Neutron computed laminography yields 3D root system architecture and complements investigations of spatiotemporal rhizosphere patterns

Abstract

Purpose Root growth, respiration, water uptake as well as root exudation induce biogeochemical patterns in the rhizosphere that can change dynamically over time. Our aim is to develop a method that provides complementary information on 3D root system architecture and biogeochemical gradients around the roots needed for the quantitative description of rhizosphere processes. Methods We captured for the first time the root system architecture of maize plants grown in rectangular rhizotrons in 3D using neutron computed laminography (NCL). Simultaneously, we measured pH and oxygen concentration using fluorescent optodes and the 2D soil water distribution by means of neutron radiography. We co-registered the 3D laminography data with the 2D oxygen and pH maps to analyze the sensor signal as a function of the distance between the roots and the optode. Results The 3D root system architecture was successfully segmented from the laminographic data. We found that exudation of roots in up to 2 mm distance to the pH optode induced patterns of local acidification or alkalization. Over time, oxygen gradients in the rhizosphere emerged for roots up to a distance of 7.5 mm. Conclusion Neutron computed laminography allows for a three-dimensional investigation of root systems grown in laterally extended rhizotrons as the ones designed for 2D optode imaging studies. The 3D information on root position within the rhizotrons derived by NCL explained measured 2D oxygen and pH distribution. The presented new combination of 3D and 2D imaging methods facilitates systematical investigations of a wide range of dynamic processes in the rhizosphere.

Coupling non‐invasive imaging and reactive transport modeling to investigate water and oxygen dynamics in the root zone

“…The optode images represent a depth‐integrated response of O 2 dynamics across a variable portion of the root–soil system under consideration, which adds uncertainty regarding which portion of the root system triggers observed patterns at a specific point in time (Roose et al., 2016). As in a previous study using a comparable experimental set‐up, where O 2 uptake of roots growing within a distance of 7.5 mm to the optode was measured a few hours after irrigation of dry soil (Rudolph‐Mohr et al., 2021), we assumed that the roots located within this soil volume were sufficiently representative of the root system in the presented model, as we aimed to compare optode images and 2D model output. Though we find this a necessary simplification, the influence of the entire root system in terms of the observed O 2 depletion pattern merits further investigation, possibly in a 3D modeling approach.…”

“…Tilting the rotational axis of the sample reduces artefacts caused by insufficient neutron transmission through the laterally extended side of the sample (Helfen et al., 2011). In a previous experiment, we demonstrated the capability of NCL to study root systems grown in slab‐shaped rhizotrons and successfully tested the combination of NCL with 2D optode imaging (Rudolph‐Mohr et al., 2021). To image the RSA of the maize plants in 3D, we captured 600 projection images over an angular range of 300°, as well as 15 flat‐field and 15 dark‐field images, each with exposure time of 15 s, resulting in total acquisition time of approximately 4 h for NCL.…”

“…It was found that the spatial distribution of roots was skewed toward the front and back rhizotron window (Figure 4a), with the majority of roots located on the front side within 4–5 mm distance to the O 2 optode. In a previous study, it was determined that in a similar set‐up, O 2 consumption by roots in up to 7.5 mm distance to the optode (half the thickness of the rhizotron) could be detected in wet soil (Rudolph‐Mohr et al., 2021). Though it is recognized that water/solute/gas from anywhere in the rhizotron could move within the detection range of the O 2 optode, we assumed that because of the bisection observed in root growth on either side of the rhizotron, water, gas, and solute present beyond 7.5 mm from the O 2 optode would more likely be drawn toward the roots on the other side of the rhizotron (Figure 4a), and so the observed moisture content and O 2 evolution was most representative of soil/water/gas within 7.5 mm of the O 2 optode.…”

“…Noninvasive imaging techniques enable high‐resolution and spatiotemporally resolved monitoring of the distribution of soil moisture and root water uptake (RWU) (Tötzke et al., 2021; Zarebanadkouki et al., 2013), as well as the concentration patterns of gases and solutes, including exudates, in the rhizosphere (Blossfeld et al., 2013; Holz et al., 2019). Experiments combining neutron imaging and planar optodes have shown that the O 2 concentration in the root zone is closely related to the local water content (Rudolph‐Mohr et al., 2021, 2017). A critical next step to understand the transport, uptake, and reaction processes taking place in the root zone in more detail, is to use data gained from high resolution imaging experiments for the set‐up, calibration, and validation of numerical models (Roose et al., 2016).…”

Quantification of root water uptake and redistribution using neutron imaging: a review and future directions

“…Bimodal imaging, for instance, using neutrons and X‐rays (Kaestner et al., 2017) on the same sample offers information on soil structure, root distribution, and water distribution. Using neutrons and planar optodes (Rudolph‐Mohr et al., 2021) offers information on root architecture, water distribution, and soil oxygen and pH status simultaneously. Thus, the combination of different imaging techniques could be an ideal tool to explore such questions.…”