2007 Progress Report: Effects of Global Change on the Atmospheric Mercury Burden and Mercury Sequestration Through Changes in Ecosystem Carbon Pools
EPA Grant Number:
Effects of Global Change on the Atmospheric Mercury Burden and Mercury Sequestration Through Changes in Ecosystem Carbon Pools
, Johnson, Dale W.
, Lindberg, Steve
, Luo, Yiqi
Desert Research Institute
University of Oklahoma
University of Nevada - Reno
Desert Research Institute
University of Nevada - Reno
University of Oklahoma
EPA Project Officer:
May 1, 2007 through
April 30, 2012
Project Period Covered by this Report:
February 15, 2007 through February 14,2008
Consequences of Global Change For Air Quality (2006)
Global Climate Change
Terrestrial carbon (C) pools play an important role in uptake, deposition, sequestration, and emission of atmospheric mercury (Hg). Biomass and soil C pools are highly sensitive to climate and land use changes with potentially serious consequences for the fate of an estimated 50,000 Mg of atmospheric Hg associated within C pools. Our overall objective is to assess how global change during the next 100 years is likely to affect Hg cycling processes (i.e., atmospheric Hg uptake, sequestration, and emission) associated with vegetation and soil C pools. We assess how global change will affect plant-derived atmospheric Hg inputs to ecosystems via changes in plant productivity, plant senescence, and litterfall; and how global change impact on plant, litter, and soil C pools will affect sequestered Hg within these pools and feedback on the future atmospheric Hg burden.
The work is implemented during a four-year period through the following interlinked tasks:
Task 1: We systematically quantify Hg concentrations associated with vegetation, litter, and organic soil horizons in forested ecosystems across the United States.
Task 2: We assess the fate of Hg during C mineralization processes using controlled laboratory incubation and field studies to evaluate to what degree decomposition of organic matter leads to emission and re-emission of Hg to the atmosphere, increased mobilization within terrestrial ecosystems, or long-term sequestration by incorporation and accumulation of Hg in the remaining C matter fraction.
Task 3: We statistically evaluate and model the dependence of atmospherically-derived plant and organic Hg pools and fluxes on climatic conditions.
Task 4: We integrate data on measured plant, litter, and soil Hg pools and fluxes—along with the hierarchy of climatic factors—into an existing terrestrial C model (i.e., the TECO model; Luo and Reynolds 1999). We use model results to explore possible mitigation measures for atmospheric Hg and land management options to stabilize Hg pools associated with terrestrial C pools.
This proposed study includes a systematic determination of Hg in vegetation and soil C pools in terrestrial ecosystems, field and laboratory experimental studies, and modeling components. We aim to (1) collect the first systematic database on Hg pools and fluxes associated with terrestrial C pools by assessing C:Hg ratios of major ecosystem compartments in nine terrestrial ecosystems across the U.S. and combining these data with existing detailed data on plant and soil C pools and turnover rates; (2) perform controlled laboratory and field studies to clarify the fate of sequestered Hg pools during C mineralization (i.e., organic matter decomposition), specifically to what degree Hg is emitted back to the atmosphere; (3) statistically compare Hg pools and fluxes among climatically different sites to establish a hierarchy of climatic factors and drivers which control Hg pools and fluxes associated with terrestrial C; and (4) integrate acquired Hg data into an existing global change C model to scale up data to coterminous U.S. states and to enable prediction of how changes in C dynamics may affect the atmospheric Hg burden during the next 100 years.
Year 1 of the study has focused on Task 1 and 2 which will build the foundation for the modeling and scaling Tasks 3 and 4 and which will be conducted in year 3 and 4 of the study. The following progress has been accomplished in the first year of the study:
Task 1. Systematic quantification of Hg concentrations associated with vegetation, litter, and organic soil horizons and carbon pools
We have started vegetation and soil sampling in forested ecosystems across the Western United States and have analyzed samples for Hg mass concentrations, Hg/C ratios, N concentrations, and texture (for soils). Combining existing biomass and C inventories of sites with newly measured Hg concentration analyses serves to (1) quantify total Hg pools associated with terrestrial C pools; (2) estimate atmospheric Hg inputs and sequestration through leaf litterfall and plant senescence; (3) evaluate total Hg pools sequestered across forests of the United States; and (4) determine resilience and turnover times of Hg sequestered in vegetation and soil pools.
