia. Thus, rising Hg emissions from coal combustion and artisanal and small-scale gold mining (ASGM) in Asian countries determine recent atmospheric Hg levels in Central Asia, counteracting emission reductions due to control measures in Europe and North America.
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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 mid-elevation 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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Mercury (Hg) in tree wood has been overlooked, in part because concentrations are so low as to be below detection limits of some analytical methods, but it is potentially important to forest ecosystem processes and budgets. We tested methods for the preparation and determination of Hg in tree wood by analysing samples of four tree species at the Hubbard Brook Experimental Forest, New Hampshire, USA, using thermal decomposition, catalytic conversion, amalgamation and atomic absorption spectrophotometry (USEPA Method 7473). Samples that were freeze-dried or oven-dried at 65°C were suitable for determination of Hg, whereas oven-drying at 103°C resulted in Hg losses, and air-drying resulted in Hg gains, presumably due to sorption from indoor air. Mean (±SE) concentrations of Hg tree bole wood were 1.75 ± 0.14 ng g⁻¹ for American beech, 1.48 ± 0.23 ng g⁻¹ for sugar maple, 3.96 ± 0.19 ng g⁻¹ for red spruce and 4.59 ± 0.06 ng g⁻¹ for balsam fir. Based on these concentrations and estimates of wood biomass by species based on stand inventory, we estimated the Hg content of wood in the reference watershed at Hubbard Brook to be 0.32 g ha⁻¹, twice the size of the foliar Hg pool (0.15 g ha⁻¹). Mercury in wood deserves more attention and is feasible to measure using appropriate techniques.
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There exists observational evidence that gaseous elemental mercury (GEM) can be readily removed from the atmosphere via chemical oxidation followed by deposition in the polar and sub-polar regions, free troposphere, lower stratosphere, and marine boundary layer under specific environmental conditions. Here we report GEM depletions in a temperate mixed forest at Mt. Changbai, Northeast China. The strong depletions occurred predominantly at night during the leaf-growing season and in the absence of gaseous oxidized mercury (GOM) enrichment (GOM < 3 pg m−3). Vertical gradients of decreasing GEM concentrations from layers above to under forest canopy suggest in situ loss of GEM to forest canopy at Mt. Changbai. Foliar GEM flux measurements showed that the foliage of two predominant tree species is a net sink of GEM at night, with a mean flux of −1.8 ± 0.3 ng m2 h−1 over Fraxinus mandshurica (deciduous tree species) and −0.1 ± 0.2 ng m2 h−1 over Pinus Koraiensis (evergreen tree species). Daily integrated GEM δ202Hg, Δ199Hg, and Δ200Hg at Mt. Changbai during 8–18 July 2013 ranged from −0.34 to 0.91 ‰, from −0.11 to −0.04 ‰ and from −0.06 to 0.01 ‰, respectively. A large positive shift in GEM δ202Hg occurred during the strong GEM depletion events, whereas Δ199Hg and Δ200Hg remained essentially unchanged. The observational findings and box model results show that uptake of GEM by forest canopy plays a predominant role in the GEM depletion at Mt. Changbai forest. Such depletion events of GEM are likely to be a widespread phenomenon, suggesting that the forest ecosystem represents one of the largest sinks ( ∼ 1930 Mg) of atmospheric Hg on a global scale.
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The current knowledge concerning mercury dry deposition is reviewed, including dry-deposition algorithms used in chemical transport models (CTMs) and at monitoring sites and related deposition calculations, measurement methods and studies for quantifying dry deposition of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM), and measurement studies of litterfall and throughfall mercury. Measured median GOM plus PBM dry deposition in Asia (10.7 µg m−2 yr−1) is almost double that in North America (6.1 µg m−2 yr−1) due to the higher anthropogenic emissions in Asia. The measured mean GOM plus PBM dry deposition in Asia (22.7 µg m−2 yr−1), however, is less than that in North America (30.8 µg m−2 yr−1). The variations between the median and mean values reflect the influences that single extreme measurements can have on the mean of a data set. Measured median litterfall and throughfall mercury are, respectively, 34.8 and 49.0 µg m−2 yr−1 in Asia, 12.8 and 16.3 µg m−2 yr−1 in Europe, and 11.9 and 7.0 µg m−2 yr−1 in North America. The corresponding measured mean litterfall and throughfall mercury are, respectively, 42.8 and 43.5 µg m−2 yr−1 in Asia, 14.2 and 19.0 µg m−2 yr−1 in Europe, and 12.9 and 9.3 µg m−2 yr−1 in North America. The much higher litterfall mercury than GOM plus PBM dry deposition suggests the important contribution of gaseous elemental mercy (GEM) to mercury dry deposition to vegetated canopies. Over all the regions, including the Amazon, dry deposition, estimated as the sum of litterfall and throughfall minus open-field wet deposition, is more dominant than wet deposition for Hg deposition. Regardless of the measurement or modelling method used, a factor of 2 or larger uncertainties in GOM plus PBM dry deposition need to be kept in mind when using these numbers for mercury impact studies.
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This study examined for the first time the Hg isotope composition in rain samples from a single precipitation event at Lhasa City (China) on the Tibetan Plateau, the “world’s third pole”. Large variations of both mass-dependent fractionation (MDF, δ202Hg from -0.80‰ to -0.42‰) and mass-independent fractionation (MIF, Δ199Hg from 0.38‰ to 0.76‰) were observed, with the latter increasing with time. Our results demonstrated that the large variation of Hg isotope ratios likely resulted from mixing of locally emitted Hg and long-term transported Hg, which were characterized by different Hg isotope signatures and mainly leached by below-cloud scavenging and in-cloud scavenging processes, respectively. Our findings demonstrated that Hg isotopes are a powerful tool for investigating the dynamics of precipitation events and emphasized the importance of systematic monitoring studies of the chemical and isotope variability of Hg and other elements during rainfall events.
