Quantification of Soil Macropores at Different Slope Positions under Alpine Meadow Using Computed Tomography in the Qinghai Lake Watershed, NE Qinghai–Tibet

Soil macropores largely control fluid and solute transport and the runoff processes on the slopes. However, the characterization of soil macropores for different slope positions under alpine meadow conditions remains scarce, especially in the Qinghai Lake watershed. The objective of this study was to visualize and quantify the macropore characteristics of soils at different slope positions (i.e., the upper, middle and bottom slopes) in an alpine meadow in the Qinghai Lake watershed using X-ray computed tomography (CT). Nine soil cores (0–40 cm deep) were collected from the upper, middle and bottom slopes with three replicates per position, and cores were scanned with a GE HISPEED FX/I Medical Scanner. Macroporosity, length density, node density and tortuosity within the 40 cm soil profile were interpreted from the CT data to analyze the soil architecture. The results showed that the macroporosity decreased from the upper slope to the bottom slope. The macroporosity of soil on the upper slope was 50 times greater than that on the bottom slope, which can be attributed to the gravel in the former soil. The large number of continuous macropores and cracks concentrated in the deep soil layers (200–350 mm) of the upper slope can be attributed to the gravel layer. Most of the soil macropores under the middle and bottom slopes were distributed in the surface soil (0–150 mm), which can be attributed to the “mattic epipedon.” The soil macropores in the bottom slope, due to the well-developed root system, were more continuous and tubular than those in soils of the upper and middle slopes. It is expected that water can move preferentially through soil macropores on the upper and middle slopes compared to the bottom slope. Furthermore, due to its relatively low macroporosity the soil of the bottom slope contributes more to surface flow than those of middle and upper slopes.