Photosynthetic Carbon Isotope Discrimination and Effects on Daytime NEE Partitioning in a Subtropical Mixed Conifer Plantation

Abstract

Isoflux-based isotopic flux partitioning (isoflux-based IFP) separates net ecosystem exchange flux (NEE) into net photosynthesis (FA) and nonfoliar respiration (FNR), and avoids the spurious correlation of component fluxes found in statistical flux partitioning. Canopy photosynthetic discrimination (Δcanopy) is a key variable in isoflux-based IFP, but it ignores diffusional and respiratory fractionations, with unclear consequences for isoflux-based IFP. We used three versions of the Farquhar model (Δclassical, Δdiffusion, and Δsimple) to estimate Δcanopy based on in-situ measurements of CO2 isotope ratios and flux during a dry period (I; DOY 205–219, 2015) and a humid period (II; DOY 285–299, 2015) in a subtropical mixed conifer plantation. We found no significant differences in Δcanopy between Δclassical and Δdiffusion which ignored respiratory fractionations during photorespiration and dark respiration. The average differences between Δclassical and Δsimple, which ignored respiratory and diffusional fractionations across the leaf boundary layer and mesophyll, were 2.88 ± 2.34‰ and 2.16 ± 2.18‰ for the two periods. Similarly, there were no significant differences in FA with isoflux-based IFP using Δclassical and Δdiffusion to obtain Δcanopy; FA using Δsimple was 102 and 117% higher than that using Δclassical for the dry and humid periods. Moreover, using Δclassical and Δdiffusion captured the reduction in daytime respiration during the dry period; this may be associated with the Kok effect and seasonal drought. Sensitivity analysis showed that changes in mesophyll and stomatal conductances of ±50% resulted in > 15% change in FA. Diffusional fractionations in mesophyll and stoma are critical limitations to isoflux-based IFP, while photorespiratory fractionation is an unignorable component of Δcanopy and isoflux-based IFP.