Microbial Fe(III) Reduction in Subalpine Wetlands: Key Process Governing the Fate of Organic Carbon.

Wetlands as unique environments play an important role in the biogeochemical cycles of iron (Fe) and carbon (C). Both cycles are closely linked due to the sequestration of organic C by solid Fe(III) phases. The Fe cycle is controlled partly by the shifts in redox conditions leading to either mineralization of Fe(III) or mobilization of Fe(II) via reductive dissolution. As important carbon sinks, the efficiency of C sequestration in the wetlands is crucial. Climate change potentially leads to an alteration in the hydrological condition in wetland soils accompanied with changes in the redox condition and increase in the microbial Fe(III) transformation. Therefore, our aim is to quantify the kinetics of microbial Fe reduction to predict biochemical fate organic C in wetlands.

We focused our study on the subalpine wetlands of USDA Fraser Experimental Forest, Colorado (USA), where rise in temperature can have a significant impact on the mobility of organic carbon by an increase in microbial Fe(III) reduction. To study the kinetics of Fe(III) reduction, we constructed flow-through reactor (FTR) experiments with three hydrogeomorphically different wetland soils at three different temperatures (6, 12 and 18°C) encountering the temperature range at the subalpine field site. To monitor Fe(III) reduction and carbon degradation from organic matter, we measured total Fe, dissolved Fe(II) and dissolved organic carbon in the FTR effluent.

For all tested soil cores, our preliminary results showed a fast Fe(III) breakthrough with initially increasing Fe(III) in the outflow before reaching a steady state concentration. This steady-state was reached within 0.5 to 2.5 days after changing temperature or concentration of the input solution. Measurements of Fe(III) and Fe(II) revealed that extent of Fe(II) reduction was larger for the wetland soil with longer hydrological residence time. We also observed higher amount of dissolved organic carbon with increased Fe(III) reduction.