 |
| Titel |
C and N stable isotope inventories in CO2, CH4, DOC, DON, and peat solid phase in three southern Patagonian ombrotrophic bogs |
| VerfasserIn |
Tanja Broder, Klaus-Holger Knorr, Christian Blodau |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053261
|
|
|
|
|
|
| Zusammenfassung |
| Peatlands in Southern Patagonia have become the focus of a small, yet increasing number of
studies. Large, pristine bogs and mires provide archives of the past climate and environmental
conditions. Therefore, researchers have investigated peat accumulation, decomposition, and
vegetation changes to elucidate the history of this landscape.
In peatlands, analysis of stable isotopes of carbon and nitrogen has been increasingly used
to study peat accumulation, decomposition, past environmental conditions, and
biogeochemical processes. Yet available studies so far were almost exclusively carried out in
northern peatlands and there is limited knowledge about carbon and nitrogen isotope
patterns in southern peatlands. To interpret shifts in isotopic composition of solids and
solutes, typical profiles and levels of variation at various locations need to be known.
Therefore we investigated three ombrotrophic bogs in Southern Patagonia for isotopic
composition of dissolved CO2 and CH4, DOC, DON, and peat solid phase C and N. The
three sites differed in terms of precipitation, vegetation, and exposition to sea-spray
input.
Values of δ13C in the peat of the driest site were between -27 to -26 per mil with a slightly
increasing trend with depth down to 150 cm, and decreased to -28 per mil below (sampling
depth 300 cm). DOC had almost similar isotopic composition as the solid phase (diff. < 0.5
per mil) in the upper 150 cm and was about 1 per mil enriched in 13C below that depth.
At the site with intermediate precipitation, we found a δ13C in the solid phase of
about -26 per mil in the upper profile, slightly decreasing with depth to about -27
per mil in 300 cm. Here, DOC was depleted in 13C (0.5 to 1 per mil) in the upper
part, but adjusted to the signatures of the solid phase in 200 cm depth. This pattern
was also observed at the wettest site. Nevertheless, we could neither identify clear
common trends, nor attribute certain factors to the shifts in isotopic composition.
All variations were within reported ranges for typical peatland vegetation even
under similar environmental conditions. Values of δ15N in the peat solid phase were
around 0 per mil at all sites, regardless of depth. Contrarily, in the DON there was an
obvious enrichment of 15N, ranging from 1 to >15 per mil, that increased with depth.
We attribute this enrichment of 15N in DON to multiple recirculations and a high
proportion of microbial N in this pool, while the light isotopes are preferentially
mineralized.
Isotopic composition of dissolved gases ranged from -23 to +4 per mil for CO2,
increasing with depth, and from -75 to -63 per mil for CH4, more or less following the CO2
signature. The δ13C of CO2 was thus dominated by methanogenic activity, leading to residual
13C enrichment. This was further supported by fractionation factors aC between CO2
and CH4 in a range of 1.050 to 1.075, suggesting dominance of hydrogenotrophic
methanogenesis. Interestingly, at all sited fractionation factors converged to about 1.070 at a
depth of 100 cm, which is probably linked to a thermodynamic limitation of the
methanogens, according to available microbiological studies.
In summary, this study showed relatively small variations in carbon isotopic signatures of
solids and DOC, despite obvious differences of the sites in terms of climate, decomposition,
and vegetation. Interpretation of such data in light of paleoclimatic conditions thus needs
to be done with care. While δ15N in the solid phase was relatively constant, the
shifts in DON may merit more attention. Isotopic composition of dissolved gases
was comparable to previous studies and typical for methanogenic environments. |
|
|
|
|
|
|