Mid-season gas exchange of an alpine grassland under elevated CO₂

Ecosystem net CO₂ uptake, evapotranspiration (ET) and night-time CO₂ efflux were measured in an alpine grassland dominated by Carex curvula, treated with doubled ambient partial pressure of CO₂ via open-top chambers. One quarter of the plots were treated with mineral nutrients to simulate the effect of lowland nitrogen deposition rates. Depending upon fertilizer supply, ecosystem net CO₂ uptake per ground area in full sunlight (NCE_max) was 41-81% higher in open-top chambers supplied with doubled ambient partial pressure (p _a) of CO₂ than in plots receiving ambient CO₂. Short-term reversals of the CO₂ level suggest that the extent of downward adjustment of canopy photosynthesis under elevated CO₂ was 30-40%. ET tended to decline, while water use efficiency (WUE), expressed as the NCE_max:ET ratio, increased more than twofold under elevated CO₂. Night-time ecosystem CO₂ efflux did not respond to changes in CO₂ p _a. NCE_max and night-time CO₂ efflux were more responsive to mineral fertilizer than the doubling of CO₂. This suggests that in these alpine plant communities, atmospheric nutrient input may induce equal or greater effects on gas exchange than increased CO₂.