Transient enhancement of carbon uptake in an alpine grassland ecosystem under elevated CO2

We investigated the carbon uptake and release of a Central European alpine grassland community subjected to doubled ambient CO2 during the third (1994) and fourth (1995) season of CO2 enrichment. Within this period net carbon uptake under elevated CO2 declined successively, providing evidence of carbon saturation in this high-elevation environment. Third year data were used to calculate a CO2 balance for the 13-wk growing season and indicated that the grassland still served as net carbon sink in 1994. Integrated over the growth period, plots exposed to doubled ambient CO2 fixed 22% more CO2 than control treatments receiving ambient CO2. Increased carbon uptake under elevated CO2 was entirely due to a stimulation of daytime net CO2 uptake, since nighttime CO2 release remained unaffected. However, enhancement of net canopy CO2 uptake showed a distinct seasonal response: following substantial net CO2 gains from snowmelt until attainment of peak biomass (ca. 6 wk), the relative effect of elevated CO2 declined over the remainder of the season. In contrast to controls, the C balance became negative under CO2 enrichment during the final weeks of the growth period. Estimates of wintertime respiratory CO2 losses of unfertilized plots (ca, 9 mo during which soils remain thawed under the snow) indicate a release of 73 to 89% of the amount of CO2 fixed during the snow-free period. Under elevated CO2 an estimated mean surplus of 41 g C m(-2) accreted during the third year of CO2 enrichment, which we hypothesize must been transferred belowground, since aboveground biomass remained unchanged. Moderate additions of mineral fertilizer (NPK) alone had a strong positive effect on seasonal net CO2 balance (57% increase) mediated by enhanced plant biomass. NPK-treated plots under elevated CO2 had a 38% higher seasonal CO2 balance, relative to NPK-plots at ambient CO2 concentration. Fourth-year (1995) data indicate no further stimulation of daytime net ecosystem CO2 flux under elevated CO2, both in unfertilized plots and plots treated with NPK. Hence, it is unlikely that alpine grasslands will serve as carbon sinks in a CO2-rich world in the long term.