Composition, stability and turnover of soil organic matter: investigations by off-line pyrolysis and direct pyrolysis-mass spectrometry

Abstract

The composition, stability and turnover of soil organic matter (SOM), mostly of agricultural origin, have been investigated by C- and N-analyses, off-line pyrolysis under nitrogen, and pyrolysis-field ionization mass spectrometry using two samples of typical organic materials such as hot water extracts and soil microbes, 13 organo-mineral particle-size fractions and 18 whole soils. For in-source pyrolysis-field ionization mass spectrometry (Py-FIMS) (< 1 mPa, about 5 mg sample, heating rate 1 K s −1, 700 °C final temperature) an average volatilization of 57% and 79% of sample-C and -N, respectively, has been determined, which is in good agreement with the weight losses observed for off-line Py-FIMS (7 kPa N 2, 500 mg sample, heating rate 2–3 K s −1, 700 °C final temperature). Using the two independent thermal methods, decreased ion intensities per sample-C and lower proportions of volatilized matter were clearly indicative of larger SOM stability. The stabilization of SOM by minerals was reflected in selective enrichments of compound classes by mineralogically different particle-size fractions, preferential associations of lipids, alkylaromatics and lignin dimers with pedogenic Al- and Fe-oxides and by the intercalation of largely aliphatic molecules into swelling clay minerals. Finally, a novel concept of SOM turnover in relation to organo-mineral particle size fractions is proposed, which explains short-term, seasonal, as well as medium-term, management induced, changes of the composition and stability.