Fire regimes in the western US are changing, with patterns in burn area and burn severity becoming disconnected from historical fire regimes (Haugo et al., 2019). These shifting fire patterns are of increasing concern for watershed biogeochemical processes when considering the known impacts of wildfire on water quality and aquatic ecosystem health, which can persist for years post-fire (Bladon et al., 2014;Emelko et al., 2016;Niemeyer et al., 2020). The intermediate-and long-term influences of wildfires on water quality are diverse across affected watersheds Santos et al., 2019;Sherson et al., 2015). Post-fire stream water chemistry, for example, is thought to result from an interplay between biogeochemical and hydrologic processes impacted by fire, such as water availability and soil water repellency (Niemeyer et al., 2020). In fact, the environmental fate of fire-impacted materials-also termed pyrogenic organic matter (PyOM)-is thought to be determined by its first interactions in water post-fire (Masiello & Berhe, 2020). The short-term hydrological response to fires is also regionally dependent. For example, high hydrological connectivity between hillslopes and streams in highland regions of the western US accelerates the delivery of water to streams post-fire (Hallema et al., 2017). This immediate hydrologic response is dependent on a complexity of factors that alter the chemical and physical properties of the watershed soils, including burn severity (Moody et al., 2016).
Warmer and drier climate has contributed to increased occurrence of large, high severity wildfires in the Pacific Northwest, drawing concerns for water quality and ecosystem recovery. While nutrient fluxes generally increase post-fire, the composition of organic matter (OM) transported to streams immediately following a fire is poorly constrained, yet can play an integral role in downstream water quality and biogeochemistry. Here, we quantified spatiotemporal patterns of dissolved OM (DOM) chemistry for five streams burned by wildfires in Oregon, USA in 2020. We sampled over a 24-hour storm event one month after the fire, revealing variable temporal behavior in DOM dynamics. DOM chemistry was directly related with burn severity spatially. Specifically, nitrogen and aromatic character of DOM increased in streams burned at greater severity. Our results suggest a spatial overprinting of DOM dynamics immediately following fire activity and highlight a key gap in our knowledge of post-fire DOM transport to streams.
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