Root‐induced solubilization of phosphate in the rhizosphere of lowland rice

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Abstract
SUMMARY: 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 O2, 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.