Investigating soil-root interaction and changes in soil physical properties under different soil managements using X-ray computed tomography

Tomography technologies including X-ray computed tomography (CT) and neutron tomography have been increasingly used by both research and industrial communities in many fields to non-invasively visualize opaque materials in three dimensions. Their applications in soil science over the last decades have substantially improved our understanding of many fundamental processes which would otherwise remain unknown. With the development in both technologies, we are now able to visualize and quantify 3D pore gnomery in porous materials at a spatial resolution as fine as a few microns. Combining these with process-based pore-scale simulations would enable us to elucidate how soil structure responds to agronomic practice changes as well as its consequence for soil physical properties such as permeability, water retention and tortuosity at different scales, which are important for soil functions. My PhD project aims to build a new system to investigate soil-root interaction and soil property changes induced by both agronomical practice changes and root-soil interaction. In this thesis, I present two experimental results to analyse root growth and its consequent impact on soil structure under different managements and environments. The first experiment is to investigate how water and salinity stresses affect development of root architecture of maize in three dimensions. Maize seedlings were grown in pots packed with a loamy sand soil for three weeks under water and salinity stresses both in isolation and combination. I then scan them using X-ray CT at resolution of 110 microns to analyse the impact of these stresses on root architecture traits, including root length, root branching angle as well as diameters of the crown and primary roots. These could help us understand how plants respond to abiotic stresses by adjusting their root morphology. In the meantime, I also took soil aggregates adhered to the roots and scanned them using X-ray tomography at resolution of 4 microns to analyse the change in pore geometry under different stresses as well as its consequence for soil permeability and porosity as these parameters were important to estimate how roots take up water and nutrients from soil. The results showed that both abiotic stresses reduced the porosity and permeability of the rhizosphere when they worked in isolation. Their combination could reduce rhizosphere permeability by threefold. The second experiment was to quantify the structural change in soil aggregates following land use changes 70 years ago in long-term experiment at Rothamsted Research. Soil samples were taken from the topsoil in a field under different land usages: continuous grass, arable and fallow, for more than 70 years. I then scanned them using X-ray CT at a resolution of 1.5 microns. Image analysis showed that the land usage change has a significant impact on intra-aggregate structure in that the soil under continuous grassland is more porous while the soil having been fallowed for 70 years is least porous. Based on the X-ray images, I also analysed how the cropping systems affect the ability of soil to retain (water retention curves) and conduct water (permeability), as well as to move nutrients (diffusion coefficient). All these have direct implications for improving our understanding of root uptake of water and nutrients as well as hydrological cycle