Objectives  Excessive application of fertilizer will lead to excess soil nutrient accumulation, which will be washed into rivers and lakes by rainfall runoff, resulting in surface water pollution. This study aims to clarify the relationship between fertilization in agricultural land and nitrate (NO₃^–-N) pollution downstream for effectively controlling agricultural non-point source pollution.

  Methods  In the intensive sugarcane growing area of Guangxi, we selected Nala Watershed flowing into Kelan Reservoir as case study watershed. This watershed includes three sub-watersheds, S1, S2 and S3. We identified the source proportion of NO₃^–-N from three sub-watersheds into stream by using double stable isotope tracer (δ¹⁵N-NO₃^–, δ¹⁸O-NO₃^–) technique, and determination of main ions in combined surface water. Integrating with watershed monitoring, we analyzed the spatial-temporal variations in NO₃^–-N sources contributing to the stream and their response to fertilization in wet and dry seasons.

  Results  The concentration of NO₃^–-N in the stream of Nala Watershed ranged from 1.24 mg/L to 27.90 mg/L, exceeding the national standard for Type III surface water quality. A larger application of fertilizer was in wet season than in dry season resulted in significantly higher NO₃^–-N concentration of stream during wet season (mean 22.01 mg/L) than that in dry season (mean 5.64 mg/L). NO₃^–-N concentration gradually decreased from upstream to downstream in both wet and dry seasons, which might be caused by the cascade interception experiment applying vegetated ditches in the downstream area. The composition of δ¹⁵N-NO₃^– and δ¹⁸O-NO₃^– indicated that fertilization (chemical fertilizer and organic fertilizer) and soil nitrogen were the main sources of riverine nitrate pollution, accounting for more than 90% of all nitrate pollution contributions in the river. The contribution of NO₃^–-N sources to the stream in Nala Watershed decreased in the following order: fertilizer (42.9%) > soil nitrogen (28.6%) > organic fertilizer (19.2%) > atmospheric deposition (9.3%) in wet season, and soil nitrogen (39.7%) > chemical fertilizer (37.6%) > organic fertilizer (15.3%) > atmospheric deposition (7.4%) in dry season.

  Conclusions  Riverine nitrate pollution is positively correlated with fertilization in the intensive sugarcane growing watershed. Irrational application such as fertilizer spreading was the main cause for NO₃^–-N export. Thus, fertilizer deep application is strongly recommended for improving fertilizer utilization and reducing nitrogen inflow pollution. This may be an important practice to ensure the water safety of both people and livestock and agriculture green development.