Quantitative Imaging of Infiltration, Root Growth, and Root Water Uptake via Neutron Radiography

Water infiltration into vegetated soils is affected by interactions between soil properties and plant activity. Water uptake by plant roots depends on the soil hydraulic properties and infiltration rate. In turn, roots are important water movers and induce nonuniform water content distributions with consequent impact on the infiltration rate. Our goal was to use neutron radiography to investigate root water uptake during rapid infiltration events and subsequent water redistribution. Neutron radiography is an imaging technique capable of measuring with high spatial and temporal resolution root density, soil structure, and water distributions in the vicinity of roots. We used time-series neutron radiography to image infiltration events in 0.17- by 0.15- by 0.013-m boxes filled with a sand mixture and that contained either lupine (Lupinus L.) or maize (Zea mays L.) plants. We measured four replications at two stages of plant growth. Neutron radiography can detect small variations in water content with high accuracy and high spatial and temporal resolution. Due to their high water content, roots were easily visible in the radiograms before infiltration. Following calibration, we quantified the water content in the sample during and after each infiltration event. Effects of soil heterogeneities such as thin soil layers were visible, as was water uptake by roots. We observed lower water contents along the main root of lupine during the early stage of infiltration, and slower and more diffuse uptake during the later stages of water redistribution, when the wetting front had propagated downward and the water content in the soil decreased. The observed root water uptake was not uniform but rather was localized in certain regions. For maize we observed a loss of water mainly in the upper part of the root system, closer to the stem rather than at root tips.