The enrichment with heavy nitrogen ( 15 N) was carried out by using isotopically enriched each enrichment experiment. This tracer solution was released via a Duran glass Mariotte's bottle (Schott, Mainz, Germany) which assured a consistent release of the solution independent of the hydrostatic pressure within the bottle. In May and June 2021, a bottle of the labeled material was exposed to the river in order to allow a constant portion to leach into the stream over the course of six weeks. Water samples were collected at designated locations on a weekly basis at distances of 50, 100, 200, 300, 500, 750, 1000, 1500, and 2000 meters downstream from the administration point. Additionally, one sample was collected 50 meters upstream as a reference. The samples were subsequently frozen and stored until further analysis. Given that the modified LC-Isolink TM interface was not suitable for direct injection and measurement of nitrate from water samples, we proceeded to pre-concentrate the water samples after thawing in a vacuum evaporator. This involved evaporating 50 mL of each water sample at 60°C and under 50 mbar to approximately 1 mL, thereby enriching the samples by a factor of 50. Separate spectroscopic measurements of ammonium, nitrite, and nitrate (Tabel S6) demonstrated that the nitrate concentration in the river was approximately 7 mg L -1 , which corresponds to 1.58 mg L - 1 N. Enriching the nitrate concentration by vacuum evaporation 50-fold would result in a concentration of ~80 mg L -1 N, which should fall within the previously determined measurement range of the modified interface. Although the system still exhibits a non-linear shift in δ N values and peak areas, this shift should not impede the ability to detect enriched nitrogen isotope signatures. We proceeded to evaporate and measure a 1:50 diluted sample of the in-house standard at an initial concentration of 50 mg L -1 N. This was done to ascertain the impact of isotope fractionation effects during evaporation. Following evaporation, all samples were filtered through 0.2 µm PTFE filters (FisherScientific, Schwerte, Germany), and 10 µL of the resulting sample volume was directly injected into the system.