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| Titel |
Biological N2 fixation mainly controlled by Sphagnum tissue N:P ratio in ombrotrophic bogs |
| VerfasserIn |
Tatjana Zivkovic, Tim R. Moore |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138594
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| Publikation (Nr.) |
 EGU/EGU2017-1656.pdf |
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| Zusammenfassung |
| Most of the 18 Pg nitrogen (N) accumulated in northern nutrient-poor and Sphagnum-dominated
peatlands (bogs and fens) can be attributed to N2-fixation by diazotrophs either associated
with the live Sphagnum or non-symbiotically in the deeper peat such as through methane
consumption close to the water table. Where atmospheric N deposition is low (< 0.2 g m−2
y−1), ombrotrophic bogs rely on N2-fixation as the primary source of N that sustains
primary production. Due to high energetic requirements, N2-fixation depends on the
available phosphorus (P). Anthropogenic impacts in the last 200 years increased
atmospheric N deposition, resulting in a switch from N to P limitation in Sphagnum,
suggested by the increase in tissue N:P to >16. It is unclear how Sphagnum-hosted
diazotrophic activity may be affected by N deposition and thus changes in N:P
ratio.
First, we investigated the effects of long-term addition of different sources of nitrogen (0,
1.6, 3.2 and 6.4 g N m−2 y−1as NH4Cl and NaNO3), and phosphorus (5 g P m−2 y−1as
KH2PO4) on Sphagnum nutrient status (N, P and N:P ratio), net primary productivity (NPP)
and Sphagnum-associated N2fixation at Mer Bleue, a temperate ombrotrophic bog. We
show that N concentration in Sphagnum tissue increased with larger rates of N
addition, with a stronger effect on Sphagnum from NH4 than NO3. The addition of P
created a 3.5 fold increase in Sphagnum P content compared to controls. Sphagnum
NPP decreased linearly with the rise in N:P ratio, while linear growth declined
exponentially with increase in Sphagnum N content. Rates of N2-fixation determined in
the laboratory significantly decreased in response to even the smallest addition of
both N species. In contrast, the addition of P increased N2 fixation by up to 100
times compared to N treatments and up to 5-30 times compared to controls. The
change in N2-fixation was best modeled by the N:P ratio, across all experimental
treatments.
Secondly, to test the role of N:P ratio on N2-fixation across a range of bogs, eight study
sites along the latitudinal gradient from temperate, boreal to subarctic zone in eastern Canada
were selected. From each bog, two predominant microptopographies, hummocks and
hollows, were tested for both N2-fixation activity in the laboratory and Sphagnum
tissue concentrations of N, P and N:P ratio. We found that 65% of the variance in
the increase of N2-fixation activity was explained by the decrease in N:P ratio in
hollows (n = 73) but only about 20% in hummocks (n = 78). Changes in neither
N or P concentration alone explained the increase in N2-fixation better than N:P
ratio. We interpret that the difference between hollows and hummocks results from
the availability of moisture that further limits N2-fixation. When moisture is not a
limiting factor, i.e. in hollows, N:P ratio will be the best predictor of N2-fixation in
bogs.
Both studies suggest that the relative P availability to Sphagnum-associated diazotrophs,
measured as a tissue N:P ratio, best describes N2-fixation activity in bogs, especially ones
exposed to larger N deposition. |
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