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Abstract
Ferrihydrite (Fh) mineral associated soil organic matter (SOM) is considered to be protected. But the multiplicity of interactions between Fh and OM suggest OM protection can be disrupted. Under anoxic conditions, Fh that comes in contact with aqueous Fe(II) can be transformed into more stabilized iron minerals such as lepidocrocite, goethite, hematite, etc. While previous studies have shown that the presence of OM and the ratio of C/Fe can impact this transformation process, it is unknown how reaction with Fe(II) will impact the bioavailability of OM in the Fh-OM association. To investigate this, I first synthesized C isotope-labeled Fh-OM coprecipitates with a range of molar C/Fe ratios and reacted them with Fe(II) for the different time periods. Changes in the coprecipitate were then characterized using mössbauer spectroscopy (MBS) and ultrahigh-resolution scanning transmission electron microscopy (STEM). I found that reaction with Fe(II) drove a localized increase in Fh crystallinity, especially in the low C/Fe coprecipitates and also resulted in the partial release of OM from the coprecipitates. Higher C/Fe containing coprecipitates forestalled this transformation, but longer reaction times resulted in the higher release of OM and in a localized increase in Fh crystallinity. Then I further incubated the Fh-OM coprecipitates with microbes under oxic conditions and measured the mineralization of CO2-C, and changes in dissolved organic carbon (DOC) and solid phase C. I found that the Fe(II) reacted coprecipitates protected more of the coprecipitated C than the unreacted coprecipitates when the C/Fe ratio of the coprecipitates was low. While the significant OM was solubilized from the coprecipitates only a small amount of OC was mineralized to CO2. Following the microbial incubation, the crystallinity of the Fe in the coprecipitates decreased. Lastly, I examined the OM bioavailability in naturally occurring Bacteriogenic Fe-OM coprecipitates using the same methods and found the native OM was more protected than the OM in the synthesized Fe-OM coprecipitates.