Compaction effects on soil macropore geometry and related parameters for an arable field

Soil compaction decreases soil pores are important for root growth as well as infiltration of water and nutrients. A study was conducted to evaluate the effects of soil compaction on macropore parameters measured using X-ray computed tomography (CT). Macropore parameters included number of pores, number of macropores (> 1000 μm diam.), number of coarse mesopores (200 to 1000 μm diam.), porosity, macroporosity, coarse mesoporosity, area of largest pore, pore circularity, and fractal dimension of macroporosity. A field experiment was conducted on Mexico silt loam (fine, smectitic, mesic Vertic Epiaqualfs) with field treatments including four replicates of uniformly Compacted (C) and Non-Compacted (NC) plots arranged in a randomized complete block design. Soil cores (76.2 mm diam. by 76.2 mm long) were removed from three selected depths (0 to 10 cm, 10 to 20 cm, and 20 to 30 cm). Cores were scanned using a medical X-ray CT scanner with four scans taken in each sample at 15 mm spacing starting at 25 mm from the core surface. Images were analyzed using Image-J software. The C treatment was found to increase bulk density by 8% (1.34 to 1.45 g cm−3) and decrease saturated hydraulic conductivity by 69% (47.1 to 14.6 cm hr− 1). CT-measured number of pores decreased by 71%, number of macropores by 69%, and coarse mesopores by 75% with the C treatment used in the study. Compaction was also found to significantly decrease CT-measured porosity and macroporosity by 64%. Differences between treatments for the parameters were most pronounced in the upper 10 cm; differences between treatments were not significant below 20 cm. A regression equation with CT-measured macroporosity, area of largest pore and porosity explained most of the variability in saturated hydraulic conductivity (R2 = 0.79). Efforts should be made to minimize soil compaction due to its harmful effects on soil pores and subsequent challenges for plant root growth and enhanced runoff of water and nutrients.

Research Highlights
► Soil compaction preferentially removes large soil pores called macropores which are critical for water transport in soils. Soil cores were removed from replicated compacted and non-compacted field treatments and scanned using a medical X-ray computed tomography scanner.
► The compacted treatment for this study was found to increase the density of the soil by 8% and decrease the soil permeability by 69%. The number of macropores and the relative area of macropores were decreased by 69% and 64%, respectively.
► Differences between treatments for the porosity parameters were most pronounced in the upper 10 cm soil depth.
► A regression equation was developed which predicted soil permeability with CT-measured macroporosity and porosity.
► Efforts should be made to minimize soil compaction due to its harmful effects on soil pores and subsequent challenges for plant root growth and enhanced runoff of water and nutrients.