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

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 m 3 m −3 ) as compared with the row crop (0.014 m 3 m −3 ) and deposition positions (0.006 m 3 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.