Occurrence of Biomarkers and Straight-Chain Biopolymers in Humin: Implication for the Origin of Soil Organic Matter

Abstract

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.

Opportunities and limitations related to the application of plant-derived lipid molecular proxies in soil science

“…Depending on the research question, this may pose a problem; for instance, it might obscure chronology when molecules are used as proxies to reconstruct changes over time. Lichtfouse et al (1998b) showed that straight-chain lipids can become encapsulated in larger organic macromolecules, thus being protected against degradation. In addition, physical protection in (the micropores of) aggregates and/or through association with clay minerals has been identified as an important pathway for the stabilization of soil organic matter in general, including molecules used as molecular proxies (Tonneijck et al, 2010).…”

Knowledge, conservation and sustainable use of soil: agricultural chemistry aspects

“…Il maggior sequestro di C si verifica nelle tesi in cui il materiale labile è associato ai materiali umificati caratterizzati dalla maggiore idrofobicità, confermando che l'uso di sostanze umiche stabilizzate potrebbe ritardare la decomposizione di tutto il pool di C del suolo (Whiteley e Pettit, 1994). Questa interpretazione è in accordo con altri studi che indicano le parti idrofiliche arricchite in gruppi alcolici come le più sensibili alla decomposizione e alle pratiche agronomiche (Wander e Traina, 1996;Almendros e Dorado, 1999;Zalba e Quiroga, 1999), mentre i pools più resistenti alla degradazione sono quelli derivanti da una preservazione selettiva dei biopolimeri con carattere più apolare, prevalentemente di natura alchilica (Almendros et al, 1996;Hatcher e Clifford, 1997;Lichtfouse et al, 1998;Spaccini et al, 2000;Cerli et al, 2008).…”

A novel model of humin

“…Resistant aliphatic biopolymer showing typical very thin laminae (25 µm thick) were indeed present [4], thus strengthening the biopolymer hypothesis. Further pyrolysis of humin followed by gas chromatography coupled to mass spectrometry (GC-MS) revealed the occurrence of C 11 -C 24 linear alkanes and alkene doublets [5,6] which are typical breakdown products of aliphatic biopolymers [3]. Therefore, a part of soil organic matter is indeed composed of straight-chain biopolymers (Fig.…”

“…Gas chromatography-mass spectrometric analysis of the humin pyrolysate revealed the occurrence of hopanoid and steroid biomarkers [6]. Biomarkers have been widely used to assess the biological sources of dead matter [12].…”