Relating Spatiotemporal Rhizosphere Patterns to 3D Root System Architecture

Dynamic processes occurring at the soil-root interface crucially influence soil physical, chemical, and biological properties at local scale around the roots that are technically challenging to capture in situ. Combining 2D optical fluorescence imaging, neutron radiography (NR) and 3D neutron computed laminography (NCL), we developed a new imaging approach capable of simultaneously quantifying H₂O-, O₂-, and pH-distribution around living plant roots in soil while additionally capturing the root system architecture in 3D. The interrelated patterns of root growth and distribution in soil, root respiration, root exudation, and root water uptake can be studied non-destructively at high temporal and spatial resolution.

We analyzed rhizosphere patterns of young maize plants grown in soil-filled rhizoboxes (15 x 15 x 1.5 cm³). Optodes were attached to opposite inside glass walls of the rhizoboxes to capture pH and O₂ maps by fluorescence imaging while the water distribution was measured by NR. Complementary, NCL, a tomographic approach specially adapted to samples with large lateral extension, was applied to visualize the 3D root system architecture. NCL made it possible to quantify the distance of roots from the container walls and thus from the optodes, which enhances the interpretation of observed pH and oxygen patterns.

The older part of the root system with higher root density was associated with fast decrease of water content and rapid change in oxygen concentration. Lateral roots acidified their rhizosphere by a quarter of a pH unit and crown roots even induced acidification of up to one pH unit compared to bulk soil. The benefit of NCL is that we can extract the root structure in 3D, identify root age and root types and relate this to spatiotemporal changes in water content distribution, oxygen concentration and pH values.