Manganese transformations in soils during waterlogging and subsequent drying
Abstract
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 fungal diseases. Manganese solubility (and hence bioavailability) 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 moist soils, and correlated this with Mn solubility obtained from a sequential extraction procedure that consisted of: (1) 1 M pH 7 NH4OAc, (2) 0.5 M pH 2.9 CuSO 4 or 1 M pH 3 NH4OAc, (3) 0.018 M quinol in 1 M pH 7 NH4OAc, and (4) dithionite-citrate-bicarbonate (DCB). We followed changes in Mn oxidation state and solubility upon microbial reduction and subsequent re-oxidation using 4 Indiana surface soils with total Mn contents of 300–1050 mg kg−1 and pHs of 5.7–6.8. In moist, aerated soil, most of the Mn was present as Mn(IV) and XANES spectroscopy tracked the progressive removal of Mn(IV) by increasingly aggressive extractants. There is evidence that lowering the pH from 7 to 3 resulted in solid state reduction of Mn(IV) to Mn(III) within the soil Mn-oxide minerals without release of appreciable Mn to solution. After a 7-day microbial reduction treatment, most of the Mn occurred as Mn(II). About 2/3 of this Mn(II) was extracted by pH 7 NH4-acetate, but the remaining 1/3 was extracted by pH 3 NH4-acetate. The XANES spectra suggest that this acid soluble Mn(II) phase is not rhodocrosite (MnCO3) as predicted by thermodynamic calculations, but its exact form, whether as a solid solution in calcite ((Ca, Mn)CO3), as an Mn(II)-phosphate phase, or as some other solid, still remains to be determined. In both aerated and reduced soils, XANES spectroscopy showed that a considerable fraction of the total Mn occurred as Mn(III), presumably incorporated into the structure of iron oxide or phyllosilicate clay minerals. Re-oxidation of Mn(II) following drying of waterlogged soil was found to be primarily a microbially mediated process.
Degree
Ph.D.
Advisors
Schulze, Purdue University.
Subject Area
Soil sciences
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