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 in a 34-year-old pot experiment with grass cultivation on loamy marl using 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 particular, the enrichment of heterocyclic N-containing compounds with time was remarkable. For organomineral size fractions, even larger differences in the composition of SOM were found. The changes between the 13th 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 evolution of individual compound classes which is a measure of the strength of humic- and organomineral bonds. At lower temperatures (< 400-degrees-C), the enrichment of thermally less stable and/or loosely bound organic matter with cultivation time in clay and fine silt is due to carbohydrates, N-containing compounds, phenols and lignin monomers. Shifts of evolution maxima toward a higher pyrolysis temperature (> 400-degrees-C) in clay, fine silt and medium silt are explained by a higher thermal stability of humic and/or organomineral bonds of lignin dimers, alkylaromatics and lipids, that developed during the last two decades of the experiment.