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| Titel |
Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment |
| VerfasserIn |
U. Gamnitzer, A. B. Moyes, D. R. Bowling, H. Schnyder |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 8, no. 5 ; Nr. 8, no. 5 (2011-05-26), S.1333-1350 |
| Datensatznummer |
250005825
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| Publikation (Nr.) |
 copernicus.org/bg-8-1333-2011.pdf |
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| Zusammenfassung |
The carbon isotopic composition (δ13C) of CO2 efflux
(δ13Cefflux) from soil is generally interpreted to represent the actual
isotopic composition of the respiratory source (δ13CRs). However, soils contain a large
CO2 pool in air-filled pores. This pool receives CO2 from belowground
respiration and exchanges CO2 with the atmosphere (via diffusion and advection) and
the soil liquid phase (via dissolution). Natural or artificial modification of
δ13C of atmospheric CO2 (δ13Catm) or
δ13CRs causes isotopic disequilibria in the soil-atmosphere system. Such
disequilibria generate divergence of δ13Cefflux from δ13CRs
(termed "disequilibrium effect").
Here, we use a soil CO2 transport model and data from a
13CO2/12CO2 tracer experiment to quantify the disequilibrium between
δ13Cefflux and δ13CRs in ecosystem respiration. The model accounted for diffusion of
CO2 in soil air, advection of soil air, dissolution of CO2 in soil water, and
belowground and aboveground respiration of both 12CO2 and
13CO2
isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a
δ13Catm of −46.9 ‰ during daytime for 2 weeks. Nighttime
δ13Cefflux from the ecosystem was estimated with three independent methods:
a laboratory-based cuvette system, in-situ steady-state open chambers, and in-situ closed
chambers.
Earlier work has shown that the δ13Cefflux measurements of the laboratory-based and steady-state systems were consistent,
and likely reflected δ13CRs. Conversely, the
δ13Cefflux measured using the closed chamber technique differed from these by
−11.2 ‰. Most of this disequilibrium effect (9.5 ‰) was predicted by the
CO2 transport model. Isotopic disequilibria in the soil-chamber system were
introduced by changing δ13Catm in the chamber headspace at the onset of the
measurements. When dissolution was excluded, the simulated disequilibrium effect was only
3.6 ‰. Dissolution delayed the isotopic equilibration between soil CO2 and
the atmosphere, as the storage capacity for labelled CO2 in water-filled soil pores
was 18 times that of soil air.
These mechanisms are potentially relevant for many studies of δ13CRs in soils
and ecosystems, including FACE experiments and chamber studies in natural
conditions. Isotopic disequilibria in the soil-atmosphere system may result from temporal
variation in δ13CRs or diurnal changes in the mole fraction and
δ13C of atmospheric CO2. Dissolution effects are most important
under alkaline conditions. |
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