Phosphate solubilization by organic anion excretion from rice growing in aerobic soil: rates of excretion and decomposition, effects on rhizosphere pH and effects on phosphate solubility and uptake

Rice (Oryza sativa) plants were grown with their roots sandwiched between t hin layers of phosphorus-deficient soil from which they were separated by f ine mesh, and root-induced changes in the soil affecting phosphate solubili ty were measured. The concentrations of low molecular weight organic anions in the thin layers, particularly citrate, increased in the presence of the plants. Apparent rates of citrate excretion from the roots, calculated fro m the quantities in the soil and rates of decomposition calculated with a f irst order rate constant measured independently, varied from 337-155 nmol g (-1) root f. wt h(-1) over the course of plant growth, equivalent to 2-3 % of plant d. wt. Rates of excretion were similar for NH4+ and NO3--fed plant s. The soil pH decreased from its initial value by up to 0.6 units for the NH4+-fed plants and increased by up to 0.4 units for the NO3--fed ones. The contribution of organic anion excretion to the pH changes was small compar ed with that of the inorganic cation-anion balance in the plants. The exten t to which the observed excretion of citrate and root-induced pH changes co uld account for the observed phosphate solubilization and uptake was assess ed using a mathematical model. Previous work had shown that phosphate solub ilization by rice in this soil could not be explained by enhanced phosphata se activity in the rhizosphere, and the roots were not infected with mycorr hizas. The model allows for the diffusion of the solubilizing agent (citrat e or H+) away from the roots, its decomposition by soil microbes (citrate o nly); its reaction with the soil in solubilizing phosphate and diffusion of the solubilized phosphate to the roots. The model contains no arbitrary as sumptions and uses only independently measured parameter values. The agreem ent between the measured time course of phosphorus uptake and that predicte d for solubilization by citrate was good. Root-induced acidification by NH4 +-fed plants resulted in additional solubilization, the acidification enhan cing the solubilizing effect of citrate; However, the final phosphorus upta ke by NH4+-fed plants was no greater than that of NO3--fed plants, presumab ly because the acidification inhibited plant growth. The mechanism of solub ilization by citrate involved formation of soluble metal-citrate chelates r ather than displacement of phosphate from adsorption sites.