Evidence of dissolved trivalent manganese in acidic forest soils

Abstract

AbstractBackgroundEvidence of trivalent manganese (Mn3+) in the aqueous phase of soils is unknown so far although this strong oxidant has large environmental implications.AimsWe aimed to modify a spectrophotometric protocol (porphyrin method) and to discriminate between Mn2+ and Mn3+ in the aqueous phase of forest soils based on kinetic modeling.MethodsWe investigated manganese speciation in 12 forest floor solutions and 41 soil solutions from an acidic forest site by adjusting pH and correcting for absorbance.ResultsThe solutions showed broad ranges in pH (3.4−6.3), dissolved organic carbon (DOC, 1.78−77.1 mg C L−1), and total Mn (MnT, 23.9−908 µg L−1). For acidic solutions, a pH‐buffer was added to increase the pH of the solutions to 7.5−8.0, and background absorption was corrected for colored solutions, that is, solutions high in DOC. This was done to accelerate the reaction kinetics and avoid overestimation of MnT concentrations. After the pH and color adjustments, the comparison of MnT concentrations between the porphyrin method and optical emission spectrometry showed good agreement. Trivalent Mn, which is stabilized by organic ligands, constitutes significant proportions in both forest floor solutions (10−87%) and soil solutions (0.5−74%).ConclusionsThe dissolved Mn3+ is present in acidic forest soils. Thus, we revise the paradigm that this species is not stable and encourage to apply the revised method to other soils.