Nuclear Magnetic Resonance Imaging of Soil Processes: Water Flux, Tracer-Transport, and Root Water Uptake
(ICG-4), Research Centre Jülich.
Magnetic Resonance Imaging (MRI) is a powerful technique to study water content, dynamics and transport in natural porous media. In contrast to medical purposes, for which MRI has been mainly developped in the past, MRI in soils and other natural porous media is complicated by lower water contents and faster relaxation times. This requires the application of dedicated scanners which allow the application of shorter echo times and faster switching of gradients as well as special ultrashort pulse sequences like single point imaging derivates (SPI, SPRITE)(1) and multi-echo methods which monitor series of echoes and allow for extrapolation to zero time(2). A further possibility is the usage of low field scanners, since the transversal relaxation time increases with decreasing field strength. This partly compensates for the reduced signal intensity compared to conventional high-field scanners.
Of special interest in soils is the imaging of flux processes. This can be performed by two ways: i) Imaging of absolute of water contents and changes due to root water uptake. Here we could show at the example of maize that during increased drought stress zones of enhanced water content persist in the immediate vicinity around the roots. ii) Slow flux processes can be visualized indirectly by tracer monitoring(3,4). This is illustrated at the examples of root water uptake of lupin and maize, and for the visualization of preferential pathways in natural soil cores.
1) Pohlmeier et al. Vadose Zone J. 7, 1010-1017 (2008)
2) Edzes et al., Magn. Res. Imag. 16, 185-196 (1998)
3) Herrmann, Pohlmeier, et al. J. Env. Quality 31, 506-514 (2002)
4) Pohlmeier et al. Magn. Res. Imag. 7, 285-292 (2009)