What was done
Small, open-top chambers (37-cm diameter) were constructed in an established high grassland ecosystem located in the Swiss Alps and subjected to atmospheric CO2 concentrations of 355 and 680 ppm for four years to determine the effects of elevated CO2 on nitrogen fixation and growth of a symbiotic nitrogen fixing species.

What was learned
After four years of atmospheric CO2 enrichment, the aboveground biomass of Trifolium alpinum, the only symbiotic nitrogen fixing species in this grassland community, was not significantly different from that of ambiently-grown control plants.  In addition, elevated CO2 did not affect symbiotic nitrogen fixation in Trifolium alpinum; thus the amount of symbiotically fixed nitrogen input into this ecosystem remained relatively low and unchanged.

What it means
As the atmospheric CO2 concentration continues to rise, nitrogen fixing species, like Trifolium alpinum, will not necessarily exhibit greater relative growth enhancements than those of non-nitrogen fixing species, which has often been postulated as a mechanism for increasing competition between and displacement of plants in heterogeneous communities.  Thus, it is likely that rising CO2 levels will at least maintain interspecies diversity in high Alpine grasslands, as elevated CO2 did not provide any inherent growth advantage to the only symbiotic nitrogen fixing species present in the studied ecosystem.