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Mercury (Hg) is a global health concern due to its toxicity and ubiquitous presence in the environment. Here we review current methods for measuring the forms of Hg in the atmosphere and models used to interpret these data. There are three operationally defined forms of atmospheric Hg: gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate bound mercury (PBM). There is relative confidence in GEM measurements (collection on a gold surface), but GOM (collection on potassium chloride (KCl)-coated denuder) and PBM (collected using various methods) are less well understood. Field and laboratory investigations suggest the methods to measure GOM and PBM are impacted by analytical interferences that vary with environmental setting (e.g., ozone, relative humidity), and GOM concentrations measured by the KCl-coated denuder can be too low by a factor of 1.6 to 12 depending on the chemical composition of GOM. The composition of GOM (e.g., HgBr2, HgCl2, HgBrOH) varies across space and time. This has important implications for refining existing measurement methods and developing new ones, model/measurement comparisons, model development, and assessing trends. Unclear features of previously published data may now be re-examined and possibly explained, which is demonstrated through a case study. Priorities for future research include identification of GOM compounds in ambient air and development of information on their chemical and physical properties and GOM and PBM calibration systems. With this information, identification of redox mechanisms and associated rate coefficients may be developed.
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The isotopic composition of mercury (Hg) is increasingly used to constrain the sources and pathways of this metal in the atmosphere. Though China has the highest Hg production, consumption and emission in the world, Hg isotope ratios are rarely reported for Chinese wet deposition. In this study, we examined, for the first time outside North America, both mass-dependent fractionation (MDF, expressed as δ202Hg) and mass-independent fractionation of odd (odd-MIF, Δ199Hg) and even (even-MIF, Δ200Hg) Hg isotopes in 15 precipitation samples collected from September 2012 to August 2013 in Guiyang (SW China). All samples displayed significant negative δ202Hg (–0.44 ∼ –4.27‰), positive Δ199Hg (+0.19 to +1.16‰) and slightly positive Δ200Hg (–0.01‰ to +0.20‰). Potential sources of Hg in precipitation were identified by coupling both MDF and MIF of Hg isotopes with a back-trajectory model. The results showed that local emission from coal-fired power plants and cement plants and western long-range transportation are two main contributing sources, while the contribution of Hg from south wind events would be very limited on an annual basis. The relatively lower Δ200Hg values in Guiyang precipitation may indicate a dilution effect by local sources and/or insignificant even-MIF in the tropopause contribution of this subtropical region. Our study demonstrates the usefulness of isotope fractionation, especially MIF for tracing sources and pathways of Hg in the atmosphere.
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We evaluated spatial patterns of mercury (Hg) deposition through analysis of foliage and forest floor samples from 45 sites across Adirondack Park, NY. Species-specific differences in foliar Hg were evident with the lowest concentrations found in first-year conifer needles and highest concentrations found in black cherry (Prunus serotina). For foliage and forest floor samples, latitude and longitude were negatively correlated with Hg concentrations, likely because of proximity to emission sources, while elevation was positively correlated with Hg concentrations. Elemental analysis showed moderately strong, positive correlations between Hg and nitrogen concentrations. The spatial pattern of Hg deposition across the Adirondacks is similar to patterns of other contaminants that originate largely from combustion sources such as nitrogen and sulfur. The results of this study suggest foliage can be used to assess spatial patterns of Hg deposition in small regions or areas of varied topography where current Hg deposition models are too coarse to predict deposition accurately. Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.
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[1] Forests mediate the biogeochemical cycling of mercury (Hg) between the atmosphere and terrestrial ecosystems; however, there remain many gaps in our understanding of these processes. Our objectives in this study were to characterize Hg isotopic composition within forests, and use natural abundance stable Hg isotopes to track sources and reveal mechanisms underlying the cycling of Hg. We quantified the stable Hg isotopic composition of foliage, forest floor, mineral soil, precipitation, and total gaseous mercury (THg(g)) in the atmosphere and in evasion from soil, in 10-year-old aspen forests at the Rhinelander FACE experiment in northeastern Wisconsin, USA. The effect of increased atmospheric CO2 and O3 concentrations on Hg isotopic composition was small relative to differences among forest ecosystem components. Precipitation samples had δ202Hg values of −0.74 to 0.06‰ and ∆199Hg values of 0.16 to 0.82‰. Atmospheric THg(g) had δ202Hg values of 0.48 to 0.93‰ and ∆199Hg values of −0.21 to −0.15‰. Uptake of THg(g) by foliage resulted in a large (−2.89‰) shift in δ202Hg values; foliage displayed δ202Hg values of −2.53 to −1.89‰ and ∆199Hg values of −0.37 to −0.23‰. Forest floor samples had δ202Hg values of −1.88 to −1.22‰ and ∆199Hg values of −0.22 to −0.14‰. Mercury isotopes distinguished geogenic sources of Hg and atmospheric derived sources of Hg in soil, and showed that precipitation Hg only accounted for ~16% of atmospheric Hg inputs. The isotopic composition of Hg evasion from the forest floor was similar to atmospheric THg(g); however, there were systematic differences in δ202Hg values and MIF of even isotopes (∆200Hg and ∆204Hg). Mercury evasion from the forest floor may have arisen from air-surface exchange of atmospheric THg(g), but was not the emission of legacy Hg from soils, nor re-emission of wet-deposition. This implies that there was net atmospheric THg(g) deposition to the forest soils. Furthermore, MDF of Hg isotopes during foliar uptake and air-surface exchange of atmospheric THg(g) resulted in the release of Hg with very positive δ202Hg values to the atmosphere, which is key information for modeling the isotopic balance of the global mercury cycle, and may indicate a shorter residence time than previously recognized for the atmospheric mercury pool.
