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| Titel |
A case study of nitrification and nitrite isotope fractionation in a eutrophic temperate river system |
| VerfasserIn |
Juliane Jacob, Kirstin Dähnke, Tina Sanders |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089172
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| Publikation (Nr.) |
 EGU/EGU2014-3365.pdf |
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| Zusammenfassung |
| Stable isotopes of nitrate are often used to assess processing of nitrate in the water column of
oceans, estuaries, and rivers. In all these environments, nitrate regeneration via nitrification is
an important source of new nitrate. The bulk isotope effect of nitrification is hard to predict: It
is a two-step-process by distinct groups of microorganisms oxidizing ammonium to nitrate
via nitrite. Both processes have divergent isotope effects, and it is even more difficult to
unravel these effects in natural environments, because nitrite usually does not accumulate and
isotope analysis is not possible.
During our routine sampling scheme at the River Elbe an exceptional flood
occurred in June 2013, and nitrite and ammonium accumulated, allowing us to
investigate isotope fractionation of nitrification in a natural river system. We measured
nutrient concentrations, dual nitrate isotopes, δ15N-NO2, and, where possible,
δ15N-NH4.
Nitrate leached from catchment area, and δ15N-NO3 and δ18O-NO3 decreased from
typical spring bloom values (9.0 oand 3.5 o respectively) to winter nitrate background
values (7.4 oand 2.1 o respectively). This indicates that riverine assimilation was minimal
during the flood. Ammonium and nitrite concentrations increased to 12.5 μM and 5.7
μM, respectively, which likely was due to remineralization and nitrification in the
water column. Ammonium δ15N-NH4 values increased up to 12 oand nitrite
δ15N-NO2 values ranged from -4.8 oand -14.2 oá¹itrite oxidation and decreasing
concentrations were coupled with a fractionation factor 15É of -8.6 o following normal, and
not inverse, isotope fractionation. This deviates from findings in pure cultures of
nitrite-oxidizing bacteria. We assume that the mechanisms responsible for inverse
fractionation apply in natural environment as well, but that the resulting trend in
δ15N-NO2 is masked by dilution with fresh nitrite stemming from ammonium
oxidation. Our data are a first approximation of the natural bulk isotope effect of
nitrite oxidation in natural environments and highlight that pure culture results
cannot easily be extrapolated to natural microbial assemblages or water bodies. |
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