Measurements of gaseous elemental mercury fluxes over intact tallgrass prairie monoliths during one full year

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Atmospheric Environment 39 (2005) 957–965 www.elsevier.com/locate/atmosenv

Measurements of gaseous elemental mercury fluxes over intact tallgrass prairie monoliths during one full year Daniel Obrista,, Mae S. Gustina, John A. Arnone IIIb, Dale W. Johnsona, David E. Schorranb, Paul S.J. Verburgb a

Department of Natural Resources and Environmental Sciences, University of Nevada, Reno, Nevada 89557, USA b Division of Earth and Ecosystem Sciences, Desert Research Institute, Reno, Nevada 89512, USA Received 8 March 2004; received in revised form 25 August 2004; accepted 10 September 2004

Abstract The atmosphere is an important pathway by which mercury is transported and distributed to pristine ecosystems. The significance of anthropogenic versus natural mercury contributions to the atmosphere is controversial, and the importance of re-emission of deposited mercury from ecosystems is not known. Here we present a continuous year-long data set of gaseous elemental mercury exchange between intact soil–plant monoliths of tallgrass prairie and the atmosphere. Mercury fluxes were measured using large open-flow gas exchange chambers (7.3  5.5  4.5 m3, L  W  D). Approximately 60 mg m 2 of elemental gaseous mercury was lost from four replicate grassland ecosystems (9 m2 surface area each) to the atmosphere over the course of 1 yr. Deposition was an important flux in the winter and emissions were dominant in spring, summer, and fall. Solar radiation and air temperature were most strongly correlated with mercury emissions. Gaseous elemental mercury losses to the atmosphere exceeded other measured fluxes of mercury in and out of the grassland ecosystems. These results indicate that mercury emissions from uncontaminated terrestrial ecosystems to the atmosphere may be a significant source of atmospheric mercury. We hypothesize that most of the mercury being emitted is previously deposited mercury and that re-emissions of mercury from terrestrial ecosystems is an important process whereby mercury is continually cycled between the air and terrestrial ecosystems. r 2004 Elsevier Ltd. All rights reserved. Keywords: Mercury; Tallgrass prairie; Flux measurements; Mesocosms; EcoCELLs

1. Introduction Mercury (Hg) inputs to terrestrial and aquatic ecosystems are of significant concern for human health and the environment (Fitzgerald et al., 1998). In aquatic ecosystems Hg can be methylated and bioaccumulated (Morel et al., 1998; Gilmour and Henry, 1991), which has Corresponding author. Institute of Environmental Geos-

ciences, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland. E-mail address: [email protected] (D. Obrist).

resulted in 2800 fish consumption advisories in 48 of the 50 states in the United States and other countries (EPA, 2003). Anthropogenic Hg emissions from coal combustion and waste incineration are important sources of atmospheric Hg (Fitzgerald et al., 1998), but an increasing number of studies have also revealed the importance of gaseous elemental Hg (Hg0) emissions from geologic substrates (Rasmussen, 1994; Gustin, 2003). Our understanding of the role of natural sources in the biogeochemical cycling of Hg, however, is hindered by the dearth of direct flux measurements over vegetated terrestrial ecosystems (Grigal, 2002; Schroeder and

1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.09.081

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Munthe, 1998; Lindberg et al., 1992). Initial exchange measurements of Hg0—the dominant atmospheric Hg species—over temperate forest ecosystems during spring and summer months indicated that Hg0 emissions from terrestrial ecosystems could be up to three times higher than modeled estimates (Lindberg et al., 1992, 1998). Because Hg0 emissions to the atmosphere are influenced by meteorological parameters such as temperature and solar radiation (Lindberg et al., 1998, 2002a, b c; Engle et al., 2001), quantification of these emissions will require long-term measurements that cover different climatic conditions and vegetation development. The first objective of this study was to measure gaseous Hg0 exchange between tallgrass prairie ecosystems and the atmosphere during one full year using the Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs, Fig. 1) at the Desert Research Institute in Reno, NV, USA. The EcoCELLs are unique open flow gas exchange systems that allow for the measurement of whole-ecosystem gas fluxes from large soil–plant monoliths under precisely controlled environmental conditions (Griffin et al., 1996). For this study, monoliths of tallgrass prairie soil and vegetation were excavated from a field site near Norman, OK, USA with

