Molecular characterization of soil organic matter in Pleistocene moraines from the Bolivian Andes

The molecular composition of soil organic matter (SOM) in Pleistocene moraines of different ages in the Bolivian Andes was investigated by elemental analyses (C, N) and by off-line pyrolysis under nitrogen at atmospheric pressure and on-line pyrolysis-field ionization mass spectrometry (Py-FIMS) in vacuum. Moraine ages of 9,000, 16,000, 20,000 and > 20,000 years were derived from C-14 datings of adjoining hard-cushion moors and were considered as a soil chronosequence, The Py-Fl mass spectra of the soil samples were characterized by intensive signals of fatty acids, sterols and suberin-derived molecules. With increasing soil age, (1) relative enrichments of resistant lipids, sterols, suberin constituents, fatty acids and nitrogen-containing compounds and (2) shifts of the volatilization onsets and maxima to higher temperatures in thermograms of total ion intensities and of lipids and fatty acids were observed. These increases in thermal stability indicated the strengthening of chemical bonds as a significant process during SOM aging. It was further concluded that Py-FIMS, complementary to other analytical methods, can be used to monitor molecular-chemical changes of SOM composition and stability in soil chronosequences.