Soil Heating from Fire Induces Bidirectional Changes in Dissolved Organic Matter Release and Organic Sulfur Speciation

Abstract Soil heating from fire influences dissolved organic matter (DOM) release but has unknown effects on organic sulfur (S) speciation, with implications for S and metal cycling in aquatic environments. Here, low and high organic carbon soils (4.80 versus 50.4%) were heated at environmentally relevant temperatures and durations (75–300 °C, 0.25–4.0 h) and subject to leaching experiments that quantified DOM and inorganic anion release, and changes in organic S speciation via S X-ray absorption near-edge structure (XANES) spectroscopy. In both soils, heating between 75 and 200 °C increased dissolved organic carbon (DOC) and inorganic sulfate (SO42–) release (by up to 24- and 88-fold, respectively) while decreasing the % highly reduced S in DOM (e.g., thiols), indicating thiol oxidation to SO42–. Soil heating at 300 °C decreased DOC release and increased SO42– release, with step increases in DOM aromaticity and % heterocyclic reduced S species (e.g., thiophenes). Across treatments, DOM aromaticity positively correlated with DOM % heterocyclic reduced S, interpreted to reflect organic S cyclization during heating. The observed bidirectional changes in organic S speciation in DOM due to soil heating, with thiol oxidation at ≤200 °C and S cyclization at 300 °C with SO42– releases, provide mechanistic insights that improve the prediction of fire effects on S, C, and metal cycling in aquatic environments.