Comparison of Manganese and Iron Oxide-Coated Redox Bars for Characterization of the Redox Status in Wetland Soils

Abstract

Characterization of the soil redox status is important for pedogenesis but simple field methods for monitoring are limited. Recently, we introduced manganese (MnIII,IV) oxide-coated redox bars as an indicator for reducing conditions in soils. In this study, we compared these redox bars with well-established iron (FeIII) oxide-coated bars. For a 5-month monitoring period, we quantified the monthly oxide removal along three wetland plots with different variations in water table. Preferential dissolution of the Mn oxide coating exceeded the Fe oxide removal by two to five times that is coherent with the thermodynamic stability of the minerals. Enhanced removal of Mn oxide coatings in the capillary fringe compared to minor depletion of Fe oxides enables to differentiate weakly (300 to 100 mV, range of MnIII,IV reduction) and moderately (100 to −100 mV, range of FeIII reduction) reducing conditions. Processes that occur under weakly reducing soil conditions, e.g. denitrification and trace metal mobilization associated with the reductive dissolution of Mn oxides, can be identified when Mn oxide removal along redox bars occurs but the Fe oxide coating remains stable. Simultaneous use of Mn and Fe redox bars results in a better temporal and spatial characterization of the soil redox status.