Sulfate Reduction Potential of Subalpine Wetland Soils: Investigations into Iron Mineral Mediated Carbon Storage in Climate Change Susceptible Ecosystems.

Co-occurrence of Sulfate Reduction and iron Reduction in Subalpine Wetland Soils: Implications for Carbon Storage Jeanne Barreyre, Institut de Physique du Globe de Paris, Paris Diderot University, Paris, France,ÊLinden Schneider, University of California-Berkeley, Berkeley, CA, Thomas Borch, Colorado State University, Colorado State University, Fort Collins, CO, Charles C. Rhoades, USDA Forest Service (FS), Fort Collins, CO and CŽline Pallud, Environmental Science, Policy, and Management, University of California-Berkeley, Berkeley, CA Wetlands contain one third of the planetÕs soil carbon (C) and are characterized by markedly different chemical environments than terrestrial ecosystems. In wetlands, transformation and movement of C and iron (Fe) are closely linked due to sorption of organic C onto solid Fe-phases. Biological reduction of Fe (III)-minerals can influence C storage. Changes in Fe-biogeochemical cycling and C dynamics should occur in the presence of competing terminal electron acceptors (TEA) such as sulfate (SO42-). Following SO42- and Fe (III)-reduction together could contribute to understanding specific processes, such as mineral sorption, controlling C storage. We examined the effect of SO42- concentration (0.05 mM to 2mM) on reduction of Fe(III)-oxides in soils with different hydrologic residence times, C contents, redox conditions, and depths. Soils were from a continually wet slope and a seasonally dry depressional subalpine wetland (USFS Fraser Experimental Forest, CO, USA). Within the depressional wetland we studied two sites with contrasting seasonal inundation time. Fe(II)-export rates, dissolved organic carbon export rates, and SO42- reduction kinetics were measured on intact cores using flow-through reactor experiments. Additionally, we characterized wetland minerology using synchrotron x-ray diffraction at Stanford Synchrotron Radiation Lightsource (Beamline 11-3). Even through sulfate is not present at our field sites, we observed a potential for sulfate reduction at all sites and depths. We found that sulfate reduction rates increased almost linearly with input sulfate concentrations, and did not follow Michaelis-Menten kinetics, which we attributed to an adaptation of microbial communities, which increased in abundance with each increase in sulfate concentration. Interestingly, we also observed that sulfate reduction and Fe(II) production concurrently occurred. Furthermore, the presence of sulfate leads to an increase by 2 to 3 orders of magnitude in Fe(II) export rates, which suggests the abiotic reduction of Fe(III)-oxides by biogenic sulfide, highlighting the highly coupled nature of the S and Fe cycles. In addition, sulfate reduction activity did not affect DOC export rates, pointing to a control on C storage beyond the Fe cycle.