Organic matter composition in sewage farm soils: Investigations by 13C-NMR and pyrolysis-field ionization mass spectrometry

The composition of soil organic matter (SOM) on sewage farms south of Berlin was investigated by solid-state CP/MAS ¹³C-NMR and pyrolysis-field ionization mass spectrometry (Py-FIMS) of freeze-dried sewage solids and soil samples of differing contamination. These were an untreated soil (USOIL), a former sewage farm used as arable land since 1990 (SF90A), and a recent sewage farm (SF1994). The CP/MAS ¹³C-NMR spectra showed enrichments of the sewage-treated soils with aliphatic C and C in OCH₃-groups and amino acids. In the Py-FI mass spectra the major markers of sewage and SOM in sewage farm soils were (i) N-containing compounds, in particular peptides, (ii) dimethylphthalate (m/z 194), (iii) sterols, and (iv) signals in the mass range m/z 502 to 554 of mono- and diaryl esters which were substituted by long aliphatic chains. The latter signals were intense in the sewage solids, increased in intensity from sample SF90A to SF1994; but they were not present in the USOIL, thus clearly indicating anthropogenic origin. Temperature-resolved Py-FIMS showed that the SOM compounds in the sewage farm soils were generally incorporated into bonds with widely different stabilities which could be relevant for SOM turnover and environmental effects. This is demonstrated for the trapping of dimethylphthalate in a modelled humic substance.