Net grassland carbon flux over a subambient to superambient CO2 gradient

Increasing atmospheric CO2 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-mu mol mol(-1) to 550 mu mol mol(-1) C O2 gradient from March to mid-December in 1998 and 1999 using two, 60-m lon g, polyethylene-covered chambers built directly onto the site. One chamber was operated at subambient CO2 concentrations (200-360 mu mol mol(-1) dayti me) and the other was regulated at superambient concentrations (360-550 mu mol mol(-1)). Continuous CO2 gradients were maintained in each chamber by p hotosynthesis during the day and respiration at night. Net ecosystem CO2 fl ux and end-of-year biomass were measured in each of 10, 5-m long sections i n each chamber. Net CO2 fluxes were maximal in late May (c. day 150) in 199 8 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 s eason. Average daily CO2 flux in 1998 was 13 g CO2 m(-2) day(-1) in the sub ambient chamber and 20 g CO2 m(-2) day(-1) in the superambient chamber; com parable averages were 15 and 26 g CO2 m(-2) day(-1) in 1999. Flux was posit ively and linearly correlated with end-of-year above-ground biomass but flu x was not linearly correlated with CO2 concentration; a finding likely to b e explained by inherent differences in vegetation. Because C-3 plants were the dominant functional group, we adjusted average daily flux in each secti on by dividing the flux by the average percentage C-3 cover. Adjusted fluxe s were better correlated with CO2 concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO2 flux increased linearly with CO2 concentration. This trend was more eviden t at subambient than superambient CO2 concentrations.