Wildfire altered storm-event dynamics of dissolved nitrogen from headwater streams in the Western Cascades, Oregon

The majority of the global population, including residents in the Pacific Northwest, USA, relies on drinking water from forested headwaters. In recent decades, forested watersheds in the region have experienced increased wildfire activity, putting water security at risk due to elevated constituents (e.g., sediment, nutrients) that can persist for years or decades. Therefore, conducting high-frequency water quality sampling during elevated streamflow events (i.e., “storms”) can provide crucial information about wildfire effects on source water quality during periods when impacts are likely to be heightened. In our study, we quantified dissolved nitrogen concentrations and hysteretic behavior during two storm events (fall and winter) following a low–moderate severity wildfire (2024 Lane 1 Fire) in the Western Cascades ecoregion, Oregon, USA. We observed greater concentrations of nitrate and total dissolved nitrogen at burned sites compared to unburned sites, and greater concentrations at both burned and unburned sites in the fall compared to the winter sampling period. While concentrations of total dissolved organic nitrogen were low (≤ 0.15 mg L -1 ), concentrations were greater at the unburned sites compared to the burned sites. We observed consistent, clockwise hysteresis and flushing behavior at both burned and unburned sites, indicating nearby sourcing of dissolved nitrogen. This pattern likely reflects the predominance of hydrologic controls over wildfire disturbance on nitrogen transport at our study sites. Our study highlights the need for continued storm-event sampling following wildfire, as first-flush conditions, despite being logistically challenging to capture, can reveal water quality impacts even after low–moderate severity burns.