European Journal of Soil Science, June 2013, 64, 279–297 doi: 10.1111/ejss.12028 Review Applications of X-ray computed tomography for examining biophysical interactions and structural development in soil systems: a review J. R. Helliwell a , C. J. Sturrock a , K. M. Grayling a , S. R. Tracy a , R. J. Flavel b , I. M. Young b , W. R. Whalley c & S. J. Mooney a a Division of Agricultural and Environmental Sciences, University of Nottingham, Sutton Bonington Campus, Leicestershire LE12 5RD, UK, b School of Environmental and Rural Sciences, University of New England, Armidale, New South Wales 2351, Australia, and c Sustainable Soils and Grassland Systems, Rothamsted Research, West Common, Harpenden AL5 2JQ, UK Summary Soil systems are characterized by the spatial and temporal distribution of organic and mineral particles, water and air within a soil profile. Investigations into the complex interactions between soil constituents have greatly benefited from the advent of non-invasive techniques for structural analysis. In this paper we present a review of the application of one such technique, X-ray computed tomography (CT), for studies of undisturbed soil systems, focusing on research during the last 10 years in particular. The ability to undertake three-dimensional imaging has provided valuable insights regarding the quantitative assessment of soil features, in a way previously unachievable because of the opaque nature of soil. A dynamic approach to the evaluation of soil pore networks, hydro-physical characteristics and soil faunal behaviour has seen numerous scanning methodologies employed and a diverse range of image analysis protocols used. This has shed light on functional processes across multiple scales whilst also bringing its own challenges. In particular, much work has been carried out to link a soil’s porous architecture with hydraulic function, although new technical improvements allowing the characterization of organic matter and the influence of soil biota on structural development are showing great promise. Here we summarize the development of X-ray CT in soil science, highlight the major issues relating to its use, outline some of the applications for overcoming these challenges and describe the potential of future technological advances for non-invasive soil characterization through integration with other complementary techniques. Introduction The physical structure of soil constituents is crucial to its functioning, with quantification of internal soil architecture being key to understanding the processes governing its development. The diverse and dynamic array of mineral particles, air- and water- filled pores and organic material over a range of scales make studies of soil a challenging task, and it is for this reason it is often described as ‘the most complex biomaterial on the planet’ (Young & Crawford, 2004). As a result, a detailed understanding regarding the interactions between soil structure formation, stabilization and degradation processes still remains incomplete. As soil is Correspondence: S. J. Mooney. E-mail: sacha.mooney@nottingham.ac.uk Received 23 April 2012; revised version accepted 17 December 2012 ordered in a complex and dynamic hierarchical manner, advances in understanding of soil behaviour will only come with in- depth spatial evaluations of its constituent configurations in three dimensions (3D) and quantification of the underlying processes that cause changes over time. Over the last 30 years, X-ray computed tomography (CT) has provided a non-destructive means for visualizing and quantifying soils in 3D, allowing researchers to gain fundamental insights regarding soil function. X-ray CT has been applied in many different studies concerning the spatial distribution of soil properties (Heeraman et al ., 1997; Young et al ., 2001; Pierret et al ., 2002; Rogasik et al ., 2003; Nunan et al ., 2006), with applications evolving to cover the extensive characterization of pore-space geometry and fractures within a soil sample with © 2013 The Authors Journal compilation © 2013 British Society of Soil Science 279