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 e lemental analyses (C, N) and by off-line pyrolysis under nitrogen at a tmospheric pressure and on-line pyrolysis-field ionization mass spectr ometry (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-cushio n moors and were considered as a soil chronosequence, The Py-Fl mass s pectra of the soil samples were characterized by intensive signals of fatty acids, sterols and suberin-derived molecules. With increasing so il age, (1) relative enrichments of resistant lipids, sterols, suberin constituents, fatty acids and nitrogen-containing compounds and (2) s hifts of the volatilization onsets and maxima to higher temperatures i n thermograms of total ion intensities and of lipids and fatty acids w ere observed. These increases in thermal stability indicated the stren gthening of chemical bonds as a significant process during SOM aging. It was further concluded that Py-FIMS, complementary to other analytic al methods, can be used to monitor molecular-chemical changes of SOM c omposition and stability in soil chronosequences.