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Dissolution of Oxides of Manganese and Iron by Root Exudate Components¹

ABSTRACT

The reactions of malic acid, an important constituent of root exudates, with the hydrous oxides of manganese (Mn) and iron (Fe) were studied to further our understanding of the processes controlling the solubility and plant intake of these elements. The reaction of malate with -MnO₂ followed either of two pathways depending on the pH-controlled adsorption of carboxylates on the oxide surface. In acid systems, the first product of the oxidation of malate (oxaloacetate) is adsorbed on -MnO₂ surfaces and hydrolyzed to a molecule each of formate and malonate. These products are further oxidized to carbon dioxide (CO₂) and water, and 6 mol of Mn²⁺ are released; the malonate is oxidized via tartronate, formate, and oxalate, and the formate is oxidized directly. Oxaloacetate not adsorbed by the -MnO₂ is rapidly decarboxylated to pyruvate, which is subsequently oxidized to acetate and CO₂, and only 2 mol of Mn²⁺ are released. Although favored by acid conditions, adsorption of oxaloacetate by MnO₂ is still significant at the pH of calcareous systems; thus, the reduction of MnO₂ can proceed via both pathways in agricultural soils.

The reduction of Fe(OH)₃ by malate was less than that of -MnO₂ and was enhanced by acidity and addition to low levels of -MnO₂. Higher levels of added -MnO₂ diminished soluble Fe due to adsorption and reoxidation of Fe²⁺ by the -MnO₂.

¹ Contribution from the Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616.

² Former Graduate Student and Professor of Soil Science, respectively. The Senior Author's present address is EERA Salta, INTA, Casilla de Correo 228, 4400-Salta, Argentina.

Received for publication June 1, 1981. Accepted for publication September 16, 1981.

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