Effects of Redox on Zinc Deficiency in Paddy Rice

Greenhouse and laboratory experiments were conducted to determine if the pH and redox relationships associated with various soil moisture conditions could help to explain the increased incidence of Zn deficiency of rice (Oryza sativa L.) in submerged soils through greater solubilization of Fe and Mn. The soil used in these studies was Sycamore loam (Aeric Haplaqucepts). Increasing the moisture content of soils increased Fe and Mn concentration and decreased that of Zn in the plant tissue. Application of Zn increased Zn concentrations and decreased those of Fe and Mn in plant tissue. Depth and duration of flooding affected both pH and redox (pe + pH). After 20 days of continuous flooding, pH approached 7.0 and pe + pH approached to 4.0 at 2 cm below the soil-water interface. Both pH and pe + pH decreased significantly with soil depth, reflecting greater accumulation of CO₂ and depletion of O₂. With alternate flooding and drying the pH fluctuated from 8.0 during drying to near 7.0 during flooding while pe + pH fluctuated from 15.0 during drying to approximatelyy 6.0 during flooding. Under controlled redox conditions in the laboratory, decreasing pe + pH from 14.99 to 4.00 decreased the Zn content of rice plants from 39 to 9 mg kg⁻¹ while the contents of Fe increased from 140 to 339 and that of Mn from 226 to 919 mg kg⁻¹. The findings reported herein support the hypothesis that Zn deficiency observed in the submerged paddy rice in this study can be partially explained by increased reduction and solubilization of Fe and Mn which had an antagonistic effect on the availability and uptake of Zn. It is also possible that increased HCO⁻₃ may also contribute to increased Zn deficiency. It is hypothesized that Fe dissolved at reduced microsites and subsequently reprecipitated as ferrosic hydroxide (Fe₃[OH]₈) in more oxidized zones. The higher solubility of Fe maintained by Fe₃(OH)₈ could depress Zn²⁺ solubility through formation of ZnFe₂O₄ or a similar franklinite-like solid material.