Wednesday November 12, 2003
David Bowling, Biology Department, University of Utah
Abstract: The isotopic characteristics of whole-ecosystem photosynthesis and respiration were examined at a high-elevation coniferous forest (the Niwot Ridge AmeriFlux site). A tunable diode laser spectrometer was used to monitor concentrations and carbon isotope ratios (d13C) of CO2 within the canopy, every 6 minutes for 3 months during summer 2003. CO2 fluxes were measured using the eddy covariance technique, and atmospheric boundary layer stability was assessed using within-canopy wind and temperature profile measurements. Net ecosystem exchange of CO2 during July 2003 varied from uptake of -10 to release of 5 mmol C m-2 d-1. The carbon isotope ratio of ecosystem respiration (d13CR, calculated using Keeling plots) at night (9pm - 4 am) varied from -26.9 to -25.3 . Changes from one night to the next were small (0.5 maximum), but multi-day trends with consistent directional changes were apparent. d13CR calculated at inlet heights below 2 m varied compared to those within the vegetation canopy (7-14m) by as much as 2.2 on the same night, indicating different isotopic content in the foliar and soil respiratory fluxes. Daytime calculations of Keeling plot intercept (reflecting the combined influence of photosynthesis and respiration) varied from -26.9 to -24.5 and were generally uncorrelated with nighttime values. Nocturnal d13CR varied from the daytime values several hours earlier by as much as 2.0 . These data suggest large temporal isotopic disequilibrium may exist between the photosynthetic and respiratory fluxes. The possibility of directly measuring the isotope disequilibrium via this approach will be explored.
For more information contact J. Keener, 1-6089