Increased Stability of Organic Matter Sorbed to Ferrihydrite and Goethite on Aging

Sorption to micro‐ and mesoporous mineral phases can stabilize organic matter (OM) against microbial decay in soil. Formation of strong bonds that reduce desorbability is one plausible explanation for that effect. With time after sorption, sorbed OM may undergo changes in configuration or may migrate into intraparticle spaces. We tested the possible effects of residence time of OM sorbed to ferrihydrite and goethite. The minerals were loaded with different amounts of water‐soluble OM from an Oa horizon, then stored moist (10% w/w water) for up to 1080 d at 4°C. We monitored the content of organic C, the desorbability and chemical stability (by extraction with 0.1 M NaOH–0.4 M NaF and treatment with 1 M NaOCl), and, after freeze‐drying, the micro‐ and mesopore volume (by N 2 and CO 2 adsorption–desorption). Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to characterize the OM on the mineral surfaces at the beginning and end of the experiment. There was no detectable decrease in sorbed organic C during the experiment; also, the micro‐ and mesoporosity of the samples remained unchanged. The proportion of desorbable organic C, however, decreased by up to 16%. This was paralleled by more pronounced bands indicative of complexed organic functional groups in the DRIFT spectra. We conclude that with increasing residence time, OM sorbed to porous minerals becomes decreasingly desorbable by the formation of additional chemical bonds to the surface via ligand exchange but not by diffusion into small pores. The decrease in desorbability was accompanied by a decrease in chemical destructibility with NaOCl. The stability of sorbed OM against biological degradation may similarly increase with residence time.