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| Titel |
The nitrogen isotope effect of benthic remineralization-nitrification-denitrification coupling in an estuarine environment |
| VerfasserIn |
M. Alkhatib, M. F. Lehmann, P. A. Giorgio |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 9, no. 5 ; Nr. 9, no. 5 (2012-05-04), S.1633-1646 |
| Datensatznummer |
250007020
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| Publikation (Nr.) |
 copernicus.org/bg-9-1633-2012.pdf |
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| Zusammenfassung |
| The nitrogen (N) stable isotopic composition of pore water nitrate and total
dissolved N (TDN) was measured in sediments of the St. Lawrence Estuary and
the Gulf of St. Lawrence. The study area is characterized by gradients in
organic matter reactivity, bottom water oxygen concentrations, as well as
benthic respiration rates. N isotope effects on the water column associated
with the benthic exchange of nitrate (εapp) and TDN
(εsed) during benthic nitrification-denitrification
coupling were investigated. The sediments were a major sink for nitrate and a
source of reduced dissolved N (RDN = DON + NH4+). We observed
that both the pore water nitrate and RDN pools were enriched in 15N
relative to the water column, with increasing δ15N downcore in the
sediments. As in other marine environments, the biological nitrate isotope
fractionation of net fixed N loss was barely expressed at the scale of
sediment-water exchange, with ϵapp values
<3‰. The strongest under-expression (i.e. lowest εapp)
of the biological N isotope fractionation was observed at the
most oxygenated sites with the least reactive organic matter, indicating
that, through their control on the depth of the denitrification zone, bottom
water oxygen concentrations and the organic matter reactivity can modulate
εapp. For the first time, actual measurements of
δ15N of pore water RDN were included in the calculations of
εsed. We argue that large fractions of the
sea-floor-derived DON are reactive and, hence, involved in the development of
the δ15N of dissolved inorganic N (DIN) in the water column. In
the St. Lawrence sediments, the combined benthic N transformations yield a
flux of 15N-enriched RDN that can significantly elevate
εsed above εapp. Calculated
εsed values were within the range of 4.6 ± 2‰ and
were related to organic matter reactivity and oxygen penetration depth in the
sediments. ϵsed reflects the δ15N of the
N2 lost from marine sediments and thus best describes the isotopic
impact of fixed N loss from sediments on the oceanic fixed N pool. Our mean
value for εsed is larger than assumed by earlier work,
questioning current ideas with regards to the state of balance of the modern
N budget. |
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