High manganese redox variability and manganate predominance in temperate soil profiles as determined by X-ray absorption spectroscopy

Manganese speciation is a key to understanding the fate of contaminants, nutrients, and organic matter in soils. To date, quantification of Mn species in bulk soils has been performed mainly by sequential extraction methods and rarely supported by spectroscopic analysis. In order to obtain quantitative information on the Mn species inventory of soils, we investigated 46 soil horizons (<2-mm fraction, 45.1-2,280 mg/kg Mn) of nine typical Central European soils (Cambisols, Chernozems, Luvisols, Podzol, Stagnosol) by chemical Mn analyses and Mn K-edge X-ray absorption spectroscopy, and related speciation results to major soil properties. Amounts of Mn2+, Mn3+, and Mn4+ and the average oxidation state of Mn were evaluated by linear combination fitting (LCF) of X-ray absorption near edge structure (XANES) spectra. Additionally, we used extended X-ray absorption fine structure (EXAFS) spectroscopy to identify and quantify major Mn species. For this, EXAFS spectra of 20 organic and mineral soil samples from five soils (Cambisols, Chernozem, Luvisol) were analyzed by LCF and shell fitting. XANES analyses revealed a high Mn redox variability in organic surface layers, with Mn2+ being most abundant (<= 100%, <(x)over bar> = 54%), followed by Mn3+ (<= 80%, (x) over bar = 32%) and Mn4+ (<= 55%, (x) over bar = 14%). Mineral soil horizons contained significantly less Mn2+ (<= 56%, (x) over bar = 23%), about equal quantities of Mn3+ (<= 68%, (x) over bar = 31%), and were enriched in Mn4+ (<= 89%, (x) over bar = 46%). EXAFS analyses implied the presence of six major Mn species groups: manganates, organically complexed Mn, Mn(III) oxyhydroxides, silicate-bound Mn, Mn oxides without tunnel- or layer structure, and physisorbed Mn. In litter horizons, Mn was mainly present in organic complexes (58-91%, (x) over bar = 78%) and as physisorbed Mn (<= 15%), but individual horizons also comprised manganates, Mn(III) oxyhydroxides, and silicate-bound Mn. Manganates, likely mixtures of phyllomanganates with hexagonal layer symmetry and tectomanganates, dominated in all mineral soil horizons (37-94%, (x) over bar = 67%). Correlation analysis showed that manganates dissolve completely during dithionite-citrate and acid ammonium oxalate extractions, and suggested that Mn4+-rich manganates preferentially form under less acidic soil conditions, partly by oxidation of organically complexed Mn(II), and that they are enriched in the soil clay fraction. Mineral soil horizons also contained minor quantities of organically complexed Mn (<= 39%, (x) over bar = 11%), silicate-bound Mn (<= 30%, (x) over bar = 8%), Mn(III) oxyhydroxides (<= 37%, (x) over bar = 7%), Mn oxides without tunnel- or layer structure (<= 18%, (x) over bar = 5%), and physisorbed Mn (<= 14%, (x) over bar <1%). The detection of Mn(III) oxyhydroxides such as feitknechtite (beta-MnOOH) or groutite (alpha-MnOOH) as well as the spinel hausmannite (Mn3O4) in acidic soils is remarkable, since their formation is normally linked to neutral or alkaline pH conditions. Minor contributions of silicate-bound Mn indicate the release of Mn from primary minerals already at early stages of soil formation, and low concentrations of physisorbed Mn suggest that exchangeable Mn is rapidly converted into manganates in oxic soils. The predominance of manganates in mineral soils has far-reaching implications for the functioning of soils and biogeochemical element cycles, as these minerals play an important role in metal binding, plant nutrition, and redox-related processes. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Manganese speciation plays a crucial role in the fate of contaminants, nutrients, and organic matter in soils. This study quantitatively analyzed the Mn species in Central European soils using chemical analysis and spectroscopic techniques. The results showed that Mn redox variability was high in organic surface layers, while manganates dominated in mineral soil horizons. The predominance of manganates has significant implications for soil functioning and biogeochemical element cycles.