Stable carbon isotope evidence for the microbial origin of C₁₄C₁₈ 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 C₁₄C₃₂ 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, C₁₆C₃₃ n-alkanes, and C₂₂C₃₀ 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.