Final Report: Effects of Global Change on the Atmospheric Mercury Burden and Mercury Sequestration Through Changes in Ecosystem Carbon Pools

EPA Grant Number: R833378
Title: Effects of Global Change on the Atmospheric Mercury Burden and Mercury Sequestration Through Changes in Ecosystem Carbon Pools
Investigators: Obrist, Daniel , Johnson, Dale W. , Lindberg, Steve , Luo, Yiqi
Institution: Desert Research Institute , University of Oklahoma , University of Nevada - Reno
Current Institution: Desert Research Institute , University of Nevada - Reno , University of Oklahoma
EPA Project Officer: Chung, Serena
Project Period: May 1, 2007 through April 30, 2012
Project Amount: $899,091
RFA: Consequences of Global Change For Air Quality (2006) RFA Text |  Recipients Lists
Research Category: Global Climate Change , Climate Change , Air

Objective:

Mercury (Hg) loads in remote terrestrial ecosystems are dominated by atmospheric deposition, and large amounts of past and present Hg pollution – or so-called “legacy” pollution – reside in vegetation, litter, and surface soils. Changes in storage, mobilization, and fate processes of these terrestrial Hg reservoirs may have significant implications for global cycling of Hg. In particular, terrestrial reservoirs are a concern for present and future atmospheric loads due to back-evasion of Hg to the atmosphere – also termed secondary emissions or legacy emissions. Such legacy emissions have been predicted to exceed current primary anthropogenic emissions, and they could become increasingly important as primary anthropogenic emissions are reduced and pollutants continue to accumulate in surface reservoirs. Legacy re-emissions thereby have the potential to lead to a continued cycling of past and present Hg pollution between surfaces and the atmosphere, and to prolong the effective exposure time of atmospheric Hg pollution in the environment.

The first objective of this study was to develop a systematic inventory of the amount of Hg – both total Hg (THg) and monomethyl-Hg (Methyl-Hg; the most toxic form of Hg) – that currently is sequestered in terrestrial ecosystems across the United States to constrain the size of legacy Hg pools contained in terrestrial surface reservoirs. The second objective was to assess relationships of Hg to soil carbon (C), which provides key sorption sites for retention of Hg in terrestrial sites, and to auxiliary ecological and climate variables to characterize the critical factors related to terrestrial Hg accumulation. Third, we conducted experimental studies to determine the fate and stability of Hg in surface soils and litter horizons, in particular quantifying re-emission potential to the atmosphere during fires and during C mineralization processes. Finally, we expanded a terrestrial C model (the Community Land Model [CLM] - CASA approach) by a novel Hg component to spatially extrapolate surface pools of Hg to the entire contiguous United States and world; and to assess the sensitivity that global change-induced alterations in terrestrial C may have on sequestration of Hg. Results have been published in 15 peer-reviewed articles, 4 additional manuscripts in review and preparation, 2 master’s theses, and 31 conference presentations and abstracts.

Summary/Accomplishments (Outputs/Outcomes):

Policy-relevant findings of this project:

