Backgrounds as a potentially important component of riverine nitrate loads.

Nitrate (NO) is a major trigger for river eutrophication. While efforts have been made to understand the anthropogenic NO pollution in rivers, the role of background NO in determining NO loads remains to be studied. In this study, we used dual-isotopes (δN/δO-NO) and an isotope-mixing model to reveal the natural and anthropogenic processes regulating the NO 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 NO-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 δN-NO was extremely depleted (as low as -14.4‰). The correlations between the NO concentrations and isotopes indicate that the nitrification of different sources (i.e., soil organic nitrogen, chemical fertilizer, manure, and sewage) dominates the NO loads. Soil organic nitrogen accounted for c.a. 60% of the riverine NO in the high-flow season, which alone exceeds China's national standard. This finding clearly shows that high NO loads in rivers could not all be ascribed to direct anthropogenic inputs, and background NO could be critical triggers. Therefore, when evaluating the NO pollution of rivers, the background NO concentrations must be considered along with the actual NO 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 NO in regulating riverine NO loads, providing important implications for understanding high riverine NO loads worldwide.