Initial formation of soil organic matter from grass residues in a long-term experiment

Abstract

The formation of soil organic matter from grass residues was studied using samples of a long-term experiment (34 years) on humus and soil formation at Rostock, Germany (Hu 3), by elemental analyses (C and N) and pyrolysis-field ionization mass spectrometry of grass residues, humus-free loamy marl, mixtures of this loamy marl with grass roots, and whole soil samples from the 2nd, 7th, 13th, 19th, 25th, 29th, and 34th year of the experiment. The pyrolysis-field ionization mass spectra of the two grass species Phleum pratense and Lolium multiflorum were similar insofar as signals characteristic of lignin dimers and phytosterols dominated at higher masses and for mono-and polysaccharides at lower masses. The most prominent differences between overand underground plant constituents were indicated by higher relative abundances of lignin dimers in the stems and leaves and of sugars and suberin-derived phytosterols in the roots. In the investigation of the influence of mineral to organic matter ratios, comparatively weak effects of the inorganic matrix were obtained: firstly, in the lower mass range (m/z<250), secondly, for organic matter concentrations between 1.0% and 2.0%, and thirdly, for certain classes of compounds such as phenols, alkanes/alkenes, N heterocycles and mono-and polysaccharides. The qualitative differences in the molecular composition of soil organic matter were clearly attributed to its rapid increase during the first 7 years of the experiment and largely originated from a relative enrichment of lignin dimers. Then, in the period of steady-state soil organic matter levels, dynamic changes were indicated by slight enrichments of mono-and polysaccharides, alkanes/alkenes, fatty acids, N heterocycles, and fluctuating data for phenols/lignin monomers, lignin dimers, and the sum of N compounds. Alkylaromatics showed a steep increase between the 13th and 19th years and remained then on a high level.