THE RESPONSES OF ALPINE GRASSLAND TO 4 SEASONS OF CO2 ENRICHMENT - A SYNTHESIS

Alpine grassland at 2 470 m altitude in the Swiss Central Alps was exp osed to elevated CO2 by using open top chambers (16 ambient, 16 elevat ed CO2). Some plots received mineral fertilizer at a rate of N-deposit ion commonly measured in low altitude parts of Europe. Here we present a summary of results and data from the final harvest. Above-ground bi omass measured after the completion of growth in the fourth season of treatment was not affected by CO2 enrichment as was found by previous biometric estimates, but mean below-ground biomass was slightly stimul ated (+ 12%, n.s.). In con trast, net CO2 uptake per unit land area wa s strongly stimulated by CO2 enrichment al the beginning of the experi ment, and during the early part of each season. However, the CO2 stimu lation decreased during the later part of each growing season. By year four, also mid-season differences in CO2 uptake per unit land area ha d disappeared. Neither microbial biomass, soil respiration in the labo ratory, nor in situ land-area-based CO2 evolution during the 10 week g rowing season increased under elevated CO2. The total biomass N-pool a nd free soil nitrate and ammonium (capture by ion exchange resin bags) remained unaffected, whereas leaf nitrogen concentration was reduced and nonstructural carbohydrate concentration increased under elevated CO2 in forbs. These differences in tissue composition largely disappea red during senescence and litter formation. Despite low CO2 responsive ness at ecosystem level, species responses differed in terms of nitrog en, carbohydrates, tillering and flowering, suggesting the possibility for long-term changes in community structure. Addition of NPK equival ent to 40 kg N ha(-1) a(-1) had massive effects on all plant traits st udied, but did not enable stimulated growth under CO2 enrichment. Howe ver, when fertilizer and CO2 enrichment were provided jointly, soil mi crobes were stimulated indicating a co-limitation by carbon and nutrie nts (most likely nitrogen). Since responses to elevated CO2 were absen t in both warm and cold growing seasons, we conclude that this late su ccessional plant community is carbon saturated at current atmospheric CO2 concentrations for reasons not directly related to nutrient supply and climate. Perhaps, contrary to our expectation, evolutionary adjus tments of this ''old'' ecosystem to the life conditions at high altitu des caused carbon to become a surplus resource today.