INORGANIC PHOSPHORUS TRANSFORMATION AND TRANSPORT IN SOILS - MATHEMATICAL-MODELING IN ECOSYS

The movement and uptake of P in soils occur primarily in the soluble p hase, so that the reliable simulation of P movement and uptake require s that the concentrations of soluble P forms be explicitly represented in mathematical models. To represent soluble P concentrations under d ynamic boundary conditions, a convective-dispersive model of P transpo rt 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 t o explain the temporal and spatial distribution of P among soluble and resin-, NaHCO3-, NaOH-, and HCl-extractable fractions in soils follow ing amendment with KH2PO4. Simulated reductions in soil pH following d ifferent P amendments caused solid-phase P in the model to be recovere d more from resin- and NaOH-extractable forms and less from HCl-extrac table 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 equili bration with 0 to 512 mg L-1 of KH2PO4 and following its irrigation fo r 205 d with 50 mg L-1 of KH2PO4. Simulated displacement of cation cop recipitates from exchange sites allowed the model to reproduce the tem poral and spatial patterns of water- and HCl-extractable P in resin co lumns of different cation-exchange capacities following a KH2PO4 surfa ce amendment. The results of model testing suggest that changes in sol uble P concentrations following P amendments may be represented from c oncurrent equilibrium reactions for adsorption-desorption, precipitati on-dissolution, and ion pairing. However, the rate at which these reac tions proceed remains uncertain.