Mobilization of ferrihydrite-associated organic carbon during Fe reduction: Adsorption versus coprecipitation

The association of organic carbon (OC) with ferrihydrite (Fh) is critical for regulating soil carbon persistence. However, the reduction of Fh compromises the stability of Fh-bound OC. So far, little is known about the impact of formation pathway (adsorption versus coprecipitation) of Fh-OC association (FOA) on the mobilization of Fh-bound OC during Fh reduction. In this study, we examined the release of Fh-adsorbed and Fh-coprecipitated OC during the abiotic reduction of FOA by hydroxylamine hydrochloride, to elucidate processes regulating the stability of OC during the redox processes. Using ESI-FT-ICR-MS, combustion-derived condensed polycyclic aromatic and vascular plant-derived polyphenols were found to be preferentially adsorbed and coprecipitated by Fh compared with aliphatics, and this structural preference was more pronounced for adsorption. Adsorbed and coprecipitated OC showed different release patterns during reduction: all the adsorbed OC was rapidly released initially, while the coprecipitated OC was slowly mobilized for almost the entire reduction period. The reductive release of OC depended upon the C/Fe ratio, with high C/Fe ratio inhibiting the release of OC. For both adsorption and coprecipitation, aliphatics were preferentially released from FOA into solution during Fe reduction. Our results are valuable for a better understanding of the role of Fh in OC stabilization in natural environments with fluctuating redox conditions.