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

Abstract

SUMMARYLowland 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 Fe"* by root-released Oj, 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 reaetion 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.

Organic phosphorus in Madagascan rice soils

“…It may therefore be an important source of phosphorus to rice growing on infertile soils. It seems to be a dynamic pool of phosphorus, because marked changes in NaOH-extractable phosphate were detected in response to mineral fertilization of rice soils of Bangladesh (Saleque and Kirk, 1995). Indeed, there is evidence that it contributes to plant nutrition, because it was depleted in the vicinity of rice roots growing on a low-fertility infertile Ultisol (Hedley et al, 1994) and in the rhizosphere of two agroforestry species growing in a Kenyan Oxisol (George et al, 2006).…”

Copper and nickel uptake, accumulation and tolerance in Typha latifolia with and without iron plaque on the root surface

“…In a nutrient solution, the resultant pH change will be dispersed throughout the solution; but in a soil or sediment this change will be propagated away from the roots more slowly and the pH at the root surface can often be 1-2 units lower than that in the soil bulk following chemical reduction. This will greatly alter the mobility of nutrients and toxins, as shown for Zn (Kirk & Bajita, 1995), cations in general (Kirk & Solivas, 1994) and P (Saleque & Kirk, 1995). A fall in pH will tend to increase the concentrations of Cu and Ni in solution in most soils and sediments.…”

Phosphorus fractions depletion in the rhizosphere of young and adult maize and oilseed rape plants

“…In contrast, HCl-P i is almost inaccessible to isotopic exchange and therefore not accessible for immediate plant uptake, except in soils with a pH (KCl) clearly above 7 (Bühler et al, 2002). It has been reported that a significant proportion of acid soluble P i can be depleted by rice roots, but only under submerged soil conditions where Fe 3+ reduction and NH 4 + uptake caused a strong acidification of the rhizosphere (Saleque and Kirk, 1995;Zhang et al, 2006).…”

“…This is because acid soluble P in acid to neutral soils is not accessible to isotopic exchange (Machold 1962(Machold , 1963 and thereby not accessible to direct plant uptake unless it would be solubilised by soil acidification (Saleque and Kirk, 1995).…”