SEASONALITY IN THE MINERAL STABILITY OF A SUB-ALPINE SPODOSOL

Mineral stability in soils can be affected by seasonal changes in parameters such as solution composition and temperature. This study investigated the effects of seasonal changes in soil temperature and soil solution composition of a subalpine Spodosol on the stability of the soil clay minerals. Soil solutions were collected monthly using two methods-centrifugation and lysimetry; these two methods were utilized to determine if collection method would affect detection of seasonal differences. Stability diagrams were developed for E, Bs, and C soil horizons. As temperature effects on mineral equilibrium constants were found to be substantial in this high-elevation system, the average seasonal temperature of each horizon was used to determine mineral equilibrium lines. Stability diagrams show differences in mineral stability among horizons and between solution types. Lysimeter solutions do not show seasonal differences in mineral stability; however, the centrifuge solutions do indicate differences in mineral stability with season. An overall pattern of mineral dissolution in spring and stability in summer was found for clay minerals using the centrifuge solutions; stability during fall and winter is mineral-dependent. Changes in solution H4SiO4 were the most important factor in mineral stability of the E horizon. The seasonal path of the centrifuge solutions relative to mineral equilibrium lines varies among horizons. This seasonal cycle of mineral stability was evident in a temperate subalpine soil where environmental changes between seasons are substantial, but such cycles of mineral stability could also occur with other climatic regions.