High‐Elevation Forest Soils of the Southern Appalachians: II. Geomorphology, Pedogenesis, and Clay Mineralogy

Abstract

AbstractWe investigated soil mineral weathering sequences in three geologically distinct high‐elevation areas of southern Appalachian spruce‐fir (Picea rubens Sarg.‐Abies fraseri [Pursh.] Poir) forests in order to elucidate the pedogenic factors that distinguish these unique soils from their low‐elevation counterparts. Four pedons representing a developmental sequence ranging from soils having distinct spodic field morphology (E plus Bhs horizons) to those having either minimal (weak E horizons) or no spodic character (umbric epipedons/cambic horizons) were selected for further study from a total of 35 excavated pits. Whereas the mineralogy of sand and silt fractions reflected inheritance from parent materials, clay mineral suites of all soils were very similar, indicating biotic and climatic controls on weathering. Surface‐horizon clay fractions were dominated by regularly interstratified mica/vermiculite and high‐charge smectite, which decreased with depth in all profiles. Subsoil clay fractions were principally composed of gibbsite and hydroxy‐interlayered vermiculite. Genesis of secondary 2:1 layer silicates in these soils occurs initially by the pseudomorphic transformation of biotite to both hydrobiotite and vermiculite. However, direct conversion of biotite to kaolinite is of little importance in these soils. The demonstrated mobility of Al‐organic complexes out of surface horizons and the inability of hydroxy‐Al interlayers to form in vermiculite in the presence of organic acids suggest that these soils are undergoing early stages of podzolization and that they are more similar to northern Appalachian Spodosols than they are to adjacent low‐elevation soils of the southern mountains.