minimal disturbance so that they remained intact. The monoliths were then transported to Reno, NV, USA and placed in the four EcoCELLs to form ecosystems with total surface areas of 9.8 m2 and soil volumes of approximately 35 metric tons. Within the EcoCELLs, monoliths were maintained under climatic conditions that tracked average annual conditions at the field site. Further objectives were to assess which environmental parameters were correlated with observed Hg0 exchange and to compare other Hg inputs and outputs of the system to Hg0 exchange. Tallgrass prairie monoliths were used in this study because grasslands are a major biome covering 30% of the earth’s terrestrial surface area (DeFries and Townshend, 1994; Wilson and Henderson-Sellers, 1985).

2. Materials and methods 2.1. EcoCELLs laboratory and gas exchange measurements In the summer of 2001, 12 large monoliths (2.4  1.2  1.7 m3, L  W  D) of tallgrass prairie vege-

Fig. 1. Field excavation and installation of the 12 grassland monoliths in the EcoCELLs. (a) Field excavation of large soil–plant monoliths (12 metric tons) near Norman, OK, USA. (b) Air-lifting of a monolith through the roof into the EcoCELLs in Reno, NV, USA. (c) View in one of the four EcoCELLs showing the three adjacent grassland monoliths that build large grassland ecosystems with a total surface area of 8.9 m2 per EcoCELL and a soil depth of 1.7 m. (d) Top view on one of the EcoCELLs and a second EcoCELL behind, each with a volume of 184 m3.

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tation and soils were excavated in Norman, OK, USA by carving vertical trenches on the four sides of the monoliths (Fig. 1a). A four-sided concrete box was slowly lowered onto the monoliths islands to a depth of 1.7 m as the excavation around the islands proceeded further down. The containerized soil–vegetation monoliths were then removed from the field after the bottom plate was welded to the side walls. The monoliths were transported to Reno, NV and then lifted into the EcoCELLs through the roof (Fig. 1b) so that each EcoCELL housed three adjacent monoliths with a total surface area of 8.92 m2 per EcoCELL (Fig. 1c and d). Disturbance effects on the plant–soil monoliths during excavation and transport were minimal (e.g., no detectable compression, changes in soil repiration, or changes in soil water contents). Soils were part of the Nash–Lucien complex and were classified as Fluvents. In October 2002, monitoring of net exchange of gaseous Hg0 between the soil–vegetation monoliths and the atmosphere was initiated by measuring Hg concentrations of incoming and outgoing air with a Tekran 2537A Hg analyzer (Tekran Inc., Toronto, Canada). Air samples were drawn through 0.2 mm poresize Teflons particulate filter membranes at the inlet and outlet lines. The Tekran 2537A analyzer is considered to predominately analyze for Hg0 (e.g., Lindberg et al., 2002a, b c). The EcoCELL flow rates of 425 l s 1 (approximately one volume exchange every 7 min) were calibrated every 2 days using CO2 standard addition tests during night when CO2 fluxes were most stable. Calibration of the Tekran 2357A Hg analyzer was performed daily at midnight using an internal Hg permeation source. All inlet and outlet lines were regularly flushed with Hg-free air to test for contamination of the Teflons sampling lines using a Tekran 1100 zero-air-generator (Tekran Inc., Toronto, Canada). A multi-valve switching unit built with Teflons parts allowed sampling of air for sequential Hg0 analysis of EcoCELLs inlet and outlet lines in 5 min intervals such that sampling of an inlet line always preceded sampling of the corresponding EcoCELL outlet line. All lines were continually flushed when not being sampled. Soil Hg0 fluxes over small bare soil locations were measured using a 1.4 l polycarbonate dynamic flux chamber (Engle et al., 2001). Prior to installing the soil–vegetation monoliths in the EcoCELLs, Hg0 concentrations of inlets and outlets were measured in two empty EcoCELLs during a 3-day period in 1999 and during a 12-day time period in 2001 to obtain system blanks.

