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DIVISION S-2—SOIL CHEMISTRY

Predicting Aluminum and Soil Organic Matter Solubility Using the Mechanistic Equilibrium Model WHAM

^a Norwegian Forest Research Institute, Hoegskoleveien 12, N-1432 Aas, Norway
^b Dep. of Chemistry, Univ. of Oslo, P.O. Box 1033 - Blindern, N-0315 Oslo, Norway
^c Dep. of Soil and Water Sciences, Agric. Univ. Norway, Box 5028, N-1432 Aas, Norway

* Corresponding author (heleen{at}nisk.no)

The mechanistic equilibrium model WHAM is used to describe solution–solid phase interactions in the forest floor with regard to Al and soil organic matter (SOM) solubility. WHAM takes into account specific and nonspecific ion-binding to humic compounds. Experimental data from a forest soil that was manipulated with respect to its Al content were obtained from batch studies and a field manipulation experiment. WHAM was parameterized using observations of pH, concentrations of inorganic Al, and DOC obtained in batch. Model fits of pH, concentrations of inorganic Al (>5 x 10^-6 mol L^-1), and DOC were good to tolerable (1, 9, and 15% deviation, respectively). Values of optimized model parameters agreed reasonably with analytically determined quantities. Using the optimized parameters, WHAM simulated addition of AlCl₃ to the same soil. Comparison between model predictions and batch observations showed a deviation for pH, Al (>5 x 10^-6 mol L^-1), and DOC of 3, 60, and 15%, respectively. We regard this as a reasonable model performance and support for the assumption of an organic complexation control of Al solubility in organic soils. Application of WHAM to predict effects of AlCl₃ addition in the field resulted in qualitative agreement between simulations and observations from tension lysimeters in the forest floor, but in a failure regarding the observed ranges of H, Al, and DOC. The discrepancy between model simulations and field observations may be explained qualitatively by a lack of equilibrium due to the diffusion-limited exchange of solutes between immobile water in micropores and mobile water in macropores.

Abbreviations: Al-qr, quickly reacting aluminum • BAR, bound Al ratio • CAL, soil content of reactive Al • CEC, cation-exchange capacity • CFA, soil contents of FA • CHA, soil contents of HA • CHS, soil content of HS • DOC, dissolved organic carbon • FA, fulvic acids • FA0, minimum concentration of dissolved FA • HA, humic acids • HS, humic substances • I, ionic strength • ICP, inductively coupled plasma optical emission spectroscopy • OM, organic matter • RDM, relative mean deviation • RMSD, root of mean squared deviation • SOM, soil organic matter • SSR, soil-to-solution ratio • TOC, total organic C • WHAM, Windermere Humic Aqueous Model • , parameter describing the distribution of FA over 10 model fractions with varying degrees of hydrophobicity

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