Runoff Nitrogen Loss Characteristics in Small Agricultural Catchments of Subtropical Dry–Hot Valleys

Nitrogen export dynamics in small watersheds of subtropical dry–hot valleys remain poorly characterized. This study investigated runoff nitrogen patterns in a typical mango planting area lacking long-term monitoring by coupling in situ monitoring with the SCS-CN model to optimize parameters and the river-entry coefficient during the research period of 2021–2022. Results showed that runoff nitrogen concentration peaked significantly from May to August (p < 0.05), with NO3-N as the dominant form, accounting for 51.50~76.56% of total nitrogen. The SCS-CN parameter λ exhibited a highly significant linear correlation with monthly rainfall (p < 0.01), ranging from 0.07 to 0.30. Spatially corrected TN river-entry coefficient (0–0.50) were elevated in riparian zones with steeper slopes. Annual TN loads were 5.902 kg/ha and 5.566 kg/ha, respectively. Overall, May to August constitutes the critical period for nitrogen export in this dry–hot valley, primarily as NO3-N. Optimizing model parameters enhances the applicability of the SCS-CN model, revealing that nitrogen influx into rivers is concentrated near steep riverbanks, providing a scientific basis for pollution control in sloping orchards.