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Soil Science Society of America Journal Abstract -

Weathering of Mt. St. Helens Tephra under a Cryic-Udic Climatic Regime


This article in SSSAJ

  1. Vol. 61 No. 5, p. 1519-1525
    Received: Mar 8, 1996

    * Corresponding author(s): radahlgren@ucdavis.edu
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  1. R. A. Dahlgren ,
  2. J. P. Dragoo and
  3. F. C. Ugolini
  1. Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616
    Dipartimento Di Scienza Del Suolo E Nutrizione Della Pianta, Universita Degli Studi, Piazzale Delle Cascine, 15, 50144 Firenze, Italy



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 Fe-humus 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.

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