Tracing the flow of carbon dioxide and water vapor between the biosphere and atmosphere: A review of optical isotope techniques and their application

Development of optical isotope techniques over the last several years has provided scientists a set of tools for tracing the transport and cycling of CO₂ and water vapor between the biosphere and atmosphere. Here, I take a micrometeorological perspective and review these technological advances, assess key instrument performance characteristics, examine how these techniques have been used in the field to improve our understanding of the processes governing the exchange of CO₂ and water vapor, and discuss future research directions. Review of the recent literature indicates that: (1) optical techniques have been used to quantify the isotope composition of biosphere-atmosphere exchange using the traditional Keeling mixing line, flux-gradient, eddy covariance, and chamber approaches under a variety of field conditions with near-continuous data records now extending to more than 5-years; (2) high frequency and near continuous isotope measurements at the canopy scale have demonstrated important new insights regarding the behaviour of kinetic fractionation at the leaf versus canopy scales and the controls on ecosystem respiration that could not have been observed previously using traditional methods; (3) based on the assessment of instrument performance, carbon isotope disequilibrium (the difference between the isotope composition of photosynthesis and respiration), carbon turnover rates, and measurement uncertainties, ¹³C-CO₂ investigations are best suited for examining the contributions and changes to ecosystem respiration with a need for more innovative ¹³C-isotope labeling experiments and compound specific isotope analyses under field conditions; (4) significant progress has been made in measuring the oxygen isotope composition of water vapor fluxes and canopy leaf water enrichment. These new data have provided an opportunity to evaluate models of leaf water enrichment and their application to the canopy scale; and (5) the use of ¹⁸O-H₂O and ¹⁸O-CO₂ as tracers of the coupled carbon-water cycle has matured significantly in recent years. Evidence from a range of ecosystems indicates that ¹⁸O-CO₂ disequilibrium (ranging from 0 to 17‰) is much larger than for ¹³C-CO₂ (typical less than 3‰ in natural C₃ ecosystems) making it a useful tracer of coupled carbon and water cycle processes. However, a better understanding of the role of carbonic anhydrase in photosynthetic and respiratory processes under field conditions is now needed in order to make further progress. Finally, the increasing use and development of isotope-enabled land surface schemes along with the acquisition of high temporal resolution isotope data is providing a new opportunity to constrain the carbon and water cycle processes represented in these models.