Organo-Mineral Interactions Between Manganese Oxides and Carbon: Low in Concentration but High in Importance

Soils store a large amount of C in the form of organic matter (OM), more than vegetation and the atmosphere combined. The association of OM with minerals, known as organo-mineral interactions, increases the residence time of OM in soils because minerals interact with OM through physical and chemical interactions. Secondary oxides, such as iron (Fe) oxides, are often poorly crystalline with high specific surface areas that can sequester or degrade OM. Manganese (Mn) is a redox-sensitive element that forms secondary oxides in soil that can sequester and degrade OM, however the balance between these processes is not well understood. The goal of this research is to evaluate processes contributing to OM storage with Fe and Mn oxides in temperate forest soils and permafrost-underlain tundra soils. Two in situ mineral bag incubations were conducted at each site using sand (Qtz) coated with either Mn oxides or Fe oxides. The mineral-coated sands were encased in mesh bags and buried in soils for 1 year at Walker Branch Watershed (WBW; temperate forest) and 28 days on the North Slope of Alaska (tundra). After harvesting, the mineral bags were analyzed for total organic C and total metals. Iron, Mn and organic C chemistry were examined using spectroscopy techniques like X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Organo-mineral interactions were examined in soil cores collected along a hillslope transect at WBW. Manganese oxides sequestered equal or greater amounts of OM than Fe oxides in the mineral bags in WBW and tundra soils. Calcium facilitated OM adsorption for Mn and Fe oxides in both environments. Across the WBW, hillslope OM was correlated with Mn, suggesting Mn oxides affect OM storage along hillslopes. Manganese oxides were enriched in surface WBW soils suggesting a greater role for OM sequestration in surface soils while Fe oxides were dominant in subsoils. This research demonstrates that Mn oxides play an outsized role in organo-mineral interactions despite low abundances in soil.