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Decomposition of organic matter strongly influences ecosystem carbon storage(1). In Earth-system models, climate is a predominant control on the decomposition rates of organic matter(2-5). This assumption is based on the mean response of decomposition to climate, yet there is a growing appreciation in other areas of global change science that projections based on mean responses can be irrelevant and misleading(6,7). We test whether climate controls on the decomposition rate of dead wood-a carbon stock estimated to represent 73 +/- 6 Pg carbon globally(8)-are sensitive to the spatial scale from which they are inferred. We show that the common assumption that climate is a predominant control on decomposition is supported only when local-scale variation is aggregated into mean values. Disaggregated data instead reveal that local-scale factors explain 73% of the variation in wood decomposition, and climate only 28%. Further, the temperature sensitivity of decomposition estimated from local versus mean analyses is 1.3-times greater. Fundamental issues with mean correlations were highlighted decades ago(9,10), yet mean climate-decomposition relationships are used to generate simulations that inform management and adaptation under environmental change. Our results suggest that to predict accurately how decomposition will respond to climate change, models must account for local-scale factors that control regional dynamics.
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The Minamata Convention on Mercury (MC) includes provisions for a global monitoring program (GMP) and effectiveness evaluation (EE) to provide information on changes in mercury sources in various environmental media. While conventional measurement and modeling techniques have limitations in explaining the changes in mercury concentrations, the measurements of natural abundances of mercury stable isotopes have become powerful tracers for distinguishing between mercury sources and for understanding biogeochemical processes in the environment. Unfortunately, it is uncertain whether mercury isotope ratios can provide globally comparable results on specific mercury sources for the GMP and trend analyses for the EE. We have compiled a dataset from the literature to evaluate large-scale patterns of mercury isotope ratios in various environmental samples and to summarize sample types that can be used for the GMP. Total gaseous mercury, precipitation, foliage, and litter can provide comparable source information regarding atmospheric mercury across a large spatial scale. Interpretation of spatially relevant information using sediment and fish mercury isotope ratios are challenging because they represent multiple mercury sources and contain mercury that has been subject to biogeochemical transformation leading to isotope fractionation. In regards to the EE, data that provides evidence of changes due to source regulation needs to be gathered from local point source regions to assess health impacts. We recommend that the measurements of particulate-bound mercury in the atmosphere and sediment mercury isotope ratios near mercury hotspots and in fish, are needed to identify ecosystems sensitive to atmospheric deposition and to evaluate the effectiveness of the MC.
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As global climate continues to warm, melting of glaciers releases a large quantity of mercury (Hg) originally locked in ice into the atmosphere and downstream ecosystems. Here, we show an opposite process that captures atmospheric Hg through glacier-to-vegetation succession. Our study using stable isotope techniques at 3 succession sites on the Tibetan Plateau reveals that evolving vegetation serves as an active “pump” to take up gaseous elemental mercury (Hg0) from the atmosphere. The accelerated uptake enriches the Hg pool size in glacier-retreated areas by a factor of ∼10 compared with the original pool size in the glacier. Through an assessment of Hg source–sink relationship observed in documented glacier-retreated areas in the world (7 sites of tundra/steppe succession and 5 sites of forest succession), we estimate that 400 to 600 Mg of Hg has been accumulated in glacier-retreated areas (5‰ of the global land surface) since the Little Ice Age (∼1850). By 2100, an additional ∼300 Mg of Hg will be sequestered from the atmosphere in glacier-retreated regions globally, which is ∼3 times the total Hg mass loss by meltwater efflux (∼95 Mg) in alpine and subpolar glacier regions. The recapturing of atmospheric Hg by vegetation in glacier-retreated areas is not accounted for in current global Hg models. Similar processes are likely to occur in other regions that experience increased vegetation due to climate or land use changes, which need to be considered in the assessment of global Hg cycling.
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Composition of stable mercury (Hg) isotopes provides important chemical signatures for tracing the source and transformation pathway of Hg in the environment. However, such measurements are challenging for natural water samples due to the ultratrace levels of Hg and therefore a preconcentration of Hg in water is needed prior to isotope measurements. In this study, we developed a preconcentration method using modulated apparatus that can be deployed for field measurements. The system includes a 3-L bubbler, a chlorine-impregnated activated carbon (ClC) trap, a zero-air filter connected to the inlet, and a vacuum pump. Hg in aqueous phase samples is first oxidized by BrCl, then reduced with SnCl2. The produced Hg0 is purged from the aqueous phase at a flow rate of 2.5 L/min for 1 hour and collected by a trap containing 550 mg of ClC. Hg collected in the ClC trap is then thermally desorbed in an argon carrying gas and preconcentrated into a 40% mixed acid solution (4 M HNO3 and 1.3 M HCl). This method was evaluated using solutions spiked with NIST SRM 3133, UM-Almadén and BCR 482 standards at Hg concentrations of 1-50 ng/L. The results showed an analytical recovery of 98.9 ± 1.6% (1SD, n = 21) in Hg concentration and no significant difference in isotope compositions between the recovered Hg and original standards (ANOVA, p-δ202Hg = 0.62, p-Δ199Hg = 0.92, and p-Δ199Hg = 0.76). Duplicated experiments on filed aqueous samples showed that the analytical precision were 0.13‰, 0.06‰ and 0.07‰ (n = 16) for δ202Hg, Δ199Hg and Δ200Hg, respectively. The developed method is reliable and efficient for concentrating Hg in natural water samples for determining the Hg isotopic compositions.
