Impacts of hydrous manganese oxide on the retention and lability of dissolved organic matter

Jason W Stuckey; Christopher Goodwin; Jian Wang; Louis A Kaplan; Prian Vidal-Esquivel; Thomas P Beebe Jr; Donald L Sparks

doi:10.1186/s12932-018-0051-x

Impacts of hydrous manganese oxide on the retention and lability of dissolved organic matter

Geochem Trans. 2018 Feb 13;19(1):6. doi: 10.1186/s12932-018-0051-x.

Authors

Jason W Stuckey^{1

2}, Christopher Goodwin³, Jian Wang⁴, Louis A Kaplan^{5

6}, Prian Vidal-Esquivel⁷, Thomas P Beebe Jr³, Donald L Sparks⁷

Affiliations

¹ Biology Department, Multnomah University, Portland, OR, 97220, USA. jstuckey@multnomah.edu.
² Department of Plant & Soil Sciences and Delaware Environmental Institute, University of Delaware, Newark, DE, 19716, USA. jstuckey@multnomah.edu.
³ Department of Chemistry and Biochemistry, University of Delaware, Newark, DE, 19716, USA.
⁴ Canadian Light Source Inc., University of Saskatchewan, Saskatoon, SK, S7N 2V3, Canada.
⁵ Stroud Water Research Center, 970 Spencer Road, Avondale, PA, 19311, USA.
⁶ Delaware Environmental Institute, University of Delaware, Newark, DE, 19716, USA.
⁷ Department of Plant & Soil Sciences and Delaware Environmental Institute, University of Delaware, Newark, DE, 19716, USA.

Abstract

Minerals constitute a primary ecosystem control on organic C decomposition in soils, and therefore on greenhouse gas fluxes to the atmosphere. Secondary minerals, in particular, Fe and Al (oxyhydr)oxides-collectively referred to as "oxides" hereafter-are prominent protectors of organic C against microbial decomposition through sorption and complexation reactions. However, the impacts of Mn oxides on organic C retention and lability in soils are poorly understood. Here we show that hydrous Mn oxide (HMO), a poorly crystalline δ-MnO₂, has a greater maximum sorption capacity for dissolved organic matter (DOM) derived from a deciduous forest composite O_i, O_e, and O_a horizon leachate ("O horizon leachate" hereafter) than does goethite under acidic (pH 5) conditions. Nonetheless, goethite has a stronger sorption capacity for DOM at low initial C:(Mn or Fe) molar ratios compared to HMO, probably due to ligand exchange with carboxylate groups as revealed by attenuated total reflectance-Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy and scanning transmission X-ray microscopy-near-edge X-ray absorption fine structure spectroscopy coupled with Mn mass balance calculations reveal that DOM sorption onto HMO induces partial Mn reductive dissolution and Mn reduction of the residual HMO. X-ray photoelectron spectroscopy further shows increasing Mn(II) concentrations are correlated with increasing oxidized C (C=O) content (r = 0.78, P < 0.0006) on the DOM-HMO complexes. We posit that DOM is the more probable reductant of HMO, as Mn(II)-induced HMO dissolution does not alter the Mn speciation of the residual HMO at pH 5. At a lower C loading (2 × 10² μg C m^-2), DOM desorption-assessed by 0.1 M NaH₂PO₄ extraction-is lower for HMO than for goethite, whereas the extent of desorption is the same at a higher C loading (4 × 10² μg C m^-2). No significant differences are observed in the impacts of HMO and goethite on the biodegradability of the DOM remaining in solution after DOM sorption reaches steady state. Overall, HMO shows a relatively strong capacity to sorb DOM and resist phosphate-induced desorption, but DOM-HMO complexes may be more vulnerable to reductive dissolution than DOM-goethite complexes.

Keywords: Dissolved organic matter; Goethite; Manganese oxide; Organo-mineral associations; Soil carbon.

Abstract

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