Saleque M A, Kirk G J D
International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines.
New Phytol. 1995 Feb;129(2):325-336. doi: 10.1111/j.1469-8137.1995.tb04303.x.
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 Fe by root-released O , 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.
低地水稻（Oryza sativa L.，品种IR74）种植在缺磷还原老成土的柱状容器中，容器通过细尼龙网分隔为上下两部分，以使根系在网的上方形成一个平面层。这使得能够测量土壤磷组分的变化以及根系诱导的根平面附近土壤的其他变化。在施磷和未施磷的土壤中，与植物吸收的磷量相比，植物易于利用的磷量可以忽略不计，因此植物必然会诱导土壤发生变化以溶解磷。生长6周后，吸收的磷中有90%来自酸溶性库。其余的来自碱溶性无机库，尽管其浓度分布包含与铁（III）积累区域相对应的积累区域，但总体上该库被耗尽。碱溶性有机磷也有少量积累。更难分解的土壤磷库没有变化。磷耗尽区域宽4 - 6毫米，随磷添加量增加而增大，并且与酸化区域重合，在该区域土壤pH从土体接近6降至根系附近小于4。酸化是由于根系释放的氧气氧化铁产生的氢离子，以及根系释放的氢离子以平衡阳离子吸收超过阴离子吸收的过量部分。随着磷缺乏程度增加，根干重与地上部干重的比值、地上部干重与植物总磷的比值、单位植物干重阳离子吸收超过阴离子吸收的过量部分以及相应向土壤释放的氢离子量、单位植物干重氧化的铁量以及相应向土壤释放的氢离子量均增加。独立的体外测量证实，添加酸会增加土壤溶液中的磷浓度以及每克土壤可解吸的磷量。一个关于酸从根系扩散、酸与土壤反应溶解磷以及磷扩散回吸收根系的数学模型表明，在根平面实验条件下，酸化溶解的磷在施磷和未施磷土壤中至少占吸收磷的80%，但溶解的磷中只有不到50%能被根系吸收。
