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DIVISION S-2—SOIL CHEMISTRY

Correlating Manganese X-Ray Absorption Near-Edge Structure Spectra with Extractable Soil Manganese

^a Dep. of Agronomy, Purdue University, West Lafayette, IN 47907
^b Dep. of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907

* Corresponding author (dschulze{at}purdue.edu)

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 CuSO₄ or 1 M pH 3 NH₄OAc, (iii) 0.018 M quinol in 1 M pH 7 NH₄OAc, 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 NH₄Ac, while the remaining 1/3 was extracted by pH 3 NH₄Ac. The XANES spectra suggest that this acid soluble Mn(II) phase is not rhodocrosite (MnCO₃), but its exact form (solid solution in calcite or MnPO₄) 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.

Abbreviations: Ac, acetate • XANES, x-ray absorption near-edge structure • DCB, dithionite-citrate-bicarbonate • PPAC, Pinney Purdue Agricultural Center soil sample • SEPAC, Southeast Purdue Agricultural Center soil sample • SWPAC, Southwest Purdue Agricultural Center soil sample • WP, West Point soil sample

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