T1/T2- and Diffusion measurements in soil samples to characterise the pore size distribution and the effects of swelling of soil organic matter on it
NMR-DIFF Wageningen/EU: WNMRC06-006
Top soil layers are influenced by strong changes in moisture, which affect sorption and transport processes. 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 1H-NMR studies in humus soil samples revealed relevance of swelling and wetting processes of soil organic matter (SOM), but also assumptions of microbial influences on 1H-NMR relaxometry. In this project we will quantify the effect of microorganisms on the relaxation time distribution and in how far (swelling of) SOM and microorganisms affects the determined pore size distribution. We hypothesize that swelling of SOM as well as soil microorganisms effect the pore size distribution and self diffusion of water molecules in soils.
In a pilot study at WNMRC, we characterized two sandy soils with different SOM contents in terms of T2, T1 and diffusion behaviour in the curse of SOM swelling. In both sandy soil samples we found:
1. Relaxation time distributions with one to three peaks for T1 and two to four peaks for T2.
2. Good T1/T2 correlations. In the low SOM content soil, T1 equals two to three times T2. In the high SOM content soil, T1 equals four to six times T2.
3. Relaxation times ranged from 1 ms to 1000 ms for T1 and from 0.1 ms to 500 ms for T2.
4. Shifts of peaks towards smaller T1 and T2 times in the curse of SOM swelling.
5. Two water components with different self diffusion coefficients Dfast=3 to 4x10-9 m2s-1 and Dslow= 10-12 to 10-9 m2s-1. Dfast is associated with peaks between 10-100 ms and Dslow with peaks between 1-10ms. Dslow values appeared as a broad band in the D/T2 diagram, representing evaluation problems due to the combination of small T2 and slow diffusion.
The results show the potential of proton NMR relaxation for the investigation of soil samples. For successful application in soil science, it is necessary to validate the interpretation of the results with respect to the huge heterogeneity of soil samples, including high amounts of paramagnetic compounds. Furthermore, improvement of the measurements is needed for the evaluation of small T2 and slow diffusion.
The aims of this full project study (Start in spring 2007) are to investigate for a broad range of different soils pore size distributions and water self diffusion in soil samples during swelling of SOM using T1, T2 and diffusion measurements. In this project, we will apply an improved probe with a more sensitive rf coil (over factor 2) and stronger gradient (again over factor two) to gain a better access to the water with slow diffusion coefficients. In that new probe, we also have temperature control (in the range -5 to around 40 C) and will perform temperature dependent measurements to investigate kinetics of swelling and wetting processes in soils. We will use a broad range of soil samples with different textures as well as SOM contents. These results will be combined with data on composition, particle size distribution and matric potential gained within our current research project (DFG HUMUS H-Relax). This knowledge will help to validate the interpretation of the NMR results of the soil samples.
Prof. Dr. Gabriele E. Schaumann (Koblenz)
Dr. Henk van As (Wageningen)
Dipl.-Ing. Fabian Jaeger