Wildfire-derived dissolved black carbon reorganizes sulfur photochemistry in freshwater systems

Wildfires release substantial dissolved black carbon into surface waters, often coinciding with elevated yet poorly explained levels of organosulfate-like dissolved organic sulfur in rivers. Here we show that wildfire-derived dissolved black carbon promotes light-driven sulfur transformation in dilute, near-neutral freshwaters. Electron paramagnetic resonance reveals sulfate-associated radical-like signals only in irradiated systems containing dissolved black carbon, with signal intensity increasing with sulfate availability and organic carbon abundance. High-resolution mass spectrometry shows enrichment of sulfur-containing molecular formulas and mass-difference patterns consistent with –SO3/–SO4-type transformation linkages within unsaturated and aromatic compositional domains. Across sulfate gradients, net dissolved organic sulfur accumulation follows a nonmonotonic trend, consistent with competing sulfur incorporation and photodegradation. Pre-irradiated samples continue to accumulate dissolved organic sulfur during subsequent dark incubation, suggesting sustained sulfur transformation activity beyond the photic period. These results indicate that wildfire-derived carbon inputs can influence sulfur cycling and photochemical reactivity in inland waters following wildfire disturbance. Across sulfate gradients, dissolved organic sulfur accumulation was nonmonotonic, while pre-irradiation sustained sulfur transformation beyond the photic period, indicating wildfire-derived carbon can alter sulfur cycling and photochemical reactivity in inland waters, based on incubation experiments.