Direct measurement of biosphere‐atmosphere isotopic CO₂ exchange using the eddy covariance technique

Abstract

[1] Quantifying isotopic CO 2 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 CO 2 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 CO 2 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 (C O transport showed that d N x was relatively independent of eddy scale for this ecosystem type. Flux loss, therefore, did not significantly bias d N x . There was excellent agreement between isofluxes (F d 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 CO 2 exchange.

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“…However, to measure the source/sink signature properly near the land surface, one should interface the isotopic analyser with plant (Barbour et al, 2007;Barthel et al, 2011) and soil chambers (e.g. Wingate et al, 2010a, b) and deploy it in the gradientdiffusion mode either over the vegetation (Griffis et al, 2004) or over the soil surface inside the canopy (Santos et al, 2010), or combine it with a sonic anemometer for direct eddy covariance measurement of isotopic fluxes (Lee et al, 2005;Griffis et al, 2008Griffis et al, , 2010 or landscape scale measurements in high elevation or airborne conditions (e.g. Tuszon et al, 2010).…”

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