Effects of prescribed burning and drought on the solute chemistry of mixed-conifer forest streams of the Sierra Nevada, California

Solute concentrations in atmospheric deposition and stream water were measured in two mixed-conifer catchments (Tharp's and Log creeks) in the Sierra Nevada of California from 1984 through 1995, a period including a 6-year drought and a prescribed burn in one catchment. The effects of prescribed burning in the Tharp's Creek catchment significantly increased the concentrations of most solutes in stream water. In the first year after prescribed burning, the VWM (volume-weighted mean) concentrations of acid anions in stream water increased proportionally more than those of the base cations, and ANC (acid neutralizing capacity) more than doubled. Sulfate and NO; increased proportionally more in stream water than any other ions after the fire, but pre-and post-burn VWM pH were not significantly different. VWM SO22- and NO3- concentrations the first year after burning occurred were about 16- and 2,000-fold above pre-burn baselines, respectively, while that of Cl- increased 4-fold. Net retention (precipitation inputs minus streamwater outputs) of H+, NO3-, NH4+, SO42- and Cl- occurred in both catchments, except after prescribed burning of the Tharp's Creek catchment in the fall of 1990, which caused a net export of SO42-, Cl- and K+ the first year after the bum. Most solutes remained above pre-disturbance concentrations by the end of the third year after burning, whereas H+ and SiO2 remained below. Periodic increases in the concentrations of Na+, Ca2+ and SO42-, and decreases in ANC and SiO2 occurred during a 6-year drought monitored in the adjacent undisturbed catchment of Log Creek.