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Top soil layers are influenced by strong changes in moisture, which affect sorption and transport processes for e.g. pollutants. 1H-NMR relaxometry may be used as a method to determine wa-ter uptake characteristics of soils, gaining information about water distribution and mobility as well as pore size distribution. The principle of this method is to observe changes of the longitu- dinal or transversal magnetization due to spin relaxation of protons back to equilibrium after e...Top soil layers are influenced by strong changes in moisture, which affect sorption and transport processes for e.g. pollutants. 1H-NMR relaxometry may be used as a method to determine wa-ter uptake characteristics of soils, gaining information about water distribution and mobility as well as pore size distribution. The principle of this method is to observe changes of the longitu- dinal or transversal magnetization due to spin relaxation of protons back to equilibrium after excitation within an external static magnetic field. The changes of magnetization can be de-scribed using exponential functions with time constants T1 and T2 (longitudinal and transversal relaxation times, respectively). In porous media, one obtains a multitude of relaxation times, which are displayed as relaxation time distributions (RTD). The correlation between the meas-ured relaxation time T1,2 and the pore size can be described with (Hinedi et al., 1997). T1,2 is the relaxation time of water confined within a pore, averaged from the bulk relaxation time (T1,2B) and the surface relaxation time (T1,2S). l is the layer thickness, in which T1,2S takes place and l/T1,2S the surface relaxivity (r1,2), S0 the water covered pore surface and V0 the water filled pore volume. Recent studies (Schaumann et al., 2004; Todoruk et al., 2003) revealed relevance of swelling and wetting processes of soil organic matter (SOM), but also assumptions of microbial influences on 1H-NMR relaxometry. The goal of this investigation was to achieve first indications to which extent microbial activity and quantity of bacteria affect T2 relaxation time distribution during rewetting of humous soil samples. We used a humous forest soil sample and added cellobiose to selected samples to enhance microbial activity (treated samples). Treated and untreated samples were moistened and investigated at several points of time during 3 weeks. The RTD of all samples showed up to three peaks. During hydration, the number of peaks de-creased, and the peaks revealed significant movement towards lower relaxation times. Microbial respiratory activities were highest after 1-3 days of hydration, with values 2 to 15 times higher in the treated as compared to the untreated samples. Total cell counts increased in all samples from 1 to 5 x 109 cells/g. We assume changes in the pore size distribution as well as of the spin relaxation mechanisms are responsible for the shifts in the RTD of the untreated samples. This can be due to wetting and swelling of SOM and increasing numbers of paramagnetic centers. In addition, we found indications that soil microorganisms may have a further substantial effect on the RTD.» weiterlesen» einklappen

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