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Vegetation uptake of atmospheric mercury (Hg) is an important mechanism enhancing atmospheric Hg deposition via litterfall and senescence. We here report Hg concentrations and pool sizes of different plant functional groups and plant species across nine tundra sites in northern Alaska. Significant spatial differences were observed in bulk vegetation Hg concentrations at Toolik Field station (52 ± 9 μg kg⁻¹), Eight Mile Lake Observatory (40 ± 0.2 μg kg⁻¹), and seven sites along a transect from Toolik Field station to the Arctic coast (36 ± 9 μg kg⁻¹). Hg concentrations in non-vascular vegetation including feather and peat moss (58 ± 6 μg kg⁻¹ and 34 ± 2 μg kg⁻¹, respectively) and brown and white lichen (41 ± 2 μg kg⁻¹ and 34 ± 2 μg kg⁻¹, respectively), were three to six times those of vascular plant tissues (8 ± 1 μg kg⁻¹ in dwarf birch leaves and 9 ± 1 μg kg⁻¹ in tussock grass). A high representation of nonvascular vegetation in aboveground biomass resulted in substantial Hg mass contained in tundra aboveground vegetation (29 μg m⁻²), which fell within the range of foliar Hg mass estimated for forests in the United States (15 to 45 μg m⁻²) in spite of much shorter growing seasons. Hg stable isotope signatures of different plant species showed that atmospheric Hg(0) was the dominant source of Hg to tundra vegetation. Mass-dependent isotope signatures (δ²⁰²Hg) in vegetation relative to atmospheric Hg(0) showed pronounced shifts towards lower values, consistent with previously reported isotopic fractionation during foliar uptake of Hg(0). Mass-independent isotope signatures (Δ¹⁹⁹Hg) of lichen were more positive relative to atmospheric Hg(0), indicating either photochemical reduction of Hg(II) or contributions of inorganic Hg(II) from atmospheric deposition and/or dust. Δ¹⁹⁹Hg and Δ²⁰⁰Hg values in vascular plant species were similar to atmospheric Hg(0) suggesting that overall photochemical reduction and subsequent re-emission was relatively insignificant in these tundra ecosystems, in agreement with previous Hg(0) ecosystem flux measurements.
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Stable isotope compositions of mercury (Hg) were measured in the outlet stream and in soil cores at different landscape positions in a 9.7-ha boreal upland-peatland catchment in northern Minnesota. An acidic permanganate/persulfate digestion procedure was validated for water samples with high dissolved organic matter (DOM) concentrations and we verified the complete breakdown of DOM through Hg spike addition analysis. We report a relatively large variation in mass-dependent fractionation (δ²⁰²Hg; from -2.12 to -1.32 ‰) and a smaller, but significant, variation of mass-independent fractionation (Δ¹⁹⁹Hg; from -0.35 to -0.12 ‰) during two years of sampling with streamflow varying from 0.003 to 7.8 L s⁻¹. Large variations in δ²⁰²Hg occurred only during low streamflow (< 0.6 L s⁻¹). These results suggest that under high streamflow conditions a peatland lagg zone between the bog (3.0 ha) and uplands (6.7 ha) becomes the dominant source of Hg and controls the isotopic signature of Hg in downstream waters. Further, we used a binary mixing model and showed that except for the spring snowmelt period, Hg in stream water from the catchment was mainly derived from dry deposition of gaseous elemental Hg (73-95 %). This study demonstrated the usefulness of Hg isotopes for tracing sources of Hg deposition, which can lead to a better understanding of the biogeochemical cycling and hydrological transport of Hg in headwater catchments.
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To determine whether trees are reliable biomonitors of air mercury (Hg) pollution concentrations were measured in bark, foliage, and tree rings. Data were developed using 4-year old Pinus and Populus trees grown from common genetic stock in Oregon and subsequently transferred to four air treatments differing in gaseous oxidized mercury (GOM) chemistry and total gaseous Hg (TGM) concentrations. Soil of a subset of trees was spiked with HgBr2 in solution to test for root uptake. Results indicate no significant effect of the soil spike or GOM compounds on tree tissue Hg concentrations. TGM treatment had a significant effect on Pinus and Populus foliage, and Pinus year 5 growth ring concentrations. Populus foliar Hg concentrations were highest in the exposure where 24 h TGM concentrations were highest, indicating the importance of the nonstomatal pathway for uptake. Pinus tree ring concentrations were correlated to day time TGM concentrations suggesting Hg accumulation into tree rings is by way of the stomata and subsequent translocation by way of phloem. Populus leaves and Pinus rings can be used as biomonitors for TGM concentrations over space. However, the use of trees as temporal proxies requires further investigation due to radial translocation observed in active sapwood tree rings.
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Terrestrial runoff represents a major source of mercury (Hg) to aquatic ecosystems. In boreal forest catchments, such as the one in northern Sweden studied here, mercury bound to natural organic...
