Cation-anion balances and chemical changes in the rhizosphere of rice in an iron toxic soil

The plant cation-anion balance and consequent release of acid or base from the roots greatly affect the chemical conditions in the rhizosphere of rice in Fe toxic soil. To understand the role played by Si in the cation-anion balance, rice (cv IR72) was grown in nutrient solutions with and without Si and with N as either NO3- or NH4+, and mineral uptake was measured. Silicon addition decreased the intakes of cations and anions as well as the ash alkalinity of the plant, regardless of N form. In the NH4+ fed plants the decrease in ash alkalinity was larger than what could be explained by the balance between unassimilated cations and anions assuming that Si was absorbed in an unchanged form. The discrepancy was equivalent to about half the Si intake. Various possible explanations for the discrepancy are discussed. Silicon addition considerably reduced the transpiration rate of NO3-fed plants. It is hypothesized that in the NH4+- fed plants Si was in part absorbed as the anion SiO(OH)3-. A system was developed for studying chemical changes and cation mobility in the rhizosphere of rice in Fe toxic soil with varying concentrations and combinations of anions in the soil solution. Nine solutions were used with combinations of Cl- and HCO3- as the balancing anion. Plant mineral contents were generally highest in Cl- treatments. A large amount of Fe accumulated in or on the roots. The profiles of Fe in the soil near the roots showed that a large quantity of mobile Fe (2) was oxidized in that region, resulting in the accumulation of immobile Fe (OH)3. The pH profiles showed a zone of acidification near the roots. The pH change was caused by a) H+ generated in Fe2+ oxidation, and b) H+ exported from the roots to balance excess intake of cations over anions. The cation profiles reflect the rate of removal of the cation by roots versus its rate of transport to the roots by mass flow and diffusion. In some treatments, especially those with the largest anion concentration in solution, the rate of delivery evidently exceeded plant demand and consequently cations accumulated near the roots. Conversely, depletion occurred where the rate of uptake exceeded the rate of transport through the soil. The soil K profiles showed that, at small Cl- and HCO3- concentrations in the soil solution, the mobility of K was decreased by soil acidification and zones of K depletion near the roots developed. It is concluded that Fe oxidation and the consequent acidification may greatly alter the mobility of cations in reduced soils, especially when the concentration of anions is low