  1. Systematic quantification of THg and Methyl-Hg concentrations across U.S. ecosystems
    • Across 14 terrestrial sites, soils show highest THg accumulation (averaging 91% of total areal mass), with lower storage found in litter (8%) and negligible amounts in aboveground biomass (<1%); as such, soils have the highest potential for Hg re-emissions and re-mobilization upon ecosystem disturbances and climate change.
    • Hg concentration, areal mass, and Hg/C ratios were found to increase with increasing latitude, soil C content, and precipitation. Hence, particularly high accumulations of Hg were found in northern and coastal sites that are high in organic C.
    • Soil C is strongly correlated with soil Hg across all sites, supporting a fundamental determining role of organic matter and C for terrestrial Hg accumulation.
    • Sites and ecosystem components high in THg also were generally high in Methyl-Hg, indicating that accumulation of THg in terrestrial sites has direct implications for exposure to this most toxic Hg form.
  2. Fate of terrestrial Hg to losses of C by fires and C mineralization
    • Fires lead to near-complete re-emission of Hg in aboveground biomass and re-mobilize significant amounts of Hg from surface litter. Wildfires and prescribed fires did not lead to significant re-emissions of the large soil Hg pools in temperate forest sites.
    • Experimental studies show a strong potential for atmospheric re-emission of Hg upon C mineralization from surface litter, indicating that up to 50% of litter-bound Hg is re-emitted to the atmosphere as gaseous Hg. In soils, losses of Hg to the atmosphere upon C mineralization were smaller, accounting for a few percent of the original soil Hg/C content. Still, such soil re-emissions are important as soils constitute the largest ecosystem Hg pools.
    • Using soil pore measurements, soils further were found to occasionally be an unexpected sink for gaseous Hg, possibly explaining the smaller re-emission potential of Hg from soils as compared to litter. Soils remain highly complex in regard to soil Hg dynamics, and large uncertainties remain in understanding the re-emission potential of soil-bound Hg to the atmosphere.
  3. Spatial extrapolation of Hg reservoirs and sensitivity to climate variables using a terrestrial C model
    • Total mass of Hg stored in top soils (upper 40 cm) is estimated at 15,230 metric tons in the contiguous United States and at more than 300,000 metric tons globally.
    • Highest soil Hg densities are predicted for the Northeastern and Northwestern United States, in accordance with observed patterns in the field; the Southwest and Midwest show lowest soil Hg densities across the country.
    • Model results suggest a strong sensitivity of soil Hg levels to changes in CO2 concentrations, precipitation, and annual temperatures due to strong underlying changes in soil C pools. Compared to present-day values, increases in CO2 concentrations to 700 ppm is expected to stimulate soil Hg storage; increases in precipitation will increase soil Hg storage while a reduction in precipitation will decrease soil Hg storage. Increased air temperatures are expected to have small, negative effects on soil Hg storage.
    • Responses of soil Hg to climate variables are expected to be highly regional. A high sensitivity to precipitation is expected to occur in the Northwestern and Northeastern United States, and effects of CO2 on soil Hg pools are strongly governed by annual precipitation.

Summary of Accomplishments (Outputs/Outcomes):

Objective 1: Systematic quantification of THg and Methyl-Hg concentrations and pool sizes in vegetation, litter, and soils across U.S. forest ecosystems; Objective 2: Spatial patterns of THg and Methyl-Hg and relationships to soil C and other environmental variables.

We conducted an intensive 3-year sampling campaign to develop a systematic database on Hg surface reservoirs across 14 forest sites in the United States (see Figure 1). We sampled all major vegetation and soil compartments (foliage, branches, bole, and bark of dominant tree species); surface litter (Oi horizon); partially decomposed litter (Oe horizon); fully decomposed litter (Oa horizon); and surface soils (two to five depths, depending on site). Concentrations of THg, Methyl-Hg, C, and nitrogen (N) were quantified in these ecosystem components, and we used detailed biomass, litter mass, soil mass, and C inventories to calculate total areal mass of THg and MeHg across all 14 sites.

In Obrist, et al., 2011 (Environmental Science & Technology 45:3974-3981), we reported that across these 14 sites, the highest concentrations of THg are observed in litter layers, which were strongly enriched in THg compared to aboveground biomass. Soil Hg concentrations across sites were lower than in litter, with highest concentrations in surface soils. The vertical Hg accumulation patterns of THg are consistent with substantial, post-depositional sorption of Hg from atmospheric deposition in soils and litter. Aboveground tissues showed no detectable spatial patterns across the 14 sites, possibly due to high species-specific variation in Hg concentrations of the 17 different tree species present across the sites. Litter and soil THg concentrations, however, increased significantly with increasing latitude, precipitation, and soil C and clay content (in soils only). A multi-regression analysis combining these four variables showed a coefficient of determination, r2, of up to 94% for THg in soils across these sites. Observed latitudinal increases of THg concentrations in soils and litter were in contrast to inverse latitudinal gradients generally observed for atmospheric deposition measures, indicating that THg accumulation in surface reservoirs may be controlled by factors other than current atmospheric deposition loads. Soil THg concentrations strongly correlated with organic C content, indicating that well known associations between organic matter and Hg determine spatial distribution of THg at continental scales. We suggest that the link between C and Hg distribution represents a legacy effect whereby old, C-rich soil and litter layers sequester and store large amounts of past atmospheric Hg depositions for long time periods.

