INFLUENCE OF THE MINERAL MATRIX ON THE FORMATION AND MOLECULAR COMPOSITION OF SOIL ORGANIC-MATTER IN A LONG-TERM, AGRICULTURAL EXPERIMENT

The formation of soil organic matter from grass residues was studied i n a 34-year-old pot experiment with grass cultivation on loamy marl us ing pyrolysis-field ionization mass spectrometry (Py-FIMS). For whole soils, the Py-FI mass spectra indicated clear changes in the molecular -chemical composition during SOM formation from grass residues. In par ticular, the enrichment of heterocyclic N-containing compounds with ti me was remarkable. For organomineral size fractions, even larger diffe rences in the composition of SOM were found. The changes between the 1 3th and 34th experimental year are partly explained by a net transfer of phenols, lignin monomers und lignin dimers from medium silt to fine silt. Moreover, it is demonstrated that temperature-resolved Py-FIMS enables the determination of the thermal energy required for the evolu tion of individual compound classes which is a measure of the strength of humic- and organomineral bonds. At lower temperatures (< 400-degre es-C), the enrichment of thermally less stable and/or loosely bound or ganic matter with cultivation time in clay and fine silt is due to car bohydrates, N-containing compounds, phenols and lignin monomers. Shift s of evolution maxima toward a higher pyrolysis temperature (> 400-deg rees-C) in clay, fine silt and medium silt are explained by a higher t hermal stability of humic and/or organomineral bonds of lignin dimers, alkylaromatics and lipids, that developed during the last two decades of the experiment.