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aboveground biomass were quantified during a harvest in August 2003 in which all aboveground biomass (410 cm) was removed to simulate grazing and to quantify biomass production. Samples of irrigation and drainage water were collected six times during the experiment and total Hg concentrations were determined using cold vapor atomic fluorescence spectrometry (Bloom and Crecelius, 1983). Volumes of irrigation water added were measured with high-precision water gauges. The volumes of drainage were quantified by weekly weight determination of drainage buckets. 2.3. Environmental conditions EcoCELL air temperatures were adjusted every 15 min to track average diel and seasonal patterns (20 yr records) for the field site in Norman, OK, USA. Starting February 11, 2003, temperatures in two of the four EcoCELLs were increased by 4 1C as part of a National Science Foundation (NSF) project investigating the influence of inter-annual climate variability on ecosystem processes, while two EcoCELLs were maintained at ambient field temperatures. Although air temperature is a factor controlling Hg0 exchange (see Section 3), this warming treatment did not significantly affect Hg0 fluxes as assessed by ANCOVA using pretreatment fluxes as co-variable. Humidity was adjusted to mimic Oklahoma conditions using large evaporative pans in the air supply ducts of the EcoCELLs. Natural rainfall patterns of the field site were replicated using a sprinkling system mounted on the ceiling during irrigation at approximately 2 m above the monoliths. Soil temperature profiles of the monoliths were maintained similar to those observed in the field by insulating the side walls of the monoliths with 0.5 m thick blocks of StyrofoamTM to minimize lateral heat flow and by using a cooling and heating system underneath each monolith to achieve set point temperatures of 161 C at 1.45 m soil depth, which corresponds to the constant annual mean temperature at this depth in the field. No significant pressure differences were observed between the EcoCELLs and the atmospheric pressure outside. Air circulation fans inside the EcoCELLs were adjusted to induce a slight wind over the canopy of the monoliths. Solar radiation was not controlled as the EcoCELLs are lit naturally by daylight. Average annual solar loads in Oklahoma and northern Nevada are similar (5–6 kW m 2 d 1).

2.2. Quantification of soil and water Hg concentrations

3. Results and discussions

Total soil Hg concentrations to a depth of 170 cm were quantified in February 2003 in each monolith using a Milestone DMA-80 Direct Mercury Analyzer (Milestone Inc., Bergamo, Italy). Total plant Hg concentrations of

3.1. EcoCELL blanks The concentration differences measured in the empty EcoCELLs during the first blank determination in 1999

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were operational; thus we were only able to quantify blanks of two EcoCELLs. Because blanks of the two EcoCELLs were not significantly different and all four EcoCELLs were identical, we adjusted Hg0 fluxes of all EcoCELLs using the 2001 blank. We also report flux numbers without blank correction (i.e., using the 1999 non-significant blank) in the text.

were small and not statistically significant from zero (average 0.017 ng m 3, Figs. 2a and b). The concentration differences measured during the second blank determination in 2001, however, were significantly different from zero and averaged 0.0547 0.014 ng Hg m 3 (Figs. 2c and d). The blank differences were not statistically different between the two EcoCELLs and showed no diel trends. The lack of diel trends was very distinct from all Hg0 measurements when the EcoCELLs housed the soil–plant monoliths which showed clear diel patterns throughout the year. During the 2001 blank determination, we observed relatively high variability in EcoCELL inlet and outlet Hg0 concentrations (and hence in the difference between the two) compared to the more stable Hg0 concentrations measured in the 1999 blank period and those obtained during the measurements from October 2002 to 2003 that were measured with a newer Tekran 2537A unit. A laboratory intercomparion between the Tekran used to make the blank measurements and other Tekran analyzers yielded very similar Hg0 concentrations despite differences in variability. At the time of the blank determinations only two of the four EcoCELLs

