Nitrite consumption and associated isotope changes during a river flood event

In oceans, estuaries, and rivers, nitrification is an important nitrate source, and stable isotopes of nitrate are often used to investigate recycling processes (e.g. remineralisation, nitrification) in the water column. Nitrification is a two-step process, where ammonia is oxidised via nitrite to nitrate. Nitrite usually does not accumulate in natural environments, which makes it difficult to study the single isotope effect of ammonia oxidation or nitrite oxidation in natural systems. However, during an exceptional flood in the Elbe River in June 2013, we found a unique co-occurrence of ammonium, nitrite, and nitrate in the water column, returning towards normal summer conditions within 1 week. Over the course of the flood, we analysed the evolution of delta N-15-NH4+ and delta N-15-NO2- in the Elbe River. In concert with changes in suspended particulate matter (SPM) and delta N-15 SPM, as well as nitrate concentration, delta N-15-NO3- and delta O-18-NO3-, we calculated apparent isotope effects during net nitrite and nitrate consumption. During the flood event, >97% of total reactive nitrogen was nitrate, which was leached from the catchment area and appeared to be subject to assimilation. Ammonium and nitrite concentrations increased to 3.4 and 4.4 mu mol L-1, respectively, likely due to remineralisation, nitrification, and denitrification in the water column. delta N-15-NH4+ values increased up to 12 parts per thousand, and delta N-15-NO2- ranged from -8.0 to -14.2 parts per thousand. Based on this, we calculated an apparent isotope effect (15)epsilon of -10.0 +/- 0.1 parts per thousand during net nitrite consumption, as well as an isotope effect (15)epsilon of -4.0 +/- 0.1 parts per thousand and (18)epsilon of 5.3 +/- 0.1 parts per thousand during net nitrate consumption. On the basis of the observed nitrite isotope changes, we evaluated different nitrite uptake processes in a simple box model. We found that a regime of combined riparian denitrification and 22 to 36% nitrification fits best with measured data for the nitrite concentration decrease and isotope increase.