Soil organic matter composition and transformation in gelic histosols of coastal continental antarctica

Abstract

Soil organic matter (SOM) of two antarctic soils was studied with special emphasis on soil formation processes under extreme climate conditions. An integrated approach of modern analytical methods, including wet-chemical analyses, cross-polarization magic-angle spinning carbon-13 nuclear magnetic resonance spectroscopy (CPMAS 13C-NMR) and pyrolysis-field ionization mass spectrometry (Py-FIMS), was applied to characterize the SOM composition at different depths in a Terri-Gelic Histosol and a Fibri-Gelic Histosol. Corresponding to the brownish-black colour of deeper soil horizons in the Terri-Gelic Histosol formed by mosses, wet-chemical analyses indicated transformation processes in the profile. Relative to the LH-horizon, the deeper horizons were characterized by an enrichment of organic-C and non-α-NH 2N. However, in the Fibri-Gelic Histosol wet-chemical analyses did not show changes of SOM composition with increasing profile depth, which is clearly in accordance with the greenish-grey colour of all soil horizons. Both, the CPMAS 13C-NMR and the Py-FIMS spectra of the Terri-Gelic Histosol were dominated by signals of carbohydrates and alkylic compounds. The 13C-NMR data suggested decomposition of carbohydrates and the enrichment of alkyl-C in the deeper horizons. Py-FIMS indicated for carbohydrates a slight increase with increasing profile depth. The relative intensities (percentage of total ion intensity) of selected Py-FIMS signals for lipids ( n-C 10 to n-C 20 alkyl-diester, n-C 30 alkenes and alkanes, n-C 16 to n-C 34 fatty acids, n-C 44 to n-C 46 alkyl-monoesters, sterols) increased in the H1-and H2-horizons compared to the top and lowest horizons. Complementary to the general enrichment of alkyl-C with increasing soil depth determined by 13C-NMR spectroscopy, the detection of specific, biologically-important lipid-derived structures by Py-FIMS enabled a deeper insight in the behaviour of alkyl structures in the profile of the Terri-Gelic Histosol. For the first time a separate recording of signals for free fatty acids and sterols is presented. Although mosses do not contain lignins, both 13C-NMR and Py-FIMS spectra have shown signals for aromatics. This indicated the formation of aromatic humic substances without lignin precursors from plants. The 13C-NMR spectra of all horizons of the Fibri-Gelic Histosol were similar and showed intensive signals for carbohydrates. This indicated only a slight decomposition of carbohydrates and no selective preservation of alkylic biomacromolecules. Likewise, aromatic signals were of minor importance. Due to the extreme climatic conditions and the high water capacity, SOM transformation processes are retarded in the Fibri-Gelic Histosol.