Applications of X-ray computed tomography for examining biophysical interactions and structural development in soil systems: a review

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 10years 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.