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| Titel |
Influence of environmental factors on dissolved nitrate stable isotopes under denitrifying conditions - carbon sources and water isotopes |
| VerfasserIn |
A. Wunderlich, R. Meckenstock, F. Einsiedl |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063479
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| Zusammenfassung |
| Stable isotopes in dissolved nitrate are regularly used to identify sources of nitrate
contamination in aquifers and water bodies. A dual isotope plot of 15N and 18O in nitrate can
provide good evidence of the origin of such pollution as various sources have different
isotopic signatures. Microbial denitrification changes both isotopic values by removing
nitrate with lighter isotopes first, thereby increasing δ18O as well as δ15N. This change can
distort the determination of sources but also has the potential to be used to identify and
quantify microbial denitrification. Previous studies found a wide range of enrichment factors
(É) that did not allow conclusions towards the extent of microbial denitrification. However,
it was found that during denitrification at each respective field site or laboratory
experiment, there was a constant ratio in increase of the values of δ18O in relation
to δ15N. That ratio was, however, not constant across field sites and the values
published range from below 0.5 to more than 1.0. The reasons for these variations in
enrichment factors and relative enrichment of oxygen compared to nitrogen are yet
unknown.
We conducted microcosm experiments with three different bacterial species to
elucidate possible influences of environmental factors on these parameters. As a result
we conclude that the type of carbon source available to denitrifying bacteria can
play a role in the value of the enrichment factors, but not in the relative enrichment
of the two isotopes. Specifically we found that complex hydrocarbons (toluene,
benzoate) produce significantly different enrichment factors in nitrate than a simple
hydrocarbon substrate (acetate). The relative enrichment of δ18O compared to δ15N was
0.86.
We hypothesise that this influence is based on a variation in process kinetics of
cross-membrane nitrate transport in relation to intracellular nitrate reduction. The core of the
hypothesis is that nitrate transport into the cell becomes rate limiting as a result of a carbon
source induced change in cell membrane composition. The apparent kinetic isotope effect
observed outside the cell is then changed as transport-related isotope effects dominate the
observations.
In addition, a possible effect of water δ18O values on the δ18O of dissolved nitrate was
researched. Intermediary nitrite is known to exchange oxygen atoms with water; a reverse
reaction of the nitrate reducing step could thus influence the oxygen isotope composition of
dissolved nitrate without changing the nitrogen isotopic composition in the same way.
Such a process was already shown for sulfate reduction. By adding 18O-labelled
water to microcosm experiments, we could show that such an exchange exists for
selected microorganisms. The environmental implications of this result is discussed. |
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