Oxygen isotope composition of evapotranspiration and its relation to C ₄ photosynthetic discrimination

The oxygen isotope of water (¹⁸O-H₂O) and carbon dioxide (¹⁸O-CO₂) is an important signal of global change and can provide constraints on the coupled carbon-water cycle. Here, simultaneous observations of ¹⁸O-H₂O (liquid and vapor phases) and ¹⁸O-CO₂ were used to investigate the relation between canopy leaf water ¹⁸O enrichment, ¹⁸O-CO ₂ photosynthetic discrimination (¹⁸Δ), isotope disequilibrium (D_eq), and the biophysical factors that control their temporal variability in a C₄ (Zea mays L.) ecosystem. Data and analyses are presented from a 74 day experiment conducted in Minnesota during summer 2009. Eddy covariance observations indicate that the oxygen isotope composition of C₄ evapotranspiration (δ_E) ranged from about -20‰ (VSMOW scale) in the early morning to -5‰ after midday. These values were used to estimate the isotope composition at the sites of leaf water evaporation (δ_L,e) assuming non-steady-state conditions and revealed a strong diurnal pattern ranging from about -5‰ in the early morning to +10‰ after midday. With the addition of net ecosystem CO₂ exchange measurements and carbonic anhydrase (CA) assays, we derived canopy-scale Δ¹⁸. These estimates typically varied from 11.3 to 27.5‰ (VPDB scale) and were shown to vary significantly depending on the steady state or non-steady-state assumptions related to leaf water enrichment. We demonstrate that the impact of turbulence on kinetic fractionation and steady state assumptions result in larger estimates of Δ¹⁸ and D_eq. Further, the results indicate that both leaf-scale and canopy-scale CO₂ hydration efficiency may be substantially lower than that previously reported for laboratory conditions. These results may have important implications for interpreting variations in atmospheric ¹⁸O-CO₂ and constraining regional carbon budgets based on the oxygen isotope tracer approach.