Flanagan, L.B., J.R. Brooks, G.T. Varney, and J.R. Ehleringer. 1997. Discrimination against C18O16O during photosynthesis and the oxygen isotope ratio of respired CO2 in boreal forest ecosystems. Global Biogeochemical Cycles 11:83-98.

Our objective was to analyze factors that influence changes in the oxygen isotope ratio (d18O) of atmospheric CO2 within boreal forest ecosystems. We made measurements in the three major forest types (black spruce, jack pine and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem Atmosphere Study (BOREAS). In terrestrial ecosystems the d18O value of atmospheric CO2 is strongly influenced by an equilibrium isotope effect that occurs between oxygen in CO2 and oxygen in soil water and plant chloroplast water. During the equilbrium reaction the oxygen isotope ratio of CO2 becomes enriched in 18O relative to that of water. As a consequence we measured seasonal changes in the oxygen isotope ratio of: (i) water input to the ecosystems (precipitation), (ii) water taken up by the major plant species from the soil (plant stem water), and (iii) water in plant leaves. We calculated the oxygen isotope ratio of CO2 in isotopic equilibrium with water in soils and plant leaves and compared these values to observations of diurnal change in the d18O value of atmospheric CO2 within forest canopies. Discrimination against C18O16O during photosynthetic gas exchange (DA) (influenced by equilibration with chloroplast water) averaged approximately 21 at midday, and was similar for all forest types. In contrast CO2 released during plant and soil respiration was significantly depleted in 18O with an average d18O value of -14.4. The estimates we obtained of the d18O of respired CO2 were, on average, enriched in 18O relative to values predicted for respired CO2 in equilibrium with soil water. Respired CO2 enriched in 18O was most pronounced in black spruce sites because of the extensive coverage of moss on the ground surface, and the observation that water in the upper moss layers can have an oxygen isotope ratio substantially different from water in deeper soil layers.