The influence of native soil organic matter and minerals on ferrous iron oxidation

Fe(II) oxidation by O2 is an important process generating Fe (oxyhydr)oxides, which play sorptive, structural and electron-transfer roles in soils. Here we explored how native minerals and organic matter (OM) affect the rate of Fe(II) oxidation and resulting de novo Fe(III) minerals in soil slurries. A topsoil was collected from the Luquillo Experimental Forest, and a topsoil and subsoil were collected from a cultivated site at the Calhoun Experimental Forest. We oxidized 57Fe(II) in these soils either untreated or with OM and/or Fe (oxyhydr)oxides removed. We measured Fe oxidation kinetics by tracking the loss of Fe(II) and characterized the de novo Fe(III) solids using 57Fe Mössbauer spectroscopy. We find that OM retarded Fe(II) oxidation, while pre-existing Fe (oxyhydr)oxides played a significant role in catalyzing Fe(II) oxidation. The non-extractable (residual) soil minerals (i.e. phyllosilicates and quartz) after removing Fe (oxyhydr)oxides, had only a minor effect on oxidation rates. In the topsoils, OM resulted in lower-crystallinity Fe(III) minerals, including nanogoethite and highly-disordered Fe phases, relative to soils with OM-removed. Goethite of varying crystallinity was promoted by the pre-existing Fe (oxyhydr)oxides in all soils, in contrast to homogenous oxidation treatments in which lepidocrocite was formed. Fe(II) oxidation in the Calhoun subsoil, which was enriched in native crystalline Fe phases and depleted in OM, resulted in large-particle goethite with the highest crystallinity of all treatments. Crystalline hematite was also formed in the Calhoun subsoil most likely due to a templating effect of pre-existing hematite. These findings suggest that the nature of de novo formed Fe minerals in soils and sediments may depend strongly on resident existing soil OM and Fe phases. This study extends similar results from previous model mineral and organic experiments to whole, complex soils and thus constitutes a significant step forward in understanding Fe transformation in natural environments.