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Environmental regulations on mercury (Hg) emissions and associated ecosystem restoration are closely linked to what Hg levels we consider natural. It is widely accepted that atmospheric Hg deposition has increased by a factor 3±1 since pre-industrial times. However, no long-term historical records of actual atmospheric gaseous elemental Hg (GEM) concentrations exist. In this study we report Hg stable isotope signatures in Pyrenean peat records (southwestern Europe) that are used as tracers of Hg deposition pathway (Δ²⁰⁰Hg, wet vs dry Hg deposition) and atmospheric Hg sources and cycling (δ²⁰²Hg, Δ¹⁹⁹Hg). By anchoring peat-derived GEM dry deposition to modern atmospheric GEM levels we are able to reconstruct the first millennial-scale atmospheric GEM concentration record. Reconstructed GEM levels from 1970-2010 agree with monitoring data, and maximum 20th Century GEM levels of 3.9±0.5 ng m⁻³ were 15±4 times the natural Holocene background of 0.27±0.11 ng m⁻³. We suggest that a -0.7‰ shift in δ²⁰²Hg during the medieval and Renaissance periods is caused by deforestation and associated biomass burning Hg emissions. Our findings suggest therefore that human impacts on the global mercury cycle are subtler and substantially larger than currently thought.
Article
The isotopic composition of atmospheric total gaseous mercury (TGM) and particle-bound mercury (PBM) and mercury (Hg) in litterfall samples have been determined at urban/industrialized and rural sites distributed over mainland China for identifying Hg sources and transformation processes. TGM and PBM near anthropogenic emission sources display negative delta Hg-202 and near-zero Delta Hg-199 in contrast to relatively positive delta Hg-202 and negative Delta Hg-199 observed in remote regions, suggesting that different sources and atmospheric processes force the mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) in the air samples. Both MDF and MIF occur during the uptake of atmospheric Hg by plants, resulting in negative. delta Hg-202 and Delta Hg-199 observed in litter-bound Hg. The linear regression resulting from the scatter plot relating the delta Hg-202 to Delta Hg-199 data in the TGM samples indicates distinct anthropogenic or natural influences at the three study sites. A similar trend was also observed for Hg accumulated in broadleaved deciduous forest foliage grown in areas influenced by anthropogenic emissions. The relatively negative MIF in litter-bound Hg compared to TGM is likely a result of the photochemical reactions of Hg2+ in foliage. This study demonstrates the diagnostic stable Hg isotopic composition characteristics for separating atmospheric Hg of different source origins in China and provides the isotopic fractionation clues for the study of Hg bioaccumulation.
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Mercury (Hg) accumulation in montane forested areas plays an important role in global Hg cycling. In this study, we used the Hg isotope tracer to understand sources and accumulation processes of Hg in 23 forest sites at the eastern fringe of the Tibetan Plateau (TP). Litter in the TP has a mean total Hg concentration of 32±12 ng g-1, and the 0-60 cm soil horizon has a mean Hg pool size of 23±9 mg m-2. Using the signature of mass independent fractionation of odd Hg isotopes (termed as Δ199Hg and Δ201Hg), Hg sources leading to the accumulation in forest ecosystems were identified. Slightly negative Δ199Hg (-0.12 to -0.05‰) at low elevations (3100 to 3600 m) reflects influence from local anthropogenic emissions, whereas larger negative Δ199Hg (-0.38 to -0.15‰) at high elevations (3700 to 4300 m) indicates impact from long range transport. The surface soil has a mean Δ199Hg of -0.19±0.12‰, close to the mean Δ199Hg of -0.18±0.09‰ in leaf litter. The Δ199Hg in surface soil has the significant correlations to leaf area index of the canopy and to soil carbon content. Litter biomass production is controlled by the precipitation and temperature. These further suggest that Hg input by litterfall dominates the Hg accumulation in the TP forest floor, whereas the precipitation and temperature likely indirectly affect the Hg accumulation by controlling the production of litter biomass.
Article
Dry deposition of atmospheric mercury (Hg) to various land covers surrounding 24 sites in North America was estimated for the years 2009 to 2014. Depending on location, multi-year mean annual Hg dry deposition was estimated to range from 5.1 to 23.8 µg m-2 yr-1 to forested canopies, 2.6 to 20.8 µg m-2 yr-1 to non-forest vegetated canopies, 2.4 to 11.2 µg m-2 yr-1 to urban and built up land covers, and 1.0 to 3.2 µg m-2 yr-1 to water surfaces. In the rural or remote environment in North America, annual Hg dry deposition to vegetated surfaces is dominated by leaf uptake of gaseous elemental mercury (GEM), contrary to what was commonly assumed in earlier studies which frequently omitted GEM dry deposition as an important process. Dry deposition exceeded wet deposition by a large margin in all of the seasons except in the summer at the majority of the sites. GEM dry deposition over vegetated surfaces will not decrease at the same pace, and sometimes may even increase with decreasing anthropogenic emissions, suggesting that Hg emission reductions should be a long-term policy sustained by global cooperation.
