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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 19
Biogeosciences, 13, 5649–5659, 2016
https://doi.org/10.5194/bg-13-5649-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 13, 5649–5659, 2016
https://doi.org/10.5194/bg-13-5649-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 11 Oct 2016

Research article | 11 Oct 2016

Nitrite consumption and associated isotope changes during a river flood event

Juliane Jacob1,2, Tina Sanders1, and Kirstin Dähnke1 Juliane Jacob et al.
  • 1Institute for Coastal Research, Helmholtz Center Geesthacht, Geesthacht, Germany
  • 2Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, Hamburg, Germany

Abstract. 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 δ15N–NH4+ and δ15N–NO2 in the Elbe River. In concert with changes in suspended particulate matter (SPM) and δ15N SPM, as well as nitrate concentration, δ15N–NO3 and δ18O–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 µmol L−1, respectively, likely due to remineralisation, nitrification, and denitrification in the water column. δ15N–NH4+ values increased up to 12 ‰, and δ15N–NO2 ranged from −8.0 to −14.2 ‰. Based on this, we calculated an apparent isotope effect 15ε of −10.0 ± 0.1 ‰ during net nitrite consumption, as well as an isotope effect 15ε of −4.0 ± 0.1 ‰ and 18ε of −5.3 ± 0.1 ‰ 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.

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During a flood in the Elbe in June 2013, a unique co-occurrence of ammonium, nitrite and nitrate in the water column was found. SPM and nutrient concentrations as well as isotopes were analysed. We calculated an isotope effect 15ε of −10.0 ± 0.1 ‰ during net nitrite and 15ε of −4.0 ± 0.1 ‰ and 18ε of −5.3 ± 0.1 ‰ during net nitrate consumption. A simple box-model calculation results in combined riparian denitrification and 22 to 36 % nitrification.
During a flood in the Elbe in June 2013, a unique co-occurrence of ammonium, nitrite and nitrate...
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