Wildfires impact the thermal stability and molecular composition but not the age of dissolved organic carbon exported by Northern streams

Understanding how wildfires impact the biogeochemistry of dissolved organic matter (DOM) in peatland catchments is important for predicting how they may respond to climate change. However, the net effects of wildfires on the composition of DOM are not yet well understood. We investigated how fire changes the age, thermal stability, and molecular composition of stream DOM in blanket peatlands in the Flow Country and the Isle of Lewis, North of Scotland. Radiocarbon measurements showed that stream DOC was predominantly modern in both bulk and ramped thermal fractions with no apparent change observed due to wildfires. Ramped thermal oxidation revealed higher thermal stability of stream DOM in wildfire impacted areas, as demonstrated by higher activation energies, a proxy for organic C bond strength. This was prominent between 350 and 470°C and was also associated with an increase in the content of thermally stable C and a reduction in bond diversity. Using ultra high-resolution mass spectrometry, we found an increase in the molecular diversity of DOM and in the relative abundance of highly unsaturated and phenolic class. There was also a higher relative abundance of highly oxygenated N- and S-containing formula, potentially from partially combusted plant and soil material, which could explain the shift in activation energy. Together, our results demonstrate ways that wildfires can impact the reactivity and composition of DOM, with implications for its stability and residence time along the terrestrial-aquatic continuum.