Direct Measurement Of Biosphere-Atmosphere Isotopic Co2exchange Using The Eddy Covariance Technique

Quantifying isotopic CO₂ exchange between the biosphere and atmosphere presents a significant measurement challenge, but has the potential to provide important constraints on local, regional, and global carbon cycling. Past approaches have indirectly estimated isotopic CO₂ exchange using relaxed eddy accumulation, the flask-based isoflux method, and flux-gradient techniques. Eddy covariance (EC) is an attractive method because it has the fewest theoretical assumptions and the potential to give a direct measure of isotopic CO₂ flux, but it requires a highly sensitive and relatively fast response instrument. To date, no such field measurements have been reported. Here we describe the use of a closed-path tunable diode laser absorption spectroscopy and eddy covariance (EC-TDL) system for isotopic (C¹⁶O₂, ¹³CO₂, C¹⁸O¹⁶O) flux measurements. Results are presented from an intensive field experiment conducted over a soybean canopy from 18 July to 20 September 2006. This experiment represents a rigorous field test of the EC-TDL technique because the transport was dominated by relatively high frequency eddies. Net ecosystem CO₂ exchange (F_N) measured with the EC-TDL system showed strong correlation (r² = 0.99) in the half-hourly fluxes with an EC open-path infrared gas analyzer (EC-IRGA) over the 60-d period. Net CO₂ flux measured with the EC-IRGA and EC-TDL systems agreed to within 9%. Flux loss associated with diminished frequency response beyond 1 Hz for the EC-TDL system was approximately 8% during daytime windy (>4 m s⁻¹) conditions. There was no significant evidence of a kinetic-type fractionation effect related to a phase shift among isotopologues due to tube attenuation. Investigation of isotopic spectral similarity in the flux ratio (δ_N^x) for both ¹³CO₂ and C¹⁸O¹⁶O transport showed that δ_N^x was relatively independent of eddy scale for this ecosystem type. Flux loss, therefore, did not significantly bias δ_N^x. There was excellent agreement between isofluxes (F_δ^x) measured using the flux-gradient and eddy covariance methods. Application of the EC-TDL technique over rougher surfaces or below canopy, where the flux-gradient approach is difficult to apply, appears promising for obtaining continuous long-term measurements of isotopic CO₂ exchange.