In Obrist, 2012 (Environmental Science & Technology 46:5921-5930), we reported distribution patterns of Methyl-Hg and areal mass of both THg and Methyl-Hg across these forest sites. Methyl-Hg concentrations increased from surface litter to intermediate and deeper, decomposed litter and were lower in soils, thereby showing similar patterns as observed for THg. Ratios of Methyl-Hg to THg were higher in litter compared to soils. Methyl-Hg concentrations in soils positively correlated with THg across sites, and Methyl-Hg concentrations also increased with C content and latitude. We calculated that areal mass of THg ranged from 41.6 g ha-1 to 268.8 g ha-1, and that the largest THg mass at all sites was sequestered in soils (average of 91%), followed by litter (8%) and aboveground biomass (<1%). Methyl-Hg mass (litter plus soils only) ranged from 75 to 443 mg ha-1, of which 88% was found in soils. These data show that by far the largest pool sizes of THg and MeHg are associated with soils, which account for the main ecosystem storage reservoirs. Both THg and MeHg mass correlated with latitude, with average mass increases of 10.6 g ha-1 (THg) and 20 mg ha-1 (Methyl-Hg) per degree latitude, indicating that the largest THg and Methyl-Hg pools are expected to be found in northern forests with large detrital and soil C pools.

In Obrist, et al., 2012 (Journal of Plant Nutrition and Soil Science 175:68-77) and Obrist, et al., 2009 (Biogeosciences 6:1-13), we reported details of THg and Methyl-Hg in several adjacent study sites and quantified how fire occurrence and different vegetation types affect THg and Methyl-Hg accumulation patterns. These results are discussed in detail under Objective 3 below.

Additional field studies that addressed terrestrial Hg dynamics by Faïn, et al., 2011 (Geochimica et Cosmochimica Acta 75:2379-2392) and Faïn, et al. (in preparation) support the importance of upland terrestrial sites for uptake, retention, and re-emission of atmospheric Hg deposition. Faïn, et al. (2011) calculated that stream run-off accounted for only 4% of total atmospheric Hg deposition in a Sierra Nevada site and concluded that a large fraction of mercury deposition is sequestered in soils rather than subject to runoff. In addition, snowpack accounted for only one-half of the Hg deposited by snowfall in winter, indicating that reduction processes lead to subsequent volatilization and substantial and immediate re-emission of snow-related Hg depositions to the atmosphere. This result also is supported by Faïn, et al. (in preparation), which will show significant redox reactions in a Rocky Mountain snowpack and that a significant fraction of Hg deposition in snow is re-emitted back to the atmosphere prior to the onset of snowmelt.

Objective 3: Assessing the fate of terrestrial Hg to losses of C induced by fires and mineralization

Since terrestrial Hg pools are strongly associated with organic matter and C, C dynamics have the potential to re-mobilize Hg that is associated with C pools. This objective aimed to quantify the fate of Hg on C losses – with a focus on the potential for atmospheric re-emissions  – as induced by fires and C mineralization.

In Obrist, 2007 (Biogeochemistry 85, 119-123), we calculated that vegetation alone accumulates as much as 1,000 Mg of atmospheric Hg annually in aboveground biomass – which is equivalent to about 20% of the global atmospheric Hg load. We hypothesized that past and future changes in biomass productivity and organic C dynamics may have significant effects on atmospheric Hg levels. Specifically, we proposed that large losses in soil and biomass C pools during the last 150 years could have contributed significantly to observed increases in atmospheric Hg pollution, and that future global-change induced alterations of C dynamics may lead to re-emissions of Hg to the atmosphere. A dearth of data on the fate of Hg on C losses made such predictions highly uncertain at the start of this project.

Effects of fires:  An important remobilization potential for terrestrial Hg occurs through fires. In Obrist et al., 2008 (Environmental Science and Technology, 42, 721-727), we showed that aboveground fuel combustion (including leaves, needles, and branches) fully re-emit Hg associated with these tissues. Emissions occur both as gaseous elemental Hg (GEM) and particulate-bound Hg (PHg): atmospheric emissions are dominated by GEM in dry fuels, but reach up to 50% as PHg in fuels with high moisture. These study results suggest that fuel moisture – possibly via underlying differences in smoldering versus flaming combustion – causes differences in emission patterns and determines to what degree fire-induced atmospheric emissions enter the global atmospheric Hg pool (as expected for GEM) or are mainly of local and regional importance (as expected for PHg).

