Zn Reactions with Poorly Crystalline Fe Oxides: Implications On Solubility and Bioavailability Under Flooded-Rice Production.

Poorly crystalline Fe oxides commonly occur in soil environments and serve as an important sink and source for trace metals, including Zn2+, in both oxidized and wetland systems. Zn, a micronutrient required by both plants and animals for normal growth, is often deficient in flooded-rice production, with resulting widespread Zn deficiencies in humans in developing countries where paddy rice is the staple food. Zn deficiency in rice can occur even under conditions where total soil Zn is relatively high. In wetland-rice production there are several mineralogical factors that could impact immobilization of Zn2+, including co-precipitation with Fe-sulfide and Fe-oxide minerals. The surface of a physiologically active rice root represents a zone of oxidation and Fe-oxide precipitation and also as a site for possible co-precipitation and immobilization of Zn2+. The current study was designed to evaluate the possible role of poorly crystalline Fe-oxide in overall Zn dynamics. Precipitation and adsorption experiments of Zn2+ with poorly crystalline Fe oxides were conducted under laboratory conditions. Comparative Fe3+ and Zn2+ release kinetics in the presence of chelates was evaluated to determine whether Zn2+ is retained as a structural or surface adsorbed component. Rice-root plaques were similarly evaluated to determine the mode of retention of associated Zn2+. Results indicated that Zn2+ can be co-precipitated and strongly occluded within the Fe-oxide matrix, resulting in limited immediate plant bioavailability. The implications of these reactions to Zn dynamics during flooded-rice production will be discussed as well as implications to crop management.