- In the field, soil components are subjected to subsequent changes of their water content. These are expected to alter especially physicochemical properties of SOM. Despite this relevance, very little is known about the process of hydration of SOM, and even less is known about its effect on SOM physicochemistry. In this study, water sorption on samples with hydrophobic and hydrophilic surfaces was investigated. We focussed on hydration and drying and their effects on the evolution of water binding, pore size distribution, water repellency and SOM macromolecular structure. The hydration of peat and of Ah samples with differing water repellency and swelling capacity is presented and discussed. While wetting can occur within wide time ranges, swelling was 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 found strong differences between hydrophobic and wettable soils. The results also point at different forms of water binding in organic matter, which depend on the water content and the state of swelling. 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. Hence, water incorporation and 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. These changes are well-known to affect sorption and transport phenomena and thus have to be taken into consideration.
