Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

Abstract An experiment was carried out to determine the effects of elevated CO 2 , elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis , a C 4 perennial grass, or Pascopyrum smithii , a C 3 perennial grass, were placed in growth chambers with 350 or 700 μL L −1 atmospheric CO 2 , and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high‐temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO 2 , with no significant difference between the C 3 and C 4 grass. The effect of elevated CO 2 on biomass was greatest at the intermediate water level. The positive effect of elevated CO 2 on shoot biomass was greater at normal temperatures in B. gracilis , and greater at elevated temperatures in P. smithii. Neither root‐to‐shoot ratio nor production of seed heads was affected by elevated CO 2 . Plant tissue N and soil inorganic N concentrations were lower under elevated Co 2 , but no more so in the C 3 than the C 4 plant. Elevated CO 2 appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO 2 effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO 2 , in spite of greater N limitation.