Biodegradation of ferrihydrite-associated organic matter

The association of organic molecules with mineral surfaces is a major mechanism to stabilize soil organic matter against biodegradation. We performed microbial incubation experiments to quantify the mineralization of soil organic matter associated with ferrihydrite by adsorption and coprecipitation. Samples were produced using either water-extractable organic matter of a Podzol forest-floor layer (FFE) or a sulfonated lignin. Incubation was carried out with an inoculum extracted from the forest-floor layer under oxic conditions at pH 4.8 over 68 days. Our data show that the association with ferrihydrite stabilized the associated organic matter: the degradation of the polysaccharide-rich FFE was slowed down, while the degradation of lignin was inhibited. Since differences in the degradability of adsorbed and coprecipitated organic matter were small, we conclude that coprecipitation did not lead to a significant formation of microbial inaccessible organic matter domains in our experiments.