3.2. Ecosystem Hg0 exchange In general, daily net Hg0 exchange was low or even negative (i.e., Hg0 deposition) in the colder months of the year, while during the warmer and more active vegetation periods Hg0 exchanges were higher with peak values ranging between 30 and 40 ng m 2 h 1 (Fig. 3a). In October and November 2002, Hg0 fluxes were positive and caused the cumulative sum of Hg0 exchange to be positive (i.e., net Hg0 emissions to the atmosphere, Fig. 3b). From December 2002 to February 2003, Hg0 fluxes were predominantly negative (with the exception of EcocCELL 4), indicating deposition processes to be the dominant pathway for Hg0 during that time period. As a result, cumulative Hg0 fluxes dropped below zero in

EcoCELL 1

EcoCELL 2

5

5

(a)

4

EcoCELL Inlet EcoCELL Outlet Difference

Hg0 concentration [ng m-3]

3

(b)

4 3

2

2

1

1

0

0

-1

-1 -2

-2 8

9

10

8

11

9

Dec 1999 5 4

11

5 (c)

4

3

3

2

2

1

1

0

0

-1

-1

-2 28

10 Dec 1999

30 Oct 2001

1

3

5

7 Nov 2001

9

11

-2 28

(d)

30 Oct 2001

1

3

5

7

9

11

Nov 2001

Fig. 2. (a) EcoCELL inlet (open symbols) and outlet (filled symbols) concentrations and differences between the two (solid line) during a 3-day blank period in 1999 when the EcoCELLs were completely empty (i.e., no soil–plant monoliths inside). (b) EcoCELL inlet (open symbols) and outlet (filled symbols) concentrations and differences between the two (solid line) during a 2-week blank period in 2001.

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three of the four EcoCELLs. By March 2003, Hg0 emissions resumed and increased continuously towards the summer months, resulting in net positive cumulative emissions for the year. The cumulative annual exchange of gaseous Hg0 for the four replicate EcoCELLs was 60.4727.1 mg Hg m 2, demonstrating that substantial amounts of Hg0 were lost from all four ecosystems to the atmosphere (Fig. 3b). While EcoCELLs 1–3 showed relatively close agreement of measured Hg0 exchange, Hg0 losses measured in EcoCELL 4 were almost three times higher than those measured in all other EcoCELLs, in spite of similar soil Hg concentrations and environmental conditions. Although plant community types differed slightly

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among the four EcoCELLs, we cannot fully explain the large discrepancy between EcoCELL 4 and the other three cells. Eliminating the emissions measured in EcoCELL 4, cumulative Hg0 emissions were 34.2710.0 mg Hg m 2. This more conservative estimate, however, would be higher without the blank adjustment (120735 mg Hg m 2). Hg0 emissions measured with the EcoCELLs compare relatively well with emissions measured over other ecosystems (Kim et al. 1995; Lindberg et al., 1992, 1998). For example, Hg0 emissions peaking at 16 ng m 2 h 1 during June 2003 (monthly mean) in our study compare well with Hg0 fluxes measured over Pinus plantations (18 ng m 2 h 1) in Tennessee in spring and

(a) Daily Hg0 exchange 80 EcoCELL 1

30 20 10 0 -10

EcoCELL 2

30 20 10 0 -10

EcoCELL 3

30 20 10 0 -10

EcoCELL 4

30 20 10 0 -10

60

20 0 -20 80 60 40

Air temperature [°C]

Hg0 exchange [ng m-2 hr-1]

