Grantee Research Project Results
Final Report: Determining the Role of Plants and Soils in the Biogeochemical Cycling of Mercury on an Ecosystem Level
EPA Grant Number: R827622E02Title: Determining the Role of Plants and Soils in the Biogeochemical Cycling of Mercury on an Ecosystem Level
Investigators: Alden, Ray , Gustin, Mae Sexauer , Johnson, Dale W. , Coleman, James , Lindberg, Steve
Institution: University of Nevada - Reno , Desert Research Institute
EPA Project Officer: Chung, Serena
Project Period: August 9, 1999 through August 8, 2003
Project Amount: $253,798
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (1998) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Objective:
The main objective of this research project was to determine the role of plants and soils in controlling the fate and transport of mercury (Hg) in the environment at the ecosystem level. Because of natural variability and environmental complexity, the use of field studies to understand mechanisms controlling contaminant fluxes between environmental compartments is difficult. One specific objective of this research project was to use experimental designs at three scales: (1) large mesocosms; (2) smaller multiple plant-exposure chambers and a single-plant gas exchange system to investigate the exchange of Hg between plants; and (3) soils and the atmosphere.
This research project utilized the state-of-the-art mesocosms or Ecologically Controlled Lysimeter Laboratories (EcoCELLs) at Desert Research Institute. The use of two (approximately 160 m3) EcoCELLs provided a unique experimental setting in which relatively unrestricted ecological interactions could be studied. The system was configured to allow for extremely accurate and precise measurement of whole-ecosystem or whole-CELL-gaseous Hg, CO2, and H2O exchange. This experimental design allowed us to study the cycling of Hg between soil and vegetation, atmosphere and vegetation, and soil and atmosphere in a controlled experimental setting. Ancillary experiments using smaller field chambers within the EcoCELLs allowed for further characterization of soil-air and plant-air exchange, and soil gas wells allowed for the assessment of the processes within the soil that could be associated with Hg release from soil. Mercury uptake was quantified as a function of time for foliage within the EcoCELLs along with leaf area. Mercury in all plant compartments was determined at the end of the experiment. Measurement of plant accumulation of Hg along with system Hg fluxes allowed for the development of a mass balance of Hg transfers within the EcoCELL ecosystem.
Additional experimental designs included the use of ecopods, which allowed
for the assessment of multiple plant exposures to different soil and air Hg
concentrations, and a single-plant gas exchange chamber, which provided information
on foliar and soil Hg fluxes as a function of soil and air Hg concentrations
and environmental conditions in almost real time. Smaller scale investigations
provided supportive information for: (1) the results at the ecosystem level;
(2) understanding the mechanisms controlling Hg fluxes; and (3) assessment
of the applicability of the data developed for scaling ecosystem responses.
The data collected were applied towards: (1) development of a model detailing
processes controlling Hg fluxes from soil versus vegetation and the relative
significance of the fluxes; (2) assessment of the influence of vegetation on
Hg flux from soils; and (3) characterization of the role that plants play in
cycling of Hg between soils and the atmosphere.
An important objective within the mission of the EPSCoR program was to demonstrate that the EcoCELLs, which had only been used twice for experiments involving "climate change" and carbon exchange objectives, could be effectively used for investigating the biogeochemical cycling of an environmental contaminant.
Summary/Accomplishments (Outputs/Outcomes):
EcoCELL Experiments
Our original hypothesis was that with plant leaf out, the whole CELL flux would increase. We hypothesized this because previous work with single-plant gas exchange systems (e.g., Hanson, et al., 1995; Leonard, et al., 1998) had demonstrated that plants move Hg from the soil to the air. Rather, whole-CELL flux declined with leaf out in the first experiment. Because of several factors that could have lead to this response (the influence of a plant canopy, the decline of surface soil Hg concentration, and the equilibration of a disturbed system) during the second year, only one CELL was planted. By shading the soil in the unplanted CELL, it was clearly demonstrated that shading of the soil by the plant canopy was the reason whole-CELL flux declined with leaf out. Foliar uptake by the vegetation was not sufficient to account for the decline in whole-CELL flux, although the deposition of atmospheric Hg to foliage occurred. During both experiments, foliar Hg uptake was on the order of 4.7 to 9 ng/m2 h after 4 weeks, and declined to 2 to 3 ng/m2 h towards the end of the experiment. The difference in the midday Hg flux between the planted and unplanted EcoCELL fluxes after leaf out in 2001 was 250 to 1,000 ng/m2 h; foliar uptake would not account for this discrepancy.