In Obrist et al., 2009 (Biogeosciences 6, 1-13), we compared THg concentrations and pools sizes of four Sierra Nevada forests, with two intact sites adjacent to two sites affected by a wildfire and a prescribed fire. We showed that aboveground pools at the post-fire sites contained about half of the Hg areal mass of adjacent intact sites (45% and 52% lower aboveground pool sizes, respectively), indicating significant re-mobilization of aboveground THg pools. Hg losses were mainly due to reductions in litter pools (for the prescribed fire site) and removal of standing aboveground biomass (at the wildfire site). No significant changes in soil Hg pools between burned and unburned sites were observed at any site, indicating that fires at these sites do not deplete the large soil THg storage pools (which represent 94–98% of total ecosystem areal mass).

In Obrist et al., 2012 (Journal of Plant Nutrition and Soil Science, 175, 68-77), we compared Hg levels in two sites in Washington state 80 years after the occurrence of stand-replacing wildfires which were re-vegetated by different tree species. Results showed that the site re-vegetated by coniferous Douglas-fir trees was characterized by significantly higher THg and Methyl-Hg concentrations in aboveground biomass and litter compared a red alder stand, with differences in litter horizons between sites reaching more than 200 µg kg-1 of THg, indicating that vegetation types can strongly shape accumulation of Hg in terrestrial sites. No differences in THg and MeHg concentrations were observed in soils, nor in areal mass of Hg between sites.

Effects of C mineralization:  In Pokharel and Obrist, 2011 (Biogeosciences, 8, 2507-2521), we performed controlled laboratory litter incubation studies and quantified changes in dry mass, C, N, and Hg mass during litter decomposition for 18 months. Using a mass balance approach, we observed significant mass losses of Hg during decomposition (5 to 23% of initial mass after 18 months), which we attributed to re-emission losses of gaseous Hg to the atmosphere. Percentage mass losses of Hg generally were less than observed dry mass and C mass losses (48% to 63% Hg loss per unit dry mass loss), although one of four litter type showed similar losses. Our results indicate large gaseous emissions, or re-emissions, of Hg associated with plant litter upon decomposition in the range of 50% of the initial Hg contained in litter. Litter types showed highly species-specific differences in Hg levels during decomposition suggesting that emissions, retention, and sorption of Hg on decomposition are dependent on litter type.

In Obrist et al., 2010 (The Science of Total Environment, 408, 1691–1700), we quantified losses of Hg on C mineralization in soils by corresponding measurements of GEM emissions and carbon dioxide (CO2) respiration under controlled laboratory conditions. Results showed a linear correlation (r2 = 0.49) between GEM and CO2 emissions across 41 soil samples, an effect we attribute to partial re-mobilization of Hg during C mineralization. Stoichiometric comparison of Hg/C ratios of emissions and underlying soil substrates, however, suggests that atmospheric re-emissions of Hg were small only accounting for about 3% of the original Hg/C ratios of the soil. We also found that changes in environmental conditions, in particular implementation of anaerobic soil conditions, lead to pronounced pulses of GEM emissions to the atmosphere, which we attribute to possible microbial reduction of Hg(II) by anaerobes or to changes in soil redox conditions.

In Pokharel, 2011 (published Master’s thesis at the University of Nevada, Reno) as well as Pokharel and Obrist (in preparation), we performed continuous, in situ measurement of soil pore GEM and CO2 concentrations to assess linkages between the behavior of these two gases on soils. We developed and operated a novel, continuous soil CO2 and GEM monitoring system and were able to provide the first-ever continuous observations of soil pore GEM concentrations. Our results showed consistently lower GEM concentrations in the soil pores compared to ambient air at two measurement sites at soil depths of 7 cm, 20 cm, and 40 cm. Soil pore GEM depth profiles showed decreasing levels with increasing soil depth, and as such were in contrast to the typical diffusion gradients observed for CO2. Our results indicate that soils served as an unexpected sink for GEM. Such a soil GEM sink could be a reason that Hg mobilized during mineralization in soils does not re-emit to the atmosphere but rather is taken up again by the soil matrix, and may explain differences in Hg re-emission potential between litter and soils (see above).

