Abstract

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.

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