Root-induced iron oxidation, pH changes and zinc solubilization in the rhizosphere of lowland rice
1995
Bajita, J.B. | Kirk, G.J.D. (Philippine Rice Research Inst., Munoz, Nueva Ecija (Philippines). Agronomy, Soils and Plant Physiology Div.)
Rice plants (cv. IR34) were grown with their roots sandwiched between cylinders of anaerobic Zn-deficient Mollisol. After periods of root-soil contact of up to 12 days - total plant age approx. 28 days - the profiles of different Zn fractions, reduced and oxidized Fe, and pH in the soil near the root "plane" were determined. The concentration of easily plant-extractable Zn in the soil was negligible, and so it was necessary for the plants to induce changes in the soil to solubilize Zn. After 6 days, there was a substantial accumulation of Zn asssociated with organic matter and amorphous ferric hydroxide within 4-5 mm of the root plane. Over the next 6 days, the accumulation continued but there was also a substantial depletion of the accumulated fractions within 2 mm of the root plane. The zones of accumulation and depletion coincided with zones of Fe (3) accumulation and soil acidification in which the pH decreased from the bulk soil value by over 0.2 unit; i.e. a two-fold increase in H+ concentration. The acidification was the result of H+ released from the roots to balance excess intake of cations over anions, and H+ generated in the oxidation of Fe (2) by root released O2. At the high pH and CO2 pressure of the experimental soil (7.3 and approx. 0.009 atm, respectively), soil acidity diffusion is fast and consequently the pH drop at the root surface was small. The rate of Fe oxidation peaked before 6 days, but the acidification and Zn accumulation continued beyond 6 days unabated. It is concluded that Fe oxidation released Zn from highly insoluble fractions, and that this Zn reabsorbed on Fe (OH3) and organic matter in forms that were acid-soluble and therefore accessible to plants
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