Differential thermal analysis, thermogravimetry and in-source pyrolysis-mass spectrometry studies on the formation of soil organic matter

Abstract

Differential thermal analysis (DTA) thermogravimetry (TG) and pyrolysis-field ionization mass spectrometry (Py-FIMS) were compared in studies of soil organic matter (SOM) in particle-size fractions in different years of a long-term soil formation experiment with loamy marl. The DTA curves and TG weight losses indicated the presence of different amounts of SOM, quartz and carbonates in the various size fractions and experimental years. The soil development from the 2nd to the 34th experimental year was characterized by the enrichment of SOM in the size fractions investigated, which also resulted in an accelerated weathering of carbonates. The comparison between DTA curves, TG weight losses and Py-FIMS thermograms and weight losses, respectively, showed a shift of the maximum of thermal decomposition of SOM to higher temperatures in Py-FIMS. This was explained by the higher heating rate (0.5 K s −1) and the particularly large sample amounts (5 mg) used in the mass spectrometric studies of particle-size fractions of whole soils with organic carbon (C org) concentrations between 0.2 and 1.9%. However, the onset of thermal evolution was in good agreement with DTA and TG data. Similar results were also obtained in the thermal behaviour of particle-size fractions and changes during soil development. By Py-FIMS, complementary information was obtained about the molecular chemical composition of SOM in the fractions by the evaluation of summed and averaged mass spectra and by the assignment of signals to the principal classes of chemical compounds. The abundances of phenols and lignin monomers and N-compounds decreased with increasing particle size; the reverse was true for lignin dimers and fatty acids. The latter two compound classes were also enriched in the samples from the 34th experimental year owing to SOM maturation. A unique feature of Py-FIMS as a novel method of whole soil analysis was its high analytical resolution which allowed monitoring of the thermal evolution of selected classes of compounds on a molecular chemical basis. This was demonstrated for the first time for phenols and lignin monomers, lignin dimers and alkylaromatics which are important soil constituents for SOM formation as well as for agricultural and environmental problems.