Effect of peat burn severity on peatland DOC concentration and DOM composition exported following wildfire

Climate change is increasing boreal biome drying, area-burned, wildfire intensity, and burn severity as evidenced by the unprecedented 2023 Canadian wildfire season (>15 Mha burned). Of particular concern in boreal wildfires are deep burning smouldering peat fires that can switch peatlands to net emitters of atmospheric carbon. Less studied are the effects of peat fires on water-borne carbon and the deleterious impacts on downstream water quality as the burned area recovers post-fire. To better understand the impacts of wildfires on northern peatlands, we investigated the effects of varying peat burn severities on the dissolved organic carbon (DOC) concentration and composition of dissolved organic matter (DOM) exported from peatlands located in Ontario's Boreal Shield ecozone. Using a paired peatlands approach with twelve peatlands of comparable size and catchment, runoff and water quality were measured within the footprint of the Parry Sound #33 wildfire (burned) and near Dinner Lake (unburned). Over three years (2021-2023), exported DOC concentrations decreased with increasing burn severity but the composition of DOM varied across burn severities. Spectral slope (SR), SUVA₂₅₄, and humification index (HIX) were utilized to assess DOM composition. Lower HIX and higher SR values were observed indicating smaller, less humified DOM as burn severity increased. SUVA­₂₅₄, however, showed no strong trends across burn severities suggesting that returning vegetation composition may have a strong control on DOM composition. Considering that climate change is increasing burn severity, the recovery of burned peatlands may play a large role in the export of DOC concentration and DOM composition post-wildfire.