Stable carbon isotope evidence for the microbial origin of C14---C18 n-alkanoic acids in soils

In order to delineate the origin of soil fatty acids, crop soil samples have been incubated for 21 days in vitro either with unlabelled or 13C-labelled glucose. Analyses of C14---C32 n-alkanoic acids from monocarboxylic acid fractions, as methyl esters, by gas chromatography-combustion-isotope ratio monitoring mass spectrometry (GC—C—IRMS) show that C14, C16 and C18 n-alkanoic acids are 13C-labelled, thus demonstrating their derivation from soil microorganisms, e.g. fungi or bacteria, growing during the experiment. Higher n-alkanoic acids, C16---C33 n-alkanes, and C22---C30 n-alkanols have not been significantly labelled, 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 transformations in soils and other systems such as sediments.