Dynamics of isotopic exchange of carbon dioxide in a Tennessee deciduous forest

The combination of isotopic measurements and micrometeorological flux measu rements is a powerful new approach that will likely lead to new insight int o the dynamics of CO2 exchange between terrestrial ecosystems and the atmos phere. Since the biological processes of photosynthesis and respiration mod ify the stable isotopic signature of atmospheric CO2 in different ways, mea surements of the net fluxes of CO2, (CO2)-C-13, and (COO)-O-18 can be used to investigate the relative contribution of each process to net ecosystem C O2 exchange. We used two independent approaches to measure isotopic fluxes of CO2 over a Tennessee oak-maple-hickory forest in summer 1998. These appr oaches involved (1) a combination of standard eddy covariance with intensiv e flask sampling, and (2) a modification to the relaxed eddy accumulation t echnique. Strong isotopic signals associated with photosynthesis and respir ation were observed and persisted in forest air despite the potential for m ixing due to atmospheric turbulence. Calm nights allowed a buildup of respi ratory CO2 below the canopy and were associated with isotopically depleted forest air in the morning. Windy nights were followed by a relatively more enriched early-morning isotopic signal. Entrainment of air from above the d ecaying nocturnal boundary layer during daytime mixed layer growth exerted strong control on isotopic composition of forest air, resulting in similar isotope ratios in the late afternoon despite different isotopic starting po ints following calm or windy nights. The influences of the convective bound ary layer and turbulent mixing within the forest cannot be ignored when usi ng isotopes of CO2 to investigate biological processes.