Collaborative Research: Effects of Fire and Subsequent Sediment Burial on Sulfur and Mercury Binding in Organic Matter of Forest Soils

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Intellectual merit: Mercury mobilized by forest fire is accumulating in aquatic organisms in lakes and reservoirs in the Rocky Mountain region. Recent research has focused on mercury emitted to the atmosphere during forest fires, but fire also releases mercury to streams, lakes, and reservoirs. Soil organic matter effectively sequesters most of the mercury deposited in forested watersheds, and fire results in the destruction or erosion of most of the soil organic matter. Fire also results in the development of anoxic, sulfate-reducing conditions in the lakes and reservoirs receiving runoff from fire-disturbed watersheds, and under these conditions, mercury is biologically converted to methylmercury, the form that readily accumulates in aquatic organisms and concentrates up the food chain. In this proposal, research is outlined to examine the effect of fire on the ability of soil organic matter to strongly bind mercury and prevent methylation and bioaccumulation. The research is driven by three hypotheses addressing the transformations caused by fire in the oxidation state of sulfur in soil organic matter and the effect of these changes on mercury binding. The hypotheses are (1) that forest fire increases the oxidation state of sulfur in soil organic matter, (2) that forest fire decreases the strong mercury binding capacity of soil organic matter because of the increase in the oxidation state of the sulfur, and (3) that sediment burial restores, or increases, the strong mercury binding capacity of soil organic matter because of incorporation of hydrogen sulfide in the lake and reservoir sediments. Soil and lake/reservoir sediment sampling will be carried out in watersheds affected by wildfire and prescribed burn. Mercury adsorption to and desorption from the soils, sediments, and extracted organic matter, will be examined using furnace heating to reproducibly assess the effect of heat on sulfur oxidation and mercury binding. In addition, the sulfur and mercury in these materials will be characterized with x?]ray absorption near edge structure (XANES) and extended x?]ray absorption fine structure (EXAFS) spectroscopy.Broader Impact: The broader impacts of this proposal involve three unique features: (1) growth of the San Juan Collaboratory, an initiative of the University of Colorado, Fort Lewis College, the Mountain Studies Institute, and federal, regional, and state land management agencies in the San Juan Mountains region, (2) recruitment of under-represented graduate students through well-established and successful programs at the University of Colorado and University of Illinois, and (3) establishment of the Colorado Water Quality Outreach Center (CWQOC) at the University of Colorado. The San Juan Collaboratory was recently formed to enhance and establish research and educational programs for the southwestern Colorado area. The Colorado Water Quality Outreach Center is being established with the support of the University of Colorado Outreach Committee to provide a state-wide avenue for watershed stakeholder groups and municipalities dealing with water quality problems to seek assistance from faculty and students with water quality problems.