Determination of respiration and photosynthesis fractionation factors foratmospheric dioxygen inferred from a vegetation-soil-atmosphere analogue ofthe terrestrial biosphere in closed chambers

The isotopic composition of dioxygen in the atmosphere is a global tracer which depends on the biosphere flux of dioxygen toward and from the atmosphere (photosynthesis and respiration) as well as exchanges with the stratosphere. When measured in fossil air trapped in ice cores, the relative concentration of O-16, O-17, and O-18 of O-2 can be used for several applications such as ice core dating and past global productivity reconstruction. However, there are still uncertainties about the accuracy of these tracers as they depend on the integrated isotopic discrimination of different biological processes of dioxygen production and uptake, for which we currently have very few independent estimates. Here we determined the respiration and photosynthesis fractionation factors for atmospheric dioxygen from experiments carried out in a replicated vegetation-soil-atmosphere analogue of the terrestrial biosphere in closed chambers with growing Festuca arundinacea. The values for O-18 discrimination during soil respiration and dark respiration in leaves are equal to -12.3 +/- 1.7 %o and -19.1 +/- 2.4 %o, respectively. In these closed biological chambers, we also found a value attributed to terrestrial photosynthetic isotopic discrimination equal to +3.7 +/- 1.3 %o. This last estimate suggests that the contribution of terrestrial productivity in the Dole effect may have been underestimated in previous studies.