The measured nitrate concentrations in the river Rotbach were between 6 to 8 mg L -1 (see Table ) and the resulting nitrogen concentrations were around 1.2 mg L -1 N-NO3 -, which could not be measured directly due to low sensitivity. Thus, we measured the nitrogen isotope signature of N-NO3 -from concentrated river water one and two weeks after administration of isotopically enriched ammonium chloride. In contrast to expectations, δ N values of river nitrate stayed between +0.4 ± 0.4 ‰ and +4.1 ± 0.3 ‰ in those two weeks, varying over a small range of 3.7 ‰ (Table ). No trends in δ 15 N values were observed between sampling points or between sampling weeks (see Figure ). After the second week, the reference point had a δ 15 N value of +2.2 ± 1.7 ‰, which was well within the observed range of δ 15 N values of nitrate downstream the administration point. The δ 15 N values of the injection peaks for all samples during that time were much higher and ranged between +164.8 ± 8.1 ‰ and +272.4 ± 1.2 ‰, but also with no observable trend either between sampling points or weeks. The chromatograms of evaporated river water showed highly enriched compound eluting from the column right after the start of the injection peak (see Figure ), as the m/z 29/28 ratio showed a strong positive shift shortly after the beginning of the chromatographic peak, which was comparable to what we also observed in nonevaporated water samples. of magnitude lower compared to nitrate, which means that the isotope composition of nitrite in the water sample would be much higher than the measured δ 15 N value of ~90 ‰, since it coelutes with the much more abundant nitrogen injection peak and typically has a δ 15 N value of ~5 ‰. This is supported by measurements of evaporated river water, where the δ N value of the injection peak is even higher and reaches over +200 ‰, which would be a direct result of concentration of nitrite alongside nitrate in the sample material. In case of the administration study, our results suggest that concentration of nitrate in water samples via evaporation under vacuum is feasible without isotope fractionation and the measured δ 15 N values of evaporated as well as non-evaporated nitrate standards are statistically not distinguishable.