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FP6

ISOCYCLE Résumé de rapport

Project ID: 42268
Financé au titre de: FP6-MOBILITY
Pays: Switzerland

Final Activity Report Summary - ISOCYCLE (Tracing the carbon and water cycle in terrestrial ecosystems with stable isotope spectroscopy)

Understanding the processes and mechanisms that control carbon and water flow through terrestrial ecosystems is essential both to estimate the current capacity of terrestrial ecosystems to absorb carbon from the atmosphere and to predict the ecosystem's responses under a changing climate. Stable isotopes of carbon and oxygen provide a powerful tool to trace the flow of carbon and water through ecosystems and to elucidate the interplay between various ecosystem compartments. So far, stable isotope research has been restricted to discontinuous field sampling due to laborious flask sampling and expensive lab analysis. In this project we built a stable isotope ecology research group using novel state-of-the-art laser spectroscopy for continuous stable carbon and oxygen isotope measurements.

We quantified carbon and water isotope fluxes at multiple levels ranging from leaf to soil and to the entire ecosystem. We combined field observations using unique custom-made chamber measurements and eddy covariance flux measurements with manipulative drought experiments under controlled laboratory conditions as well as ecosystem scale modelling. Eddy covariance (EC) measurements provide a direct measure of the isotope flux ratio of CO2 and thus indicate the ecosystem fingerprint on the isotope budget of the atmosphere. For the first time, it was possible to obtain EC flux measurements of stable CO2 isotopologues over a forest. The 18O isotope discrimination is strongly reduced after precipitation events, reflecting 18O isotope exchange of CO2 with water in various water pools of the ecosystem. The 13CO2/12CO2 flux ratio is surprisingly constant on a diurnal timescale, but the substantial random noise in the half-hourly EC flux ratio puts a limit on resolving diurnal variations of only a few permil.

Using data simulation, we explored the limits of such EC set-ups to determine directly ecosystem discrimination on hourly or diurnal time scales. Linking the carbon and water cycle, we were able - for the first time so far - to measure and model with unprecedented time resolution the exchange of 13C and 18O in CO2 and of 18O and 2H in leaf gas exchange under field conditions. The stable isotope 18O in CO2 has been recognised to reflect a unique coupling between the hydrological and the carbon cycles of terrestrial ecosystems. The cycles are linked via the enzyme carbonic anhydrase, which catalyses CO2 (de)hydration and thus the exchange of 18O isotopes of H2O and CO2 in plant leaves and the soil. We investigated the extent to which this enzyme equilibrates CO2 and H2O (theta) using a novel instrument setup at field conditions.

Our analysis revealed lower values for theta then reported by laboratory measurements. These findings were supported by a short eddy covariance campaign in 2008 for which also a lower value than previously reported was found. This was the first time that eddy covariance and chamber measurements were used to estimate theta. In order to improve the understanding of the terrestrial carbon cycle in response to drought events, we performed manipulative drought experiments under defined laboratory conditions (using climate chambers) and used 13C in CO2 and 18O in H2O label to trace carbon and water flow. Overall, our 13C tracer experiments showed that the transport of freshly assimilated carbon to belowground is largely slowed down under drought conditions.

Furthermore, a strong diel coupling of leaf metabolism with soil respiration was found for both, control and drought stressed plants, which was driven by the alternate supply of leaf mobile carbon and transient leaf starch. Those results could be reproduced and substantiated by a carbon allocation model based on Bayesian probability. Furthermore, our results obtained from the H218O watering indicated a rapid exchange between the oxygen isotopes in leaf water and CO2, indicating a strong link of the carbon and water isotope cycle on the leaf scale.

Reported by

EIDGENOESSISCHE TECHNISCHE HOCHSCHULE ZUERICH
ZUERICH
Switzerland