ROOT-INDUCED SOLUBILIZATION OF PHOSPHATE IN THE RHIZOSPHERE OF LOWLAND RICE

Lowland rice (Oryza sativa L., cv IR74) was grown in cylinders of a P-deficient reduced Ultisol separated into upper and lower cells by a fine nylon mesh so that the roots formed a planar layer above the mesh. This enabled changes in soil P fractions and other root-induced changes in the soil near the root plane to be measured. In both P-fertilized and unfertilized soil, the quantity of readily plant-available P was negligible in comparison with the quantity of P extracted by the plants, and the plants therefore necessarily induced changes in the soil so as to solubilize P. After 6 wk of growth, 90% of the P taken up was drawn from acid-soluble pools. The remainder was from an alkali-soluble inorganic pool which was on balance depleted, although its concentration profile contained zones of accumulation corresponding to zones of Fe(III) accumulation. There was also a small accumulation of alkali-soluble organic P. There were no changes in the more recalcitrant soil P pools. The zone of P depletion was 4-6 mm wide, increasing with P addition, and coincided with a zone of acidification in which the pH fell from near 6 in the soil bulk to less than 4 near the roots. The acidification was due to H+ generated in oxidation of Fe2+ by root-released O-2, and to H+ released from the roots to balance excess intake of cations over anions. With increasing P deficiency there were increases in the ratio of root:shoot d. wt; the ratio of shoot d. wt to total P in the plant; the excess intake of cations over anions per unit plant d. wt and corresponding release of H+ to the soil; and the quantity of Fe oxidized per unit plant d. wt and corresponding release of H+ to the soil. Independent, in vitro measurements confirmed that acid addition increased the P concentration in the soil solution and the quantity of P that could be desorbed per gram of soil. A mathematical model of the diffusion of acid away from the roots, acid reaction with the soil in solubilizing P, and the diffusion of P back to the absorbing roots showed that, under the conditions of the root-plane experiments, solubilization by acidification accounted for at least 80% of the P taken up in both P-fertilized and unfertilized soil, but that less than 50% of the P solubilized could be taken up by the roots.