Soil Infiltration Capacity Affected by Salinization Degree in Soda Saline–Alkali Soils of the Songnen Plain, China

Understanding the differences in the infiltration processes of soda saline–alkali soils with varying degrees of salinization and their underlying mechanisms is of great significance for the rational use of regional soil and water resources. This study was conducted in the Songnen Plain, one of the world’s three major saline–alkali soil distribution areas, where the salt composition is dominated by sodium bicarbonate and sodium carbonate. Five types of soda saline–alkali soils with different degradation levels were selected from the study area. Using a one-dimensional vertical constant-head single-ring infiltration method, characteristic parameters of the infiltration process were measured through in situ experiments. Based on principal component analysis (PCA), a comprehensive multi-parameter infiltration capacity index (SICI) was constructed. Pathway analysis was further employed to explore the potential relationship between soil physical and saline–alkali characteristics and the infiltration process. The results showed that compared to the initial infiltration rate, the steady-state infiltration rates of the five soils decreased significantly by 41.81%, 64.87%, 97.20%, 99.24%, and 99.59%, respectively. Notably, the steady-state infiltration rate of the most severely degraded saline–alkali soil (Suaeda glauca) was only 0.13 mm·h−1. Correspondingly, Suaeda glauca soil exhibited the lowest SICI. Correlation and pathway analyses indicated that SICI was significantly associated with physical and saline–alkali parameters of the soda saline–alkali soils. Besides the direct associations of the fractal dimension of particle size distribution (D), non-capillary porosity (NCP), and salt content (SC) on SICI, D was also linked to lower SICI indirectly through its relationship with NCP, sodium adsorption ratio (SAR), and SC. The findings suggest that soil physical structure, particularly the fractal dimension of particle size distribution and pore characteristics, appears to be a primary factor influencing the infiltration capacity of highly soda saline–alkali soils, and that improving soil texture structure and enhancing soil porosity could be prioritized in the restoration and management of severely degraded soda saline–alkali lands.