Objective  This paper aims to study the soil infiltration characteristics and its influencing factors of Robinia pseudoacacia forest in the loess gully region of western Shanxi Province of northern China, which could provide functional guidance for the precise improvement of stand structure.

  Method  We selected the stand age of 15, 25, and 35 years and the density of 800, 1 200, 1 600, 1 800, and 2 200 plant/ha Robinia pseudoacacia forest for double-ring infiltration test. Correlation between the physical and chemical properties of soil, such as soil porosity, bulk density, organic matter, and soil infiltration rate, was analyzed.

  Result  (1) To some extent, the steady infiltration rate increased with the increase of forest age and stand density. The initial infiltration rate and average infiltration rate increased with the increase of stand age. (2) Four models (Horton, Kostiakov, Philip, and general empirical model) were used to simulate the infiltration process of Robinia pseudoacacia forest in different stand ages and densities. The results showed that the average regression coefficient of general empirical model (0.977) > Horton model (0.958) > Kostiakov model (0.953) > Philip model (0.945). Therefore, the fitting effect of general empirical model was best. (3) According to principal component and correlation analysis, the soil infiltration performance was significantly correlated with the soil bulk density, organic matter, and water-stable aggregates. The initial soil moisture content was the main factor affecting the initial infiltration rate. The 1−2 mm water-stable aggregates, soil bulk density, and capillary porosity were the main factors affecting stable infiltration rate. The main factors affecting average infiltration rate were the initial soil moisture content and 0.5−1 mmwater-stable aggregates.

  Conclusion  In a certain range (800−2 200 plant/ha), with the increase of age and density of Robinia pseudoacacia forest, the soil structure is improved, and the soil infiltration performance is gradually improved. Under the same stand density, the 35-year Robinia pseudoacacia forest shows better infiltration performance than others.