Wildfires Change Summertime Dissolved Organic Matter in Boreal Headwater Streams

Boreal forests export large amounts of terrestrial carbon into downstream waters as dissolved organic material (DOM), but how increasing wildfire frequency affects this flux remains understudied. If more DOM is exported from land into water after wildfire and/or this DOM is more readily transformed by microbes, the net loss of carbon from terrestrial ecosystems after wildfire may be greater than currently estimated. Here we investigated how wildfire changes DOM exported from boreal forests into headwater streams in northwestern Ontario, Canada over a summer growing season. We compared the concentration and molecular composition of DOM between 10 recently burned and 10 undisturbed catchments using optical spectroscopy and ultra‐high‐resolution mass spectrometry. We found a 29% increase, on average, in DOM concentrations in the streams of burned catchments in August only. DOM in burned catchments appeared less bioavailable, as indicated by a lower H:C and higher modified aromaticity index. As expected, because of wildfire, black carbon was 55% more abundant, on average, in burned catchments compared to controls, contributing to the greater aromaticity of DOM. However, despite the lower bioavailability of DOM, compounds in burned catchments were more thermodynamically favorable for microbial degradation and as likely to be biochemically transformed as unburned DOM during July and August, but not in June. Overall, our results suggest wildfires reduce forest carbon sequestration more than currently estimated because of fluvial DOM losses. If exported DOM is mineralized, carbon sequestration may even decrease, highlighting the need to incorporate the impacts of wildfires on receiving waters into carbon accounting.