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Impact of short-term dynamics of isotopic fractionation during dark respiration: Scaling from leaf to ecosystem carbon fluxes.
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Edité par CCSD -
International audience. A functional understanding of respiratory processes and biosphere/atmosphere CO 2 exchange is important regarding global climate change. Isotopic fractionation during dark respiration is well described at the physiological scale. Much less is known on larger spatial and temporal scales, in spite of new evidence of large diurnal changes in δ 13 C of leaf (δ 13 C res) and ecosystem respired CO 2. Our broad species survey indicates marked differences between functional groups: evergreen, slow growing and aromatic species exhibit a large increase of δ 13 C res during the light period (4 to 8‰), whereas herbaceous, fast growing plants or temperate trees showed no significant diurnal changes. Utilization of positional 13 C-labelling of pyruvate proved that apparent fractionation processes in respiratory pathways caused the observed functional differences: the large diurnal enrichment of δ 13 C res in evergreens was due to increasing investment of pyruvate derived Acetyl-CoA into secondary metabolism (e.g. defence or aromatic compounds) while the Krebs-cycle activity remained constant. Fast growing herbs with a high respiratory demand exhibit no change in respiratory pathways during the day. Hence, allocation of carbon between respiratory pathways due to different metabolic demands for growth, maintenance or storage does induce large short-term variations in δ 13 C res. The impact of apparent fractionation during dark respiration for larger scales was assessed by application of Keeling plots and a rapid In-tube Incubation Technique 1 which enables high-time resolved measurements of δ 13 C res of different ecosystem components. Temporal pattern of respired δ 13 C res from ecosystem (δ 13 C R), soils, foliage and roots in a Mediterranean oak woodland reflected the response of respiratory processes to drought. Large variation in δ 13 C res occurred in foliage and roots, which were in accordance with above described pattern of post-photosynthetic fractionation. The isotopic pattern and component fluxes were used in a mass balance model to scale from leaf to ecosystem responses and partition ecosystem carbon fluxes. The model indicated that changes in the relative contribution of isofluxes from different ecosystem component induce large variation in δ 13 C R. This shows the substantial impact of apparent fractionation during dark respiration on larger scales and contributes to a process-based understanding of isotopic variation at the ecosystem level.
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