SOLUTION CHEMISTRY PROFILES OF MIXED-CONIFER FORESTS BEFORE AND AFTERFIRE

Solution chemistry profiles of mixed-conifer forests in granitic catch ments of the Sierra Nevada were measured for three years before (1987- 1990) and three years after (1990-1993) prescribed fire. Wet depositio n, throughfall and soil solution samplers were installed in both white -fir and giant-sequoia dominated forest stands underlain by poorly dev eloped inceptisols. Stream water chemistry was monitored as part of an ongoing study of catchment outputs. Calcium, NO3- and Cl- were the ma jor ions in precipitation. Canopy leaching increased mean concentratio ns of all major ions, especially K+ and Ca2+. Water flux through the s oil occurred largely during spring snowmelt. Forest floor leachate rep resented the most concentrated solutions of major ions. Interaction wi th the mineral soil decreased mean concentrations of most species and the average composition of soil solutions closely resembled stream wat er at baseflow. Bicarbonate alkalinity, Ca2+, Mg2+, and Nat were enric hed in stream water relative to precipitation whereas inputs of H+, NH 4+, NO3-, and SO42- were retained within the catchments. Burning of th e forest understory and litter layer increased solute concentrations i n soil solution and stream water. Mean soil solution Ca2+, Mg2+ and K concentrations increased more than 10 fold, but the relative predomin ance of these cations was not affected by burning. Sulfate concentrati on, which was very low in soil solutions of undisturbed stands (< 25 m mol(c) m(-3)), increased more than 100 times following fire. Ammonium concentration exhibited a rapid, short-term increase and then a decrea se below pre-bum levels. Changes in soil solution chemistry were refle cted in catchment outputs.