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Aluminum Fractionation of Soil Solutions from Unperturbed and Tephra-Treated Spodosols, Cascade Range, Washington, USA

College of Forest Resources; Univ. of Washington, Seattle, WA 98195

* Corresponding author.

ABSTRACT

Fractionation of aqueous Al was investigated in leachates from a subalpine Spodosol (Typic Cryohumod) located in the Cascade Range of Washington state. Leachates were collected from major genetic horizons with tension lysimeters during the fall, winter, and spring seasons. Solutions were collected from unperturbed profiles and from profiles that had received artificial additions of Mt. St. Helens tephra 4 yr prior to this study. Fractionation of Al was performed by a column cation exchange technique that minimized alterations in pH and ionic strength during the fractionation procedure. Organically complexed Al was the dominant form of soluble Al in the Oa, E, and Bhs horizons, while organically complexed and inorganic forms were nearly equal in the Bs horizon. These results are consistent with the metal-fulvate theory of podzolization. The effects of tephra deposition were only apparent in the Bs horizon, where inorganic Al concentrations were twice the concentrations measured in nondisturbed profiles. Laboratory experiments showed that the ability of natural organic acids in soil solutions to complex Al increased as the pH was increased from 3.8 to 5.0. Aqueous Al³⁺, added as AlCl₃ to solutions at pH = 4.5, was found primarily as organic complexes when organic C/metal atomic ratios were >50. At organic C/metal ratios <50, the additional Al was found primarily in inorganic forms.

Contribution from the College of Forest Resources, Univ. of Washington.

Received for publication April 20, 1987.