X-ray microtomography analysis of soil pore structure dynamics under wetting and drying cycles

Luiz F Pires,André C Auler,Waldir L Roque,Sacha J Mooney

doi:10.1016/j.geoderma.2019.114103

Abstract

The soil water retention curve is one of the most important properties used to predict the amount of water available to plants, pore size distribution and hydraulic conductivity, as well as knowledge for drainage and irrigation modeling. Depending on the method of measurement adopted, the water retention curve can involve the application of several wetting and drying (W-D) cycles to a soil sample. The method assumes soil pore structure is constant throughout however most of the time soil structure is dynamic and subjected to change when submitted to continuous W-D. Consequently, the pore size distribution, as well as other soil morphological properties can be affected. With this in mind, high resolution X-ray Computed micro-Tomography was utilized to evaluate changes in the soil pore architecture following W-D cycles during the procedure of the water retention curve evaluation. Two different soil sample volumes were analyzed: ROIW (whole sample) and ROIHC (the region close to the bottom of the sample). The second region was selected due to its proximity to the hydraulic contact of the soil with the water retention curve measurement apparatus. Samples were submitted to the following W-D treatments: 0, 6 and 12 W-D. Results indicated the soil changed its porous architecture after W-D cycles. The image-derived porosity did not show differences after W-D cycles for ROIW; while for ROIHC it increased porosity. The porosity was also lower in ROIHC in comparison to ROIW. Pore connectivity improved after W-D cycles for ROIHC, but not for ROIW. W-D cycles induced more aligned pores for both ROIs as observed by the tortuosity results. Pore shape showed changes mainly for ROIW for the equant and triaxial shaped pores; while pore size was significantly influenced by the W-D cycles. Soil water retention curve measurements showed that W-D cycles can affect water retention evaluation and that the changes in the soil morphological properties can play an important role in it.

Highlights

The soil water retention curve is a very important soil physical-hydraulic property, expressed by the relationship between the pressure head of the soil and its water content (Klute, 1986)
The lower image-derived porosity for the lower portion of the samples may have been influenced by the procedures utilized for collecting samples in volumetric rings; as regions close to the walls of the cylinders can be subjected to stresses which damage the soil structure
The soil close to the hydraulic contact with the sandbox as part of measurement of the water retention curve presented similar behavior to the rest of soil sample which was surprising as the pore connectivity and tortuosity measured by computed tomography imagery was greatly affected by wetting and drying (W-D) cycles for this region

Summary

Introduction

The soil water retention curve is a very important soil physical-hydraulic property, expressed by the relationship between the pressure head of the soil and its water content (Klute, 1986). The relation between the pressure head and soil water content can be obtained in two ways, desorption (drying) and sorption (wetting). Continuous curves are obtained in both methods, but in general, they are not identical due to hysteresis (Hillel, 2004). The soil water retention curve determination involves the measurement of a series of equilibria of the water in the soil sample at known pressure heads. Depending on the experimental procedure chosen samples can be submitted to several wetting and drying (W-D) cycles (Moraes et al, 1993; Kong et al, 2018; Reis et al, 2019)

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