Use of X-ray tomography for examining root architecture in soils

•When grown in soils, plant roots are notoriously difficult to observe and study.•X-ray computed tomography (CT) can be used to analyse roots in situ within the soil.•We discuss the use of X-ray CT, including its strengths and limitations.•Both conventional X-ray CT and synchrotron-based X-ray CT are discussed.•This information will assist others in using X-ray CT to solve a range of problems. Despite the critical importance of roots within soils for supporting plant growth, the assessment of root distribution in soils remains difficult and much is unknown regarding their behaviour. In this review, we examine X-ray computed tomography (CT) as a non-invasive method for examining root distribution in soils. X-ray CT enables three-dimensional reconstruction of soil cores to accurately estimate a wide range of features within the soil, including roots, not only examining changes spatially but also temporally. With the development of high-end X-ray CT systems and image processing algorithms, this approach can now be used to examine a range of factors, including root system architecture, soil-root interactions, soil pore architecture, soil biophysical interactions, and soil microorganism behaviour. In addition, we examine the use of synchrotron-based X-ray CT which has been used to provide better resolution, larger sample analysis, faster scanning, and images with greater contrast compared to conventional systems.