Partitioning net ecosystem carbon exchange into net assimilation and respiration with canopy‐scale isotopic measurements: An error propagation analysis with 13 CO 2 and CO 18 O data

Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange between terrestrial ecosystems and the atmosphere in terms of nonfoliar respiration ( F R ) and net photosynthesis ( F A ) in order to better understand the variations of this exchange. However, the accuracy of the partitioning strongly depends on the isotopic disequilibrium between these two gross fluxes, and a rigorous estimation of the errors on F A and F R is needed. In this study, we account for and propagate uncertainties on all terms in the mass balance and isotopic mass balance equations for CO 2 in order to get accurate estimates of the errors on F A and F R . We apply our method to a maritime pine forest in the southwest of France. Nighttime Keeling plots are used to estimate the 13 C and 18 O isotopic signature of F R (δ R ), and for both isotopes the a priori uncertainty associated with this term is estimated to be around 2‰ at our site. Using δ 13 C‐CO 2 and [CO 2 ] measurements, we then show that the uncertainty on instantaneous values of F A and F R can be as large as 4 μmol m −2 s −1 . Even if we could get more accurate estimates of the net CO 2 flux, the isoflux, and the isotopic signatures of F A and F R , this uncertainty would not be significantly reduced because the isotopic disequilibrium between F A and F R is too small, around 2–3‰. With δ 18 O‐CO 2 and [CO 2 ] measurements the uncertainty associated with the gross fluxes lies also around 4 μmol m −2 s −1 but could be dramatically reduced if we were able to get more accurate estimates of the CO 18 O isoflux and the associated discrimination during photosynthesis. This is because the isotopic disequilibrium between F A and F R is large, of the order of 12–17‰. The isotopic disequilibrium between F A and F R and the uncertainty on δ R vary among ecosystems and over the year. Our approach should help to choose the best strategy to study the carbon budget of a given ecosystem using stable isotopes.