A model of phosphate solubilization by organic anion excretion from plant roots

One of the mechanisms by which phosphate-efficient plants extract less readily available phosphate from soils is thought to be through the excretion of organic anions from their roots, but few studies have shown quantitatively how effective this mechanism is. This paper develops a mathematical model with which to calculate the amount of additional phosphate a root excreting an organic anion will absorb for a given release of the anion. The model allows for the diffusion of the organic anion away from the root, its decomposition by soil microbes, its reaction with the soil in solubilizing phosphate, and diffusion of the phosphate away from the solubilization zone both towards the root and away from it. The model is corroborated by comparing its predictions with measured phosphate concentration profiles in the rhizosphere of rice plants growing in an aerobic highly weathered soil. All input parameter values are measured independent of the output. A sensitivity analysis is given for the factors governing the solubilizing effect of the organic anion, its longevity in the rhizosphere, and the effect of cylindrical versus planar geometry. For the example given, organic anion excretion could account for the observed phosphate solubilization and uptake, and the model provides a satisfactory description of the processes involved.