Properties of soil organic matter and aqueous extracts of actually water repellent and wettable soil samples

The occurrence of water repellent spots can inhibit a homogeneous wetting progress in soil. Although the wettability is an important factor for sorption and transport processes, the knowledge about the reasons for water repellent behavior and its effects on other soil properties is still insufficient. In this study, water repellent and wettable soil samples from two urban locations were compared. It was examined, to which extent differences in the actual water repellency of closely neighboring spots are associated to variable factors like the water content or to stable properties of soil organic matter (SOM). In order to analyze the interrelations between the moisture status and the wettability behavior, soil samples were either subjected to drying–remoistening cycles or pre-conditioned under various conditions. For the characterization of stable SOM properties, the soil samples were investigated by ESEM (Environmental Scanning Electron Microscope) and FT-IR spectroscopy. Beside the investigation of solid soil samples, selected properties of aqueous soil extracts were measured. The results of this study indicate that the water content is not the only factor responsible for the differences in actual water repellency. For different locations, the wettability behavior probably is based on different mechanisms. For one of the examined sites, differences in actual repellency of closely neighboring samples were interrelated with differences in pH and the ionic strength of the soil solutions. For both locations, amphiphilic substances probably are an important factor of influence for the wettability behavior. Additionally, the results suggest that the time which proceeds after a change of the moisture status in soil is an important factor for water repellency.