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

• Published in: The New phytologist Vol. 142; no. 2; pp. 185 - 200
• Main Authors: KIRK, G. J. D., SANTOS, E. E., SANTOS, M. B.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 01-05-1999; Blackwell
• Subjects: Absorption. Translocation of ions and substances. Permeability; Acid soils; Agronomy. Soil science and plant productions; Anions; Biological and medical sciences; Citrates; citric acid; diffusion; Economic plant physiology; Excretion; Fundamental and applied biological sciences. Psychology; Nutrition. Photosynthesis. Respiration. Metabolism; organic acid; Organic soils; pH change; phosphate; Plant physiology and development; Plant roots; Plants; rhizosphere; rice (Oryza sativa); Soil solution; Soil water; Solubilization; Water and solutes. Absorption, translocation and permeability; diffusion; rice (Oryza sativa); solubilization; citric acid; phosphate; organic acid; pH change; rhizosphere; Phosphates; Monocotyledones; Ammonium; Excretion; Solubility; Decomposition; Nitrates; Aerobe; Solubilization; Cereal crop; Citric acid; Oryza sativa; Rhizosphere; Organic anion; Absorption; Gramineae; Angiospermae; pH; Nutrient; Spermatophyta; Application method
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1046/j.1469-8137.1999.00400.x

Rice (Oryza sativa) plants were grown with their roots sandwiched between thin layers of phosphorus-deficient
soil from which they were separated by fine mesh, and root-induced changes in the soil affecting phosphate
solubility 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 from the quantities in the soil and rates of decomposition calculated with a first 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 plants. 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 compared with that of the inorganic
cation-anion balance in the plants. The extent to which the observed excretion of citrate and root-induced pH
changes could account for the observed phosphate solubilization and uptake was assessed using a mathematical
model. Previous work had shown that phosphate solubilization by rice in this soil could not be explained by
enhanced phosphatase activity in the rhizosphere, and the roots were not infected with mycorrhizas. The model
allows for the diffusion of the solubilizing agent (citrate or H+) away from the roots, its decomposition by soil
microbes (citrate only); its reaction with the soil in solubilizing phosphate and diffusion of the solubilized
phosphate to the roots. The model contains no arbitrary assumptions and uses only independently measured
parameter values. The agreement between the measured time course of phosphorus uptake and that predicted for
solubilization by citrate was good. Root-induced acidification by NH4+-fed plants resulted in additional
solubilization, the acidification enhancing the solubilizing effect of citrate. However, the final phosphorus uptake
by NH4+-fed plants was no greater than that of NO3−-fed plants, presumably because the acidification inhibited
plant growth. The mechanism of solubilization by citrate involved formation of soluble metal-citrate chelates
rather than displacement of phosphate from adsorption sites.