Measuring the isotopic composition of ecosystem scale fluxes of CO2, water vapor and other trace gases in terrestrial ecosystems provides additional information on ecosystem-atmosphere interaction. It can reveal insights into ecosystem scale isotope discrimination for CO2 (Griffis et al. 2008, Sturm et al. 2012), provide an approximation of leaf water 18O enrichment at ecosystem scale (Lee et al. 2007a, Griffis et al. 2010) and quantify the isoforcing of ecosystems, i.e. the imprint of ecosystems to the isotopic composition of the atmosphere (Griffis et al. 2007, Lee et al. 2009). Furthermore, knowing the isotopic composition of ecosystem scale fluxes also provides independent constraints for evaluating ecosystem models (Baldocchi & Bowling 2003) and helps to partition the net ecosystem exchange of CO2 into its components, assimilation and respiration (Bowling et al. 2001, Ogée et al. 2004, Knohl & Buchmann 2005).
Measuring the isotopic composition of ecosystem scale fluxes is, however, technically challenging as it typically requires online and direct field measurements of the isotopes in the respective trace gas. Only the development of laser spectrometer for stable isotope analysis enables us to carry out such measurements. Before that people used relaxed eddy accumulation, a micrometeorological approach by conditionally sampling up and down draft of air parcels (Bowling et al. 1999), or the EC/flask approach using assumptions about constant relationships of the isotopic composition and the mixing ratio (Bowling et al. 2001, Ogée et al. 2004, Knohl & Buchmann 2005).
Using laser spectrometer two approaches are typically applied for ecosystem scale flux measurements: (a) the flux gradient approach and (b) the eddy covariance approach.
The flux gradient approach is well suited for short vegetation and when continuous, but not very fast (a few seconds) measurements are available. Isotopic measurements are carried out simultaneously or alternately in two heights above the vegetation. The isotopic composition, e.g. 18O in H2O, of the flux is then calculated (Lee et al. 2007b) as
where Rx is the ratio of the molar flux of the heavy isotopologue (a) to the molar flux of the light isotopologue (b). c1 and c2 denote the calibrated mixing ratio measurements of the respective isotopologues in height 1 and height 2. The molar flux ratio (Rx) is then converted to the delta notation ( x) in reference to an isotopic standard (Rs, e.g. VPDB or VSMOW).
The eddy covariance approach is well suited over tall vegetation, e.g. forest, but requires high frequency measurements (5-10 Hz) of the isotopic composition of the respective gas. The eddy flux Fa (μmol m 2 s 1) is calculated from the covariance of the vertical wind speed w (m s 1) and the mole fraction ca of the isotopologue (μmol mol 1) measured above the canopy (Sturm et al. 2012):
where Vm (m3 mol 1) is the molar volume of air, the covariance (cov) of w and ca is typically calculated over 30 minutes intervals. The delta value of the flux ( x) can then be derived from the eddy flux ratio of the heavy isotopologue (Fa) to the light isotopologue (Fb) in reference to an isotopic standard (Rs).
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