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

Authors: BUDHATHOKI, SUMAN - Auburn University; LAMBA, JASMEET - Auburn University; SRIVASTAVA, PUNEET - University Of Maryland; MALHOTRA, KRITIKA - Auburn University; Way, Thomas - Tom; KATUWAL, SHEELA - University Of Arkansas

Submitted to: Soil and Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2021
Publication Date: 9/28/2021
Citation: Budhathoki, S., Lamba, J., Srivastava, P., Malhotra, K., Way, T.R., Katuwal, S. 2021. Using x-ray computed tomography to quantify variability in soil macropore characteristics in pastures. Soil and Tillage Research. 215:105194. https://doi.org/10.1016/j.still.2021.105194.
DOI: https://doi.org/10.1016/j.still.2021.105194

Interpretive Summary: Water flow through soil is important for providing infiltration to deliver rain and irrigation water to crop roots, and for reducing runoff water. However, this flow can have detrimental environmental effects, as it allows nutrients and other solutes to be transported by subsurface water flow away from the field area to which they are applied. Soil macropores are cavities in soil larger than 75 micrometers. Macropores strongly affect water flow through soil. Subsurface water flow typically increases as the sizes of macropores and the connectivity of macropores increase. Cylindrical soil samples (columns), 15 cm diameter and 50 cm depth, were collected from the fine sandy loam soil of a pasture field in northeastern Alabama. The pasture has a gentle slope and the soil columns were collected at three locations along the slope, designated as the upslope, midslope, and downslope locations. A medical computed tomography (CT) scanner was used to scan each soil column and represent the soil as multiple slices, each 0.625 mm thick. The CT scan results show that the number of macropores was least and the total volume of macropores was lowest for the 0-100 mm depth soil layer at the downslope location. At that location, there was a large number of macropores in the 200-500 mm depth range. Connectivity of macropores in the 100-500 mm depth range was lowest at the upslope location and greatest at the downslope location. In contrast, connectivity of macropores in the 0-100 mm depth range was lower at the downslope location than at the midslope and upslope locations. These results provide quantitative information of different pore size-dependent soil macropore characteristics for various topographical locations and depths in a pasture field. The results are expected to be useful in analyzing the movement of water, nutrients, and other solutes in soil, and in promoting the accuracy and usefulness of computer modeling to improve agricultural productivity and reduce environmental contamination.

Technical Abstract: Soil macropores largely control the transport phenomenon of water and solutes in subsurface flows. Preferential flow via soil macropores can affect water quality substantially. 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 CT and image analysis to characterize soil pore structure at upslope, midslope and downslope positions within a 0.40 ha pasture field. A total of 18 undisturbed soil columns (15 cm diameter and 50 cm depth) were collected from the fine sandy loam soil of a pasture field at the Sand Mountain Research and Extension Center in northeastern Alabama, during May 2019. Six cores were collected from each topographical location within the field. The cores were scanned using a medical CT scanner to produce images with a voxel size of 0.3516*0.3516*0.625 mm^3. Results on variability in soil macropore characteristics, including macroporosity, macropore number, interconnectivity, and macropore equivalent diameter as a function of soil depth and slope position within the 50-cm soil profile were interpreted from X-ray computed tomography. The results indicate 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 soil for depths below 200 mm. The lowest macroporosity values in the surface layer at the downslope position can be attributed to higher soil moisture content at the downslope location, via runoff and seepage losses from the upper slopes. This contributed to a higher degree of compaction due to trampling by cattle. There was a significant (P<0.05) effect of slope position on macroporosity only for pores with diameters less than 1 mm. Likewise, it was found that the macropore characteristics differ significantly (P<0.05) with a change in the volume of soil core analyzed. 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. These results provide quantitative information of different pore size-dependent soil macropore characteristics under varying topographical locations and depths in a pasture field. In addition, this study suggests the need for future research to quantify the effect of using different sizes of soil cores at multiple resolutions and using that information to develop a more reliable and consistent sampling strategy.