Origin, occurrence and fate of extractable lipids in Dutch coastal dune soils along a pH gradient

Total lipid extracts (TLEs) of subsoils from under oak, had been subjected to natural acidification, were analysed to trace the effects of soil pH on their composition. In addition, roots were analysed as being the main source of SOM in the deeper mineral soil horizons. The TLE of the roots comprised largely triterpenoids and steroids that were barely present in the subsoils. Conversely, only small amounts of alkanoic acids, and no alkanols, ω-hydroxyalkanoic acids or α,ω-alkanedioic acids were detected in the root TLE, whereas in soil TLEs these compounds were highly abundant. The compounds are suggested to be derived from hydrolysed suberin, from the previous grass vegetation and most likely illuviated lipids (i.e. immobilised lipids that moved downwards in dissolved form from overlying horizons) from upper, organic matter-rich horizons and oxidation of lipids mediated by microorganisms. In the course of the soil acidification the amounts of TLE increased and within this fraction long-chain (>C₂₀) alkanoic acids, alkanols, ω-hydroxyalkanoic acids and α,ω-alkanedioic acids increased relatively with respect to short chain (20) alkanoic acids. The decrease in short chain (20) alkanoic acids, for a great part derived from microorganisms, may confirm that the microbial population and therefore also its activity decreased at lower soil pH.