IMMEDIATE PRESCRIBED FIRE IMPACTS ON NITRIFICATION RATES WITHIN AN INTERMITTENT STREAM CHANNEL

Fire on hillslope soils often increases soil nitrification rates and downstream nitrate concentrations in perennial stream systems in the year following fire, yet fire responses in non-perennial stream systems may differ because they dry for the part of the year. Drying exposes the channel bed to oxygenated conditions and results in variable sediment moisture throughout non-perennial stream networks. To determine the magnitude of both fire and streambed moisture variability on nitrogen dynamics, we collected pre- and post-fire non-perennial stream sediments to obtain nitrification rates, gravimetric water contents, pH, available ammonium, and available nitrate from a non-perennial stream system in southwestern Idaho that was recently subjected to prescribed fire. To investigate the driving factors behind nitrification rates, we used multivariate analyses including Principal Coordinates Analysis (PCoA) and Principal Coordinates Regression (PCR). Gravimetric water content and available ammonium had Pearson’s correlations of 0.25 and 0.41 respectively with nitrification rates, and explained approximately 50% of the variability in the first PCoA dimension. In contrast, pH was negatively correlated with nitrification rates post-fire (r = -0.10), but positively correlated pre-fire. pH explained nearly 80% of the variability in the second PCoA dimension. Nitrification rates were uncorrelated with Dimension 1 (p-value = 0.76), but were strongly associated with Dimension 2 (p-value = 0.00017), despite gravimetric water content explaining a greater percentage of the variability than Dimension 2. These results indicate that increases in sediment pH from fire lead to an initial decrease in nitrification rates within stream sediments, and later, sediment moisture and ammonium availability lead to increased nitrification rates. Nitrification rates increased two weeks after the fire, suggesting that non-perennial stream channels see increases in nitrate production quicker than perennial stream systems. Subsequent nitrate pulses may be predicted by post-fire rewetting patterns. Understanding how nitrate production interacts with rewetting will inform water quality predictions for fire and stream managers as both wildfires and non-perennial stream systems become more common due to climate and land use changes.