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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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Mercury in the Antarctic troposphere has a distinct chemistry and challenging long-term measurements are needed for a better understanding of the atmospheric Hg reactions with oxidants and the exchanges of the various mercury forms among air-snow-sea and biota. Antarctic mosses and lichens are reliable biomonitors of airborne metals and in short time they can give useful information about Hg deposition patterns. Data summarized in this review show that although atmospheric Hg concentrations in the Southern Hemisphere are lower than those in the Northern Hemisphere, Antarctic cryptogams accumulate Hg at levels in the same range or higher than those observed for related cryptogam species in the Arctic, suggesting an enhanced deposition of bioavailable Hg in Antarctic coastal ice-free areas. In agreement with the newest findings in the literature, the Hg bioaccumulation in mosses and lichens from a nunatak particularly exposed to strong katabatic winds can be taken as evidence for a Hg contribution to coastal ecosystems by air masses from the Antarctic plateau. Human activities on the continent are mostly concentrated in coastal ice-free areas, and the deposition in these areas of Hg from the marine environment, the plateau and anthropogenic sources raises concern. The use of Antarctic cryptogams as biomonitors will be very useful to map Hg deposition patterns in costal ice-free areas and will contribute to a better understanding of Hg cycling in Antarctica and its environmental fate in terrestrial ecosystems.
Article
Long-range transport and residence time of elemental Hg (Hg°) in air promote global dispersion and deposition in remote ecosystems. Many biotic and abiotic factors contribute to the photoreduction and phytovolatilization of Hg from terrestrial ecosystems, and the assessment of deposition and volatilization fluxes is very challenging. Mosses and lichens are widespread in nature and constitute the dominant vegetation in alpine and polar ecosystems. This review surveys the results of Hg biomonitoring with cryptogams in areas with different Hg sources and deposition processes. Lichen and moss ecophysiology, and factors affecting Hg uptake and bioaccumulation are discussed. Although some laboratory experiments indicate a linear accumulation of Hg in cryptogams exposed to Hg°, without any significant release, in nature the Hg accumulated in cryptogams is in a dynamic equilibrium with Hg in air and decreases when organisms are transplanted to clean environments. Mercury concentrations in mosses and lichens have often been used to estimate concentrations and deposition fluxes of atmospheric Hg; however, Hg° exchanges between cryptogams and air, and the time necessary for mosses and lichens to equilibrate elemental composition with changing atmospheric chemistry, preclude reliable estimates. Biological processes of Hg uptake and exchange with air cannot be reproduced by mechanical collectors, and comparisons between Hg concentrations in biomonitors and those in atmospheric deposition are scarcely reliable. However, the Hg biomonitoring with mosses and lichens is easy and cheap and allows to locate "hot spots" of natural or anthropogenic emissions and to assess spatio-temporal changes in Hg deposition patterns. Climate change is affecting the global Hg cycle through the melting of sea-ice in coastal Polar Regions, and modifying Hg sequestration in mountain ecosystems. Despite limitations, large-scale monitoring of Hg with mosses and lichens may be used as a tool to evaluate the impact of global processes in remote ecosystems.
Article
There is a large uncertainty in the estimate of global dry deposition of atmospheric mercury (Hg). Hg deposition through litterfall represents an important input to terrestrial forest ecosystems via cumulative uptake of atmospheric Hg (most Hg0)to foliage. In this study, we estimate the quantity of global Hg deposition through litterfall using statistical modeling (Monte Carlo simulation) of published datasets of litterfall biomass production, tree density and Hg concentration in litter samples. Based on the model results, the global annual Hg deposition through litterfall is estimated to be 1180±710 Mg yr-1, more than two times greater than the estimate by GEOS-Chem. Spatial distribution of Hg deposition through litterfall suggests that deposition flux decreases spatially from tropical to temperate and boreal regions.Approximately 70% of global Hg0 dry deposition occurs in the tropical and subtropical regions. A major source of uncertainty in this study is the heterogeneous geospatial distribution of available data, more observational data in regions (Southeast Asia, Africa, and South America) where few data exist will greatly improve the accuracy of the current estimate. Given that the quantity of global Hg deposition via litterfall is typically 2-6 times higher than Hg0 evasion from.
Article
An ecological and biological investigation on the bryophyte communities of the Mediterranean temporary ponds was carried out in Italy. Here, the occurrence of a rich set of liverworts is very significant, as several species of Riccia characterize the communities from a physiognomical, ecological and syntaxonomical point of view. The moss component is represented only by acrocarpous mosses, e.g. Archidium alternifolium and Pleuridium acuminatum which play an important phytosociological role. In this study, the bryophyte communities are used to identify a pattern, corresponding to a “functional type” which reflects the correlated response of the bryophyte communities toward the prevalent ecological parameters of the habitat. The individuated functional type assembles life strategy, life form, lifespan, spore size, dispersal strategy, reproduction effort, independently from the taxonomical relationship of the species characterizing the communities. It represents an adaptative pattern of coevolved characters, adopted by the bryophyte communities for the habitat establishment and re-establishment following the seasonal water fluctuations typical of the investigated habitat.
Article
The current knowledge concerning mercury dry deposition is reviewed, including dry deposition algorithms used in chemical transport models (CTMs) and at monitoring sites and related deposition calculations, measurement methods and studies for quantifying dry deposition of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM), and measurement studies of litterfall and throughfall mercury. Measured median GOM plus PBM dry deposition in Asia (10.7 μg m−2 yr−1) almost double that in North America (6.1 μg m−2 yr−1) due to the higher anthropogenic emissions in Asia. Measured median litterfall and throughfall mercury are 22.3 and 56.5 μg m−2 yr−1, respectively, in Asia, 12.8 and 16.3 μg m−2 yr−1 in Europe, and 11.9 and 7.0 μg m−2 yr−1 in North America. The much higher litterfall mercury than GOM plus PBM dry deposition suggests the important contribution of gaseous elemental mercy (GEM) to mercury dry deposition to vegetated canopies. Over all the regions, including the Amazon, dry deposition, estimated as the sum of litterfall and throughfall minus open-field wet deposition, is more dominant than wet deposition for Hg deposition. Regardless of the measurement or modelling method used, a factor of two or larger uncertainties in GOM plus PBM dry deposition need to be kept in mind when using these numbers for mercury impact studies.
