Backgrounds as a potentially important component of riverine nitrate loads.

Nitrate (NO3-) is a major trigger for river eutrophication. While efforts have been made to understand the anthropogenic NO3- pollution in rivers, the role of background NO3- in determining NO3- loads remains to be studied. In this study, we used dual-isotopes (δ15N/δ18O-NO3-) and an isotope-mixing model to reveal the natural and anthropogenic processes regulating the NO3- loads in a forest river that acts as a headwater source for the China's South to North Water Transfer Project. Even though the basin is sparsely populated, the mean NO3--N concentration (0.6 ± 0.2 mg/L) was much higher than the median concentration of global rivers (0.3 ± 0.2 mg/L). Meanwhile, the δ15N-NO3- was extremely depleted (as low as -14.4‰). The correlations between the NO3- concentrations and isotopes indicate that the nitrification of different sources (i.e., soil organic nitrogen, chemical fertilizer, manure, and sewage) dominates the NO3- loads. Soil organic nitrogen accounted for c.a. 60% of the riverine NO3- in the high-flow season, which alone exceeds China's national standard. This finding clearly shows that high NO3- loads in rivers could not all be ascribed to direct anthropogenic inputs, and background NO3- could be critical triggers. Therefore, when evaluating the NO3- pollution of rivers, the background NO3- concentrations must be considered along with the actual NO3- loads. In the low-flow season, the contribution from manure and sewage (c.a. 34%) increases. This study highlights the potentially important role of background NO3- in regulating riverine NO3- loads, providing important implications for understanding high riverine NO3- loads worldwide.