Elevated CO₂ Enhances Productivity and the C/N Ratio of Grasses in the Colorado Shortgrass Steppe

Elevated CO₂ Enhances Productivity and the C/N Ratio of Grasses in the Colorado Shortgrass Steppe

Atmospheric CO₂ concentrations have been increasing since the industrial revolution, and are projected to double within this century over today's concentration of 360 µmol mol^-1 . This study used six open-top chambers in the Colorado, USA shortgrass steppe to investigate how increasing CO₂ will affect productivity and C and N status of indigenous perennial grasses and forbs. From March until October, chambers were placed on two plots in each of the three blocks. In each block, one chamber was assigned an ambient CO₂ treatment (~360 µmol mol^-1), the other an elevated CO₂ treatment (~720 µmol mol^-1). Each block also had an unchambered control plot. Growth under elevated CO₂ increased above-ground phytomass an average 31% in 1997 and 47% in 1998, with no differences in relative growth responses of C₃ and C₄ grasses and forbs. Growth in chambers was greater than non-chambered control plots, presumably due to warmer temperatures in chambers and a longer growing season. Shoot N concentrations were reduced 21% and C/N ratios increased 23% in elevated compared to ambient chambers. Variation in aboveground phytomass due to year, CO₂ and chamber effects correlated well to % shoot N and C/N ratios, although for both traits different regression lines were required for green plant material (harvested in July) and senescent plant material (harvested in October). Results suggest increased growth and reduced N concentrations in this mixed C₃/C₄ grassland in an elevated CO₂ environment.