In the first year of the study, we have (i) developed sampling, analysis, and scaling protocols including purchase and implementation of a new total mercury analyzer; (ii) determined sites with existing biomass/C pool inventories and contacted site managers for permission for sampling and use of existing C/biomass data, (iii) finished vegetation and soil sampling on four forest sites (Jeffrey Pine forest in Nevada, Jeffrey Pine forest in California, California Blue Oak Savanna forest, and Colorado Rocky Mountain Pine forest; (iv) finished Hg mass and Hg/C analyses of 3 of the 4 initial sites and scaled up Hg pools to the ecosystem level.
First results indicate significant ecosystem Hg storage and sequestration ranging from 45 to 80 g Hg ha-1 in the two Jeffrey Pine forests sites and of 220 g Hg ha-1 in the California Oak Savanna forest. Hg was highly associated with ecosystem C pools with higher Hg accumulation associated with high C pools. Most of the Hg in ecosystems was stored in the soils (>95%) with the remaining stocks mainly associated with the litter horizon, tree foliage, and tree bark. A strong relationship between Hg and C accumulation was found in the soils where soil C concentrations explained > 80% of the variability of soil Hg concentrations (across all sites and soil horizons!). Hg/C and Hg mass concentrations of the above-ground compartments increased from Green Needles/Leaves < Dry Needles/Leaves < Oi horizon (undecomposed litter) < Oe horizon (partially decomposed litter) < Oa (decomposed organic humus). These patterns indicate that as tree foliage ages and decomposes after litterfall, these components experience stark increases in Hg levels. Further site sampling and current experimental flux studies (see next section) will help to identify to what degree concentration increases are due to atmospheric depositions or C losses/Hg retention in the decomposing litter. Initial atmospheric deposition fluxes by mean of leaf litterfall and plant senescence in the first forest sites are estimated at 5 to 11 μg m-2 yr-1 on these sites.
Task 2. Assessing the fate of Hg during C mineralization processes using controlled laboratory incubation and field studies
To determine the fate of Hg during C losses we are performing a series of field and laboratory flux studies. The tasks include field and laboratory flux components to concurrently measure of CO2 and Hg0 emissions from various organic ecosystem compartments (i.e. litter and soil horizons), and long-term laboratory incubations to assess fate of mercury during C mineralization.
In the initial year of the study, we have started controlled laboratory flux studies with soil substrates and optimized the technical and experimental set-up for concurrent flux measurements of Hg and CO2 exchange. We developed a laboratory system that allows concurrent measurements of 6 replicate soils chambers under controlled environmental conditions and where air is supplied by pressurized tanks which allows for stable inlet air concentrations (and hence stable fluxes). We further evaluated field sites to conduct field flux measurements and received permission for installation of field flux measurements in fall 2008. Finally, we determined sites and experimental set-ups for the proposed long-term biomass/litter incubation experiments.
Initial results show an excellent control on soil fluxes and highly accurate and replicable measurements of soil Hg and CO2 (i.e., mineralization) fluxes. Current experimental treatments of flux samples include manipulations of C mineralization rates (by means of O2 depletion, sterilization, etc.) to assess the dependence of soil Hg emissions to soil C mineralization rates.
Systematic quantification of Hg levels in major ecosystems will provide the first comprehensive database on Hg pools, plant Hg uptake, sequestration, and Hg turnover associated with terrestrial C pools in the U.S. Fate processes of Hg sequestered in C pools are fundamental to prediction of how anticipated C pool changes in the future will feedback on the future atmospheric Hg burden. Integration of Hg data into an existing global change C model will enable for prediction on how various global change scenarios will affect C-related Hg cycling. Our results will support formulation of strategies by land managers, policy makers, and stakeholders to mitigate atmospheric Hg and stabilize Hg sequestered in terrestrial C pools.
The activities of the second year will focus on further field sample collections and analyses of ecosystem Hg and C pools as part of the systematic quantification of Hg pools sequestered in US forest ecosystems. The next sites planned in the sampling are sites in Washington State and California which will be followed by sites in the Mid-Western and Eastern United States. A further emphasis will be placed on continuing laboratory flux studies, initiating field flux measurements, and begin of long-term incubation of litter in the laboratory to assess fate processes of Hg during C mineralization.
on this Report
: 1 Displayed | Download in RIS Format
, RFA, Scientific Discipline, Air, climate change, Air Pollution Effects, Environmental Monitoring
Progress and Final Reports:
2008 Progress Report
2009 Progress Report
2010 Progress Report