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| Titel |
Linking carbon isotope signatures of nighttime leaf-respiratory and daytime assimilatory CO2 fluxes observed with laser spectrometry under field conditions |
| VerfasserIn |
Lydia Gentsch, Jerome Ogee, Lisa Wingate, Patrick Sturm, Rolf Siegwolf, Roland A. Werner, Nina Buchmann, Alexander Knohl |
| 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 |
250101789
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| Publikation (Nr.) |
 EGU/EGU2015-1021.pdf |
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| Zusammenfassung |
| The 13C/12C ratio (δ13C) of atmospheric CO2 is a valuable tool for constraining the impact of
the terrestrial biosphere on atmospheric CO2 dynamics. Alterations of the 13C signal of
terrestrial net CO2 fluxes are generally attributed to variations in photosynthetic 13C
discrimination. Yet, over the past decade, evidence has emerged that plant metabolism and
respiration modify the initial δ13C signature of recent photosynthetic assimilates. Such
postphotosynthetic δ13C modifications were reported for all plant organs, but leaf respiratory
metabolism may play a central role as it impacts carbon turnover in other plant tissues.
Leaf-respired CO2 is frequently 13CÂenriched with respect to leaf organic matter.
Mechanisms potentially explaining this enrichment include the differential use of carbon
sources, metabolite fragmentation or the expression of kinetic isotope effects of respiratory
enzymes. For global and ecosystem-scale applications of δ13C, it is now important to study,
under field conditions, the variability of δ13C in leaf-respired CO2 (δ13CRES) and the
deviation of the latter from δ13C of recent assimilates (δ13CAS). Here, we present 74 days
of hourly δ13C measurements for daytime assimilatory and nighttime respiratory
CO2 fluxes on leafy branches of three mature Fagus sylvatica trees in a temperate
forest. Measurements were conducted with a laser spectrometer (QCLAS-ISO,
Aerodyne Research Inc.) measuring CO2 isotopologue mixing ratios in ambient
and sampling air from photosynthetic gas exchange chambers. We used daytime
measurements of photosynthetic 13C discrimination for diurnally flux-weighted estimates of
δ13CAS, and found that flux-weighted δ13CRES roughly tracked previous-day shifts in
δ13CAS. Deviations between flux-weighted δ13CAS and δ13CRES were further
robustly predicted by previous-day assimilation, with δ13CRES displaying 13C
enrichment on low and 13C depletion on high assimilation days. On the hourly
timescale, δ13CRES either significantly decreased by more than 0.2‰ hr-1 or
remained relatively stable during one third of all nights each. For the other third
of all nights, observed δ13CRES patterns were highly variable. These nighttime
trends were not related to any monitored environmental (e.g. leaf temperature) or
physiological (e.g. previous-day assimilation) variable, nor to trend measures of the
respective variables. Given that nighttime leaf respiration is fully fuelled by starch
accumulated during the previous day, we simulated daytime synthesis and nighttime
degradation of layered starch granules, in which 13C signal and layer thickness
depended on daytime-measured δ13CAS and assimilation strength. Albeit disregarding
any potential metabolic and respiratory modification of the δ13CAS signature, the
simulation frequently produced δ13CRES patterns similar to the ones measured. In
conclusion, the results indicate that the observed δ13CRES variability on hourly
timescales probably originated in leaf catabolic processes, or it could also reflect hourly
variability of previous-day photosynthetic 13C discrimination. The relationship
between flux-weighted means of δ13CAS and δ13CRES asserted the strong link
between assimilatory and respiratory 13C signals reported in several ecosystem studies. |
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