Iron oxide solubilization by organic matter and its effect on iron availability

The solubility of Fe in soils is largely controlled by Fe oxides; ferrihydrite, amorphous ferric hydroxide, and soil-Fe are generally believed to exert the major control. Fe(III) hydrolysis species constitute the major Fe species in solution. Other inorganic Fe complexes are present, but their concentrations are much less than the hydrolysis species. Organic complexes of Fe including those of organic acids like citrate, oxalate, and malate contribute slightly to increased Fe solubility in acid soils, but not in alkaline soils.The most important influence that organic matter has on the solubilization of Fe is through reduction. Respiration of organic matter creates reduction microsites in soil where Fe²⁺ concentrations increase above those of the Fe(III) hydrolysis species. Fluctuating redox conditions in these microsites are conducive to the formation of a mixed valency ferrosic hydroxide. This metastable precipitate maintains an elevated level of soluble inorganic Fe for prolonged periods and increases Fe availability to plants. The release of reducing agents and acids next to roots, as well as the production of siderophores by microorganisms within the rhizosphere, contribute to the solubilization and increased availability of Fe to plants.