Neutron Imaging of Plant-Soil Systems at Oak Ridge National Laboratory.

Plant water relations have been extensively studied over the last century. In particular, aboveground plant response to drought has been monitored in situ using innocuous measurements of foliar water release. Non-destructive analysis of belowground dynamics has not been similarly achieved since the nature of analysis requires indirect or invasive, and often destructive measurements. Recently, advances in imaging techniques (X-ray, MRI, neutron) have been able to provide non-invasive imaging capabilities of soil-root systems in-situ. Of these techniques, neutron imaging has been less commonly used, but has demonstrated excellent potential for further development based on recent work at the JRR-3M facility in Japan, the PSI NEUTRA facility in Switzerland, and the NIST and MNRC facilities in the USA. Neutrons are sensitive to light atoms such as Hydrogen and can provide high spatial resolution (~50 microns or better) at relevant temporal scales (seconds, minutes, hours) for analysis of soil-plant water systems. A neutron imaging program has been initiated at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. Preliminary Time-Of-Flight (TOF) neutron imaging experiments were conducted in 2008 on plant and soil samples at the Spallation Neutrons And Pressure (SNAP) diffractometer beamline of SNS. Experiments will continue at the High Flux Isotope Reactor (HFIR) using a monochromatic beamline. The HFIR prototype imaging beamline also has white beam capabilities (cold neutron range). Plant species examined included switchgrass, maize, grape and poplar. Plants were successfully cultivated in sand in aluminum chambers, with up to 50% D2O. Fine leaf veins (<200 um) and roots were readily visible using a 1.7 cm2, <50 um resolution detector. Initial results suggest the importance of selective neutron energy to optimize the image contrast; better contrast was achieved in the cold energy range ( 3-4 angstroms). Current experiments are focusing on water flux through porous media, including soil-plant systems.