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

Abstract

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 moss N concentrations were increased by about 40% throughout the study period, thus representing the aquatic analog to N retention via increased plant N uptake. The lack of impact on aquatic N dynamics after spring prescribed burning indicated N retention of post-fire available N within the terrestrial ecosystem. However, healthy forest ecosystems are adapted to periodic pulses of high post-fire availability of inorganic N in soil and streams. Therefore, from a N cycling perspective, rehabilitation measures are not required after wildfire and managers should target higher burn severities for prescribed burning at both the plot and watershed scale in order to stimulate N cycling and to reduce fuel loads more substantially.