Aluminum speciation and equilibria in soil solutions of a Haplorthod in the Adirondack Mountains (New York, U.S.A.)

Aluminum chemistry was evaluated in throughfall and leachates from O-, E- and B-horizons of a Spodosol (Typic Haplorthod) at hardwood and conifer sites in the Adirondack Mountains, New York. Organic and fluoride complexes were the dominant forms of aluminum in all soil solutions and throughfall, whereas sulfate and hydroxide complexes were less prevalent. Aquo-aluminum concentrations were extremely low in O-horizon leachates (6 and 7% of total inorganic aluminum at conifer and hardwood sites, respectively), and increased to 28 and 26% of total inorganic aluminum in solutions from B-horizons at conifer and hardwood sites, respectively. Acid-soluble aluminum was 51 and 54% of total aluminum in O-horizon leachates at the conifer and hardwood sites, respectively, and 88% of total aluminum in throughfall. Like aqueous aluminum, soil-bound aluminum was largely associated with organic matter. Solutions were generally undersaturated with respect to the solubility of aluminum-sulfate and aluminum-hydroxide mineral phases. Elevated concentrations of organic and non-labile aluminum in forest floor solutions suggest that biocycling by vegetation may be an important mechanism of aluminum transport in forest ecosystems.