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Aims Roots need to be in good contact with the soil to take up water and nutrients. However, when the soil dries and roots shrink, air-filled gaps form at the rootsoil interface. Do gaps actually limit the root water uptake, or do they form after water flow in soil is already limiting? Methods Four white lupins were grown in cylinders of 20 cm height and 8 cm diameter. The dynamics of root and soil structure were recorded using X-ray CT at regular intervals during one drying/wetting cycle. Tensiometers were inserted at 5 and 18 cm depth to measure soil matric potential. Transpiration rate was monitored by continuously weighing the columns and gas exchange measurements.Results Transpiration started to decrease at soil matric potential = between −5 kPa and −10 kPa. Air-filled gaps appeared along tap roots between =0−10 kPa and =0−20 kPa. As = decreased below −40 kPa, roots further shrank and gaps expanded to 0.1 to 0.35 mm. Gaps around lateral roots were smaller, but a higher resolution is required to estimate their size. Conclusions Gaps formed after the transpiration rate decreased. We conclude that gaps are not the cause but a consequence of reduced water availability for lupins.

X-ray CT is a powerful technology to study root growth in soil in-situ. Root systems can be studied in its true 3D geometry over time. Hence, the same plant can be scanned multiple times during development. A downside is the potential of X-rays to interfere with biological processes and therefore plant growth. The aim of this study is to evaluate the influence of cumulative X-ray dose on Vicia faba and Hordeum vulgare during a growth period of 17 days. One control treatment without X-ray scanning was compared to two treatments being scanned every two and four days, respectively. Scanned treatments received a maximum cumulative dose of less than 8 Gy. Plant species differed in their susceptibility to X-ray dose. For Vicia faba, mean total root length was reduced significantly. Leave growth was reduced as well. Number and length of second order laterals was reduced significantly, as well as length of first order laterals. Hordeum vulgare showed no negative impact of X-ray dose on any of the root parameters. Large differences between the two species investigated were detected in respect to susceptibility to X-ray dose. Results indicate that for X-ray CT studies involving temporal resolution a control treatment without scanning is required.

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The rhizosphere, the fraction of soil altered by plant roots, is a dynamic domain that rapidly changes during plant growth. Traditional approaches to quantify root growth patterns are very limited in estimating this transient extent of the rhizosphere. In this paper we advocate the analysis of root growth patterns from the soil perspective. This change of perspective addresses more directly how certain root system architectures facilitate the exploration of soil. For the first time, we propose a parsimonious root distance model with only four parameters which is able to describe root growth patterns throughout all stages in the first 3 weeks of growth of Vicia faba measured with X-ray computed tomography. From these models, which are fitted to the frequency distribution of root distances in soil, it is possible to estimate the rhizosphere volume, i.e., the volume fraction of soil explored by roots, and adapt it to specific interaction distances for water uptake, rhizodeposition, etc. Through 3D time-lapse imaging and image registration it is possible to estimate root age dependent rhizosphere volumes, i.e., volumes specific for certain root age classes. These root distance models are a useful abstraction of complex root growth patterns that provide complementary information on root system architecture unaddressed by traditional root system analysis, which is helpful to constrain dynamic root growth models to achieve more realistic results.

A new method combining online nano solid phase extraction coupled with Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was developed to extract and analyze organic matter (OM) from microliter volumes of salt containing soil solution samples. This approach allows the reproducible analysis of only minute amounts of organic carbon (down to 10 ng C) without the need of further sample preparation. The new method was applied to unravel developing small-scale patterns of dissolved organic matter (DOM) in soil solutions of a soil column experiment in which Zea mays plants were grown for 3 weeks. Soil solution was sampled by micro suction cups from the undisturbed soil-root system once a week. Growth of the root system and, hence, position of individual roots relative to the suction cups was followed by X-ray computed tomography (X-ray CT). Our method makes it possible to resolve the chemical complexity of soil solution OM (up to 4300 molecular formulas from 2.5 μL sample). This allows to observe chemical gradients in the rhizosphere on a molecular level over time. The increasing influence of roots on soil solution OM is visible from higher molecular masses, an increasing degree of oxygenation and a higher fraction of formulas containing heteroatoms. The online nano solid phase extraction-FT-ICR-MS method provides novel insight into the processes affecting DOM in the rhizosphere, such as root exudation, microbial processes, and soil organic matter stabilization.

Aims Root shrinkage in drying soil has been shown repeatedly. The aim of this study was to investigate the dynamics of root-soil contact and its relationship with plant water status during soil drying. Methods The development of root-soil contact of Vicia faba L. during a drying period was studied. Plants (N = 4) were grown in cylinders filled with a sandy soil. Samples were repeatedly scanned with an X-ray CT scanner to visualize root-soil contact. Soil matric potential, transpiration rate, and stomatal conductance were measured daily. Results Root-soil contact was lower in taproots than in lateral roots at any time. Transpiration rate and stomatal conductance decreased before roots started to shrink. Root-soil contact decreased significantly over the course of the drying period, starting at soil matric potentials below −20 kPa. Root shrinkage did not differ significantly between taproots and laterals. Conclusions This study confirms previous findings with Lupinus albus roots in that roots shrink after transpiration rate decreases. The dynamics of root shrinkage are governed by soil water availability and transpirational demand.

Radiation damage to plants through X‐ray exposure has been reported to impair root growth. The literature on the critical dose for growth impairment is inconclusive, partly as dose measurements in soil are scarce. Here we fill this gap and show that the dose in a typical single pot scan amounts to 1.2 Gy. In addition, we demonstrate the shortcomings of estimating the dose from scan settings using the RadPro Calculator and highlight the efficient reduction of X‐ray exposure by a lead shield.