An associated study performed by collaborators Moore and Castro, 2012 (The Science of the Total Environment 419, 136-143) supports the finding of the above study: using manual collections of soil pore GEM concentrations at other sites, they showed that GEM was regularly depleted in soils compared to ambient air, although at times and particularly in upper horizons they also observed increased GEM levels. These authors also reported that soil pore GEM concentrations correlated with soil temperature and that redox potential played a key role in GEM soil pore concentrations, with potential consequences for soil Hg emissions to the atmosphere.

An associated study  by Co-PI Lindberg (Zhang et al., 2008, Atmospheric Environment 42:5424-5433) further reported unexpected behavior or GEM emissions from soils, in particular indicating consistent, re-occurring diel cycles of emissions from dry soils held at constant temperature in the dark in the laboratory. Observed Hg emission cycles coincided well with ozone, indicating that near-ground atmospheric chemistry may exert an additional influence on GEM emissions from soils. This study emphasizes the complex and currently poorly understood behavior of GEM emissions from soils.

Clearly, re-emissions of Hg to the atmosphere as addressed in this study is not the only environmental concern in regard to mobilization of large Hg pools contained in terrestrial ecosystems. In Teisserenc et al. (Limnology and Oceanography, in review), we show how reservoir impoundment causes substantial leaching of Hg from upland, inundated soils. Such mobilization of Hg from soils can substantially increase Hg loads in watersheds for decades after disturbances. This study serves as a strong example that large Hg pools accumulating in soils due to present and past atmospheric pollution may have substantial consequences for ecosystem health when these pools become mobilized by disturbance.

Objective 4: Spatial extrapolation of Hg surface reservoirs and assessing the sensitivity of global change on Hg sequestration storage via changes in soil C pools

In Obrist et al., 2012 (Environmental Science and Technology, 45, 3974–3981) and Hararuk et al., (Biogeosciences, in review), we present results from the development of an Hg component for a commonly employed global C simulation model (Community Land Model; CASA version 3.5 [CLM-CASA]). The model allows simulation of soil C concentrations and densities (areal mass in g m-2) across the contiguous U.S. and globe; a novel Hg component then implements Hg/C ratios observed in the systematic field study of Objective 1 of this study to simulate soil Hg densities. We used this model to spatially extrapolate surface soil Hg levels in top soil (top 40 cm) reservoirs, and to assess the sensitivity of Hg levels to changes in soil C as predicted to occur under various global change scenarios.

In Hararuk et al. (Biogeosciences; in review), model simulations showed that current sequestration of Hg across the contiguous U.S. accounts for 15,230 metric tons of Hg in the top 0–40 cm of soils. Predicted and observed Hg mass densities at 14 field observation sites plus additional published data found in sites across the U.S., showed relatively good agreement between observed and predicted soil Hg densities (generally within 15.6% of each other; r=0.47, P=0.024). Predicted spatial distribution patterns of Hg generally agreed with those of soil Hg densities observed across field sites. In particular, model results reflect a clear trend of increasing Hg concentrations with increasing latitude, and strong effects of soil C content and precipitation on Hg distribution patterns. Highest soil Hg densities were predicted for the northeastern and northwestern U.S., in accordance with observed patterns that showed highest soil Hg densities in two sites located in Washington state and a site in Maine. Southwest and Midwest pool sizes showed among the lowest soil Hg densities of all sites. If Hg soil densities observed and modeled for the contiguous U.S. are extrapolated to the global scale, we predict a total global surface pool of Hg in the top 40 cm of the soils of 300,000 metric tons.

The model uses statistical, present-day relationships of Hg to soil C and other environmental variables to estimate soil Hg loads, and we used these relationships to assess the potential sensitivity of Hg pools to climate change. Model results predict a strong sensitivity of soil Hg densities to increases in CO2 concentrations (from 380 to 700 ppm) which generally stimulates both soil C densities and soil Hg concentrations. This model result is in support of experimental observations in Free-Air-CO₂-Enrichment (FACE) experiments showing that CO₂ increases soil Hg content, likely through expanded soil C pools. Soil Hg pools also were predicted to increase beyond present-day values following an increase in precipitation and decrease following a reduction in precipitation; the effects of precipitation were strongly regional, however, with highest sensitivity along the West and East Coast. Increased air temperatures had small, negative effects on both soil C and Hg densities.

