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

As an 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 multisphere 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 rocks and soil has been exacerbated. This study aimed to analyze the impact of nitrogen input on the infiltration characteristics of rocks and soil in the Taihang Mountains. To this end, indoor soil column simulation experiments was conducted. The effects of nitrogen input on the infiltration performance of two different textured soils, clastic rock and brown soil, were analyzed under six different nitrogen concentration solutions (0, 25, 50, 75, 100, 125 mg·L^–1 treatment, the infiltration time for clastic rock was shorter than that for brown soil, with the greatest difference observed at a nitrogen concentration of 25 mg·L^–1. Significant differences in infiltration times were observed under different nitrogen concentrations. For both brown soil and clastic rock, the infiltration time decreased as the nitrogen concentration increased, although the extent of reduction varied slightly. brown soil showed a sharp initial decrease followed by a gradual reduction, whereas clastic rock exhibited a small initial decrease followed by a sharp decline. Under the same nitrogen concentration, the cumulative infiltration of clastic rock was greater than that of the brown soil (excluding N5). Noticeable differences were observed in the cumulative infiltration under different nitrogen concentrations, with the cumulative infiltration increasing as the nitrogen concentration increased. The cumulative infiltration of the control group fell between the low-concentration nitrogen treatments (25, 50, and 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 the wetting-front advancement distance was consistent with that of the 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 associated with the type of rock and soil texture.