Interactions Between Organic Carbon and Synthetic Manganese and Iron Oxides in Acidic Forest Soil

Soils are one of the largest carbon (C) reservoirs on Earth. Secondary iron (Fe) and manganese (Mn) oxides in soils can sequester organic C and protect it from degradation or stimulate C oxidation and release carbon dioxide (CO2). Previous studies that examine the interactions between organic C and Mn oxides found that Mn oxides influence both the sequestration and degradation of organic C. The purpose of this research is to better understand the roles Mn and Fe play in sequestering or degrading C in soils and the potential impacts of these interactions on soil C storage. Synthetic hydrous Mn oxides (HMO) and ferrihydrite (Fh) were coated onto quartz (Qz) sand, encased in 5 µm mesh bags, and buried under the litter layer of a temperate forest soil in the Walker Branch Watershed in Oak Ridge, TN, USA. Uncoated sands were buried for comparison. The minerals were harvested after 7, 14, 28, 90, 180, and 365 days. In addition, minerals were buried at 10 cm and 30 cm and harvested after 90 days. The mineral-coated sands were analyzed for total C on an elemental analyzer and for total Fe and Mn extracted with buffered sodium dithionite solution before analysis using Inductively Coupled Plasma Optical Emission spectroscopy. The average oxidation state (AOS) of Fe and Mn in the minerals was analyzed using X-ray Absorption Near Edge Structure spectroscopy. The organic C functional groups of organic matter (OM) that associated with the minerals were characterized using Near Edge X-ray Absorption Fine Structure spectroscopy. The total C associated with the HMO-coated sand, Fh-coated sand, and Qz sand increased throughout the year with Fh-coated sand and HMO-coated sand accumulating 2.6× and 1.6× more organic C, respectively, than Qz at the soil surface. When normalized to the metal concentration (i.e., molar ratio of C/Mn or C/Fe), C storage for HMO was equal to or higher than for Fh. Total C associated with all minerals decreased with depth. Mn (19 mmol/kg) and Fe (67 mmol/kg) concentrations in the HMO- and Fh-coated sand remained constant over time. Mn AOS in HMO increased from +2.9 to +3.3, indicating possible reactivity with C, but Fe in Fh remained constant at +2.9. OM associated with the Fh- and HMO-coated sands were dominated by carboxylic-C and O-alkyl-C. These results suggest that both HMO and Fh can store C, and that HMO may store more C than Fh.