Magnetic Resonance Imaging of Soils and Root Soil Interactions.

Magnetic Resonance Imaging (MRI) is a most versatile imaging technique for visualization processes in soil, roots and their mutual interactions. It bases upon the nuclear magnetic resonance effect of (mostly) water protons in an external magnetic field and analyzes the spatially resolved signal after excitation. By the choice of appropriate parameters in combination with knowledge about the specific relaxation properties of soil material and roots MRI allows visualization of root features like anatomy and architecture, water content distribution in the surrounding soil, and fluxes in soil and roots. Mostly these fluxes are relatively slow, but they can be visualized by monitoring the transport of contrast agents. For our studies we have chosen Gd-DTPA for reasons of its chemical inertness and conservative transport properties which are proven by batch equilibrium experiments and by comparison of forward modeling with MRI for transport in a model soil core. This agent accelerates local relaxation times and therefore MRI can be made either tracer-sensitive by using short repetition time tR or moisture-sensitive by long tR while keeping the echo delay tE very short. Furthermore, quantification of tracer concentrations in the porous media is possible by determination of the specific relaxivity parameters. This enabled us to investigate local tracer redistribution in the topmost soil layer during evaporation. Here lateral fluxes from high to low conductive compartments were observed which were predicted independently from numerical simulation. In a third series the tracer method was applied to lupinus albus plant grown in a sandy soil under steady-state irrigation. GdDTPA is taken up rapidly but it is also enriched homogeneously in the surrounding soil. This proves that its uptake is hindered, but also that the redistribution in bulk soil is sufficiently high to prevent strong local enrichment around the roots.