Interactions of manganese oxides with natural dissolved organic matter: Implications for soil organic carbon cycling

Metal (oxyhydr)oxides are key factors for the transformation and stabilization of organic carbon (OC) in soil. While sorptive interactions between Al and Fe (oxyhydr)oxides and dissolved organic matter (DOM) are well studied, the role of Mn(IV) oxides (manganates) in DOM sorption, fractionation, and oxidation has not been extensively investigated. Therefore, we examined sorptive interactions between manganates (δ-MnO2, birnessite, cryptomelane; 5 g L-1) and different DOM types (beech litter and Oa/Oe material under pine; ∼100 mg L-1 dissolved organic carbon, DOC) at pH 4 and 7 in different background electrolytes (BGE; no salt, 0.01 M NaCl or CaCl2) for up to 32 hours. Changes of DOM solutions and mineral phases were assessed by solid, liquid, and gas analyses, and statistically evaluated for the effects of manganate, DOM type, pH, and BGE on DOC sorption, fractionation, and oxidative transformation. Manganates sorbed 0.2–8.7 mg OC g-1. Per unit mass, δ -MnO2 was least effective in DOC retention due to its high DOC oxidation capacity. Manganates sorbed more DOC at acidic pH and more aromatic pine than more aliphatic beech DOC, with Ca2+ generally facilitating DOC sorption. Up to 56 % of added DOC (average: 25 %) was oxidized to CO2, which is comparable to the extent of DOC respiration by soil microorganisms. DOC oxidation by δ-MnO2 and cryptomelane exceeded that of birnessite, which had the lowest specific surface area of all manganates. However, we found no difference in the efficiency of manganates to oxidize DOC, implying a similar redox activity of phyllo- and tectomanganates. More beech than pine DOM was oxidized to CO2 and an acidic pH facilitated DOC decomposition. While the presence of Na-BGE increased DOC oxidation to CO2 relative to no-salt treatments, Ca-BGE had the opposite effect, as Ca2+ apparently impeded the electron transfer from sorbed OC to structural Mn(III/IV). Contact of DOM with manganates also produced high concentrations of dissolved low-molecular-weight organic acids (mainly formate, acetate, and oxalate), accounting for up to 19 % of the initial DOC concentration. In addition, reduced specific ultraviolet absorbance of DOM solutions at 280 nm indicated preferential sorption of aromatic moieties, especially in Ca-BGE. However, in the absence of Ca2+ and at neutral pH, manganates increased the aromaticity of beech DOM, most likely due to polymerization reactions. No mineral transformations occurred after reaction of manganates with DOM, despite reductive manganate dissolution. Our results imply that soil manganates accumulate more OC in acidic soils and in presence of more aromatic DOM. However, manganates oxidatively destabilize DOC by generating CO2 and low-molecular-weight organic compounds, which is presumably more relevant in acidic soils with low concentrations of polyvalent cations and for more aliphatic DOM. The produced low-molecular-weight organic compounds may promote microbial activity and foster mineral weathering. Our results suggest a pivotal role of manganates in soil OC cycling, including the fate and bioavailability of nutrient elements associated with DOM, and in supporting ligand-promoted mineral weathering and soil formation.