Department of Micrometeorology

The determination of net ecosystem CO₂ exchange (NEE) has become a fundamental tool for the investigation of the carbon balance of terrestrial ecosystems. The derivation of the annual sum of NEE is complicated by characteristic diurnal and seasonal variation in the governing gross flux components of assimilation and respiration. Therefore, additional information is required to achieve both the partitioning of NEE into its component fluxes and an understanding of the corresponding processes. This might be provided by a tracer, which could identify the individual contributions to the net flux at ecosystem scale. The isotopic signature of CO₂—that is, the CO₂ isotope ratios with respect to ¹³CO₂—can serve as a tracer because photosynthetic uptake discriminates against the heavier isotope. Isotope mass balances may be determined by measurements of ¹³CO₂ iso-fluxes. A sufficient signal-tonoise ratio for the isotopic parameters is essential for the determination of ¹³CO₂ iso-fluxes. The hyperbolic relaxed eddy accumulation (HREA) method allows for maximized differences of the isotopic signatures in samples for appropriate laboratory analysis. HREA measurements of ¹³CO₂ iso-fluxes were conducted at FLUXNET station DE-Bay (Waldstein-Weidenbrunnen) during three intensive measuring campaigns in 1999, 2000, and 2003. Furthermore, HREA ¹³CO₂ iso-fluxes were measured at the extensively managed submontane grassland site “Voitsumra,” near the Waldstein-Weidenbrunnen site, during an intensive measuring campaign in 2010. Results of ¹³CO₂ isoflux measurements highlight their capabilities for NEE flux partitioning as well as for the examination of CO₂ exchange mechanisms over forests.