Studies on the composition and formation of soil organic matter in terrestrial soils by wet-chemical methods and pyrolysis—field ionization mass spectrometry

Abstract

Soil organic matter (SOM) in soils from the Northwest Lower Plain in Germany was investigated to characterize its composition and formation. Using the combination of wet-chemical methods and pyrolysis—field ionization mass spectrometry (Py—FIMS) two problems of soil science are discussed. Firstly, the composition of SOM of an Orthic Luvisol and selected Bh-horizons of Haplic Podzols was analysed to examine the suitability of a wet-chemical distinction between plant constituents (litter) and humic compounds for the explanation of soil formation. The distribution of litter and humic compounds determined by wet-chemical methods was compared with the assignment of Py—FIMS signals to compound classes of SOM. The observed chemical structural changes of plant materials during formation of SOM were less distinct than formerly supposed on the basis of previously published investigations. Py—FIMS showed that differences between litter and humic compounds are reflected to a lesser extent by changes of chemical units, but can be clearly recognized by differences in the thermal stability of SOM, i.e. by intra/intermolecular bonds of humic compounds and organo-mineral complexes. Secondly, a preliminary comparison of wet-chemical results and Py—FIMS intensities of compound classes of SOM of a Podzolic Cambisol is presented. Depth functions of compound classes obtained by both analytical methods showed corresponding trends. Moreover, for seven soil horizons, results of the wet-chemical determination of litter and humic compounds correlated significantly with intensities of Py—FIMS signals typical for selected soil constituents. Thus, data of thermally evolved marker molecules obtained by the direct Py—FIMS of whole soils are suitable for qualitative descriptions and quantitative comparisons of whole soils and soil horizons.