Effect of Climate Change on Coupled Biogeochemistry of Carbon, Sulfur, and Mercury in Northern Peatlands.

Peatlands are sinks for Hg and S from atmospheric deposition, and potential sources of methyl-Hg through the activity of microorganisms, such as sulfate-reducing bacteria. Projected climate changes may alter internal cycling of Hg, S, and C as well as fluxes of Hg, S, and C to the atmosphere and surface waters. In this context, we are annually sampling peat soil depth profiles from experimental plots of the Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) Project; an extensive study of the response of peatlands to climatic manipulation in northern Minnesota. In addition, porewater is collected twice monthly from piezometers and Hg(0) fluxes to the atmosphere are measured using passive sampling approaches. SPRUCE warming treatments range from 0 to +9° C, and elevated CO₂ treatments include ambient or +500 ppm. We are using a suite of analytical methods to track the biogeochemical processes that govern Hg, S, and C cycling. These techniques include concentrations of total- and methyl-Hg, total CNS, δ¹³C of microbial biomarkers, δ³⁴S, as well as S X-ray absorption spectroscopy. We hypothesize that low water tables and oxidizing conditions in elevated temperature treatment plots will: (1) result in a build-up of oxidized S compounds, such as sulfate, ester-sulfate, sulfonate, and sulfone in peat; (2) during subsequent wetter periods, a pulse of oxidized organic S will be released to porewaters and δ³⁴S will indicate faster microbial cycling rates; (3) oxidized S in peat and porewaters will stimulate production of methyl-Hg in porewaters; and 4) microbial growth and carbon fractionation will be sensitive to warming, mediating changes in S and Hg release. We are finding that biogeochemical cycling rates are strongly driven by depth and temporal gradients, and that surface fungal communities responsible for C cycling appear more sensitive than bacterial communities to changes in temperature.