Julia Hurraß, Gabriele Ellen Schaumann

• The dynamics of precipitation and the resulting changes of soil moisture strongly affect the physico-chemical characteristics of SOM , , . This interrelation between moisture status and soil properties has to be known in order to understand transport processes in soil. Often, varying hydrophobic and wettable regions arise from the changing moisture conditions in soil. The immobilization grade of water is an important influencing factor for transport processes in soil. Recent results show that the immobilization grade of soil water is exposed to permanent dynamics because of the changes of moisture conditions. In the presented study, phase transitions of soil water in wettable and hydrophobic soil samples were investigated with Differential Scanning Calorimetry. The parameters, that describe the phase transitions, can be used as indicators for the water binding in soil. Reactions of the freezing and the evaporation process on changes of the moisture conditions were observed. The results of this study show that in hydrophobic soil samples a part of the water does not participate in the freezing process. A fraction of the soil water seems not to be available as free water, but to be strongly bound to the soil matrix. In hydrophobic samples the temperature range of the melting and the freezing process is significantly larger than in wettable samples. In addition, the shapes of the freezing and the melting peaks show that the immobilization grade of water in hydrophobic samples spreads over a larger range than in wettable samples. Slight differences of the Onset-temperature of the freezing and melting peaks also hint on a greater immobilization of water in hydrophobic soil samples. There are differences of the evaporation process between wettable and hydrophobic samples, too. Probably, these differences can be explained likewise with the immobilization grade of the soil water and with the surface properties of SOM depending on the moisture conditions.