40

20 0

Oct-03

Sep-03

Jul-03

Aug-03

Jun-03

Apr-03

May-03

Mar-03

Jan-03

Feb-03

Dec-02

Oct-02

Nov-02

Oct-03

Sep-03

Jul-03

Aug-03

Jun-03

Apr-03

May-03

Mar-03

Jan-03

Feb-03

Dec-02

Oct-02

Nov-02

-20

(b) Cumulative Hg0 exchange (1 year)

Hg0 exchange [µg m-2]

160 140

all EcoCELLs CELL 4

120 100 80

CELL 2

60 40

CELL 1

20

CELL 3

0 Oct-03

Sep-03

Aug-03

Jul-03

Jun-03

May-03

Apr-03

Mar-03

Feb-03

Jan-03

Dec-02

Oct-02

Nov-02

-20

Fig. 3. (a) Net daily Hg0 exchange (ng m 2 h 1) between the grassland ecosystems and the atmosphere in the four EcoCELLs. Positive fluxes (40) denote net emission of gaseous Hg0 from the ecosystems to the atmosphere while negative fluxes (o0) denote net Hg0 uptake by the ecosystems. Inserts are mean daily air temperature (1C) in the corresponding EcoCELLs. (b) Cumulative gaseous Hg0 exchange (mg m 2) between grassland ecosystems and the atmosphere over 1 yr. Cells 1 and 3 experienced a +4 1C temperature increase starting February 11, which had no significant effects on Hg0 exchange.

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summer, while our peak fluxes were about half of those measured in a mature deciduous forests (Lindberg et al., 1992, 1998). The relatively close agreement of measured fluxes among these studies confirm the potential for substantial Hg0 emissions from terrestrial, vegetated ecosystems to the atmosphere, especially given the different methodologies applied (large dynamic flow chamber versus modified Bowen ratio approach). If the elemental Hg0 emissions (34–60 mg Hg m 2) measured in our study are scaled up to the area of the world’s grasslands—roughly estimated at 3.75  107 km2 (DeFries and Townshend, 1994; Wilson and HendersonSellers, 1985)—gaseous Hg0 emissions to the atmosphere would be in the range of 1300–2300 ton yr 1, and grasslands alone would already exceed the current estimates of all terrestrial biogenic Hg emissions of 20–1200 ton yr 1 (Nriagu, 1989). This very initial estimate, together with a similar estimate of Hg0 emissions over forest ecosystems (850–2000 ton yr 1; Lindberg et al., 1998) indicate the potential importance of terrestrial ecosystems as sources of Hg to the atmosphere. If the emissions measured in our and in Lindberg’s study are corroborated in other ecosystems, the contribution of uncontaminated terrestrial ecosystems to global Hg emissions would be underestimated. 3.3. Ecosystem Hg0 exchange in relation to environmental controls and soil Hg0 fluxes Hg0 fluxes showed pronounced diel patterns throughout the year. The diurnality of measured fluxes is

apparent in Fig. 4, which presents linear regressions between hourly means of Hg0 flux and solar radiation for each month of the year (using only data of the first three EcoCELLs). Solar radiation exhibited the best correlation with Hg0 exchange and explained between 5% (December) and 37% (June) of the variability in measured fluxes. The rather low correlation coefficients (r2 ) observed during the winter months were caused by the smaller range of solar load during this time compared to the summer months (e.g., solar load in December ranged between 0 W m 2 at night and a maximum of 450 W m 2 during the day, which is about half of the range observed in the summer). The slopes of the linear regressions were surprisingly similar throughout the year and generally ranged between 0.02 and 0.03 (0.02670.002, mean7SE), indicating that an increase in solar radiation of 100 W m 2 increased Hg0 emissions (or reduced Hg0 deposition) by 2–3 ng m 2 h 1. The significant and relatively constant slopes between solar radiation and Hg0 fluxes throughout the year indicate that physical or chemical processes might be more important for Hg0 exchange over these ecosystems than plant physiological processes (e.g., plant transpiration). If plant physiological processes were a major controlling variable for Hg0 exchange, a distinctly different response to solar radiation could be expected in winter when leaf areas and plant activity are low compared to summer months. First direct measurements of plant-level Hg exchange showing only small plant Hg exchange fluxes seem to support this suggestion (Stamenkovic and Gustin, personal communication). Similar linear

