Net grassland carbon flux over a subambient to superambient CO₂ gradient

Increasing atmospheric CO 2 concentrations may have a profound effect on the structure and function of plant communities. A previously grazed, central Texas grassland was exposed to a 200‐µmol mol −1 to 550 µmol mol −1 CO 2 gradient from March to mid‐December in 1998 and 1999 using two, 60‐m long, polyethylene‐ covered chambers built directly onto the site. One chamber was operated at subambient CO 2 concentrations (200–360 µmol mol −1 daytime) and the other was regulated at superambient concentrations (360–550 µmol mol −1 ). Continuous CO 2 gradients were maintained in each chamber by photosynthesis during the day and respiration at night. Net ecosystem CO 2 flux and end‐of‐year biomass were measured in each of 10, 5‐m long sections in each chamber. Net CO 2 fluxes were maximal in late May ( c. day 150) in 1998 and in late August in 1999 ( c. day 240). In both years, fluxes were near zero and similar in both chambers at the beginning and end of the growing season. Average daily CO 2 flux in 1998 was 13 g CO 2 m −2 day −1 in the subambient chamber and 20 g CO 2 m −2 day −1 in the superambient chamber; comparable averages were 15 and 26 g CO 2 m −2 day −1 in 1999. Flux was positively and linearly correlated with end‐of‐year above‐ground biomass but flux was not linearly correlated with CO 2 concentration; a finding likely to be explained by inherent differences in vegetation. Because C 3 plants were the dominant functional group, we adjusted average daily flux in each section by dividing the flux by the average percentage C 3 cover. Adjusted fluxes were better correlated with CO 2 concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO 2 flux increased linearly with CO 2 concentration. This trend was more evident at subambient than superambient CO 2 concentrations.