Controls on Soil Solution Chemistry in a Subalpine Forest in North‐Central Colorado

Abstract

AbstractHigh‐elevation ecosystems in the western USA are potentially susceptible to increased inputs of strong acids. A long‐term research project was established to identify the processes controlling surface water chemistry and to evaluate the sensitivity of Loch Vale Watershed in Rocky Mountain National Park, Colorado, to acid precipitation. Using lysimeters, we estimated the concentration and flux of major solutes in the Oie and B horizons in an old‐grown Englemann spruce (Picea engelmannii Parry ex Engelm.) and subalpine fir [Abies lasiocarpa (Hook.) Nutt.] forest, and in an adjacent site disturbed by a snow avalanche. In the forested site, most solutes were highly concentrated in soil solutions during the initial stages of snowmelt, and concentrations declined rapidly to low levels in the first 4 to 6 wk of snowmelt. Surface water chemistry in Loch Vale Watershed is controlled principally by mineral weathering. During the early stages of snowmelt, however, the flushing of solutes, especially N, S, and C, from forest soils (which comprise only 6% of the study area) exerts an important influence. Rates of cationic denudation per unit area were 3.5 times higher in the forest (131 µmolc m−2 yr−1) than in the whole watershed (38 µmolc m−2 yr−1) probably because of H+ exudation from roots during nutrient uptake and the generation of organic acids in the forest soils. Rates of N mineralization and nitrification as well as concentrations of NO3 in the soil solution were higher in the second year after forest disturbance from a snow avalanche than immediately following the disturbance, indicating a delayed nitrification response. Significant amounts of NH4 and NO3 were temporarily stored in extractable forms in the soil during the first year after the disturbance. Although precipitation inputs and theoretical mineral weathering can explain the total annual solute flux from Loch Vale Watershed, the effects of forest soil solutes may be important during the initial stages of snowmelt and following large‐scale disturbance.