Fig. 4. Scatter plots and linear regression lines between hourly mean solar radiation and hourly Hg0 exchange for each month of the year (data of EcoCELLs 1–3). P-values for all regressions o0.01. Correlation coeffiecients (r2 ) and slopes are inserted in each figure. Note: Clusters of solar radiation observed in June, July and August are due to similar diel patterns of solar radiation throughout the months (e.g., in June, solar radiation at 07:00 was always between 450 and 480 W m 2).

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relationships were observed between air temperature and Hg0 exchange (Fig. 5), although the regression coefficients (0.08–0.40) were generally lower than those observed for solar radiation. Measurements of soil Hg0 exchange using a small dynamic flux chamber (volume, 1.4 l) over small vegetation-free locations in each monolith indicated similar Hg0 exchange patterns as observed at the ecosystem level (i.e., deposition processes during winter, emissions during spring until fall, strong diurnality of fluxes throughout the year). Measured soil Hg0 fluxes, however, only ranged between 10% and 30% of those measured at the ecosystem-level fluxes. While the low soil Hg0 fluxes might suggest that the majority of Hg0 passed through plants—i.e., with the transpiration stream—the observed constant relationships of solar radiation to Hg0 exchange throughout the year do not support this notion (see above). Furthermore, if measured soil solution Hg concentrations (6.1–11.8 ng l 1) are used along with plant transpiration rates to calculate potential plant emissions via transpiration, the large differences between whole-ecosystem fluxes and soil chamber fluxes are not accounted for. A more likely explanation of the large differences between soil chamber and EcoCELL fluxes is that the small soil chamber might greatly underestimate Hg0 fluxes in comparison to the EcoCELLs, which has also been observed in another study (Gustin, in review) and was found in an intercomparison of micrometeorological measurement and field chamber measurement (Gustin, 1999). Such differences could be especially

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pronounced when the soil chambers are operated at low flow rates (Lindberg et al., 2002a, b c) as was done in this study (1.5 l min 1 flowrate) so that we could detect the small concentration differences between chamber inlets and outlets. Although we are aware that an inherent problem of all chamber methods is the disturbance of boundary layer and environmental conditions, we think that the use of very large chambers such as the EcoCELLs with reasonable wind speeds and tightly controlled environmental conditions yields a better approximation of real field conditions than is possible with small soil chambers. 3.4. Annual Hg mass balance The gaseous losses of Hg0 greatly exceeded all other measured Hg fluxes in and out of the ecosystem (Table 1). Total Hg concentrations of the irrigation water ranged between 1.0 and 5.2 ng l 1, and the 860 mm of irrigation water applied during the year yielded a Hg input of 2.5 mg m 2 yr 1 to the ecosystem (i.e., wet deposition). Thus, wet deposition rates of Hg were more than one order of magnitude smaller than Hg0 losses to the atmosphere. The use of tap water for irrigation in our study was certainly not ideal and might have resulted in relatively low wet depositions to these ecosystems. Even higher wet deposition rates commonly observed in field studies (5–15 mg m 2 yr 1, Grigal, 2002), however, would not have balanced the measured gaseous Hg0 losses. Hg losses via drainage were very small (0.0870.03 mg m 2) because only minimal

Fig. 5. Scatter plots and linear regression lines between hourly mean air temperature (1C deviation from monthly mean) and hourly Hg0 exchange for each month of the year (data of EcoCELLs 1–3). Arrows indicate monthly mean temperatures. P-values for all regressions o0.01. Correlation coeffiecients (r2 ) and slopes are inserted in each figure.

