Assessing the influence of the rhizosphere on soil hydraulic properties using X-ray computed tomography and numerical modelling.

Keith R Daly,Sacha J Mooney,Malcolm J Bennett,Tiina Roose,Neil M J Crout,Saoirse R Tracy

doi:10.1093/jxb/eru509

Keith R Daly, Sacha J Mooney + Show 4 more

Open Access

https://doi.org/10.1093/jxb/eru509

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Abstract

Understanding the dynamics of water distribution in soil is crucial for enhancing our knowledge of managing soil and water resources. The application of X-ray computed tomography (CT) to the plant and soil sciences is now well established. However, few studies have utilized the technique for visualizing water in soil pore spaces. Here this method is utilized to visualize the water in soil in situ and in three-dimensions at successive reductive matric potentials in bulk and rhizosphere soil. The measurements are combined with numerical modelling to determine the unsaturated hydraulic conductivity, providing a complete picture of the hydraulic properties of the soil. The technique was performed on soil cores that were sampled adjacent to established roots (rhizosphere soil) and from soil that had not been influenced by roots (bulk soil). A water release curve was obtained for the different soil types using measurements of their pore geometries derived from CT imaging and verified using conventional methods, such as pressure plates. The water, soil, and air phases from the images were segmented and quantified using image analysis. The water release characteristics obtained for the contrasting soils showed clear differences in hydraulic properties between rhizosphere and bulk soil, especially in clay soil. The data suggest that soils influenced by roots (rhizosphere soil) are less porous due to increased aggregation when compared with bulk soil. The information and insights obtained on the hydraulic properties of rhizosphere and bulk soil will enhance our understanding of rhizosphere biophysics and improve current water uptake models.

Highlights

The concept of the ‘rhizosphere’, proposed by Hiltner (1904), refers to the volume of soil adjacent to a plant root over which the root has influence
In this paper we demonstrate the application of this method to quantify water distribution in soil pores for bulk and rhizosphere soil in contrasting soil textures
The water release characteristic (WRC) was obtained via conventional methods and the imaging method (Figure 2) for bulk and rhizosphere soil in the two soil textures

Summary

Introduction

The concept of the ‘rhizosphere’, proposed by Hiltner (1904), refers to the volume of soil adjacent to a plant root over which the root has influence. The rhizosphere is created from root-soil-microbe interactions and the compression of soil due to root expansion (Aravena et al, 2011; Aravena et al, 2014; Dexter, 1987; Whalley et al, 2005). The root secretes chemical compounds, referred to as exudates, into the surrounding soil. These exudates can be divided into three categories: (1) mucilage, which is usually found at the root tips and consists of polysaccharides and uronic acids; (2) molecules excreted by the root hairs such as amino acids, organic acids and simple sugars; and (3) cellular organic substances produced by root epidermis senescence (Tan, 2000). Gases are released from roots, including carbon dioxide and methane, some researchers (Grayston et al, 1997; Swinnen et al, 1995) do not define them as exudates as they diffuse into the atmosphere

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