Christian Tötzke

Background and aims: Root system architecture (RSA) shapes biogeochemical concentration patterns in the rhizosphere. Root-soil studies are often conducted on plants cultivated in rectangular rhizotrons, including when using 2D hydrochemical analysis methods. However, roots naturally expand in three dimensions, with the rhizosphere extending accordingly. Three-dimensional neutron imaging can enhance interpretation of such studies, yet imaging flat, slab-shaped rhizotrons is technically challenging. This study presents a methodological comparison between conventional neutron tomography (NT) and neutron computed laminography (NCL) to assess whether NT under high-flux conditions can achieve image quality sufficient for 3D root segmentation, comparable to NCL, without requiring tilting of the rotation axis. Methods: NT and NCL were applied to maize plants grown in rectangular rhizotrons. Imaging artifacts and their impact on root segmentation were assessed for two plants representing low and high soil moisture conditions suitable for neutron imaging. Results: Both methods produced 3D tomograms of comparable quality across the tested moisture range, enabling effective segmentation of primary and seminal roots. Lateral root detection was more challenging and depended on soil moisture. NCL captured a greater number of horizontally oriented lateral roots while NT was more effective in resolving vertically oriented roots. Conclusions: NCL is not required to resolve 3D RSA of maize plants in flat rhizotrons. Under high-flux neutron beam conditions, NT is preferable as it simplifies sample handling, reduces plant stress, avoids soil water redistribution and enables direct integration with timeseries of 2D chemical and neutron radiographic imaging.

The continuous input of microplastics into terrestrial environments is altering the physico-chemical properties of soils. The wide variety of microplastic particles in terms of particle shape, size, polymer type and additives makes microplastic pollution a multifaceted problem. Recent research efforts aim to improve the mechanistic understanding of how microplastics change soil structure and function and how this affects plants and other soil biota. A number of analytical detection methods are now available, but these typically involve sampling or processing steps that destroy the integrity of the sample. As a result, essential information about the soil structure and the spatial distribution of microplastics in the sample is irretrievably lost. Non-invasive tools are needed to directly study the interplay between microplastic particles and the 3D structure of the soil matrix. We introduce a combination of neutron and X-ray tomography as a non-invasive method capable of detecting and localizing microplastic particles in sandy soils (by neutrons) and simultaneously analyzing the 3D microstructure of the surrounding soil (by X-rays). The feasibility and limitations were tested in a series of sandy soil samples containing organic matter and microplastics of different plastic types and shapes, including particles, films, or fibers. Pretreatment with H2O2 was tested to facilitate the image analysis for samples with higher organic content.Our three-dimensional imaging approach can provide detailed information about the spatial distribution of microplastics in the sample and can reproduce the size, shape, and orientation of particles, although it cannot distinguish between plastic types. Visualization of embedded polyethylene film fragments as well as fibers revealed perturbations in the soil matrix that can clearly affect its hydraulic and mechanical properties. Finally, we analyzed microplastics in the spatial context of plant-soil interactions for the root system of a lupine plant, demonstrating that it is also an attractive tool for in-situ studies of soil microplastic effects on plant roots. Overall, this approach offers the opportunity to study the impact of microplastics on soil hydromechanical properties, the interaction of biota with microplastics, and possibly also microplastics local fate in sandy soil, albeit not as a screening or high-throughput tool, but suited as powerful tool for dedicated process studies.

Oxygen (O ₂ ) availability in soils is vital for plant growth and productivity. The transport and consumption of O ₂ in the root zone is closely linked to soil moisture content, the spatial distribution of roots, as well as structure and heterogeneity of the surrounding soil. In this study, we measure three‐dimensional root system architecture and the spatiotemporal dynamics of soil moisture (θ) and O ₂ concentrations in the root zone of maize ( Zea mays ) via non‐invasive imaging, and then construct and parameterize a reactive transport model based on the experimental data. The combination of three non‐invasive imaging methods allowed for a direct comparison of simulation results with observations at high spatial and temporal resolution. In three different modeling scenarios, we investigated how the results obtained for different levels of conceptual complexity in the model were able to match measured θ and O ₂ concentration patterns. We found that the modeling scenario that considers heterogeneous soil structure and spatial variability of hydraulic parameters (permeability, porosity, and van Genuchten α and n ), better reproduced the measured θ and O ₂ patterns relative to a simple model with a homogenous soil domain. The results from our combined imaging and modeling analysis reveal that experimental O ₂ and water dynamics can be reproduced quantitatively in a reactive transport model, and that O ₂ and water dynamics are best characterized when conditions unique to the specific system beyond the distribution of roots, such as soil structure and its effect on water saturation and macroscopic gas transport pathways, are considered.

