Critical evaluation of micrometeorological methods for measuring ecosystem–atmosphere isotopic exchange of CO2

Isotopic net ecosystem exchange (isofluxes, or flux densities of View the MathML source) can be combined with standard eddy covariance methods to partition net ecosystem exchange of carbon dioxide (F) into its component one-way fluxes, photosynthesis and respiration. At present, the approaches used to estimate isotopic fluxes are labor-intensive and dependent on several assumptions. To assess the relative utility of the available methods, we studied an ecosystem associated with large CO2 fluxes and maximal isotopic exchange. Three independent techniques were used to measure isotopic flux densities over an irrigated alfalfa field: (1) a combination of standard eddy covariance and flask sampling; (2) the flux-gradient method; and (3) hyperbolic relaxed eddy accumulation (HREA). Consistent isotopic flux results were obtained via the three methods, with similar diurnal patterns and peak midday isotopic flux densities of 600–700 μmol m−2 s−1 ‰. Air samples were collected over a wide range of CO2 mole fractions (325.3–597.5 μmol mol−1) and isotopic composition (−5.9 to −15.4‰). The relationship between isotopic composition (View the MathML source) and CO2 mole fraction was consistent among types of samples, except for HREA samples during the morning boundary layer transition. Total ecosystem respiration was estimated based on a regression against soil temperature, and the flux and isotopic flux measurements were used to examine whole-canopy photosynthetic discrimination (Δcanopy) and the isotopic composition of the photosynthetic flux. Δcanopy weighted by net ecosystem exchange was 17.9‰. The isotopic content of total ecosystem respiration, soil respiration, and foliar respiration, and View the MathML source of various organic components (leaves, roots, soil organic matter) were examined and evaluated relative to Δcanopy. The View the MathML source of organic components does not appear to be a good predictor of View the MathML source of ecosystem CO2 fluxes.