- Under field conditions, the upper soil layers are exposed to dynamics of moisture and temperature. In the course of change of moisture, soil organic matter (SOM) changes its water content and its state of swelling. This affects especially physicochemical properties of SOM. In contrast to the SOM of naturally grown soils, SOM of strongly anthropogenically influenced soils has often been less adapted to the climatic conditions. This is also true for the anthopogenically influenced soil flora, fauna and microbiology, which themselves also react on moisture dynamics. Recent results show that, with the state of swelling, SOM gradually changes its physicochemical properties, such as sorbent properties, macromolecular structure, thermal characteristics or the binding of hydrophobic organic chemicals. 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. In this contribution, we will give an overview on effects of drying and re-moistening on physicochemical properties of soil organic matter of a forest soil and anthropogenically influenced soils. We focus on the evolution of pore size distribution, macromolecular structure, colloidal composition and binding of contaminants. Water uptake is probably occuring in at least two steps. While wetting can occur within wide time ranges, swelling is found to be slow, lasting for up to several weeks. During swelling, effects on the glass transition behaviour, on colloidal composition in the soil solution and on the extractibility of contaminants were observed. We also found strong differences between hydrophobic and wettable soils. Hence, the kinetics of swelling and wetting of SOM are important factors for understanding ecologically relevant processes in organic soil layers, especially when processes in field studies are in the center of interest. They have to be taken into consideration for understanding sorption and transport phenomena in urban areas as well as structure and relevance of solid SOM. Due to this relevance, swelling and wetting of SOM opens a wide and challenging field of research with high demand.
