Changes in organic compound composition in soil following heating to maximum soil water repellency under anoxic conditions I. Atanassova, A,B S. H. Doerr A,D and R. Bryant C A College of Science, Department of Geography, Swansea University, Singleton Park, Swansea, SA2 8PP, UK. B ‘N. Poushkarov’ Institute of Soil Science, 7 Shosse Bankya, Sofia 1080, Bulgaria. C College of Engineering, Swansea University, Singleton Park, Swansea, SA2 8PP, UK. D Corresponding author. Email address: s.doerr@swan.ac.uk Environmental context. Heating of soils under wildfires can substantially reduce their ability to absorb rainfall, causing reduced vegetation recovery and increased erosion and flooding. This study examines, for the first time, the chemical changes in soil organic matter associated with heating in the oxygen-limited conditions typical under many wildfires. There was a noticeable tendency for production of non-polar compounds, which may ultimately contribute to a more persistent form of soil water repellency with important implications for managing fire affected terrain. Abstract. Soil heating, as experienced during wildfires or management burns, can lead to extreme soil water repellency (WR). Previous work has focussed on the chemical composition of soil organic matter (SOM) that may be associated with WR in natural soil samples or samples heated in air. Under wildfires, however, oxygen supply is typically reduced and previous work has shown that the extreme WR induced under such conditions resists eventual destruction at temperatures ,200 8C higher than that of the same soil heated in air. This study examines, for the first time, the chemical changes in SOM associated with extreme WR following heating under oxygen limited conditions. Extracts obtained by accelerated solvent extraction (ASE), using mixtures of isopropyl alcohol/aqueous ammonia (IPA/NH 3 ) and dichloromethane/ methanol (DCM/MeOH), were analysed using gas chromatography–mass spectrometry (GC/MS). The data were compared with the SOM composition of the same soil unheated and following heating in air. In the absence of oxygen during soil heating, phthalic acid esters, substituted benzaldehydes, unsaturated amides and organophosphate esters were produced. In comparison with extracts of the same soil heated in air, there was a decreased methoxyphenol/phenol ratio, suggesting progressive demethoxylation and synthesis of new aromatic structures likely to promote extreme WR in soil. Additional keywords: amides, aromatics, eucalyptus, fire, phthalates. Received 4 October 2011, accepted 3 May 2012, published online 6 July 2012 Introduction It is well established that heating of soil, as experienced during severe wildfires or land management fires, may induce or enhance water repellency (WR) in the top few centimetres of soil (see reviews [1,2] ). This has been reported as a factor contributing to increased run-off, erosion, debris flow and flooding events that have occurred in many areas following fire and affecting post-fire vegetation recovery. [2,3] Enhanced WR is likely to arise from in-situ chemical changes in soil organic matter (SOM) by production of specific compounds of aromatic character derived from degradation of lignin, carbohydrate and lipids, and by alteration of amino acids. [1,3,4] In addition, condensation of volatilised components derived from litter on to underlying soil particles may also contribute to increased WR (see reviews [1,2] ). Relationships established between heating temperature (T) and WR for soils heated in air indicate that intense soil WR arises between 175 and 205 8C, but is typically eliminated following exposure above 270–300 8C, with some dependency on the exposure time at the scale of minutes. [5–7] However, availability of atmospheric oxygen is typically insufficient to support complete combustion during fires, as indicated by the ubiquitous presence of pyrolysed organic matter (OM) (char or black carbon) at burned sites. This material is formed from incomplete combustion of biomass in the limited supply or absence of oxygen and spans a continuum from partly charred plant residues to highly graphitised soot. [8–10] According to Knicker, [11] pyrolysed OM formed in vegetation fires is a heterogeneous mixture of heat-altered biopolymers with domains of relatively small polyaromatic clusters and considerable substitution with N, O and S functional groups. Recently it was shown that, in the absence of oxygen, the T required to induce maximum WR and its elimination in soil increases to between 400 and 510 8C, (again) depending on exposure time. [12] The structural changes in soil organic matter (SOM) compo- sition, in air, at T . 300 8C (the threshold for WR elimination in air), are thought to be associated with decarboxylation, an increase in the proportion of aromatic structures [13,14] and CSIRO PUBLISHING Environ. Chem. http://dx.doi.org/10.1071/EN11122 Journal compilation Ó CSIRO 2012 www.publish.csiro.au/journals/env A Research Paper