Quantitative imaging of sodium concentrations in soil-root systems using magnetic resonance imaging (MRI)

Aims: Demonstrating the potential of MRI as a 3D, non-invasive and continuous measurement technique to map Na⁺ concentration distributions in soil and around roots. Methods: Dissolved NaCl in soil and soil-plant systems was mapped by 3D ²³Na-MRI. The lower limit of detectability in saturated and unsaturated porous media was evaluated, followed by evaporation experiments to test the quantification. Finally, Na⁺ enrichment around tomato roots, irrigated with saline solution under low/high transpiration rates (LT, HT), was imaged in parallel to the root system,. Results: A spin echo pulse sequence allowed the quantitative mapping of the volume concentration of NaCl in sandy porous medium. Evaporation experiments showed slight enrichment in the top surface layer, plus uniform temporal enrichment in the deeper layers. In the tomato experiments, enrichment was more distinct under HT than under LT. Concentration-distance correlation curves revealed thin enrichment zones ranging a few mm around the roots. Conclusions: MRI can map Na⁺ non-invasively in 3D at relevant concentrations for root activity. Visualizing water content, roots and Na⁺ on the same scale is possible, despite limitations of different scanning times and resolution. This opens a route for further quantitative investigations of salt enrichment processes in soil and soil-plant systems.