Modeling Aluminum and Organic Matter Solubility in the Forest Floor Using WHAM
1999
de Wit, Helene A. | Kotowski, Marek | Mulder, Jan
Several studies suggest that solution concentrations of Al in organic surface soils are controlled by complexation with organic matter. We applied the mechanistic Windermere humic aqueous model (WHAM) to describe the solubility of Al and organic matter as observed in a batch equilibrium study with a forest floor Oe horizon. WHAM is unique in that it considers interactions of soil organic matter with protons and metals. We also compared WHAM with a previously proposed linear regression model that describes Al solubility. A range of soil Al contents was established by adding different amounts of Al in batch prior to titration with acid or base. The soil Al content was described by the bound Al ratio (BAR), defined as the equivalent ratio of organically bound Al and carboxyl groups. The bound Al ratio and pH ranged from 0.1 to 3, and from 1.7 to 6.3, respectively. Solutions were undersaturated with respect to Al(OH)₃, except at BAR ≥ 2 and pH ≥ 4.5. Aluminum solubility increased with increasing BAR. Organic matter solubility was greatest at low BAR and high pH. WHAM reproduced the observed pH and Al concentrations, using parameters derived from experimental data. At BAR < 0.7, pH–pAl relationships were approximately linear. At BAR ≥ 0.7, there was a nonlinear increase in ΔpAl/ΔpH with pH. WHAM simulated the changing slope of the pH–pAl curves satisfactorily and reproduced observed trends in dissolved organic C (DOC) concentrations. This supports the hypotheses and assumptions concerning mechanisms for binding Al to soil organic matter in a forest floor, as embodied in WHAM.
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