Dissolution of Oxides of Manganese and Iron by Root Exudate Components

Abstract

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 δ‐MnO 2 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 δ‐MnO 2 surfaces and hydrolyzed to a molecule each of formate and malonate. These products are further oxidized to carbon dioxide (CO 2 ) and water, and 6 mol of Mn 2+ are released; the malonate is oxidized via tartronate, formate, and oxalate, and the formate is oxidized directly. Oxaloacetate not adsorbed by the δ‐MnO 2 is rapidly decarboxylated to pyruvate, which is subsequently oxidized to acetate and CO 2 , and only 2 mol of Mn 2+ are released. Although favored by acid conditions, adsorption of oxaloacetate by MnO 2 is still significant at the pH of calcareous systems; thus, the reduction of MnO 2 can proceed via both pathways in agricultural soils. The reduction of Fe(OH) 3 by malate was less than that of δ‐MnO 2 and was enhanced by acidity and addition to low levels of δ‐MnO 2 . Higher levels of added δ‐MnO 2 diminished soluble Fe due to adsorption and reoxidation of Fe 2+ by the δ‐MnO 2 .