Elevated Atmospheric Sulfur Deposition Affects Predominant Forms of Sulfur in Ombrotrophic Peatlands

Abstract Long‐term transformations of sulfur from atmospheric deposition in ombrotrophic peatlands have rarely been studied, although the potential impact on carbon mineralization and particularly methane formation is acknowledged. To elucidate the long‐term fate of sulfur in peat, we therefore applied sulfur K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy to investigate peatlands with either natural or anthropogenic atmospheric sulfur deposition. A peatland in central Europe (Germany, BBM) experienced high sulfur deposition during the Industrial Revolution, and an oceanic peatland (Chilean Patagonia, PBr) continuously exposed to aerosols from sea spray, were investigated. During early phases of site BBM, when sulfur deposition was constantly low, wet‐chemical extractions indicated that 98% ± 0.7% of sulfur was present as organic sulfur. The S K‐edge XANES fitting suggested that sulfur redox transformations mainly occurred near the water table, at which reduced sulfur forms (primarily organic sulfides, and thiols) increased from ∼40% to and stabilized at 65% ± 4% in anoxic peat. An increased contribution of reduced sulfur forms was observed in the polluted section of BBM and entire peat profile of PBr. While increases in reduced inorganic sulfur (TRIS) likely depended on available Fe, rises in the ratio of reduced organic sulfur to total organic sulfur from both sites were pronounced. This increase in reduced organic sulfur forms likely resulted from abiotic sulphurization of organic carbon after sulfate reduction. Our study highlights the long‐term fate of elevated sulfur in ombrotrophic peatlands, being mainly transformed into reduced organic sulfur.