Two current limitations of the models are that statistical relationships between soil C and soil Hg are based on present-day observations and therefore do not simulate process-driven and biogeochemical changes in Hg cycling under climate change; and that changes in soil Hg are linearly related to changes in soil C, which does not allow simulation of different fate processes of Hg to C dynamics such as reported above (e.g., C losses through wildfires, litter decomposition, versus soil C mineralization). A key finding of our modeling component is that responses of Hg to climate change show pronounced regional differences; for example, the climate sensitivity of soil Hg is expected to be particularly high in the northwestern and northeastern U.S. due to high soil Hg densities resulting from large precipitation amounts and large soil C pools. The combined effects of increased CO2 are governed strongly by precipitation and are different by region. Based on the modeling results, we conclude that precipitation and CO2 should be emphasized as main factors when assessing how climate-induced changes in soil C may affect sequestration of Hg in soils.

Conclusions:

 Our study shows that very high pools of Hg – exceeding 15 kilotons in the contiguous U.S. alone – are presently stored in terrestrial ecosystems, of which the largest pool is located in surface soils. Hg pools are strongly associated with organic C and have a tendency to partially re-emit to the atmosphere upon losses of C both via fires and mineralization. The potential for re-emissions is highest for Hg associated with aboveground biomass and litter (leading up-to-complete re-emissions during fires), and are relatively smaller (in a few percent range) for Hg sequestered in soils. Modeling studies suggest that terrestrial Hg storage is sensitive to climate change due to expected changes in soil C distribution, and that changes are expected to be highly regional. The absolute magnitude of climate change on re-emissions of legacy Hg pollution from terrestrial systems, and their effects on the atmospheric Hg burden, is difficult to predict because re-emission from soil – the largest terrestrial Hg pool – is highly complex and will require further experimental studies to understand respective mechanisms. Still, data of this study have been are used in complex global atmospheric models (e.g, the GEOS-Chem Global Terrestrial Mercury Model) where they provide key constraints on Hg reservoir sizes, spatial distribution patterns, and re-emission potentials of Hg to the atmosphere.


