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| Titel |
Effect of photosynthesis on the abundance of 18O13C16O in atmospheric CO2 |
| VerfasserIn |
Magdalena E. G. Hofmann, Thijs L. Pons, Martin Ziegler, Lucas J. Lourens, Thomas Röckmann |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250134894
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| Publikation (Nr.) |
 EGU/EGU2016-15671.pdf |
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| Zusammenfassung |
| The abundance of the isotopologue 18O13C16O (Δ47) in atmospheric air is a promising new
tracer for the atmospheric carbon cycle (Eiler and Schauble, 2004; Affek and Eiler, 2006;
Affek et al., 2007). The large gross fluxes in CO2 between the atmosphere and biosphere are
supposed to play a major role in controlling its abundance. Eiler and Schauble (2004) set up a
box model describing the effect of air-leaf interaction on the abundance of 18O13C16O in
atmospheric air. The main assumption is that the exchange between CO2 and water
within the mesophyll cells will imprint a Δ47 value on the back-diffusing CO2 that
reflects the leaf temperature. Additionally, kinetic effects due to CO2 diffusion
into and out of the stomata are thought to play a role. We investigated the effect of
photosynthesis on the residual CO2 under controlled conditions using a leaf chamber
set-up to quantitatively test the model assumptions suggested by Eiler and Schauble
(2004).
We studied the effect of photosynthesis on the residual CO2 using two C3 and one C4
plant species: (i) sunflower (Helianthus annuus), a C3 species with a high leaf conductance
for CO2 diffusion, (ii) ivy (Hedera hibernica), a C3 species with a low conductance, and
(iii), maize (Zea mays), a species with the C4 photosynthetic pathway. We also
investigated the effect of different light intensities (photosynthetic photon flux density of
200, 700 and 1800 μmol m2s−1), and thus, photosynthetic rate in sunflower and
maize.
A leaf was mounted in a cuvette with a transparent window and an adjustable light source.
The air inside was thoroughly mixed, making the composition of the outgoing air equal to the
air inside. A gas-mixing unit was attached at the entrance of the cuvette that mixed air with a
high concentration of scrambled CO2 with a Δ47 value of 0 to 0.1‰ with CO2 free air to set
the CO2 concentration of ingoing air at 500 ppm. The flow rate through the cuvette was
adjusted to the photosynthetic activity of the leaf so that the CO2 concentration at
the outlet was 400 ppm and varied between 0.6 and 1.5 L min−1. CO2 and H2O
concentrations in air were monitored with an IRGA and air was sampled at the outlet with
flasks.
We found that the effect on Δ47 of the residual CO2 for the C3 species sunflower and ivy
was proportional to the effect on δ18O of the residual CO2. The difference in Δ47 between
the in- and outgoing CO2 was between -0.07 and 0.49‰ varying with the CO2
concentration in the chloroplasts relative to the bulk air (Cc/Ca). The Cc/Ca depends on
conductance and photosynthetic activity, and was different for the two species and
was manipulated with the light intensity. For the C4 species maize, a Δ47 value of
-0.08±0.02‰ was observed. The slightly negative effect on Δ47may be related to its lower
Cc/Ca ratio and possibly a lower carbonic anhydrase activity causing incomplete
exchange with leaf water. We will discuss these results in light of the suggested
fractionation processes and discuss the implication for the global Δ47 value of atmospheric
CO2.
References
Affek H. P. and Eiler J. M., GCA 70, 1–12 (2006).
Affek H. P., Xu X. and Eiler J. M., GCA 71, 5033–5043 (2007).
Eiler J. M. and Schauble E., GCA 68, 4767–4777 (2004). |
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