Probing Soil-Root Heterogeneity by Nuclear Magnetic Resonance (NMR).

Soils as heterogeneous systems possess a broad range of functions in the environment, most of them related to water content and fluxes. Among the non-invasive techniques, nuclear magnetic resonance (NMR) methods are particularly convenient since they probe directly the substance of interest: Water. Besides spectroscopy, NMR can be applied in soils as relaxometry to obtain volume-averaged information about dynamics of water in pores which allows further conclusions about pore sizes and connectivity. Furthermore NMR is used for imaging (MRI), which can resolve relaxation times spatially by using multi-echo pulse sequences. The drawback of the so-called spin echo sequences is the requirement of long echo times greater than some milliseconds so that fast relaxation processes are already decayed [1]. In this contribution, conventional relaxation time maps obtained by MRI of soil – root regions are extended by local NMR relaxometry measurements using microcoils. Due to an encapsulation they are placed directly into the soil at different positions in the neighborhood of the roots. Therefore the common rf-coil for volume excitation in a Halbach magnet is replaced by the microcoil which probes an area of few mm2. With this setup echo times of < 0.2 ms are now achievable and the window of possible relaxation times is extended on the short time scale.