Journal Articles on this Report : 20 Displayed | Download in RIS Format

Other project views: All 64 publications 21 publications in selected types All 21 journal articles
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Journal Article Fain X, Obrist D, Pierce A, Barth C, Gustin MS, Boyle DP. Whole-watershed mercury balance at Sagehen Creek, Sierra Nevada, CA. Geochimica et Cosmochimica Acta 2011;75(9):2379-2392. R833378 (2010)
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  • Journal Article Fain X, Helmig D, Hueber J, Obrist D, Williams MW. Mercury dynamics in the Rocky Mountain, Colorado, snowpack. Biogeosciences 2013;10(6):3793-3807. R833378 (Final)
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  • Journal Article Graydon JA, St. Louis VL, Hintelmann H, Lindberg SE, Sandilands KA, Rudd JW, Kelly CA, Hall BD, Mowat LD. Long-term wet and dry deposition of total and methyl mercury in the remote boreal ecoregion of Canada. Environmental Science & Technology 2008;42(22):8345-8351. R833378 (2009)
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  • Journal Article Graydon JA, St. Louis VL, Hintelmann H, Lindberg SE, Sandilands KA, Rudd JWM, Kelly CA, Tate MT, Krabbenhoft DP, Lehnherr I. Investigation of uptake and retention of atmospheric Hg(II) by boreal forest plants using stable Hg isotopes. Environmental Science & Technology 2009;43(13):4960-4966. R833378 (2010)
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  • Journal Article Graydon JA, St. Louis VL, Lindberg SE, Sandilands KA, Rudd JWM, Kelly CA, Harris R, Tate MT, Krabbenhoft DP, Emmerton CA, Asmath H, Richardson M. The role of terrestrial vegetation in atmospheric Hg deposition: pools and fluxes of spike and ambient Hg from the METAALICUS experiment. Global Biogeochemical Cycles 2012;26(1):GB1022 (14 pp.). R833378 (Final)
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  • Journal Article Hararuk O, Obrist D, Luo Y. Modelling the sensitivity of soil mercury storage to climate-induced changes in soil carbon pools. Biogeosciences 2013;10(4):2393-2407. R833378 (Final)
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  • Journal Article Moore CW, Castro MS. Investigation of factors affecting gaseous mercury concentrations in soils. Science of The Total Environment 2012;419:136-143. R833378 (Final)
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  • Journal Article Obrist D. Atmospheric mercury pollution due to losses of terrestrial carbon pools? Biogeochemistry 2007;85(2):119-123. R833378 (Final)
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  • Journal Article Obrist D, Moosmuller H, Schurmann R, Chen L-WA, Kreidenweis SM. Particulate-phase and gaseous elemental mercury emissions during biomass combustion:controlling factors and correlation with particulate matter emissions. Environmental Science & Technology 2008;42(3):721-727. R833378 (Final)
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  • Journal Article Obrist D, Hallar AG, McCubbin I, Stephens BB, Rahn T. Atmospheric mercury concentrations at Storm Peak Laboratory in the Rocky Mountains: evidence for long-range transport from Asia, boundary layer contributions, and plant mercury uptake. Atmospheric Environment 2008;42(33):7579-7589. R833378 (2009)
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  • Journal Article Obrist D, Johnson DW, Lindberg SE. Mercury concentrations and pools in four Sierra Nevada forest sites, and relationships to organic carbon and nitrogen. Biogeosciences 2009;6(5):765-777. R833378 (2009)
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  • Journal Article Obrist D, Fain X, Berger C. Gaseous elemental mercury emissions and CO2 respiration rates in terrestrial soils under controlled aerobic and anaerobic laboratory conditions. Science of The Total Environment 2010;408(7):1691-1700. R833378 (2009)
    R833378 (2010)
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  • Journal Article Obrist D, Johnson DW, Lindberg SW, Luo Y, Hararuk O, Bracho R, Battles JJ, Dail DB, Edmonds RL, Monson RK, Ollinger SV, Pallardy SG, Pregitzer KS, Todd DE. Mercury distribution across 14 U.S. forests. Part I: Spatial patterns of concentrations in biomass, litter, and soils. Environmental Science & Technology 2011:45(9):3974-3981. R833378 (2010)
    R833378 (Final)
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  • Journal Article Obrist D, Johnson DW, Edmonds RL. Effects of vegetation type on mercury concentrations and pools in two adjacent coniferous and deciduous forests. Journal of Plant Nutrition and Soil Science 2012;175(1):68-77. R833378 (2010)
    R833378 (Final)
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  • Journal Article Obrist D. Mercury distribution across 14 U.S. forests. Part II: Patterns of methyl mercury concentrations and areal mass of total and methyl mercury. Environmental Science & Technology 2012;46(11):5921-5930. R833378 (Final)
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  • Journal Article Obrist D, Pokharel AK, Moore C. Vertical profile measurements of soil air suggest immobilization of gaseous elemental mercury in mineral soil. Environmental Science & Technology 2014;48(4):2242-2252. R833378 (Final)
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  • Journal Article Pokharel AK, Obrist D. Fate of mercury in tree litter during decomposition. Biogeosciences 2011;8(9):2507-2521. R833378 (2010)
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  • Journal Article Tas E, Obrist D, Peleg M, Matveev V, Fain X, Asaf D, Luria M. Measurement-based modeling of bromine-induced oxidation of mercury above the Dead Sea. Atmospheric Chemistry and Physics 2012;12(5):2429-2440. R833378 (Final)
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  • Journal Article Teisserenc R, Lucotte M, Canuel R, Moingt M, Obrist D. Combined dynamics of mercury and terrigenous organic matter following impoundment of Churchill Falls Hydroelectric Reservoir, Labrador. Biogeochemistry 2014;118(1-3):21-34. R833378 (Final)
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  • Journal Article Zhang H, Lindberg SE, Kuiken T. Mysterious diel cycles of mercury emission from soils held in the dark at constant temperature. Atmospheric Environment 2008;42(21):5424-5433. R833378 (2007)
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  • Supplemental Keywords:

    RFA, Scientific Discipline, Air, climate change, Air Pollution Effects, Environmental Monitoring

    Progress and Final Reports:

    Original Abstract
  • 2007 Progress Report
  • 2008 Progress Report
  • 2009 Progress Report
  • 2010 Progress Report