PS 15-218 - Novel developments in neutron imaging of root and rhizosphere water dynamics in vivo

Abstract: Plant-soil exchanges of carbon, nutrients and water are critical processes that have not been fully elucidated due to difficulty in directly accessing and quantifying their dynamics under changing edaphic conditions. The primary goal of this research is to improve representation of root function in earth system models. The belowground portion of the plant system is typically treated as a black box, but new efforts to understand mechanistic soil-root processes are underway, including development of global root trait databases that can be used to parameterize and test the models. This project was focused on understanding root-specific water uptake across species, and if we can assess these uptake dynamics in 3D.

Using plant-soil samples placed into a neutron beamline, rhizosphere and root water dynamics were assessed in vivo via attenuation-based 2D radiography or 3D computed tomography (CT). Results with maize, switchgrass and woody plants, indicate variability in water dynamics across the soil-rhizosphere-root pathway, including root water uptake and hydraulic redistribution, hysteresis in water release curves and soil wettability. Measured root water extraction rates by cottonwood ranged from 0.003 to 0.02 g cm^-2 h^-1, with lower rates for larger roots. Across species, the root rhizosphere thickness increased with root size, stabilizing as roots reached ~2 mm in diameter. Analysis required development of a novel image analysis software to identify and segment roots, and analyze root, rhizosphere and soil water dynamics. These experiments also indicated significant root and mycorrhizal impacts to soil hydraulic parameters, including hydraulic conductivity and residual water content. Initial testing of CT imaging of water dynamics using ultra-fast tomography illustrated that timing of experiments can be reduced from hours to minutes. A particular future focus is to measure pathway resistance between soils and roots to understand the development of gaps across the rhizosphere that suddenly limit root water availability during drying, then leverage results to inform plant-hydraulics models, such as the US Department of Energy’s E3SM FATES model. New capabilities are planned, including the VENUS beamline that will enable advanced root imaging (https://neutrons.ornl.gov/users). A key objective for future research is to expand neutron imaging capabilities to assess symbiotic root-microbial interactions and critically, nutrient or ion uptake dynamics.