Effects of elevated CO2 on gas exchange characteristics of alpine grassland

The ecosystem-level gas exchange characteristics of an alpine grassland treated with a combination of elevated CO2 and moderate additions of NPK fertilizer during the third season of experimental treatments are described. Mid-season maximum daytime net ecosystem CO2 flux (NEC)increased significantly under elevated CO2 (+45%), whereas nighttime NEC was unaffected by the CO2 treatment. Since daytime NEC under elevated CO2 underwent a seasonal decline, only moderate carbon surpluses accumulated under elevated CO2. The observed seasonal decline in daytime NEC may be due to reduced sink strength once maximum aboveground biomass is attained, and appears to be a reg ulatory mechanism of ecosystem carbon accumulation. Moderate additions of NPK fertilizer stimulated both day- (+39%) and nighttime NEC (+29%) due to increased plant biomass, independent of CO2 treatment. Yet there is no indication that enhanced mineral nutrient status will increase ecosystem responsiveness to elevated CO2.