When short-range order (SRO) iron (Fe) mineral are formed in presence of dissolved organic carbon (OC), OC is often incorporated in the mineral structure creating Fe-C coprecipitates. This coprecipitation serves as an important mechanism for retaining carbon (C) in soil and sediments. However, the presence of Fe (II) can promote changes in the Fe mineral structure depending on the length of exposure and ratio of C/Fe in the co-precipitate. The aim of this study was to assess the impact of reaction time length of Fe (II) with different C/Fe ratio containing coprecipitates on the availability of carbon using a microbial incubation under oxic conditions. To do this, 13C labeled natural organic matter (NOM) was co-precipitated with Fh (Fh-NOM had a molar C/Fe ratio of 1.8 and 0.8) and then reacted with Fe (II) for 1 d and 14 d. The reacted co-precipitates and unreacted controls were incubated under oxic conditions for 45 d with soil microbial inoculum. Biodegradation was tracked by measuring the headspace CO2 (g) in the reactors (both CO2 (g) and δ13CO2 (g)) and the total C content of the coprecipitates were measured before and after the incubation study. CO2 (g) production was greater from coprecipitates reacted with Fe (II) than from the unreacted controls and highest at C/Fe ratio of 0.8. This could be due to the increases in crystallinity of the coprecipitate structure after reacting with Fe(II), which eventually reduces the available mineral surface area and could lead to lower adsorption of carbon and more emission of CO2. Moreover, this effect is more prominent for coprecipitates with C/Fe ratios of 0.8, which are known to undergo transformation than coprecipitates with C/Fe ratios of 1.8. The study suggests that the Fe(II) may decrease the stability of organic matter in Fh- NOM coprecipitates and increase its bioavailability in natural systems.
Noor, Nadia, Zhe Zhou, Drew E. Latta, Michelle M. Scherer, Alain F. Plante, Thomas Borch, and Aaron Thompson (2019): Effect of Reaction Time of Fe (II) with Different C/Fe Ratio Containing Ferrihydrite-Natural Organic Matter Coprecipitate on the Bioavailability of Carbon. ASA-CSSA-SSSA International Annual Meeting, 10-13 Nov. 2019, San Antonio, Texas.