SUMMARY: Quantifying root water uptake is essential to understanding plant water use and responses to different environmental conditions. However, non‐destructive measurement of water transport and related hydraulics in the soil–root system remains a challenge. Neutron imaging, with its high sensitivity to hydrogen, has become an unparalleled tool to visualize and quantify root water uptake in vivo. In combination with isotopes (e.g., deuterated water) and a diffusion–convection model, root water uptake and hydraulic redistribution in root and soil can be quantified. Here, we review recent advances in utilizing neutron imaging to visualize and quantify root water uptake, hydraulic redistribution in roots and soil, and root hydraulic properties of different plant species. Under uniform soil moisture distributions, neutron radiographic studies have shown that water uptake was not uniform along the root and depended on both root type and age. For both tap (e.g., lupine [Lupinus albus L.]) and fibrous (e.g., maize [Zea mays L.]) root systems, water was mainly taken up through lateral roots. In mature maize, the location of water uptake shifted from seminal roots and their laterals to crown/nodal roots and their laterals. Under non‐uniform soil moisture distributions, part of the water taken up during the daytime maintained the growth of crown/nodal roots in the upper, drier soil layers. Ultra‐fast neutron tomography provides new insights into 3D water movement in soil and roots. We discuss the limitations of using neutron imaging and propose future directions to utilize neutron imaging to advance our understanding of root water uptake and soil–root interactions.

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.

Root water uptake is an essential process for terrestrial plants that strongly affects the spatiotemporal distribution of water in vegetated soil. Fast neutron tomography is a recently established non-invasive imaging technique capable to capture the 3D architecture of root systems in situ and even allows for tracking of three-dimensional water flow in soil and roots. We present an in vivo analysis of local water uptake and transport by roots of soil-grown maize plants—for the first time measured in a three-dimensional time-resolved manner. Using deuterated water as tracer in infiltration experiments, we visualized soil imbibition, local root uptake, and tracked the transport of deuterated water throughout the fibrous root system for a day and night situation. This revealed significant differences in water transport between different root types. The primary root was the preferred water transport path in the 13-days-old plants while seminal roots of comparable size and length contributed little to plant water supply. The results underline the unique potential of fast neutron tomography to provide time-resolved 3D in vivo information on the water uptake and transport dynamics of plant root systems, thus contributing to a better understanding of the complex interactions of plant, soil and water.

Purpose: Microplastics have become a ubiquitous pollutant in marine, terrestrial and freshwater systems that seriously affects aquatic and terrestrial ecosystems. Common methods for analysing microplastic abundance in soil or sediments are based on destructive sampling or involve destructive sample processing. Thus, substantial information about local distribution of microplastics is inevitably lost. Methods: Tomographic methods have been explored in our study as they can help to overcome this limitation because they allow the analysis of the sample structure while maintaining its integrity. However, this capability has not yet been exploited for detection of environmental microplastics. We present a bimodal 3D imaging approach capable to detect microplastics in soil or sediment cores non-destructively. Results: In a first pilot study, we demonstrate the unique potential of neutrons to sense and localize microplastic particles in sandy sediment. The complementary application of X-rays allows mineral grains to be discriminated from microplastic particles. Additionally, it yields detailed information on the 3D surroundings of each microplastic particle, which supports its size and shape determination. Conclusion: The procedure we developed is able to identify microplastic particles with diameters of approximately 1 mm in a sandy soil. It also allows characterisation of the shape of the microplastic particles as well as the microstructure of the soil and sediment sample as depositional background information. Transferring this approach to environmental samples presents the opportunity to gain insights of the exact distribution of microplastics as well as their past deposition, deterioration and translocation processes.

Complex plant-soil interactions can be visualized and quantified by combined application of different non-invasive imaging techniques. Oxygen, carbon dioxide and pH gradients in the rhizosphere can be observed with fluorescent planar optodes, while neutron radiography detects small-scale heterogeneities in soil moisture and its dynamics. Respiration and exudation rates can vary between roots of different types, such as primary and lateral roots, as well as along single roots among the same plant. The 3D root system architecture is therefore a key information when studying rhizosphere processes. It can be captured in detail with neutron tomography, but so far only for plants grown in small, cylindrical containers.Combined non-invasive imaging of biogeochemical dynamics, soil moisture distribution and 3D root system architecture is a technical challenge. Thin, slab-shaped rhizotrons with relatively large vertical and lateral extension are well suited for optical fluorescence imaging, allowing for spatially extended observation of biogeochemical patterns. This rhizotron geometry is, however, unfavorable for standard 3D tomography due to reconstruction artefacts triggered by insufficient neutron transmission when the long side of the sample is aligned parallel to the beam direction.We therefore applied neutron laminography, a method where the rotational axis is tilted, to measure the root systems of maize and lupine plants grown in slab-shaped glass rhizotrons (length = 150 mm, width = 150 mm, depth = 15 mm) in 3D. In parallel, we investigated rhizosphere oxygen dynamics and pH value via fluorescence imaging and assessed soil moisture distribution with neutron radiography.Neutron laminography enabled the 3D reconstruction of the root systems with a nominal spatial resolution of 100 µm/pixel. Reconstruction quality strongly depended on root-soil contrast and hence soil moisture level. After reconstruction of the root system and co-registration with the fluorescence images, first results indicate that observed oxygen concentrations and pH gradients depend on root type and individual distance of the roots from the planar optode.In conclusion, neutron laminography is a novel 3D imaging method for root-soil systems grown in slab-shaped rhizotrons. The method allows for determining the precise 3D position of individual roots within the rhizotron and can be combined with 2D imaging approaches. Following experiments will address X-ray laminography as a possible attractive further application.