Abstract:

There is considerable interest in the use of atmospheric C¹⁸O¹⁶O and ¹⁸O¹⁶O as a tracer for resolving the role of the terrestrial biosphere in the global carbon cycle. Leaf transpiration will result in the enrichment of the heavy H₂¹⁸O isotopes. The δ¹⁸O of leaf water at the evaporating site in the stomatal cavity directly influences the C¹⁸O¹⁶O and ¹⁸O¹⁶O exchanges, instead of that of the bulk leaf water. How to best quantify this enrichment effect remains an active area of research. In the past, a closed form of the Craig-Gordon model was obtained by invoking the steady-state assumption (δ¹⁸O of the transpired water is identical to δ¹⁸O of the xylem water). For the purpose of verification, the predictions of Craig-Gordon model are compared with δ¹⁸O of the bulk leaf water after appropriate corrections for the Péclet effect. On small time scales of minutes to hours,δ¹⁸O of the transpired water is variable in field conditions, implying that the steady state assumption is invalid. Recently, in-situ δ¹⁸O and δD measurement technology has been developed that has potential for improving our understanding of isotopic exchanges between the Earth’s surface and the atmosphere. The precision of hourlyδ¹⁸O and δD is comparable to the precision of mass spectrometry. It has the potential to improve prediction of δ¹⁸O of leaf water at the evaporating site within the stomatal cavity for the temporal dynamics of atmospheric water vapor δ¹⁸O and the δ¹⁸O of the transpired water, especially if its measurement is made in a non-destructive manner and on a continuous basis. Because the isotopic flux of δ¹⁸O and δD is influenced by a similar set of biological and meteorological variables, simultaneous observations of δ¹⁸O and δD will provide additional constraints on the hydrological and ecological processes of the ecosystem. We review the theory and measurement techniques for the enrichment of H₂¹⁸O in leaves and focus on the recently developed in-situ measurement technology and its potential for improving our understanding of H₂¹⁸O enrichment in leaf water and C¹⁸O¹⁶O and ¹⁸O¹⁶O exchanges between the ecosystem and atmosphere.

Key words: stable isotope, Craig-Gordon model, steady versus nonsteady state