Organic compounds at different depths in a sandy soil and their role in water repellency

Abstract

The causes of soil water repellency are still only poorly understood. It is generally assumed that hydrophobic organic compounds are responsible, but those concerned have not previously been identified by comparison between samples taken from a water repellent topsoil and the wettable subsoil. In this study we separated, characterised, and compared the organic compounds present at 4 different depths in a sandy soil under permanent grass cover that is water repellent in the upper 0.30 m but wettable below this. Soil samples were extracted using a mixture of isopropanol and aqueous ammonia (7 : 3 v : v). Samples were wettable after extraction and re-application of the extract from each sample onto wettable sand induced water repellency. The chloroform-soluble portions of the extracts were analysed by gas chromatography and gas chromatography-mass spectrometry. The compounds identified at all soil depths included long-chain carboxylic acids (C16–C24), amides (C14–C24), alkanes (C25–C31), aldehydes or ketones (C25–C29), and more complex ring-containing structures. 1H and 13C nuclear magnetic resonance spectroscopy, and the carbon/hydrogen ratio as determined by microanalysis, confirmed the predominantly aliphatic character of the extracts. Both wettable and water repellent samples contained hydrophobic compounds. The 3 water repellent samples contained far more organic material, although the amount extracted was not related to the degree of water repellency. Perhaps more importantly, they contained polar compounds of high relative molecular mass, which were almost absent from the wettable subsoil. It may be speculated that these are the compounds in this soil whose presence in significant amounts is necessary for water repellency to be exhibited.