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

Isotopic net ecosystem exchange (isofluxes, or flux densities of CO2) 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 mol m−2 s−1 ‰. Air samples were collected over a wide range of CO2 mole fractions (325.3–597.5 mol mol−1) and isotopic composition (−5.9 to −15.4‰). The relationship between isotopic composition (δ13C) 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 (∆canopy) and the isotopic composition of the photosynthetic flux. ∆canopy weighted by net ecosystem exchange was 17.9‰. The isotopic content of total ecosystem respiration, soil respiration, and foliar respiration, and δ13C of various organic components (leaves, roots, soil organic matter) were examined and evaluated relative to ∆canopy. The δ13C of organic components does not appear to be a good predictor of δ13C of ecosystem CO2 fluxes. © 2003 Elsevier Science B.V. All rights reserved.