SOLUTION CHEMISTRY PROFILES OF MIXED-CONIFER FORESTS BEFORE AND AFTER FIRE

Solution chemistry profiles of mixed-conifer forests in granitic catchments of the Sierra Nevada were measured for three years before (1987-1990) and three years after (1990-1993) prescribed fire. Wet deposition, throughfall and soil solution samplers were installed in both white-fir and giant-sequoia dominated forest stands underlain by poorly developed inceptisols. Stream water chemistry was monitored as part of an ongoing study of catchment outputs. Calcium, NO3- and Cl- were the major ions in precipitation. Canopy leaching increased mean concentrations of all major ions, especially K+ and Ca2+. Water flux through the soil occurred largely during spring snowmelt. Forest floor leachate represented the most concentrated solutions of major ions. Interaction with the mineral soil decreased mean concentrations of most species and the average composition of soil solutions closely resembled stream water at baseflow. Bicarbonate alkalinity, Ca2+, Mg2+, and Nat were enriched in stream water relative to precipitation whereas inputs of H+, NH4+, NO3-, and SO42- were retained within the catchments. Burning of the forest understory and litter layer increased solute concentrations in soil solution and stream water. Mean soil solution Ca2+, Mg2+ and K+ concentrations increased more than 10 fold, but the relative predominance of these cations was not affected by burning. Sulfate concentration, which was very low in soil solutions of undisturbed stands (< 25 mmol(c) m(-3)), increased more than 100 times following fire. Ammonium concentration exhibited a rapid, short-term increase and then a decrease below pre-burn levels. Changes in soil solution chemistry were reflected in catchment outputs.