Inorganic phosphorus transformation and transport in soils: Mathematical modeling in ecosys

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.