Effects of different nitrogen concentrations on the infiltration performance of rock and soil in the Taihang Mountains region

As the ecological barrier for the North China Plain and an important water conservation area for the Beijing-Tianjin-Hebei region, the Taihang Mountains have a profound impact on regional water security and ecosystem service functions through their water-soil-atmosphere-biology multi-sphere interface processes. In recent years, intensive agricultural activities, mineral exploitation, and climate change have led to a sharp increase in nitrogen input in this region. Coupled with the development of fractured rock masses, shallow soil layers, and significant heterogeneity within the critical zone of the mountains, the complexity of water infiltration into the rock and soil has been further exacerbated. To analyze the impact of nitrogen input on the infiltration characteristics of rock and soil in the Taihang Mountains, this study conducted indoor soil column simulation experiments. The effects of nitrogen input on the infiltration performance of two different textured soils—debris and cinnamon soil—were analyzed under six different nitrogen concentration solutions (0, 25, 50, 75, 100, 125 mg·L⁻¹). The results show that under the same nitrogen concentration treatment, the infiltration time for debris was shorter than that for cinnamon soil, with the greatest difference observed at a nitrogen concentration of 25 mg·L⁻¹. There were significant differences in infiltration times under different nitrogen concentration treatments. For both cinnamon soil and debris, the infiltration time decreased as the nitrogen concentration increased, though the extent of reduction varied slightly: cinnamon soil showed a sharp initial decrease followed by a gradual reduction, while debris exhibited a small initial decrease followed by a sharp decline. Under the same nitrogen concentration treatment, the cumulative infiltration of debris was greater than that of cinnamon soil (excluding N5). There were noticeable differences in cumulative infiltration under different nitrogen concentration treatments, with cumulative infiltration increasing as the nitrogen concentration rose. The cumulative infiltration of the control group (CK) fell between the low-concentration nitrogen treatments (25, 50, 75 mg·L⁻¹) and the high-concentration nitrogen treatment (125 mg·L⁻¹), and its trend was similar to that of infiltration time. The fitting relationship between cumulative infiltration and time followed a power function infiltration model. The variation pattern of wetting front advancement distance was consistent with that of cumulative infiltration. The results indicate that the nitrogen concentration in the infiltrating water significantly affects the infiltration performance of rock and soil, and the extent of this impact is closely related to the type of rock and soil texture.