Inhaltszusammenfassung

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. 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 chemicals. These ...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. 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 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 (6). In the study presented here, we investigated the process of drying and rewetting in selected humic soil samples. We focussed on the mobility of water molecules and the properties of dissolved organic matter. The soil samples were observed before and after drying and during cycles of re-wetting. As observed by NMR Relaxation, the state of mobility of water seems to change slowly gradually during re-wetting. This can be explained by slow changes of the pore size distribution within the sample, induced by swelling of soil organic matter. The results show that this pro-cess- is slow, with time constants of up to 6 days and thus indicating a high ecological relevance. Also, strong effects were observed in soil extracts obtained from soil samples in different states of moisture. The state of moisture affected the surface tension and the amount and quality of the dissolved organic matter, which was indicated by the molecular size distribution, by the UV absorption of the size fractions, and probably also by the particle size distribution itself. From these observations, we conclude that the state of moisture of humic soil samples strongly affects many physicochemical characteristics which are relevant for transport processes, including both solute and colloidal transport. Swelling of SOM is an important factor for understanding many ecologically relevant processes in humic soil layers, especially when processes under dynamic moisture conditions in field studies are in the center of interest. 1 S. Altfelder, T. Streck and J. Richter, J. Environ. Qual. 1999, 28, 1154. 2 E. J. LeBoeuf and W. J. Weber Jr., Environ. Sci. Technol. 2000, 34, 3632. 3 E. J. LeBoeuf and W. J. Weber Jr., Environ. Sci. Technol. 2000, 34, 3623. 4 G. E. Schaumann, C. Siewert and B. Marschner, J. Plant Nutr. Soil Sci. 2000, 163, 1. 5 G. E. Schaumann and O. Antelmann, J. Plant Nutr. Soil Sci. 2000, 163, 179. 6 G. E. Schaumann and J. Hurraß, Mitteilgn. Dt. Bodenkundl. Gesellsch. 2000, 92, 25- 28» weiterlesen» einklappen

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