ISOFLUXES: Direkt eddy covariance measurements of the stable isotope composition of CO2 and H2O fluxes between forest and atmosphere

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The stabile isotopes 13C and 18O can be used to constrain carbon and water fluxes at local to global levels, to provide independent data to challenge biogeochemical models, and to inform us about sink and sources as well as underlying physiological processes. The interpretation of stable isotope data in carbon and water fluxes between terrestrial ecosystem and the atmosphere is, however, limited by the missing availability of long-term and continuous datasets. Due to technical challenges, simultaneous measurements of 13C and 18O in CO2 and H2O fluxes have not been carried out so far on annual time scale. Here we propose to overcome this knowledge gap and to investigate the variability of isotopologue fluxes in CO2 and water vapor at ecosystem scale over two years using the eddy covariance technique and state-of-the-art laser spectrometry. We will employ a continuous wave quantum cascade laser spectrometer for CO2 isotopologues and an off-axis integrated cavity output spectrometer for H2O vapor isotopologues for high frequency online measurements (up to 10 Hz). Measurements will be carried out at one of our flux towers above a managed 30 m tall beech forest in Central Germany. Along with meteorological measurements, we will identify environmental drivers on isotopologue flux variability. We will further use the water isotopologue flux measurements to quantify leaf water enrichment at ecosystem scale, which will be validate against direct leaf water isotope measurements. For that leaf, xylem, soil water, and precipitation will be sampled with three short-term intensive campaigns and a seasonal biweekly approach. The Péclet-modified Craig-Gordon leaf-water enrichment model will be used to link the leaf water isotope composition with the isotope composition of ecosystem scale water vapor fluxes. Additionally, the data will be used to develop and constrain a multi-layer numerical model for water isotope fluxes at ecosystem scale. Finally, we want to characterize the exchange of 18O in CO2 and H2O aiming to improve our understanding of the role of the enzyme carbonic anhydrase (CA) at ecosystem scale. CA is an important enzyme for photosynthetic processes and facilitates the exchange of 18O between CO2 and H2O. Measuring ecosystem scale fluxes of 18O in CO2 and H2O will provide us the unique opportunity to indirectly estimate CA activity integrated over the ecosystem continuously over the entire growing season. Overall, this project will provide worldwide unique data that will improve our understanding of stable isotope related processes at ecosystem scale and that provides independent constrains for ecosystem carbon and water models.