In the EcoCELLs, the net Hg flux associated with all foliar surfaces, including vegetation grown in low and high Hg soils, was deposition. The Hg content in foliage from trees grown within the EcoCELLs in soil with background Hg concentration (0.03 µg g-1) was similar to that of foliage from trees in Hg-enriched soil (12.3 µg g-1). These findings demonstrate that nearly all of the Hg in foliage was clearly derived from the atmosphere. Methyl Hg constituted approximately 1 percent of the total Hg found in leaves. Considering that atmospheric Hg concentrations are generally homogenous due to turbulent mixing, and that tree foliage is exposed primarily to air from aloft, foliage could act as a significant sink for atmospheric Hg, and the Hg content in litterfall would mainly represent a new Hg input to terrestrial ecosystems. The implications are that recently assimilated Hg in foliage is a potential significant source of Hg for forested catchments. With this experiment, it was demonstrated that Hg in foliage originated from the atmosphere, and that less than 3 percent of that Hg would have been removed with precipitation (leaf washing experiments). The implication of these findings is that litterfall measurements may be a simple and effective way to estimate Hg deposition rates in deciduous forests. This information is important for watershed management decisions regarding Hg-impacted ecosystems.
This research project demonstrated that the use of Teflon filters as a surrogate surface for Hg deposition to foliage does not provide adequate representation of that deposited to the leaf surface or leaf interior.
Dominant factors driving emission of Hg from the soils in the EcoCELLs were incident light, precipitation, and the presence of vegetation. Soil temperature and soil moisture also influenced flux.
Based on the data developed in this research project, soil gas Hg efflux is not a diffusion-driven process, except during certain periods of the day (midday seems to have the biggest concentration difference between the deep and shallow gas wells). Therefore, soil gas concentrations are not an effective predictor soil Hg flux. Soil gas concentrations were increased by watering, which corresponds with the fact that watering of surface soils also increases soil Hg flux. Soil gas concentrations were not influenced by plants, indicating that roots and rhizosphere processes are not important in influencing Hg efflux. An observed diel pattern is that Hg in soil gas was thought to be driven by temperature variations, which in turn, caused variations in the rate of Hg flux from soils.
The small-soil gas wells employed in this research project proved to be much easier to install, and they provided much better data on the spatial and temporal resolution of Hg and CO2 soil gas behavior in soils, relative to the large gas wells.
Ecopod Experiments
Ecopod experiments supported the results derived in the EcoCELL experiments. Mercury concentration in foliage increased as a function of time and air exposure concentration. Soil Hg concentration also exhibited an influence on foliar Hg concentration. With this experimental design, the exact amount of Hg entering foliage derived from the soil versus the air could not be determined; however, the dominant contributing source was the atmosphere.
Gas Exchange Experiments
Research results indicated that net foliar Hg flux was not significantly influenced by Hg concentration in the soil, and that physiological factors such as stomatal conductance and transpiration rate, influenced net flux, but was species specific. That Hg is transported from the soil via the transpiration stream; this was supported by the fact that with first light, there was a pulse in water vapor and Hg being emitted from the foliage. The compensation point, or air concentration where there was no net flux, differed between light and dark conditions, and was significantly less than that reported for other plant species by Hanson, et al. (1995). The net foliar flux response to changes in air concentrations was immediate with changing air concentrations. Because deposition was measured in the dark, passive adsorption of Hg also is a process important in foliar uptake; this has not been suggested by others.
