Predicting soil water repellency using hydrophobic organic compounds and their vegetation origin

It is widely accepted that soil water repellency (SWR) is mainly caused by plant-derived hydrophobic organic compounds in soils; such hydrophobic compounds are defined as SWR markers. However, the detailed influence of SWR markers on SWR is yet unclear and the knowledge of their original sources is still limited. The aims of this study are to select important SWR markers to predict SWR based on their correlation with SWR and to determine their origin. In our study, sandy soils with different SWR were collected, along with their covering vegetation, i.e. plant leaves/needles and roots. A sequential extraction procedure was applied to the soils to obtain three organic fractions: dichloromethane (DCM)/MeOH soluble fraction (D), DCM/Me0H insoluble fraction of isopropanol/ammonia solution (IPA/NH3) extract (AI) and DCM/Me0H soluble fraction of IPA/NH3 extract (AS), which were subdivided into 10 dominant SWR marker groups: D fatty acid, D alcohol, D alkane, AI fatty acid, AI alcohol, AI omega-hydroxy fatty acid, AI alpha,omega-dicarboxylic acid, AS fatty acid, AS alcohol and AS omega-hydroxy fatty acid. Waxes and biopolyesters of the vegetation were also sequentially extracted from plants. The soils with higher SWR have significantly higher relative concentrations of AS alcohols. A number of indications suggest that AS alcohols are mainly derived from roots and most likely produced by microbial hydrolysis of biopolyesters (mainly suberins). In addition, the strong correlation between the biomarkers of plant tissues and SWR markers in soils suggests that it is more accurate to predict SWR of topsoils using ester-bound alcohols from roots, and to predict SWR of subsoils using root-derived omega-hydroxy fatty acids and alpha,omega-dicarboxylic acids. Considering the sandy soils studied here, the relationships we obtained need to be tested for other types of soils. Our analysis indicates that plant roots have a primary role influencing SWR relative to plant leaves.