Soil nitrogen distribution, vertical migration, and leaching vulnerability in a typical shallow soil mountainous catchment: Evidence from the Chaohe River catchment, northern China

<title>Abstract</title> <p> Mountainous catchments, characterized by strong spatial topography and complex landscapes, directly influenced the nutrient migration, leaching and biogeochemistry. Whether agricultural soil nitrogen was stably stored in mountainous catchments, and how soil physicochemical properties affected nitrogen migration, were critical for assessing these catchments scale soil nitrogen balance. This study investigated the soil nitrogen distribution, transformation and environmental factors in the typical small mountainous catchment, the Chaohe River Catchment in northern China. Results indicated that nitrogen was primarily enriched in the 0–20 cm soil layer, whereas 27% of NO ₃ ⁻ -N remained below 30 cm in maize fields. Although cropland accounted for only 14.6% of the catchment area, NO ₃ ⁻ -N concentration in maize fields were significantly higher than in shrub-grassland, and the average NO ₃ ⁻ -N accumulation reached 230.7 ± 94.5 kg·ha ^− 1 , which was higher than that in shrub-grassland and close to the reported average level for maize fields in the Beijing region. However, because the Chaohe River catchment had shallow soil profiles and limited deep storage capacity, this accumulated NO ₃ ⁻ -N indicated leaching vulnerability rather than stable deep storage. Isotopic analysis revealed that soil NO ₃ ⁻ -N primarily originated from chemical fertilizer inputs and soil nitrogen mineralization, undergoing vertical migration, while denitrification contributed only weakly to nitrogen removal. Among the factors influencing nitrogen migration, soil sand content, NH ₄ ⁺ -N, and SON concentration positively regulated NO ₃ ⁻ -N migration, whereas pH primarily exerted a negative regulatory effect. These findings provided a scientific basis for assessing soil nitrogen balance and controlling agricultural non-point source pollution in the Chaohe River Catchment. </p>