Phosphate solubilization by organic anion excretion from rice (Oryza sativa L.) growing in aerobic soil

Abstract

A mathematical model of P solubilization by organic anion excretion from roots is described and used to account for P solubilization by rice (Oryza sativa L.) plants growing in aerobic soil. In previous experiments with rice in an aerobic, highly-weathered, P-deficient soil, we found that the plants were able to solubilize P from an alkali-soluble pool and thereby increase their P uptake. The solubilization could not be explained by pH changes nor by the release of phosphatases. In subsequent experiments we found excretion of citrate from rice roots into nutrient solutions, and the synthesis and excretion of citrate tended to increase under P starvation. The model allows for the diffusion of the organic anion away from a root, its decomposition by soil microbes, its reaction with the soil in solubilizing P, and diffusion of the solubilized P back towards the root as well as away from it. We calculated the rate of citrate excretion from rice roots growing in soil based on measured steady-state citrate concentrations in the rhizosphere and calculated rates of decomposition. Calculations using these and other model parameters obtained independently showed that the observed solubilization and increased P uptake by rice growing in soil could be accounted for. A sensitivity analysis of the model is given.