Neutron radiography and tomography of water distribution in the root zone

Water uptake by roots and resulting water redistribution along the soil profile depend on soil hydraulic properties and root distribution, as well as on the physical, chemical, and biological soil plant interactions that occur in the root zone. The hydraulic properties of the soil in the root zone are difficult to investigate in situ at the needed high spatial resolution, and they still present important open questions. For instance, is there more or less water at the root soil interface compared to the bulk soil? Neutron radiography (2-D) and tomography (3-D) are efficient methods to answer such questions, providing the possibility to image simultaneously water distributions and root structure in situ at high spatial resolution. We planted a lupin and a maize in rectangular boxes filled with sandy soils. The plants were grown for 3 weeks at controlled conditions. Infiltrated water and subsequent water redistribution were imaged for 5 d at regular intervals by means of neutron radiography and tomography. Soil water-content distributions were quantified from the radiographs after correcting for neutron scattering. The radiographs showed that the water content in the root zone was higher than in the bulk soil both during and after infiltration Similarly, the tomograms showed localized regions of high water content around some locations of the roots, in particular near the tips of the lupin. Local regions of water depletion, which are expected as a consequence of water uptake, were visible along the main root where laterals branched. These results reflect the complexity of soil plant water relations, showing the different properties of bulk soil and root zone, as well as the varying moisture gradients along the root system.