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

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 Oi, Oe, and Oa 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.Electronic supplementary materialThe online version of this article (10.1186/s12932-018-0051-x) contains supplementary material, which is available to authorized users.

Dynamic interactions at the mineral–organic matter interface

“…The adsorption, fractionation, and oxidative transformation upon reaction of dissolved organic matter with minerals are dependent on mineral surface chemistry, including the composition and abundance of terminal bonds 123,147 . Despite typically lower adsorptive capacities, Mn oxides are stronger oxidants than Fe oxides, in large part owing to differences in redox potential 123,148,149 . Indeed, Mn oxides are considered the strongest naturally occurring oxidants, and thus considerable attention has been placed in investigating the role of Mn oxides in organic carbon transformations 143 .…”

“…Nevertheless, correlations between oxidized Mn and oxidized OM point to a causal link between Mn oxidation and carbon transformations 113,149,151 . In fact, the intimate and diverse association of OM with Mn oxides 149,152 leads to a continuum of reactions, including mineralization and polymerization within layers coating the oxide surface 129 157,161 .…”

Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands

“…In the case of Fe, for example, Fe-coprecipitates may form in environments subjected to redox oscillations (Coward et al, 2018). But in most soils, nanoCLICs likely incorporate a large number of major cations other than Fe and Al, such as Si, Mn (Stuckey et al, 2018), or to a lesser extent more soluble species such as Ca (Rasmussen et al, 2018), Mg or even monovalent species as K (Grand and Lavkulich, 2015), depending on the physical-chemical conditions (concentrations, pH, redox, etc.). Recent studies showed in particular the important role played by Ca in ternary complexation mechanisms (Rowley et al, 2018;Sowers et al, 2018) and its importance in stabilizing OM in rather arid soils and in soils with a pH above 7 (Kramer and Chadwick, 2018;Rasmussen et al, 2018).…”

Composition of organic-mineral complexes (OMC) migrating in the reclaimed sod-podzolic soils (Umbric Albeluvisol Abruptic)

“…Water-soluble organic substances (WOS) are able to form stable compounds of complex nature with aluminum, manganese, and iron ions [1][2][3][4][5]. There is a close correlation between soil acidity and the mobilization of aluminum and iron from the crystal lattice of minerals [6][7].…”