Computed‐Tomographic Measurement of Soil Macroporosity Parameters as Affected by Stiff‐Stemmed Grass Hedges

Abstract

Planting stiff‐stemmed grass hedges in a watershed may reduce water runoff and soil erosion, in part by altering soil macroporosity. The objective of this study was to characterize macroporosity of soils under a perennial grass hedge system for 12 yr using x‐ray computed tomography (CT) and to compare CT‐macroporosity results with macroporosity estimated from water retention data. Three positions were sampled: grass hedge position, deposition zone position 0.5 m upslope from grass hedges, and row crop position 7 m upslope from the hedges. Intact core samples (76 mm × 76 mm) were collected from two depths, 0 to 100 and 100 to 200 mm, with five replicates per position per depth. Number of pores (macro‐ and meso‐), averaged across depths, in the grass hedge were nearly 2.5 times greater than those in the row crop and five times greater than in the deposition positions; however their circularity was 8.8% lower than in the row crop and 2.6% lower than in the deposition positions. The CT‐measured macroporosity was significantly greater (P < 0.01) for the grass hedge position (0.056 m3 m−3) as compared with the row crop (0.014 m3 m−3) and deposition positions (0.006 m3 m−3). The fractal dimension (D) was significantly greater (P < 0.01) for the grass hedge position (D = 1.56) than in the row crop (D = 1.31) and the deposition (D = 1.12) positions. The values of all measured pore characteristics decreased with depth. Computed tomography‐measured macroporosity data were comparable with macroporosity estimated from water retention data. These findings suggest that grass hedge systems have created more pores and a greater volume of macroporosity.

Coupling X-ray microtomography and macroscopic soil measurements: a method to enhance near-saturation functions?

“…Kumar et al (2010) and Kim et al (2010) tried to explain saturated hydraulic conductivity (K sat ) differences by pore parameter measurements with computed tomography, and found that most of these parameters were correlated with K sat . Rachman et al (2005) and Quinton et al (2009) studied macroporosity through X-ray tomography and water retention curves; they concluded that these methods lead to comparable results for porosity distributions. Dal Ferro et al (2012), for their part, analysed soil porosity with mercury intrusion porosimetry and X-ray microtomography.…”

Application of X-ray computed tomography to soil science: A literature review

“…) and packed soil cores (Anderson et al 1990;Peyton et al 1994a). Various properties of soil macropores and macropore networks have been estimated using X-ray CT imagery: porosity (Anderson et al 1990;Peyton et al 1992;Rachman et al 2005), pore diameter (Anderson et al 1990;Peyton et al 1992), perimeter and area (Grevers et al 1989;Adderley et al 2001), circularity (Gantzer and Anderson 2002;Rachman et al 2005), equivalent cylindrical diameter (Gantzer and Anderson 2002), tortuosity, hydraulic radius in three-dimensions (pore volume/wall area), numerical density of networks, connectivity (Perret et al 1999). A series of models have been created for the quantification of the pore network structure, viewed as the pore objects and their links (Delerue et al 2003), or as pore-bodies and pore-throats (Al-Raoush and Willson 2005).…”

“…Discrimination of soil structural characteristics by CT imaging depends to a great extent on the spatial resolution of the instrument utilized (Aylmore 1994;Rachman et al 2005). Based on statistical and geostatistical analysis of data obtained in X-ray CT imagery of soils with different textures, Cı´slerova´and Votrubova ( 2002) concluded that, in the evaluation of the porosity of heterogeneous samples, the scanning direction (vertical or horizontal) seemed to have a greater influence than image resolution (determined by the size of the field of view).…”

“…Methods for 3D investigation of the mass fractal dimension of macropore network in X-ray CT images were developed by different authors (Peyton et al 1994a;Gantzer and Anderson 2002;Perret et al 2003;Rachman et al 2005). As a rule, a strong positive correlation was found between macroporosity and fractal dimension for all the analysed soils.…”

“…The selection of a pre-established value as threshold, based on the pore presence in the analyzed samples, was utilized by many authors, such as Joschko et al (1991Joschko et al ( , 1993, Øygarden et al (1997), Je´gou et al (1999), Langmaack et al (1999), Rogasik et al (2003). Some authors employed the thresholding tool of the utilized image analysis software (Gantzer and Anderson 2002;Rachman et al 2005). Other authors calculated a thresholding value using histogram peaks.…”