Assessing the influence of the rhizosphere on soil hydraulic properties using X - ray 1 Computed Tomography and numerical modelling

Short title for page headings: Influence of the rhizosphere on soil hydraulic properties 16 17 Short statement: Using non-destructive imaging techniques and numerical modelling we 18 quantify differences in hydraulic and structural properties of bulk and rhizosphere soil for 19 sand and clay loam soils. 20 21 22 2 1. Abstract 23 Understanding the dynamics of water distribution in soil is crucial for enhancing our 24 knowledge of managing soil and water resources. The application of X-ray Computed 25 Tomography (CT) to the plant and soil sciences is now well established. However, few 26 studies have utilised the technique for visualising water in soil pore spaces. Here we utilise 27 this method to visualise the water in soil in situ and in three-dimensions at successive 28 reductive matric potentials in bulk and rhizosphere soil. The measurements are combined 29 with numerical modelling to determine the unsaturated hydraulic conductivity, providing a 30 complete picture of the hydraulic properties of the soil. The technique was performed on soil 31 cores that were sampled adjacent to established roots (rhizosphere soil) and from soil that had 32 not been influenced by roots (bulk soil). A water release curve was obtained for the different 33 soil types using measurements of their pore geometries derived from CT imaging and verified 34 using conventional methods e.g. pressure plates. The water, soil and air phases from the 35 images were segmented and quantified using image analysis. The water release 36 characteristics obtained for the contrasting soils showed clear differences in hydraulic 37 properties between rhizosphere and bulk soil, especially in clay soil. The data suggests that 38 soils influenced by roots (rhizosphere soil) are less porous due to increased aggregation when 39 compared to bulk soil. The information and insights obtained on the hydraulic properties of 40 rhizosphere and bulk soil will enhance our understanding of rhizosphere biophysics and 41 improve current water uptake models. 42 43 44 45 46 3