Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2010
Publication Date: 3/23/2011
Publication URL: http://handle.nal.usda.gov/10113/57477
Citation: Griffis, T.J., Lee, X., Baker, J.M., Billmark, K., Schultz, N., Erickson, M., Fassbinder, J.J., Xiao, W., Hu, N. 2011. Oxygen isotope composition of evapotranspiration and its relation to C4 photosynthetic discrimination. Journal of Geophysical Research-Biogeosciences. DOI:10.1029/2010JG001514. Interpretive Summary: The oxygen isotopic ratios of water and carbon dioxide are key clues to unraveling and understanding the cabon and water cycles in studies of global climate change. We have used recently developed portable isotope measurement tools to study the relationship between isotopic enrichment of plant canopy water and discrimination against the heavier 18-0 CO2 molecule during photosynthesis, as well as the isotopic disequilibrium between photosynthesis and respiration, and the biophysical factors affecting these processes. These data were collected during a 74 day field experiment over a corn field in Minnesota during the summer of 2009. We found that the isotope ratio of evapotranspiration varied substantially during the day in a repeatable way. These data were used to estimate the isotope composition of water inside the leaf at the site of evaporation, which in turn was used with CO2 exchange measurements and measurements of carbonic anhydrase enzyme activity to estimate thecanopy scale discrimination against 18-O CO2. The results show the importance of turbulence in determining canopy discrimination, and also suggest that the enzymatic efficiency of carbonic anhydrase may be lower than previously suspected. The data collected and the methodology that was developed should be useful in broader studies of global climate change.
Technical Abstract: The oxygen isotope ratio of water (18 O-H2O) and carbon dioxide (18 O-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 18 O-CO2 were used to investigate the relation between canopy leaf water 18O enrichment, 18O-CO2 photosynthetic discrimination (18delta), isotopic disequilibrium (Deq) 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 (_E) ranged from about -20 per mil (VSMOW scale) in the early morning to -5 per mil after midday. These values were used to estimate the isotope composition at the sites of leaf water evaporation (_L,e) assuming non-steady state conditions and revealed a strong diurnal pattern ranging from about -5h in the early morning to +15h after midday. With the addition of net ecosystem CO2 exchange measurements and carbonic anhydrase (CA) assays, we derived canopy scale 18 delta. These estimates typically varied from 11.3 to 27.5h (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 18 delta and Deq. 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 18 O-CO2 and constraining regional carbon budgets based on the oxygen isotope tracer approach.