Rapid molecular assessment of the bioturbation extent in sandy soil horizons under pine using ester-bound lipids by on-line thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry

Each plant species has a unique chemical composition, and also within a given plant the various tissues differ from one another in their chemistry. These different compositions can be traced back after decay of the plant parts when they are transformed into soil organic matter (SOM). As a result, the composition of SOM reflects not only the plant origin, but also the various tissues, and the composition consequently provides an estimate of the contribution of above-ground vs. below-ground litter. From the latter distribution the extent of bioturbation (mixing of above-ground litter with the mineral soil) can be assessed. Application of thermally assisted hydrolysis and methylation (THM) using tetramethylammonium hydroxide (TMAH) and subsequent analysis by gas chromatography/mass spectrometry (GC/MS) releases all typical cutin- and suberin-derived aliphatic monomers (mono-, di- and trihydroxyalkanoic acids, α,ω-alkanedioic acids) as their methyl esters and/or ethers in a rapid manner. Using the distribution of ω-hydroxyalkanoic acids that are present in pine needle cutin (C ₁₂ and C₁₄) and not in root suberin, and those that are present in roots but not in needles (C₂₀ and C22), the extent of bioturbation (mixing of above-ground plant litter with the mineral soil) can be assessed. Similarly, the (9,16-dihydroxyhexadecanoic acid+9,10,18-trihydroxyoctadecanoic acid)/(C₂₀ + C₂₂ α,ω-alkane-dioic acids) ratio reflects the degree of bioturbation. Three mineral soil profiles under Corsican pine with an A horizon that exhibited extensive bioturbation phenomena, and underlying C horizons with hardly any or no bioturbation, were investigated in order to examine the applicability of such an approach. It appeared that the A horizons contained all four mentioned ω-hydroxyalkanoic acids, while the C horizons contained virtually only the C₂₀ and C₂₂ members. The results not only suggest that bioturbation occurs in the A horizons, but also that possible illuviation or other transport mechanisms of ω -hydroxyalkanoic acids seem hardly ever or never to occur, which is a prerequisite for applying this biomarker approach in assessing degrees of bioturbation.