CHARACTERIZATION OF SOM IN HOLOCENE PALEOSOLS

Stable carbon isotope ratios in paleosol soil organic matter (SOM) serve as a proxy for the relative abundances of C3 and C4 plants in the standing vegetation of paleolandscapes. The effects of degradation in the buried environment on ¹²C/¹³C ratios of SOM are not well known. We are studying a succession of alluvial paleosols at a northeastern Kansas locality to investigate the degradation of buried SOM over the past 9,000 years. Bulk, silt-, and clay-size fractions of SOM from various A horizons of four buried soils and a modern surface soil have been analyzed for total C, total N, and ¹²C/¹³C ratios. SOM was also examined using Fourier transform infrared spectroscopy (FTIR), to characterize the key functional groups in the clay-size fraction of the SOM. Nitrogen concentrations in the bulk and silt fractions were very low, but HF-treated clay fractions contain 0.5 – 1% N. Preliminary FTIR characterization of the clay-associated SOM shows no evidence of aromatic ring vibrations that would indicate lignin residues, suggesting a dominant presence of microbial residues in the paleosol SOM. The results from C/N ratios and preliminary FTIR spectra also indicate that it is unlikely charcoal residues occur in the clay fraction SOM. These initial results suggest differences in the composition of paleosol and surface soil SOM that could impact paleovegetation interpretations derived from ¹²C/¹³C ratios.