The spatio-temporal pattern of excess N 2 production due to denitrification and gaseous nitrogen (N) removal via N 2 emission were investigated for a large agricultural river (the Jiulong River) in southeast China. During 2010–2011, direct measurement of excess dissolved N 2 (ΔN 2 , denitrification product) using the N 2 :Ar ratio method was carried out along the river network (North River and West River). The results showed that ΔN 2 ranged from 24 to 71μmolN 2 L −1 in the West River, and from 3 to 160μmolN 2 L −1 in the North River. Areal mean N 2 water–air flux was 8.66kgNha −1 yr −1 for the whole river network. “Hot spots” of denitrification were found in the upper North River and lower West River, where animal/human wastes dominated the riverine N source. Dissolved inorganic nitrogen (DIN) and dissolved oxygen are the key factors controlling the spatial variation of ΔN 2 . ΔN 2 generally peaked in January and during the warm-wet season from May through October, indicating an interactive effect of water temperature and DIN concentration on denitrification. Gaseous N removal fraction (E d =22% of [DIN]; annual N removal=24% of riverine N export) through net denitrification is comparable with other river-estuaries around the world. Gaseous N removal effectiveness generally decreased with N enrichment and was constrained in those hyper-N rich river reaches. Management of N loss should consider the terrestrial and aquatic systems, in order to reduce N export from watershed to coast and mitigate eutrophication in this region.