The sagebrush-steppe ecosystem covers more than 36 million ha and could play an important role in the global carbon cycle; however, quantitative estimates of CO2 fluxes on these extensive ecosystems are not available. The Bowen ratio/energy balance technique (BREB) was used to continuously monitor CO2 fluxes during the 1996 to 1999 growing seasons at a sagebrush-steppe site near Dubois, Idaho, USA. The daytime and night-time CO2 fluxes were modeled to provide estimates of occasionally missing or aberrant data points so that daily (24-h) integrals across the entire growing season could be quantified. Depending on the particular time of the season, daytime fluxes were best described by a rectangular hyberbolic, nonrectangular hyperbolic, or hysteresis-type functions that included radiation, relative humidity, and soil temperature. Night-time CO2 fluxes exhibited greater variability than daytime fluxes and were not closely correlated with any single meteorological characteristic. Night-time fluxes were predicted using a nonlinear parameter identification technique that estimated values of daytime respiration, which were significantly correlated with night-time fluxes. For the four growing seasons of our study, the integrated seasonal fluxes ranged from 284 to 1,103 g CO2 m-2 with an overall average of 635 g CO2 m-2. Respiratory losses during the non-growing season were estimated to be about 1.5 g CO2 m-2 day-1 or a total of 270 g CO2 m-2. This gives an annual net positive flux (carbon sequestration) estimate of 365 g CO2 m-2 (or 1.0 t C ha-1). These results suggest that the combination of BREB measurements and modeling techniques can be used to provide estimates of CO2 fluxes on important rangeland ecosystems.
