Ground surface heating, (e.g. forest fire) often leads to an increase or above a critical threshold, elimination of water repellency (hydrophobicity) in soils. Here we report on the characterisation of the chemical changes of organic compounds associated with heat-induced increases and elimination of water repellency. A eucalypt forest soil of sandy texture was heated in the laboratory at temperatures between 320°C – 330°C. Laboratory heating in this temperature range resulted in elimination of repellency, with the exception of one replicate, in which repellency increased. Soil organic compounds were extracted by accelerated solvent extraction (ASE) with an isopropanol/ammonia mixture. Organic compounds were measured in solvent/solvent mixtures of increasing polarity in order to solubilise the residue. Before heat treatments, the total lipid extracts were dominated by alkanols, palmitic acid, C alkane, β-sitosterol and 29 polar compounds. A profound change in lipid patterns occurred in the extracts following heating irrespective of whether or not water repellency was destroyed. Heat-induced changes resulted in: a) the loss of main lipid signatures and b) a high abundance of benzene carboxylic acids, as well as levoglucosan, simple sugars and glycosides, compared to the unburned soil. Our data demonstrate that many of the compounds detected in the originally unheated, water repellent soil were lost after heating in the temperature range 320 – 330 °C. We suspect that the increased water repellency in one of the heated samples is associated with compounds of different chemical structure and properties, and that saccharides and aromatic acids can play a role in repellency expression in soils afterheating.
I. Atanassova, S. Doerr