Weathering of Mt. St. Helens tephra under a cryic-udic climatic regime

The initial stages of chemical weathering in tephra were examined during a 10-yr period under field conditions in a cryic-udic climatic regime. Unleached tephra from the 1980 eruption of Mt. St. Helens was applied to the surface of soils in 5- and 15-cm depths to simulate natural tephra deposition on subalpine forest ecosystems. Solid-phase tephra samples were collected 10 yr after addition and compared chemically and mineralogically to the original, unweathered tephra. The solid-phase pH of the tephra decreased by approximately one unit, with the 5-cm treatment being 0.2 units lower than the 15-cm treatment. The accumulation of Al and Fe as alteration products in the weathered tephra was detected by selective dissolution analyses. Aluminum accumulated as Al-humus complexes, hydroxy-Al polymers in the interlayer of 2:1 layer silicates, and possibly as hydroxy-Al precipitates external to the interlayer space; Fe accumulated primarily as a noncrystalline oxyhydroxide (e.g., ferrihydrite) and Fehumus complexes. The increase in the active Fe and Al fraction (i.e., oxalate extractable Fe and Al) contributes to a sevenfold increase in phosphate retention. Hydroxy-Al interlayering of detrital 2:1 layer silicates present in the tephra was suggested from x-ray diffraction and selective dissolution analyses. There was no evidence of opaline silica, allophane, or imogolite forming in the tephra. Preferential incorporation of Al into Al-humus complexes and hydroxy-Al interlayers of 2:1 layer silicates appears to inhibit formation of allophane and imogolite during the initial stages of weathering.