- 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 water uptake characteristics of soils, gaining information about water distribution and mobility as well as pore size distribution. Recent NMR studies in humous soil samples revealed relevance of swelling and wetting processes of soil organic matter (SOM), as well as microbial influences on 1H-NMR relaxometry. The objective of this investigation was to achieve first indications to which extent microbial activity and quantity of bacteria affect 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 to 43relaxation time distributions were at 2 MHz (Maran 2, Resonance, UK). Microbial respiratory activity was determined with a Respirocond system (Nordgreen Innovation, Norway) detecting conductivity changes of KOH-solution caused by CO2 absorption. Total cell counts were determined by DAPI staining (4,6-diamidino-2-phenylindol) after bacterial extraction with a Na4O7P2-solution. The initial relaxation time distribution of all samples showed up to three peaks (see figure). During hydration, the number of peaks decreased, 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 that changes in the pore size distribution and in spin relaxation mechanism are responsible for the shifts in the relaxation time distribution. This can be due to wetting and swelling of SOM and increasing numbers of paramagnetic centres on surfaces and in the bulk phase. In addition, production and release of extracellular polymeric substances and bacterial association within biofilm may form new pore systems and reduce interparticular pore diameter.
