Effects of spring prescribed burning and wildfires on watershed nitrogen dynamics of central Idaho headwater areas

► Spring prescribed burns have lower burn severity than wildfires. ► Soil N concentrations tend to be higher after wildfire than after spring burns. ► Streamwater nitrate is positively correlated with dNBR only in wildfire sites. ► Upland plants and in-stream moss sequester post-fire excess inorganic N. ► Managers should target higher spring prescribed burn severities. Fire is known for its potential to profoundly affect nitrogen (N) dynamics in both terrestrial and aquatic ecosystems. However, few studies have investigated fire effects on several important watershed N pools simultaneously or have directly compared effects of spring prescribed burns and wildfires that occurred in the same geographic area. We simultaneously measured N concentrations in soil, understory plant foliage, streamwater, and in-stream moss in paired (burned/unburned) headwater watersheds of replicated spring prescribed burns and wildfires in several growing seasons after fire. Watershed-scale burn severity, estimated via the satellite image-derived, reflectance-based delta Normalized Burn Ratio (dNBR), was significantly higher ( P < 0.05) after wildfires than prescribed burns. Fire effects were found in the terrestrial and aquatic ecosystems after wildfire but were limited to the terrestrial ecosystem after prescribed burns. Soil ammonium (NH 4 +) and nitrate (NO 3 −) concentrations were increased several-fold ( P < 0.05) in burned relative to corresponding unburned plots in prescribed burn and wildfire sites. In the first post-fire growing season, soil NO 3 − concentrations in wildfire-burned plots were one order of magnitude higher than in prescription-burned plots reflecting differences in burn severity. Plants resprouting after spring prescribed burns and wildfires sequestered post-fire available soil N, as indicated by higher foliar N concentrations. The relative increase in foliar N concentrations of four upland understory plant species or ecological groups averaged 47% in burned plots relative to unburned plots. Fire effects tended to be strongest and statistically significant in the first growing season after fire in the terrestrial ecosystem. In the headwater streams, effects of wildfire persisted, seemingly unattenuated, throughout the first three growing seasons after fire. Streamwater NO 3 − concentrations were one order of magnitude higher in wildfire-burned watersheds than in unburned watersheds and were positively correlated with dNBR during spring runoff. In-stream