Fertilization and pH effects on processes and mechanisms controlling dissolved inorganic phosphorus in soils

We used of a set of mechanistic adsorption models (1-pK TPM, ion exchange and Nica-Donnan) within the framework of the component additive (CA) approach in an attempt to determine the effect of repeated massive application of inorganic P fertilizer on the processes and mechanisms controlling the concentration of dissolved inorganic phosphorus (DIP) in soils. We studied the surface layer of a Luvisol with markedly different total concentrations of inorganic P as the result of different P fertilizer history (i.e. massive or no application for 40years). Soil pH was made to vary from acid to alkaline. Soil solutions were extracted with water and CaCl₂ (0.01M). The occurrence of montmorillonite led us to determine the binding properties of P and Ca ions for this clay mineral. Satisfactory results were obtained using generic values for model parameters and soil-specific ones, which were either determined directly by measurements or estimated from the literature. We showed that adsorption largely controlled the variations of DIP concentration and that, because of kinetic constrains, only little Ca-phosphates may be precipitated under alkaline conditions, particularly in the P fertilized treatment. The mineral-P pool initially present in both P treatments did not dissolve significantly during the course of the experiments. The adsorption of Ca ions onto soil minerals also promoted adsorption of P ions through electrostatic interactions. The intensity of the mechanism was high under neutral to alkaline conditions. Changes in DIP concentration as a function of these environmental variables can be related to changes in the contribution of the various soil minerals to P adsorption. The extra P adsorbed in the fertilized treatment compared with the control treatment was mainly adsorbed onto illite. This clay mineral was the major P-fixing constituent from neutral to alkaline pH conditions, because the repulsion interactions between deprotonated hydroxyl surface sites and P ions were sufficiently counterbalanced by Ca ions. The drastic increase of DIP observed at acid pH was due to the effect of the lower concentration of surface sites of Fe oxides and kaolinite. In addition to confirming the validity of our approach to model DIP concentrations in soils, the present investigation showed that adsorption was the predominant geochemical process even in the P fertilized soil, and that Ca ions can have an important promoting effect on P adsorption. However the influence of the dissolution of the mineral-P pool under field conditions remained questionable.