Manganese Oxides Incubated in Acidic Forest Soil Retain More Organic Carbon Than Iron Oxides.

Iron (oxyhydr)oxides are well-recognized contributors to soil carbon (C) storage, but the effects of manganese oxides on carbon storage and transformation are relatively unexplored. Here, the relative capacities of Fe and Mn oxides to bind and stabilize soil organic C were directly compared using an in situ incubation experiment. Quartz sands coated with either poorly crystalline Mn(III/IV) oxides or Fe(III) oxides, or left uncoated, were buried in a temperate forest soil for up to one year. Chemical extractions, X-ray absorption and photon spectroscopies, scanning electron microscopy, and neutron scattering were used to evaluate organic and mineral associations and transformations during the incubation. At the end of the 1-year study, Mn oxide stored ∼11× more carbon (42.0 ± 14.3 mg C/m2 oxide) than Fe oxide (3.72 ± 0.37 mg C/m2 oxide) per unit surface area. Manganese oxidation state changed over time, likely reflecting reactions between organic C and Mn oxide, while Fe oxidation state remained unaltered. Total organic C that was associated with Mn oxide was relatively enriched in carboxylic and O-alkyl-C compared to Fe oxides and quartz, either due to preferential sorption or oxidation on the mineral surface. Mn oxides also sorbed high amounts of Ca despite being incubated in an acidic, base-cation depleted soil, which potentially facilitated C sorption. Our study demonstrates that Mn oxides, with the assistance of Ca-bridging, have a greater potential to stabilize organic C than Fe oxides in acidic environments despite higher reactivity.