ABSTRACT
Under field conditions, the upper soil layers are exposed to variations in moisture and temperature. In the course of moisture fluctuation, the soil organic matter (SOM) changes its water content and its state of swelling. Recent results show that with the state of swelling, SOM gradually changes its physicochemical properties, such as sorbent properties [1,2], macromolecular structure [3,4] thermal characteristics [5] or the binding of hydrophobic organic compounds. These changes are well-known to affect sorption and transport phenomena and thus have to be taken into consideration. Despite this relevance, very little is known about the process of swelling of SOM, and even less is known about its effect on SOM physicochemistry [6], This may be due to methodical challenges. In order to observe and describe swelling, SOM has to be maintained in its original state, which means that SOM has to be investigated within the whole soil sample. An extraction of humic fractions would change the composition and the macromolecular structure of SOM, both of which are important factors for the process of swelling. Methods applied to investigate swelling of homogeneous materials cannot easily be transferred to highly heterogeneous soil samples. Thus, the following questions arise: Which methods can be used to describe, quantify and understand swelling of SOM in soil samples containing organic matter? How fast is swelling and which swelling kinetics can be observed? Which physicochemical properties of soil and SOM are affected by swelling of SOM? To what extent does swelling affect sorption and transport phenomena?
