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| Titel |
Oxygen isotope exchange and isotopic fractionation during N2O production by denitrification |
| VerfasserIn |
Dominika Lewicka-Szczebak, Reinhard Well, Jan Kaiser |
| 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 |
250097389
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| Publikation (Nr.) |
 EGU/EGU2014-12965.pdf |
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| Zusammenfassung |
| Stable isotopic analyses of N2O may help in quantification of soil denitrification processes.
N2O reduction to N2 during denitrification is associated with significant isotopic
fractionation. Theoretically, this would allow the product ratio (N2O/(N2+N2O)) to be
calculated based on the isotopic signature of the remaining N2O if the isotopic
signature of the produced N2O were precisely predictable. Oxygen isotopes are
especially useful, in particular the 17O excess, Δ(17O), to quantify any oxygen
isotope exchange between soil water and intermediate products (NO2-, NO). This
significantly influences the δ(18O) values of the N2O produced. Previous studies showed
this exchange to be nearly complete, up to 90% 1,2. However, there are very few
studies on the associated oxygen isotopic fractionation and its potential coupling to
the magnitude of oxygen isotope exchange with soil water 2,3. We hypothesize
that for high oxygen exchange, the oxygen isotopic fractionation will show little
variability.
In previous experiments, an 18O-enriched tracer was applied to quantify the magnitude of
oxygen exchange 1,2. Such an approach does not permit determining any oxygen isotope
fractionation. Here, we applied two novel experimental approaches: (1) waters of two
different δ(18O) values within the natural range (–1.5 and –14.8 orelative to VSMOW)
were used for rewetting the incubated soils and the oxygen exchange was calculated from
comparing the relative isotope ratio difference between N2O and H2O for these two water
treatments; (2) soils were amended with Chile Saltpeter characterized by a high
17O excess (20.2 o) and the 17O excess of the N2O product was measured. Both
approaches were applied simultaneously on the same soil samples, which allowed
quantifying the oxygen isotope exchange with two independent methods at the same time.
The N2O reduction was inhibited with acetylene method and δ(18O) values of the
N2O produced were measured to determine the oxygen fractionation during N2O
production.
Both methods showed nearly complete (95-99%) oxygen exchange with soil water for all
soil types (loamy sand and silt loam), water contents (50 to 80% water-filled pore space),
temperatures (8 and 22 ºC), and N2O production rates. The oxygen isotope fractionation
É(18O, N2O-H2O), defined as the relative isotope ratio difference between product
(N2O) and substrate (soil water), was quite stable between 16 and 20 o. This
confirms that δ(18O) values may be useful in determination of the product ratio.
Moreover, the very low 17O excess found in N2O indicates that the hypothesis of soil
denitrification contributing to the oxygen isotope excess of atmospheric N2O can be rejected
4,5.
1. Kool, et al., 2009. Rapid Communication in Mass Spectrometry 23: 104-108
2. Snider, et al., 2013. Geochimica et Cosmochimica Acta 112: 102-115
3. Lewicka-Szczebak, et al., 2014. submitted to Geochimica et Cosmochimica
Acta
4. Kaiser, et al., 2004. Journal of Geophysical Research-Atmospheres 109 (D3):
D03305
5. Michalski, et al., 2003. Geophysical Research Letters 30 (16): 1870 |
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