Characterising and linking X-ray CT derived macroporosity parameters to infiltration in soils with contrasting structures

Abstract

Soils deliver the regulating ecosystem services of water infiltration and distribution, which can be controlled by macropores. Parameterizing macropore hydraulic properties is challenging due to the lack of direct measurement methods. With tension-disc infiltrometry hydraulic properties near saturation can be measured. Differentiating between hydrologically active and non-active pores, at a given water potential, indirectly assesses macropore continuity. Water flow through macropores is controlled by macropore size distribution, tortuosity, and connectivity, which can be directly derived by X-ray computed tomography (CT). Our objective was to parameterize macropore hydraulic properties based on the imaged macropore network of three horizons of an Andosol and a Gleysol. Hydraulic conductivity Kunsat was derived from infiltration measurements. Soil cores from the infiltration areas were scanned with X-ray CT. Kunsat was significantly higher in the Andosol than in the Gleysol at all water potentials, and decreased significantly with depth in both soils. The in situ measurements guided the definition of new macroporosity parameters from the X-ray CT reconstructions. For the Andosol, Kunsat was best predicted using the imaged-limited macroporosity. A low total macroporosity, coupled with a high macropore density, indicated the abundance of smaller macropores, leading to homogeneous matrix flux. Imaged macropores were not well connected. In contrast, the Gleysol had a bi-modal macropore system with few very-large, but well-connected macropores. Kunsat was best predicted using the imaged macroporosity consisting only of macropores with diameters between 0.75 and 3 mm. Our research demonstrates that linking traditional soil physical measurements with soilvisualization techniques has a huge potential to improve parameterizing macropore hydraulic properties. The relevance of the relationships found in this study for larger scales and other soil types still needs to be tested, for example by a multi-scale investigation including a much wider range of different soils.

Impacts of land use on hydrodynamic properties and pore architecture of volcanic soils from the Mexican Highlands

“…The variability was especially large for AF and AMW-Sub, explaining the low R 2 of the fitting with the Gardner equation. Müller et al (2018) also found lower variations in unsaturated hydraulic conductivity (between -0.7 and -0.1 m pressure head) and higher values of unsaturated hydraulic conductivities in Andosol (silt-loam) than in Gleysol (clay loam). The soils showed K s in the range of 0.37 Â 10 À5 to 1.90 Â 10 À5 m s -1 , usually associated with sandy-loam and silty-loam textures; it was higher in AM-Top and -Sub and AMW-Top and lower in AMW-Sub, AF, TF and TT (Table 3).…”

“…Duwig et al (2008) coupled dye infiltration with image analysis to illustrate finger-type flows in the same Mexican allophanic Andosols. More recently, X-ray tomography was used to quantify pore morphological parameters of Andosols and was coupled with hydrological analyses (Vingiani et al 2015;Müller et al 2018).…”

Saturated water conductivity estimation based on X-ray CT images – evaluation of the impact of thresholding errors

“…CT may also be used for imaging the whole soil core samples where a voxel size between 20 µm and 50 µm is achieved (Jarvis et al, 2017;Rab et al, 2014;Vaz et al, 2011). A resolution of this order allows for macropore observation (Jarvis, 2007;Katuwal et al, 2018;Müller et al, 2018) and root visualization (Daly et al, 2018;Sander et al, 2008). However, it can also be used for imaging soil aggregates with a voxel size of 1 µm or smaller (Józefaciuk et al, 2015;Voltolini et al, 2017b).…”

“…A 3D image obtained with this technique, following its processing, allows researchers to examine the inner structure of soil, and to collect information about pore distribution (Hu et al, 2015) and pore properties, such as shape factors (Elliot et al, 2010), connectivity (Sleutel et al, 2008) or pore circularity (Yang et al, 2018). The link between soil cores macroporosity and saturated water conductivity has been of special interest (Gerke, 2012;Larsbo et al, 2016;Müller et al, 2018).…”

Macropore flow in soils and pesticide risk assessment

“…The estimation of dag and the change of wf over depth is not solved yet and, considering the outcomes of this study, should be topics of future research. Some methods have been proposed in the literature for dag (Kroes et al, 2017;van Schaik et al, 2010), and the variation in field conditions of wf over depth may be measured by X-ray computer tomography (Müller et al, 2018).…”