Phosphate availability in the soil-root system : integration of oxide surface chemistry, transport and uptake

A study is presented on the adsorption of phosphate on goethite, the interaction of phosphate with other adsorbing ions at the goethite surface, and the resulting availability of phosphate to plants. The plant-availability of sorbed phosphate was determined from phosphorus uptake of plants growing on an artificial substrate containing goethite with phosphate. Uptake can be predicted from zero sink behaviour of a growing root system, diffusion and mass flow in soil, and measured non linear adsorption of phosphate on goethite. With high phosphate loading of goethite, the equilibrium phosphate concentration in solution increased, which resulted in larger phosphate availability. Competitive interaction between phosphate and sulphate on goethite caused only a small decrease in phosphate adsorption at low pH, where sulphate adsorption is strongest, but a considerable increase in the phosphate concentration in solution.Experiments showed that phosphorus uptake by plants growing on suspensions of goethite in the presence of sulphate was seven times larger at pH 3.7 than at pH 5.5. Citrate competes more strongly with phosphate than sulphate, and shows strongest interaction at pH 4.5-5. On account of the non-linear adsorption behaviour of phosphate, the relative increase in phosphate in solution upon competition is much larger at low than at high phosphate loading of goethite. Therefore, competition results in an apparent lower affinity of phosphate sorption on goethite.Adsorption of the individual anions and competitive adsorption was described with the CD-MUSIC ion adsorption model, which is based on a detailed description of the adsorbing surface and the use of surface complexes identified in spectroscopic studies. The combination of the ion adsorption model with the uptake model is a powerful tool to predict the phosphate availability to plants. This was illustrated with a simulation study in which the effect of citrate exudation from roots on the uptake of phosphate was predicted.