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Inorganic Phosphorus Transformation and Transport in Soils: Mathematical Modeling in ecosys

Dep. of Renewable Resources, Univ. of Alberta, Edmonton, Alberta, Canada T6G 2E3

D. J. Heaney

Plant Industry Division, Alberta Agriculture, Edmonton, Alberta, Canada T6H 4P2

*Corresponding author.

ABSTRACT

The movement and uptake of P in soils occur primarily in the soluble phase, so that the reliable simulation of P movement and uptake requires that the concentrations of soluble P forms be explicitly represented in mathematical models. To represent soluble P concentrations under dynamic boundary conditions, a convective-dispersive model of P transport has been coupled to a model of P transformation in which adsorption-desorption, precipitation-dissolution, and ion pairing are explicitly represented as concurrent equilibrium reactions. This model is used to explain the temporal and spatial distribution of P among soluble and resin-, NaHCO₃-, NaOH-, and HCl-extractable fractions in soils following amendment with KH₂PO₄. Simulated reductions in soil pH following different P amendments caused solid-phase P in the model to be recovered more from resin- and NaOH-extractable forms and less from HCl-extractable forms as solution P concentration increased. These changes were consistent with those observed experimentally using a P fractionation procedure on a Malmo silt loam (Typic Cryoborall) following its equilibration with 0 to 512 mg L^-1 of KH₂PO₄ and following its irrigation for 205 d with 50 mg L^-1 of KH₂PO₄. Simulated displacement of cation coprecipitates from exchange sites allowed the model to reproduce the temporal and spatial patterns of water- and HCl-extractable P in resin columns of different cation-exchange capacities following a KH₂PO₄ surface amendment. The results of model testing suggest that changes in soluble P concentrations following P amendments may be represented from concurrent equilibrium reactions for adsorption-desorption, precipitation-dissolution, and ion pairing. However, the rate at which these reactions proceed remains uncertain.

Received for publication March 25, 1996.

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