SPATIAL PATTERNS IN BASE CATION RELEASE FROM SOIL MINERAL WEATHERING IN VERMONT

Forest ecosystem sensitivity to the effects of acid deposition depends in part on the rate of release of the base cations Ca2+, Mg2+, K+, and Na+ from the weathering of primary minerals in the soil root zone. Base cations are periodically removed from forest ecosystems via harvesting and continuously removed by leaching. Chemical weathering of primary minerals in the soil profile is the dominant process by which lost base cations are replaced. Atmospheric contributions are relatively small. We evaluated spatial patterns in the relative rates of base cation release due to the influence of soil parent material composition and landscape position. Parent material mineral composition was estimated from an analysis of bedrock mineralogy and a model of glacial till source areas based on pebble-train observations. The influence of landscape position was investigated using a kinetic model to evaluate the effects of temperature and moisture regimes on mineral weathering rates. The dominant source of spatial variation in the weathering rate is the geologic distribution of reactive primary mineral phases such as calcite, hornblende and plagioclase. The landscape factors were of secondary but significant importance, imparting substantial spatial variation within similar parent material types.