EPSCoR Objective
This research project met the objectives of the EPSCoR program by demonstrating that the EcoCELLs and ecopod facilities at Desert Research Institute could be successfully used to study the biogeochemical cycling of a contaminant at the whole-ecosystem level. Because of the success of this research project, a National Science Foundation (NSF) grant was awarded to the principal Investigator to continue to use the facilities at Desert Research Institute and to the University of Nevada to continue to study Hg plant-air-soil exchange. This project, which is ongoing, "piggybacks" off of a global climate-change project funded by the NSF-Integrated Research Challenge in Environment Biology program, utilizing all four EcoCELLs. This project is studying Hg cycling associated with intact monoliths of tall grass prairie vegetation and soils that were brought from Oklahoma. This project affords us the unique opportunity to study Hg cycling between the soil, air, and vegetation associated with a pristine ecosystem type that covers one-third of the earth's surface. The experimental design of this project is unique with respect to other projects that have been conducted in the EcoCELLs in that the monoliths provide a relatively undisturbed system; a diel temperature change is being regulated that mimics daily average temperatures in Oklahoma; and seasonal temperature changes also are being imposed. The EcoCELLs, ecopods, and the plant gas exchange system are being used for this project.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 24 publications | 6 publications in selected types | All 6 journal articles |
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Type | Citation | ||
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Ericksen JA, Gustin MS, Schorran DE, Johnson DW, Lindberg SE, Coleman JS. Accumulation of atmospheric mercury in forest foliage. Atmospheric Environment 2003;37(12):1613-1622. |
R827622E02 (Final) |
not available |
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Ericksen JA, Gustin MS. Foliar exchange of mercury as a function of soil and air mercury concentrations. Science of the Total Environment 2004; 324(1-3): 271-279. |
R827622E02 (Final) |
not available |
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Frescholtz TF, Gustin MS, Schorran DE, Fernandez GC. Assessing the source of mercury in foliar tissue of quaking aspen. Environmental Toxicology and Chemistry 2003;22(9):2114-2119. |
R827622E02 (Final) |
not available |
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Frescholtz TF, Gustin MS. Soil and foliar mercury emission as a function of soil concentration. Water, Air, and Soil Pollution 2004; 155(4): 223-227. |
R827622E02 (Final) |
not available |
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Gustin MS, Biester H, Kim CS. Investigation of the light-enhanced emission of mercury from naturally enriched substrates. Atmospheric Environment. 2002;36(20):3241-3254. |
R827622E02 (Final) R827634 (Final) |
not available |
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Johnson D, Benesch JA, Gustin MS, Schorran DS, Lindberg SE, Coleman JS. Experimental evidence against diffusion control of Hg evasion from soils. Science of the Total Environment 2003;304(1-3):175-184. |
R827622E02 (Final) |
not available |
Supplemental Keywords:
biogeochemical cycling, mercury, Hg, atmospheric mercury, foliar mercury, plant, soil, fate and transport, ecosystem, environment, mesocosm, plant-exposure chamber, single-plant gas exchange system., RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Air, Waste, TREATMENT/CONTROL, Water, Ecosystem Protection/Environmental Exposure & Risk, Air Quality, Treatment Technologies, Contaminated Sediments, climate change, Air Pollution Effects, Fate & Transport, Analytical Chemistry, Hazardous Waste, Molecular Biology/Genetics, Bioremediation, Ecological Risk Assessment, Hazardous, Atmosphere, microbiology, degradation, fate and transport, environmental monitoring, bioavailability, biodegradation, contaminated sediment, transgenic plants, contaminated soil, emissions, contaminants in soil, bioremediation of soils, natural recovery, biogeochemical cycling, biochemistry, phytoremediation, contaminated soils, atmospheric mercuryRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.