Correlating manganese X-Ray absorption near-edge structure spectra with extractable soil manganese

Manganese redox chemistry plays an important role in Mn uptake by plants, ecotoxicology of trace elements associated with Mn oxides, and the etiology of some soil-borne plant fungal diseases. Manganese solubility is closely related to oxidation state, but direct measurement of Mn oxidation state has been difficult. We used x-ray absorption near-edge structure (XANES) spectroscopy to determine the relative proportions of Mn(II), Mn(III), and Mn(IV) in four soils, and correlated this with Mn solubility obtained from a sequential extraction procedure that consisted of: (i) 1 M pH 7 ammonium acetate (Ac), (ii) 0.5 M pH 2.9 CuSO 4 or 1 M pH 3 NH 4 OAc, (iii) 0.018 M quinol in 1 M pH 7 NH 4 OAc, and (iv) dithionite-citrate-bicarbonate (DCB). In moist aerated soil, most of the Mn was present as Mn(IV) and XANES spectroscopy tracked its progressive removal with increasingly aggressive extractants. Lowering the pH from 7 to 3 resulted in solid state reduction of Mn(IV) to Mn(III) within the soil Mn minerals without release of Mn to solution. After a 7-d microbial reduction treatment, most of the Mn occurred as Mn(II), 2/3 of which was extracted by pH 7 NH 4 Ac, while the remaining 1/3 was extracted by pH 3 NH 4 Ac. The XANES spectra suggest that this acid soluble Mn(II) phase is not rhodocrosite (MnCO 3 ), but its exact form (solid solution in calcite or MnPO 4 ) still remains to be determined. In both aerated and reduced soils, a considerable fraction of the total Mn occurs as Mn(III), presumably incorporated into the structure of Mn and Fe oxides. Rapid air drying of a microbially reduced soil prevented reoxidation of the reduced Mn and preserved the distribution of Mn in the different extractable Mn pools.