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DIVISION S-2-SOIL CHEMISTRY

Modeling Aluminum and Organic Matter Solubility in the Forest Floor Using WHAM

^a Norwegian Forest Research Inst., 1432 Ås, Norway
^b Dep. of Environmental Protection Engineering, Technical Univ. of Lublin, Lublin, Poland

heleen{at}nisk.no

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 pAl/pH 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.

Abbreviations: AAS, atomic absorption spectroscopy • calc, calculated • CFA, soil content of fulvic acids • CHA, soil content of humic acids • CHS, soil content of humic substances • DOC, dissolved organic carbon • FA, fulvic acids • HA, humic acids • HS, humic substances • I, ionic strength • meas, measured • optim, optimized • RMSD, root of the mean squared deviation • WHAM, Windermere humic aqueous model • , parameter describing the distribution of CFA within 10 model-defined fractions of fulvic acids with specified hydrophobicities

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