STABLE CARBON-ISOTOPE EVIDENCE FOR THE MICROBIAL ORIGIN OF C-14 C-18 N-ALKANOIC ACIDS IN SOILS

In order to delineate the origin of soil fatty acids, crop soil sample s have been incubated for 21 days in vitro either with unlabelled or C -13-labelled glucose. Analyses of C-14-C-32 n-alkanoic acids from mono carboxylic acid fractions, as methyl esters, by gas chromatography-com bustion-isotope ratio monitoring mass spectrometry (GC-C-IRMS) show th at C-14, C-16 and C-18 n-alkanoic acids are C-13-labelled, thus demons trating their derivation from soil microorganisms, e.g. fungi or bacte ria, growing during the experiment. Higher n-alkanoic acids, C-16-C-33 n-alkanes, and C-22-C-30 n-alkanols have not been significantly label led, thus suggesting their derivation from other sources, e.g. higher plants. This short-term tracer experiment using stable carbon isotopes represents a novel and fruitful approach to study organic matter tran sformations in soils and other systems such as sediments.