The ecosystem-level gas exchange characteristics of an alpine grasslan
d 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)inc
reased 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 accum
ulated under elevated CO2. The observed seasonal decline in daytime NE
C may be due to reduced sink strength once maximum aboveground biomass
is attained, and appears to be a reg ulatory mechanism of ecosystem c
arbon accumulation. Moderate additions of NPK fertilizer stimulated bo
th day- (+39%) and nighttime NEC (+29%) due to increased plant biomass
, independent of CO2 treatment. Yet there is no indication that enhanc
ed mineral nutrient status will increase ecosystem responsiveness to e
levated CO2.