Litter decomposition and humification in acidic forest soils studied by chemical degradation, IR and NMR spectroscopy and pyrolysis field ionization mass spectrometry

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Abstract
Two forest soils (Typic Dystrochrept, Entic Haplorthod) with mor and moder were investigated by chemical degradation, IR and CPMAS 13C NMR spectroscopy and pyrolysis (Py) field ionization (FI) mass spectrometry (MS). Chemical analyses show that during litter decomposition, humification, and podzolisation, cellulose and lignin structures decrease considerably, whereas no distinct changes were found for the hemicellulose and protein fractions. These results are consistent with current hypotheses on the conversion of plant residues to stable humic substances, but the sum of chemically identified organic soil components of the litter layers only accounts for 40–50% of total organic carbon. The amounts of different carbon types were estimated by the integration of CPMAS 13C NMR spectra. For the L layers this calculation assigns 56–58% as O‐alkyl‐C, 20–22% as alkyl‐C, 14–16% as aryl‐C, and 6–8% as carboxyl‐C. With increasing soil depth O‐alkyl‐C (with polysaccharides as main source) decrease to 31–42%, aliphatic C increases to 36–43%, and aryl‐ and carboxyl‐C show no distinct changes. The hypothesis of an increasing aromaticity during humification in soils therefore is questionable. Data from Py‐FIMS confirm and extend the results' of chemical methods as well as IR and 13C NMR spectroscopy. In particular, the Fi mass spectra of the generated pyrolysates show that the increase in polymethylene carbon during the biodegradation and humification of beech and spruce litter is partly due to an increase of saturated fatty acids. This means, Py‐FIMS is able to describe the structure of wet‐chemically unaccounted, individual humus constituents and thus improves the knowledge about the genesis of humic substances.