Article
Alpine lake sediments and glacier ice cores retrieved from high mountain regions can provide long-term records of atmospheric deposition of anthropogenic contaminants such as mercury (Hg). In this study, eight lake sediment cores and one glacier ice core were collected from high elevations across the Himalaya-Tibet region to investigate the chronology of atmospheric Hg deposition. Consistent with modeling results, the sediment core records showed higher Hg accumulation rates in the southern slopes of the Himalayas than those in the northern slopes in the recent decades (post-World War II). Despite much lower Hg accumulation rates obtained from the glacier ice core, the temporal trend in the Hg accumulation rates matched very well with that observed from the sediment cores. The combination of the lake sediments and glacier ice core allowed us to reconstruct the longest, high-resolution atmospheric Hg deposition chronology in High Asia. The chronology showed that the Hg deposition rate was low between the 1500s and early 1800, rising at the onset of the Industrial Revolution, followed by a dramatic increase after World War II. The increasing trend continues to the present-day in most of the records, reflecting the continuous increase in anthropogenic Hg emissions from South Asia.
Article
Gaseous elemental mercury (GEM) is the dominant form of mercury in the atmosphere. Its conversion into oxidized gaseous and particulate forms is thought to drive atmospheric mercury wet deposition to terrestrial and aquatic ecosystems, where it can be subsequently transformed into toxic methylmercury. The contribution of mercury dry deposition is however largely unconstrained. Here we examine mercury mass balance and mercury stable isotope composition in a peat bog ecosystem. We find that isotope signatures of living sphagnum moss (Δ(199)Hg = -0.11±0.09 ‰, Δ(200)Hg = 0.03±0.02 ‰, 1σ) and recently accumulated peat (Δ(199)Hg = -0.22±0.06 ‰, Δ(200)Hg = 0.00±0.04 ‰, 1σ) are characteristic of GEM (Δ(199)Hg = -0.17±0.07 ‰, Δ(200)Hg = -0.05±0.02 ‰, 1σ), and differs from wet deposition (Δ(199)Hg = 0.73±0.15 ‰, Δ(200)Hg = 0.21±0.04 ‰, 1σ). Sphagnum covered during three years by transparent and opaque surfaces, which eliminate wet deposition, continue to accumulate Hg. Sphagnum Hg isotope signatures indicate accumulation to take place by GEM dry deposition, and indicate little photochemical re-emission. We estimate that atmospheric mercury deposition to the peat bog surface is dominated by GEM dry deposition (79%) rather than wet deposition (21%). Consequently, peat deposits are potential records of past atmospheric GEM concentrations and isotopic composition.
Article
Despite 30 years of study, gaseous elemental mercury (Hg(0)) exchanges between terrestrial surfaces and the atmosphere still remain uncertain. We compiled data from 132 studies, including 1,290 reported fluxes from more than 200,000 individual measurements, into a database to statistically examine flux magnitudes and controls. We found that fluxes were unevenly distributed, both spatially and temporally, with strong biases toward Hg-enriched sites, daytime and summertime measurements. Fluxes at Hg-enriched sites were positively correlated with substrate concentrations, but this was absent at background sites. Median fluxes over litter- and snow-covered soils were lower than over bare soils, and chamber measurements showed higher emission compared to micrometeorological measurements. Due to low spatial extent, estimated emissions from Hg-enriched areas (217 Mg∙a(-1)) were lower than previous estimates. Globally, areas with enhanced atmospheric Hg0 levels (particularly East Asia) showed an emerging importance of Hg(0) emissions accounting for half of the total global emissions estimated at 607 Mg∙a(-1). The largest uncertainties in Hg(0) fluxes stem from forests (37.5(th) and 62.5(th) percentiles were -513 and +1,353 Mg∙a(-1), respectively), largely driven by a shortage of whole-ecosystem fluxes and uncertain contributions to leaf-atmosphere exchanges, questioning to what degree ecosystems are net sinks or sources of atmospheric Hg(0).
Article
Past emissions of the toxic metal mercury (Hg) persist in the global environment, yet these emissions remain poorly constrained by existing data. Ice cores are high-resolution archives of atmospheric deposition that may provide crucial insight into past atmospheric Hg levels during recent and historical time. Here we present a record of total Hg (HgT) in an ice core from the pristine summit plateau (5340 m asl) of Mount Logan, Yukon, Canada, representing atmospheric deposition from AD 1410 to 1998. The Colonial Period (~1603-1850) and North American "Gold Rush" (1850-1900) represent minor fractions (8% and 14%, respectively) of total anthropogenic Hg deposition in the record, with the majority (78%) occurring during the 20th Century. A period of maximum HgT fluxes from 1940 to 1975 coincides with estimates of enhanced anthropogenic Hg emissions from commercial sources, as well as with industrial emissions of other toxic metals. Rapid declines in HgT fluxes following peaks during the "Gold Rush" and the mid-20th Century indicate that atmospheric Hg deposition responds quickly to reductions in emissions. Increasing HgT fluxes from 1993 until the youngest samples in 1998 may reflect the resurgence of Hg emissions from unregulated coal burning and small-scale gold mining.
