Abstract 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.
Plain Language Summary Wildfires can change the amount and type of carbon naturally exported from land into water as part of the wider carbon cycle. We compared streams from boreal forests that were recently burned with nearby undisturbed streams in Ontario, Canada. We discovered higher carbon concentrations in streams from burned forests, and this carbon remained available for microorganisms to degrade despite a greater chemical complexity. These findings suggest that wildfires may result in larger carbon losses from land than expected, which has important implications for forest management aimed at mitigating climate change, such as through efforts to increase carbon sequestration.
Key Points Wildfire increases dissolved organic matter concentrations in boreal headwater streams Dissolved organic matter was less bioavailable but more thermodynamically favorable for degradation in streams from burned catchments Declines in carbon sequestration after wildfire may have been underestimated without considering organic matter export into receiving waters