Interactions of dissolved soil organic matter and manganese oxides

Secondary minerals such as iron and aluminum (oxyhydr)oxides are a well-known key factor determining the accumulation and persistence of organic carbon (OC) in soil. Manganese (Mn) oxides, although being less abundant in soil than other oxide minerals, may also bind and stabilize organic matter. In addition, they exhibit a high redox activity that may promote oxidation of refractory organic compounds into substrates easily available to microorganisms. However, little is known about the adsorption and oxidation of dissolved OC (DOC) by Mn oxides. Therefore, we investigated the adsorption of dissolved organic matter (DOM) to vernadite, acid birnessite, and cryptomelane, by varying DOM type (beech and pine-derived), pH (4 and 7), and background electrolyte composition (no salt addition, 0.01 M NaCl or CaCl₂). Preliminary results show that the extent and kinetics of DOM adsorption as well as oxidative DOM transformation strongly differed with Mn oxides and sorption conditions. Overall, DOM adsorption was higher at pH 4 than at pH 7. Vernadite was most sorptive, retaining 68% to 85% of added DOC at pH 4. At pH 7, on average 30% less DOC was adsorbed by Mn oxides. After reaction, reduced specific ultraviolet absorbance at 280 nm of DOM indicates preferential adsorption of aromatic moieties. Contact of DOM with Mn oxides also resulted in high concentrations of dissolved low-molecular-weight (LMW) organic acids, consisting mainly of formic, acetic, oxalic, and citric acid. In addition, we will present results from liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry and X-ray diffraction on the molecular transformation of reacted DOM and reductive changes of reacted Mn oxides, respectively. Consequently, interactions of DOM and Mn oxides may promote selective sorptive stabilization of organic matter as well as support microbial growth due to oxidative production of easily available organic compounds.