Hot-Water-Soluble Organic Compounds Related to Hydrophobicity in Sandy Soils

Soil water repellency (WR) reduces mineralization of soil organic matter (SOM) and has the potential to sequester soil carbon. However, predicted climate change events such as decreased rainfall and droughts can cause changes in WR influencing water storage and plant productivity. Hot-water-soluble carbon (HWSC) is a sensitive indicator of ecosystem changes. It contains binding agents influencing soil aggregate stability, which is mainly controlled by soil WR levels. Here we characterize the link between WR and the organic compounds composition in hot water extracts from accelerated solvent extraction (ASE) of sandy soils. Extracts were lyophilized, fractionated and measured by GC/MS. Dominant compounds were phenolic acids, short chain dicarboxylic acids (C-4-C-9), saccharides, glycosides, (C-8-C-18) fatty acids, and esters of oleic, stearic and palmitic acids. We speculate that the complete elimination of WR by hot water was due to: (i) critical quantity of HWSC extracted, necessary to disaggregate soil particles; (ii) removal of sugars and aromatics; (iii) removal of hydrophobic fatty acid esters (C-16 and C-18); (iv) enhanced desorption of complex DOC fractions in water at high T and pressure; (v) exposure of greater proportion of hydrophilic sites. The polarity and aromaticity of HWSC can play a critical role in stabilization and destabilization of soil organic matter (SOM), particle wettability and C dynamics in soils.