Applications of Neutron Imaging In Soil-Water-Root Systems.

Neutron imaging provides an exceptional non-invasive tool for studying root architecture and water distribution in root-soil systems in situ and without interfering with the root-zone processes. This unique property of neutron imaging results from the high sensitivity of neutrons to hydrogen nuclei. Since water and roots are the main hydrogen-bearing materials in soil systems, they both can be visualized. The neutron attenuation coefficient of water is an order of magnitude larger than that of most soils; therefore soil water content distributions can be quantified from neutron images with high precision. Depending on whether neutron radiography or tomography is performed, a two-dimensional or three-dimensional image of the root-soil system is obtained. Neutron radiography can be performed in an order of seconds; therefore, quasi-real time dynamics of water in the system can be obtained. Neutron tomography, however, requires typically a few hours. Spatial resolution in neutron images is usually in the range of 100-300 µm, but for small samples can be finer. Rapid advancements in imaging and processing tools have helped both visualization and quantification, and along with increased availability of neutron beams, have made this method viable in the recent years. These improvements can be expected to continue leading to increasing numbers of studies with this method. At present the method is limited to laboratory scale and access to neutron imaging facilities. In this chapter, we provide a comprehensive overview on the methodological approach and application of neutron imaging in soil science, particularly on root growth, root water uptake and soil hydraulic properties.