Occurrence of biomarkers and straight-chain biopolymers in humin: implication for the origin of soil organic matter.
Résumé
Soil organic matter is a very complex mixture of compounds derived from the decay of living organisms such as plants, fungi and bacteria. The mechanisms that control the preservation of soil organic matter are poorly known. This is notably due to our scarce knowledge of the molecular structure of humin, the insoluble part of soil organic matter. Three major hypotheses have been proposed to explain the formation of soil organic matter: the plant alteration hypothesis, the microbial synthesis hypothesis and the chemical polymerisation hypothesis. Here we used a "biomarker" approach, developped mainly in sedimentary geochemistry, to gain some insight on the mechanims of preservation of soil organic matter. We analysed the compounds occurring in soil humin. We isolated humin from a maize crop soil by solvent extraction to remove free lipids, then by NaOH extraction to remove humic and fulvic acids, then by HF/HCl treatment to remove minerals. The humin residue was then pyrolysed at 600°C. The pyrolysate was fractionated into alkane/alkene and aromatic fractions by silica gel column chromatography. The fractions were analysed by gas chromatography coupled to mass spectrometry (GC-MS). We identified n alkanes, n-alkenes, prist-1-ene, sterenes, hopenes and aromatic steroids in the humin pyrolysate. The results show that the stabilisation of soil humic substances may be achieved by selective preservation of lipids, involving three processes: encapsulation of small apolar molecules, chemical bonding of functionalized biomarkers, and preservation of straight-chain microbial polymers.
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