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| Titel |
Can Canopy Uptake Influence Nitrogen Acquisition and Allocation by Trees? |
| VerfasserIn |
Richard Nair, Mike Perks, Maurizio Mencuccini |
| 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 |
250105333
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| Publikation (Nr.) |
 EGU/EGU2015-4851.pdf |
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| Zusammenfassung |
| Nitrogen (N) fertilization due to atmospheric deposition of anthropogenic nitrogen (NDEP)
may explain some of the net carbon (C) sink (0.6-0.7 Pg y-1) in temperate forests, but
estimates of the additional C uptake due to atmospheric N additions (δCδN) can vary by
over an order of magnitude (~ 5 to 200 δCδN). High estimates from several recent studies
[e.g. Magnani (2007), Nature 447 848-850], deriving δCδN from regional correlations
between NDEP and measures of C uptake (such as eddy covariance -derived net
ecosystem production, or forest inventory data) contradict estimates from other
studies of 15N tracer applications added as fertilizer to the forest floor. A strong
δCδN effect requires nitrogen to be efficiently acquired by trees and allocated to
high C:N, long-lived woody tissues, but these isotope experiments typically report
relatively little (~ 20 %) of 15N added is found above-ground, with < 5 % of the total
15N applied found in wood. Consequently the high correlation-derived δCδN
estimates are often attributed to co-variation with other factors across the range of sites
investigated.
However 15N-fertilization treatments often impose considerably higher total N loads than
ambient NDEP and almost exclusively only apply mineral 15N treatments to the soil, often in
a limited number of treatment events over relatively short periods of time. Excessive N
deposition loads can induce negative physiological effects and limit the resulting δCδN
observed, and applying treatments to the soil may ignore the importance of canopy nitrogen
uptake in overall forest nutrition. As canopies can directly take up nitrogen, the chronic,
(relatively) low levels of ambient NDEP inputs from pollution may be acquired without some
of the effects of heavy N loads, obtaining this N before it reaches the soil, and allowing
canopies to substitute for, or supplement, edaphic N nutrition. The strength of this effect
depends on how much N uptake can occur across the canopy under field conditions, and
if this extra N supplies growth in woody tissues such as the stem, as well as the
canopy.
To test these ideas, we applied a low (~ 2.5 % above ambient NDEP) 15N
treatment to Picea sitchensis saplings, targeting the soil or the canopy in monthly
fertilizations for 16 months, and investigating 15N return in different age classes of
biomass and over time. While soil-targeted deposition treatments agreed well with
existing knowledge of N partitioning from this source, we could infer 2-3 times
more 15N was retained above-ground in canopy-targeted treatments, including a
relative increase in 15N allocation to stem and woody biomass when compared
to the soil treatment. These results suggest that existing forest 15N-fertilization
experiments could under-estimate the overall δCδN effect of atmospheric deposition. |
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