Spatio-temporal Response of Pyrogenic Dissolved Organic Matter in Five Streams Following a Wildfire

There is growing concern in the Western United States of increasing wildfire activity as shifting climate regimes are prompting longer and drier fire seasons. An increase in the production of pyrogenic organic matter (PyOM), concurrent with increasing fire activity, is expected to have substantial implications on terrestrial to aquatic biogeochemical cycles. PyOM is chemically complex and represents of a range of bioavailable to highly refractory compounds. While nutrient fluxes generally increase post-fire, controls on the export and composition of PyOM in aquatic systems are still poorly constrained. Here, we describe the spatio-temporal patterns in PyOM chemistry for five streams within the perimeter of the 2020 Holiday Farm fire that burned ~700 km2 of predominately forested land along the McKenzie River in Oregon, USA. We used high resolution mass spectrometry and select fire-derived chemical markers to describe PyOM composition across sites. Sampling over a 24-hour storm period at each site revealed highly variable temporal responses in PyOM behavior, suggesting significant spatial complexity in mechanisms driving the transport of pyrogenic materials to streams soon after fire activity. Across all sites, spatial variation in PyOM behavior was most directly related to burn severity land cover classification, with elevated N-containing molecular formulae identified within the most severely impacted sites. Collectively, these results suggest an overprinting of spatial controls on PyOM dynamics relative to temporal storm-related variation, highlighting a key gap in our knowledge of the post-fire mobilization of PyOM. Furthermore, this study highlights the need to better elucidate immediate post-fire controls on PyOM transport to streams and the implications for downstream biogeochemical processes and water quality.