GENESIS OF NON-ALLOPHANIC E-HORIZON IN TEPHRA-INFLUENCED SPODOSOLS

Holocene volcanic ash from the cataclysmic eruption of Mount Mazama in southwestern Oregon is an important component of many soils of the Pa cific Northwest region. High-elevation forested soils of northern Idah o containing Mazama tephra have developed E-Bhs horizon sequences and are accordingly classified as Spodosols. In this study, we report on t he formation and expression of E horizons in a developmental sequence of volcanic-ash-influenced Spodosols from this region. Results suggest that development of E horizons in these volcanic-ash-influenced soils proceeds systematically along an elevational gradient. The lightest c olored, thickest E horizons are associated with the highest elevation soils of the subalpine fir [Abies lasiocarpa (Hook.) Nutt.] forest zon e, and E horizons are more weakly expressed and then absent with decre asing elevation. Microprobe analysis of volcanic glass shows that E an d Bhs horizons contain the same tephra, indicating that substantial pe dogenesis has taken place in these horizons. Formation and eluviation of AI-humus complexes has given rise to E horizons whose mineralogy an d chemistry are consistent with that of non-allophanic Andisols and di ffer markedly from underlying horizons. Beidellite, an Al-rich smectit e, dominates the clay fraction of the well-developed, very strongly ac id E horizons at the expense of allophanic clays and metal-humus compl exes; other than beidellite, opaline SiO2 is the major secondary Si-be aring phase. Chemical and mineralogical properties of illuvial B horiz ons are consistent with those of allophanic soils; short-range-order a luminosilicates and metal-humus complexes dominate the clay fractions. Results indicate that coupled eluvial-illuvial processes associated w ith podzolization are responsible for the non-allophanic character of E horizons and the allophanic character of Bhs horizons in these fores ted, volcanic-ash-influenced ecosystems.