Article
Soils comprise the largest terrestrial mercury (Hg) pool in exchange with the atmosphere. To predict how anthropogenic emissions affect global Hg cycling and eventually human Hg exposure, it is crucial to understand Hg deposition and re-emission of legacy Hg from soils. However, assessing Hg deposition and re-emission pathways remains difficult because of an insufficient understanding of the governing processes. We measured Hg stable isotope signatures of radiocarbon-dated boreal forest soils and modeled atmospheric Hg deposition and re-emission pathways and fluxes using a combined source and process tracing approach. Our results suggest that Hg in the soils was dominantly derived from deposition of litter (~90% on average). The remaining fraction was attributed to precipitation-derived Hg, which showed increasing contributions in older deeper soil horizons (up to 27%) indicative of an accumulation over decades. We provide evidence for significant Hg re-emission from organic soil horizons most likely caused by non-photochemical abiotic reduction by natural organic matter, a process previously not observed unambiguously in nature. Our data suggest that Histosols (peat soils), which exhibit at least seasonally water-saturated conditions, have re-emitted up to one third of previously deposited Hg back to the atmosphere. Re-emission of legacy Hg following reduction by natural organic matter may therefore be an important pathway to be considered in global models, further supporting the need for a process-based assessment of land/atmosphere Hg exchange.
Article
Virtually all biotic, dark abiotic, and photochemical transformations of mercury (Hg) produce Hg isotope fractionation, which can be either mass dependent (MDF) or mass independent (MIF). The largest range in MDF is observed among geological materials and rainfall impacted by anthropogenic sources. The largest positive MIF of Hg isotopes (odd-mass excess) is caused by photochemical degradation of methylmercury in water. This signature is retained through the food web and measured in all freshwater and marine fish. The largest negative MIF of Hg isotopes (odd-mass deficit) is caused by photochemical reduction of inorganic Hg and has been observed in Arctic snow and plant foliage. Ratios of MDF to MIF and ratios of 199Hg MIF to 201Hg MIF are often diagnostic of biogeochemical reaction pathways. More than a decade of research demonstrates that Hg isotopes can be used to trace sources, biogeochemical cycling, and reactions involving Hg in the environment.
Article
Quantifying the concentration of gaseous oxidized mercury (GOM) and the chemical compounds in the atmosphere is important for developing accurate local, regional, and global biogeochemical cycles. The major hypothesis driving this work was that relative humidity affects collection of GOM on KCl-coated denuders and nylon membrane both currently being applied to measure GOM. Using a laboratory manifold system and ambient air, GOM capture efficiency on three different collection surfaces, including KCl-coated denuders, nylon membranes, and cation-exchange membranes, was investigated at relative humidities ranging from 25 to 75%. Recovery of permeated HgBr2 on KCl-coated denuders declined by 4-60 % during spikes of relative humidity (25 to 75%). When spikes were turned off GOM recoveries returned to 60±19% of permeated levels. In some cases, KCl-coated denuders were gradually passivated over time after additional humidity was applied. In this study, GOM recovery on nylon membranes decreased with high humidity and ozone concentrations. However, additional humidity enhanced GOM recovery on cation-exchange membranes. In addition, reduction and oxidation of elemental mercury during experiments was observed. The findings in this study can help to explain field observations in previous studies.
Article
The importance of Hg emissions for deposition will be scrutinized in the future as new legislation to control emissions of Hg to the atmosphere comes into effect. We show that mercury (Hg) concentrations in rainfall are closely linked to organic matter (OM) with consistent Hg/TOC ratios over large spatial scales decreasing from that in an open field (OF, 1.5 μg g–1) to that in throughfall (TF, 0.9 μg g–1). The leaf area index was positively correlated with both TF [Hg] and total organic carbon ([TOC]), but not the Hg/TOC ratio. This study shows that the progression in the Hg/TOC ratio through catchments starts in precipitation with Hg/TOCbulk dep > Hg/TOCsoil water > Hg/TOCstreamwater. These findings raise an intriguing question about the extent to which it is not just atmospheric [Hg] but also OM that influences [Hg] in precipitation. This question should be resolved to improve the ability to discern the importance of changing global Hg emissions for deposition of Hg at specific sites.
Article
High-elevation ecosystems of the northeastern United States are vulnerable to deposition and environmental accumulation of atmospheric pollutants, yet little work has been done to assess mercury (Hg) concentrations in organisms occupying montane ecosystems. The authors present data on Hg concentrations in ground-foraging insectivorous songbirds, a terrestrial salamander, and forest floor horizons sampled along a forested elevational gradient from 185 m to 1273 m in the Catskill Mountains, New York, USA. Mean Hg concentrations in Catharus thrushes and the salamander Plethodon cinereus increased with elevation, as did Hg concentrations in all forest floor horizons. Mean Hg concentrationsin in organic soils at approximately 1200 m elevation (503.5 ± 17.7 ng/g, dry wt) were 4.4-fold greater than those at approximately 200 m. Montane ecosystems of the northeastern United States, and probably elsewhere, are exposed to higher levels of atmospheric Hg deposition as reflected in accumulation patterns in the forest floor and associated high-elevation fauna. This information can be used to parameterize and test Hg transport and bioaccumulation models of landscape-specific patterns and may serve as a monitoring tool for decision makers considering future controls on Hg emissions. Further investigation is needed into the potential effects of increased Hg concentrations on high-elevation fauna. Environ Toxicol Chem 2014;33:XX-XX. © 2013 SETAC.