Humus Composition and Transformations in a Pergelic Cryohemist of Coastal Antarctica

Abstract

Soil organic matter (SOM) of an Antarctic Pergelic Cryohemist was studied with special emphasis on soil formation processes under extreme climate conditions. An integrated approach using 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. Dead mosses material are the source of fresh organic matter for soil formation in the Pergelic Cryohemist. Polysaccharides are the primary organic compounds. This composition is very similar to that of litter from peat soils in temperate climate regions. In addition protein and alkyl carbon are found in the mosses, whereas in contrast to temperate climate zones lignin was of minor importance. The alkyl compounds consist of lipids, fatty acids, and sterols. A high amount of carboxyl groups probably develops from fatty acids and proteins and the aromatic structures mainly from aromatic amino acids. Changes in the SOM morphology were not confirmed with the classical wet chemical analysis but were confirmed by CPMAS-13C-NMR and Py-FIMS. Carbohydrates are less mineralized than under temperate climate conditions. Alkyl carbon units are enriched due to selective preservation. During advanced humification the alkyl carbon units are probably incorporated into the complex humic matter. The drastic increase of alkyl-C with peat depth has been described also for peat soils in temperate climate zones. The aromatic structures identified demonstrate that lignin input is not necessary for the formation of aromatic humics. The chemical degradation of the organic matter is not as intensive as in soils of temperate climate regimes. The extremely cold climate conditions retard the transformation of fresh organic residues. Therefore in this Antarctic peaty soil, carbohydrates dominate in the composition of the SOM also in the deeper horizons.