Composition of clay-fraction organic matter in Holocene paleosols revealed by advanced solid-state NMR spectroscopy

Abstract Identifying which components of organic matter are preserved over millennia in paleosols is important for understanding the stabilization mechanisms of soil organic matter (SOM) and in evaluating the portion of the terrestrial carbon stock that might be attributed to paleosols. Using advanced solid-state nuclear magnetic resonance (NMR) techniques, we examined the chemical composition of clay-associated SOM in a chronosequence of four paleosols and one modern soil collected from a site in the eastern Great Plains in Kansas, USA. The NMR spectra of clay SOM of the paleosols indicated the presence of OCH and CH2 moieties, which represent polysaccharides and lipids, respectively. The paleosol SOM also showed an appreciable peptide component, as indicated by signals of NCH and NCO. As in the modern soil, lignin residues in the paleosols contributed little to the SOM, and condensed aromatics contributed modestly to the clay SOM. The Roberts Creek paleosol samples were exceptions. Their NMR spectra included a considerable nonprotonated aromatic carbon component, reflecting fused aromatic rings and suggesting the presence of clay-fraction charcoal residues, probably originating from paleofires. In the clay SOM of both the paleosols and the modern soil, the polysaccharides and peptides indicated by the NMR spectra are consistent with the presence of microbially derived peptidoglycan and perhaps chitin residues. We conclude that lipids, charcoal, and polysaccharides, probably in the form of microbial residues, are likely to be the dominant SOM components in the paleosol clay fractions.