Using the X-ray computed tomography method to predict the saturated hydraulic conductivity of the upper root zone in the Loess Plateau in China

The recovery of natural vegetation significantly improves soil macroporosity through root decay and biological activity. Macropores are preferential pathways for the movement of water to deep soil. However, the quantification of soil macropore structure and its relationship with soil hydraulic conductivity (K-sat) are not well understood. We characterized the macropores under different vegetation types to evaluate the effects of macropores on K-sat. Undisturbed soil cores were collected from four treatments: areas dominated by Quercus liaotungensis Koidz.(QLI), Pinus tabuliformis Carriere (PTA), Caragana korshinskii Kom. (CKO), and Medicago sativa L.(MSA). A medical computed tomography (CT) scanner was used to acquire images with a voxel size of 0.977 by 0.977 by 1.000 mm for depths of 0 to 360 mm. We used ImageJ software to quantify the macropore properties. Soil structure and K-sat improved with the succession of vegetation. The mean macropore volume fraction across the soil cores for the QLI, PTA, CKO, and MSA treatments were 6.6, 3.5, 1.3, and 0.6% within a 4900-mm(2) area, respectively. Macropore quantity (volume fraction and number) had a higher R-2 for predicting K-sat than macropore morphology (branch density, connectivity density, and junction density). Moreover, the grayscale values were negatively correlated with K-sat and accounted for 78.8% of the variation in K-sat. Grayscale values, volume fraction, and the number of macropores were the best combination of CT-measured parameters for predicting K-sat, accounting for 81.9% of the variation in K-sat. The CT-measured parameters could be used to estimate K-sat in the upper root zone in the Loess Plateau.