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| Titel |
High potential of nitrogen fixation in pristine, ombrotrophic bogs in Southern Patagonia |
| VerfasserIn |
Klaus-Holger Knorr, Marcus A. Horn, Nelson A. Bahamonde Aguilar, Werner Borken |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105263
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| Publikation (Nr.) |
 EGU/EGU2015-5876.pdf |
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| Zusammenfassung |
| Nitrogen (N) input in pristine peatlands occurs via natural input of inorganic N through
atmospheric deposition or biological dinitrogen (N2) fixation. However, N2 fixation is to date
mostly attributed to bacteria and algae associated to Sphagnum and its contribution
to plant productivity and peat buildup has been often underestimated in previous
studies. Based on net N storage, exceptionally low N deposition, and high abundance
of vascular plants at pristine peatlands in Southern Patagonia, we hypothesized
that there must be a high potential of non-symbiotic N2 fixation not limited to the
occurrence of Sphagnum. To this end, we chose two ombrotrophic bogs with spots
that are dominated either by Sphagnum or by vascular, cushion-forming plants
and sampled peat from different depths for incubation with 15N2 to determine N2
fixation potentials. Moreover, we analyzed 15N2 fixation by a nodule-forming,
endemic conifer inhabiting the peatlands. Results from 15N2 uptake were compared to
the conventional approach to study N2 fixation by the acetylene reduction assay
(ARA).
Using 15N2 as a tracer, high non-symbiotic N2 fixation rates of 0.3-1.4 μmol N g-1 d-1
were found down to 50 cm under micro-oxic conditions (2 vol.%) in samples from both plots
either covered by Sphagnum magellanicum or by vascular cushion plants. Peat N
concentrations suggested a higher potential of non-symbiotic N2 fixation under
cushion plants, likely because of the availability of easily decomposable organic
compounds as substrates and oxic conditions in the rhizosphere. In the Sphagnum plots,
high N2 fixation below 10 cm depth would rather reflect a potential fixation that
may switch on during periods of low water levels when oxygen penetrates deeper
into the peat. 15N natural abundance of live Sphagnum from 0-10 cm pointed to N
uptake solely from atmospheric deposition and non-symbiotic N2 fixation. 15N
signatures of peat from the cushion plant plots indicated additional N supply from N
mineralization.
Nitrogen fixation by the conifer Lepidothamnus fonkii was exceptionally high,
reaching 3.1 μmol N g-1 d.w. d-1 detected in roots, stems, and green biomass. For L.
fonkii, we could identify a specific association with Beijerinckiaceae as N2 fixing
bacteria in the root nodules, whereas the rhizosphere peat was dominated by other
diazotrophs.
The ARA considerably underestimated N2 fixation and can thus not be recommended for
peatland studies.
Our findings suggest that non-symbiotic or associative N2 fixation overcomes N
deficiency in different vegetation communities and has great significance for N cycling and
peat accumulation in pristine peatlands. |
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