Changes in soil organic compound composition associated with heat‐induced increases in soil water repellency

Soil heating, as for example experienced during vegetation fires, often increases soil water repellency; however, no detailed analysis of the soil chemical changes associated with this increase has been conducted to date. Here we characterize the changes in organic compound composition associated with heat‐induced increases in water repellency for three Australian eucalypt‐forest soils (one sandy loam, two sands). Laboratory heating (300°C) strongly increased water drop penetration times (WDPTs) in all soils. Soils were extracted by accelerated solvent extraction (ASE) with an iso ‐propanol/ammonia mixture (IPA/NH 3 95:5) and pure iso ‐propanol (IPA). Extracts were fractionated into less and more polar fractions and analysed by GC‐MS. Water repellency was eliminated in unheated and heated soils by IPA/NH 3 , but not by pure IPA. Before heating, total solvent extracts were dominated by n ‐alkanols, terpenoids, C 16 acid, C 29 alkane, β ‐sitosterol and polar compounds. After heating, dominant compounds were aromatic acids, aldehydes, levoglucosan, simple sugars and glycosides. Heating resulted in a sharp absolute decrease of homologous aliphatic series of alkanols and alkanes, a shift of fatty acid signature to members <C 20 and an increase in total content of aromatic compounds. Heating also caused the formation of complex high‐molecular‐weight compounds detected in the more polar fractionated extracts and low‐molecular‐weight oxo‐ and hydroxyacids and aromatics in the IPA/NH 3 solvent. We speculate that these compounds in conjunction with fatty acids of <C 12 interact with organic and mineral soil surfaces and cause the observed strong increases in soil water repellency following heating.