Isotopic evidence on multiple sources of nitrogen in the northern Jiulong River, Southeast China

• The external sources change all the year around depending on environmental conditions.
• Riverine nitrate import of the northern Jiulong River is dominated by external processes.
• Manure and sewage show an increasing proportion of contribution to the downstream section and the estuary.
• The isotopic composition of riverine NO3−-N provides effective evidence for identifying nitrate sources in catchments.

Riverine export accounts for a large portion of estuarine and coastal nutrients and could lead to severe eutrophication. However, nitrogen (N) sources at the catchment scale remain unclear because of spatial and temporal variations. The stable isotope of 15N, which has been proven to be effective in deducing sources and testing biogeochemical behaviours, is applied in this study to explore multiple sources of riverine N and their nutrient concentrations in the northern section of Jiulong River catchment seasonally. Results show that drastic seasonal variation in external nitrate sources occurs in the river; manure and sewage dominate during base flows, whereas organic N in soil and atmospheric deposition dominate during storm flows. The external sources change throughout the year depending on the environmental conditions. Furthermore, riverine nitrate import in the northern Jiulong River is dominated by an external process because of the increasing NO3−-δN15 value accompanied by an increasing NO3−-N concentration (p   < 0.05). Manure and sewage show an increasing proportion of contribution to the downstream section and proximity to the estuary with high NO3−-δN15 values. The findings in this study highlight the fact that the isotopic composition of riverine NO3−-N provides effective evidence for identifying nitrate sources in catchments. The study could serve as a critical reference for nutrient management and eutrophication reversal.