ALBERT

Description: In oceans, estuaries, and rivers, nitrification is an important nitrate source, and stable isotopes of nitrate are often used to investigate recycling processes in the water column. The bulk isotope effect of nitrification is hard to predict: It is a two-step-process, where ammonia is oxidized via nitrite to nitrate. Nitrite usually does not accumulate in natural environments, which makes it even more difficult to unravel the divergent isotope effects of both processes. However, during an exceptional flood in the Elbe River in June 2013, ammonium and nitrite accumulated in the water column for a short period, returning towards normal summer conditions within one week. Concentrations were sufficient for the analysis of δ15N-NH4+ and δ15N-NO2− evolution, which has not been studied before in a major European river like the Elbe River. In the concert with changes in SPM and δ15N-SPM, as well as nitrate concentration, δ15N-NO3− and δ18O-NO3−, we calculated the isotope fractionation effect during nitrification. We found that in the water column, ammonium and nitrite derived from internal recycling processes, whereas nitrate mainly leached from catchment area. Ammonium and nitrite concentrations increased to 3.4 µmol L−1 and 4.5 µmol L−1, respectively, due to remineralization and ammonium oxidation in the water column. δ15N-NH4+ values increased up to 12 ‰, and δ15N-NO2− ranged from −8.0 ‰ to −14.2 ‰. As water column nitrite concentration decreased, we calculated an isotope effect 15ε of −9.3 ‰ for nitrite oxidation. This isotope effect does not correspond to the inverse isotope fractionation with a positive 15ε proposed by pure culture studies. We hypothesize that the molecular mechanisms that lead to inverse fractionation also apply in natural environments, but that the resulting trend in δ15N-NO2− in this natural environment is masked by dilution with fresh nitrite stemming from ammonium oxidation. Our data are a first approximation of the isotope effect of nitrite oxidation in natural environments and highlight that pure culture results cannot readily be extrapolated to natural microbial assemblages or water bodies.