What was done
A FACE experiment using 18-m diameter plots was established on a fertile grassland ecosystem consisting of Lolium perenne swards near Zurich, Switzerland.  Plots were fumigated with atmospheric CO2 concentrations of 350 and 600 ppm and were additionally subjected to low and high nitrogen fertilization regimes to determine the effects of elevated CO2 and soil nitrogen availability on long-term biomass production.

What was learned
After one-year of differential CO2 exposure, there were no significant dry matter differences between Lolium perenne swards.  However, dry matter yield was significantly greater in CO2-enriched swards from year two to the final year (six) of the experiment, indicating that a large step increase in the atmospheric CO2 concentration does not necessarily produce ecosystem-level results in the short-term, even for periods of a single year.

At high nitrogen availability, atmospheric CO2 enrichment caused a continual increase in dry matter production from approximately 8% in year one to 25% at the close of year six.  At low soil nitrogen, however, after an initial 5% stimulation, the CO2-induced dry matter yield response dropped to -11% in year two, after which it continually climbed to ultimately reach a 9% stimulation at the conclusion of the experiment.

What it means
As the air's CO2 content increases, it is likely that fertile grasslands dominated by swards of Lolium perenne will produce greater biomass than they do at present ambient CO2 concentrations in a manner dependent upon soil nitrogen availability.  The results of this paper suggest that biomass production rates and amounts will increase to greater degrees in soils containing higher, rather than lower, amounts of available soil nitrogen.  Nonetheless, even when soil nitrogen is limiting to plant growth, it is expected that the increasing CO2 content of the air will still cause measurable increases in biomass production in these ecosystems.