Soil Genesis and Mineral Transformation Across an Environmental Gradient on Andesitic Lahar

Soils derived from andesite are regionally important in the western USA, and expression of andic soil properties may be directly related to climate. The objective of this research was to quantify mineral transformation on andesitic lahar across an environmental gradient on the western slope of the Sierra Nevada of California. We hypothesized that the dominance of short-range-order (SRO) materials would increase with increasing elevation as precipitation increased and temperatures decreased. Seven pedons were sampled across an elevation gradient (150-2800 m) having large variations in mean annual soil temperature (3-17°C) and mean annual precipitation (45-150 cm). The soil mineral assemblage was characterized by x-ray diffraction, selective dissolution, total elemental analysis, and microprobe analysis. Weathering and soil development displayed maxima in the zone just below the permanent winter snowline (∼1590 m), with a sharp decrease at higher elevations. Rainfall-dominated soils at lower elevations had a clay fraction dominated by kaolin. In the snowfall-dominated zone, SRO (allophane and imogolite) dominated the clay fraction, except for the soils in the cryic soil temperature regime, where interlayered 2:1 layer silicates dominated. The 2:1 mineral is probably inherited, based on the presence of a chlorite-like mineral in the andesite parent material. With increasing elevation, soil development followed the order Mollisols → Alfisols → Ultisols → Andisols → Inceptisols. Weathering, mineralogical transformations and soil development are limited by water availability at low elevations, whereas low soil temperature is the major limitation at high elevations.