Isotope and biosynthetic evidence for the origin of long-chain aliphatic lipids in soils
Résumé
Soil organic matter is a very complex mixture of compounds derived from numerous living organisms such as plants, fungi, algae, microfauna and microbes. During humification those biological compounds are transformed by processes such as biotic and abiotic degradation, polymerisation, reduction and aromatisation. Thus, the biological origin of soil organic molecules is often very difficult to assess. Knowledge of the biological origin of soil organic molecules is important, for instance, to understand the mecanisms of preservation of soil organic matter, which are still debated. Here I studied the origin of three classes of soil compounds, n-alkanes, n-alkanols (fatty alcohols), and n-alkanoic acids (fatty acids) that can be derived from numerous biological and anthropogenic sources. Those waxy compounds are of agronomical interest because they are believed to play a role in soil aggregates stability, as a glue, and in soil water retention due to their hydrophobic properties. Since the molecular structure of n-alkanes, n-alkanols and n-alkanoic acids does not yield enough information to assess their precise origin, I measured their 13C isotope composition using a recently developped method, gas chromatography coupled to isotope ratio monitoring mass spectrometry (GC-IRMS) to gain more insight. The results show that long-chain n-alkanes (C27, C29), n-alkanols (C28, C30) and n-alkanoic acids (C28, C30) have similar 13C isotope values averaging at -34.2‰. This finding evidences their common origin from leaf cuticular waxes of C3 plants, in agreement with their biosynthetic pathway. Short-chain n-alkanoic acids (C16-C18) are 13C-enriched with 13C isotope values averaging at -26.6‰. This finding supports their microbial origin demonstrated in a previous report. Short-chain n-alkanes (C16-C18) are also 13C-enriched with 13C isotope values averaging at -28.3‰. This finding supports their fossil fuel origin demonstrated in a previous report. Overall the findings imply that plant leaf waxes are well preserved in soil, despite soil conditions that favors their degradation. Agricultural practices such as incorporating plant leaf residues in soil during tillage and amendment with plant-derived compost should therefore improve aggregate stability and water-holding capacity, on molecular grounds.
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