No abstract

Background: Evidence of trivalent manganese (Mn 3+ ) in the aqueous phase of soils is unknown so far although this strong oxidant has large environmental implications. Aims:We aimed to modify a spectrophotometric protocol (porphyrin method) and to discriminate between Mn 2+ and Mn 3+ in the aqueous phase of forest soils based on kinetic modeling. Methods:We investigated manganese speciation in 12 forest floor solutions and 41 soil solutions from an acidic forest site by adjusting pH and correcting for absorbance. Results:The 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 (Mn T , 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 Mn T concentrations. After the pH and color adjustments, the comparison of Mn T 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%). Conclusions:The dissolved Mn 3+ 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.