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Table 1 Summary of annual Hg inputs and annual Hg losses (in mg m 2) of the four EcoCELLs Cell 1

Cell 2

Cell 3

Cell 4

Mean

Hg inputs to ecosystem Wet deposition (total Hg) Particulate Hg deposition (HgP) Dry deposition (RGM)

3 ? ?

3 ? ?

3 ? ?

3 ? ?

3

Hg losses from ecosystem Gaseous Hg loss (Hg0) Drainage (total Hg)

40 o1

48 o1

15 o1

139 o1

60 o1

Net balance (inputs–losses) Loss Gain

37 —

46 —

12 —

137 —

58 —

Hg stocks Soil Hg pool (0–170 cm, total Hg) Above ground biomass ( internal cycling, total Hg) Time to ‘‘deplete’’ soil pools (years)

15,505 14 416

14,741 17 323

14,687 9 1180

14,965 17 110

14,974 14 258

Net balance represents difference between measured inputs and losses.

amounts of water (7.673.1 mm) drained from the monoliths at 1.70 m depth, which is consistent with the field site where high evapotranspiration rates nearly balance precipitation. Soil Hg concentrations (total Hg) were highest in the soil near the top of the soil profile (20.071.4 ng Hg g 1 at 1 cm depth) and much lower in deeper soils (e.g., 3.770.2 ng Hg g 1 in 135 cm depth), resulting in a total Hg pool of 15 mg m 2 (0–170 cm depth). Hence, the measured gaseous Hg0 emissions, corrected by Hg inputs by wet deposition and Hg losses by drainage, would potentially deplete the soil Hg pools in only 507 yr (Table 1). This large imbalance of the Hg cycle in the four grassland ecosystems indicates that (1) other inputs of Hg like particulate Hg or reactive gaseous Hg (RGM) deposition—which are not accounted for in this study—might potentially be large; (2) Hg0 emissions are significantly overestimated in our study; or (3) these ecoystems are true net sources of Hg and currently exhibit a large imbalance in the Hg cycling. This latter possibility could support a suggestion of Lindberg et al. (1998) that emissions of Hg0 from terrestrial ecosystems could be due to re-emission of the 200,000 ton of Hg that were mobilized during the last 150 yr, of which 95% is estimated to reside in surface soil pools (Expert Panel on Mercury Atmospheric Processes, 1994). We hypothesize that decreasing atmospheric Hg concentrations since the 1980s (e.g., Slemr et al., 2003) might now enable such reemissions, which is similar to the concept of a compensation point at the ecosystem level. Such compensation points have in fact been observed with plants in laboratory studies which changed from sources to sinks of atmospheric Hg0 when Hg0 concentrations were lowered (Hanson et al., 1995).

4. Conclusions This year-long data set for Hg exchange between a terrestrial ecosystem and the atmosphere indicates that vegetated terrestrial ecosystems are a net source of Hg to the atmosphere. In this study, gaseous Hg0 loss was the dominant flux of Hg in and out of the ecosystem. Even if the Hg0 being emitted is a recycling of previously deposited Hg of anthropogenic origin, it is significant that natural ecosystems might continually recycle this Hg between terrestrial ecosystems and the atmosphere. Only more direct measurements of Hg exchange between ecosystems and the atmosphere in a variety of different climatic zones and geologic/pedologic settings and the development and intercomparison of different methods to quantify in situ ecosystem-level Hg exchange processes, however, are likely to clarify the current contributions of Hg from terrestrial ecosystems to the atmosphere.

Acknowledgments Funding of this project was provided by the National Science Foundation with grants from the Atmospheric Sciences Division and the Integrated Research Challenges in Environmental Biology Program (IRCEB). Research was also supported by the Nevada Agriculture Experiment Station (NAES 52042949). We thank M. Markee, R. Bergin, L. Martindale, and H. Weatherly for laboratory analyses and daily tasks in the EcoCELLs. We are grateful to C. Alewell, F. Conen, M. Engle, S. Lindberg, and R. Zehner for valuable scientific inputs and comments to this manuscript.

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