Using X-ray computed tomography to quantify variability in soil macropore characteristics in pastures

Abstract

Soil macropores largely control the transport of water and solutes in subsurface flows. Preferential flow via soil macropores can substantially affect water quality. Hence, it is important to quantify soil macropore characteristics and link this information with the preferential flow behavior in soils. However, whether macropore structure at one slope position within a field is different than that at another is unclear. With differences in the macropore characteristics, each slope position can contribute differently to the runoff and subsurface flows. The objective of this study was to use X-ray computed tomography (CT) and image analysis to characterize soil pore structure (> 0.75 mm diameter) at upslope, midslope, and downslope positions within a 0.40 ha pasture field. A total of 18 undisturbed soil columns (150 mm diameter and 500 mm depth) were collected from a pasture field located at the Sand Mountain Research and Extension Center, Alabama, during May 2019. The results indicated that both the macropore number and macroporosity values were lowest at the downslope position in the 0–100 mm soil layer. In contrast, a large number of macropores was observed at the downslope soils for depths below 200 mm. The lowest macroporosity values in the surface layer at the downslope position can be attributed to higher degree of trampling induced compaction due to higher soil moisture content at the downslope location, via runoff and seepage losses from the upper slopes. Likewise, it was found that using smaller diameter soil cores for sampling can cause bias in the soil macropore measurements due to considerably high coefficient of variation compared to the larger diameter soil cores. Macropore interconnectivity at the subsurface layer (100–500 mm) increased from the upslope to the downslope position, whereas at the soil surface (0–100 mm), the interconnectivity was lowest at the downslope position as compared to the upslope and midslope locations. The results of this study provide quantitative information of different soil macropore characteristics under varying topographical locations and depths in a pasture field. The relevance of topographical variation of different macropore characteristics found in this study should be tested by investigating a wider range of soil types and slopes in pastures.