Effects of land use types on the macropore characteristics of saline-alkali soil in Inner Mongolia Region of China

Macropore structures can directly influence the hydraulic properties and solute transport in saline-alkaline soil, particularly in the stability and complexity. It is very crucial to explore the pore structure and its characteristic parameters for the better quality of the arable land. Therefore, this study aims to quantitatively analyze the structural parameters of the soil pores under different land-use types. Computed tomography and AVIZO image processing were used for the three-dimensional reconstruction and visualization of the soil pore structures in the paddy, sunflower field, and saline Wasteland. The results indicated that the paddy fields were characterized primarily by small and compact pores, indicating a relatively simple pore structure. In contrast, the sunflower field and saline wasteland shared sparse, large, and interconnected pores, indicating the more complex structures of the soil pore. Furthermore, the vertical extent of the soil pores in the sunflower field was lower than that in the saline wastelands. The total number of the macropore, total macropore volume, total macropore surface area, and macropore length in the paddy field were all higher than those in the sunflower field and saline wasteland. The porosity of all three land-use types decreased with the increasing soil depth, which was primarily concentrated in the 3.8-15 cm surface layer. The porosity of the 3.8-15 cm layer was 2.58 to 5.95 times that of the 15~38.6 cm layer. Across the 3.8-38.6 cm soil layer, the paddy field exhibited the highest macroporosity of 9.82×10⁻³-9.90×10⁻³ mm³/mm³, while the saline wasteland shared the widest macroporosity range of 6.07×10⁻³-10.33×10⁻³ mm³/mm³. The greater fractal dimension (1.93-1.94) and tortuosity (1.15-1.19) were observed in the paddy field, compared with the sunflower field and saline wasteland. The connectivity of the soil macroporosity in the sunflower field was superior to that in the paddy field and saline wasteland, with a branching node density of 2.05 number/cm³. The number of segments also reached up to 8 472. The equivalent hydraulic radius of the saline wasteland reached up to 1.37 mm. Furthermore, spatial heterogeneity was observed in the soil structural parameters among the samples of the same land-use type. The complexity of the soil pores was highly correlated with the total pore surface area, porosity, total volume, total length, and total number of pores, with correlation coefficients ranging from 0.84 to 0.97. Different land use types significantly dominated the pore structure of the saline soil. These findings can also provide valuable insights to explore the effects of the soil pore structure on the soil water and salt transport in the saline-alkali land.