The Impact of Redox Fluctuations on Iron-Organic Carbon Associations.

Iron (Fe)-bound organic carbon (OC) contributes an important component in global C cycles. Fe-OC complexes appear highly vulnerable to variable redox effects, and can rapidly solubilize and re-precipitate in response to local redox conditions. Microbial Fe reduction under anaerobic condition may result in increased OC release and mineralization. When soils are subject to a transition from anaerobic to oxic conditions, OC mineralization could be inhibited through co-precipitation with Fe(III) oxides during Fe(II) oxidation, whereas Fe(II) oxidation by O₂ leads to the generation of OH radicals, which could stimulate abiotic OC mineralization. However the role of redox fluctuation in Fe-OC associations remains poorly understood in natural soils. Therefore, the goal of this study is to investigate how redox-mediated Fe reduction and oxidation affects OC mineralization and Fe mineral transformation in natural soils during the redox fluctuations. We hypothesize that shifts in redox conditions and Fe reduction/ re-precipitation will lead to altered mineralization of OC compounds and Fe (hydr)oxide mineral crystallinity. To test the hypothesis, ¹³C-DOC (from ¹³C-labled grass) and ⁵⁷Fe(II) are added to soils. Then we expose soils to either constantly oxic conditions or redox fluctuation cycles with 5d-oxic, 11d-anoxic, and 5d oxic conditions. Throughout, we track Fe(II), Fe isotopes, CO₂, and δ¹³CO₂ to quantify Fe oxidation/reduction and OC mineralization rates. NanoSIMS is used to assess the co-localization of the spiked ¹³C and ⁵⁷Fe in soils following Fe(II) oxidation. In addition, newly-formed ⁵⁷Fe mineral phases are characterized using Mössbauer spectroscopy.