The Effects of Legacy Sediment on Microbially-Mediated Redox Cycling in an Isolated Floodplain

Abstract

Post-settler colluvium or legacy sediment is a layer of fine sediment that has accumulated through improper farming practices of colonial settlers. The legacy sediment is eroded from hillslopes and deposited down onto active soil and covers it up. We hypothesized that an additional layer of material would modify the concentration of oxygen in the soil and as a result, affect the redox cycles of major nutrients. We constructed soil mesocosms and subjected them to varying degrees of oxygen concentrations and gravimetric water content via an anaerobic chamber to simulate the unique conditions of our field site. The iron and manganese in the mesocosms exhibited behavior of direct electron transfer, with the more electropositive manganese acting as an electron acceptor for iron oxidation. The more soluble reduced manganese was able to facilitate redox reactions while stable in the anoxic treatments. Additionally, the increased reduction of iron over time is driven by reductive-dissolution of iron oxides that recrystallize into structurally weak amorphous iron oxides that are more readily reduced. The anaerobic chamber provided a pathway for nitrogen fixation into nitrate which was reduced by microbial organisms with iron and manganese acting as electron donors. There was overall significant fractionation between the nutrient concentrations of the oxygen treatments, especially with manganese and nitrate. The carbon contents of the mesocosms that had any anoxia were all increased over the oxic, with a noticeable increase in nitrogen as well. Overall, the results indicate that manganese is exerting a larger control on the redox reactions in the buried soils than previously thought, and the method of destructive sampling of a lab-controlled buried soil is viable and scalable.