Phosphorus and nitrogen dynamics in streams associated with wildfire: A study of immediate and longterm effects

Stream nutrient data were collected both during a wildfire and over a subse quent five-year period. Sampling was from a series of paired watersheds loc ated within and outside of the wildfire. Phosphorus and nitrogen concentrat ions increased from 5 to 60 fold over background levels during the first fe w days of the month-long wildfire with maximum recorded concentrations as h igh as 135 mu g/L soluble reactive phosphorus, 261 mu g/L ammonium, and 61 mu g/L nitrate. Total phosphorus and total nitrogen during this same time p eriod increased up to 206 mu g/L and 349 mu g/L, respectively. Nutrient con centrations declined to background levels (<5-10 mu g/L) within a few weeks after the fire. Over the following five years we observed dissolved nutrie nt concentrations reaching >40 mu g/L soluble reactive phosphorus and >125 mu g/L nitrate in impacted streams, concentrations >5 fold over those obser ved in control streams. In high gradient watersheds, all nutrients were eas ily transported to the streams with most notable impact during the early ye ars after the fire. In less steep terrain, soluble reactive phosphorus conc entrations were significantly higher 3 to 5 years after the fire, which als o corresponded to relatively high spring hydrographs. In contrast, nitrogen compounds were observed to be significantly higher in concentration in fir e impact streams in the years immediately following the fire. We attribute these differences in the rate of nutrient loads to be the result of the dif ferent mobilization mechanisms of phosphorus and nitrogen and the different soil and geomorphic settings of the watersheds drained by the different st reams.