What was done
The authors grew six perennial plants common to The Netherlands in giant steel pots containing soil of high or low nitrogen content.  All were located within greenhouse compartments receiving 354 or 566 ppm CO2 for the first half of a two-year study designed to investigate the interactive effects of these variables on biomass production and water use.  In the second year, two levels of water stress were additionally imposed upon the plants to investigate three-way interactions.

What was learned
Elevated CO2 increased biomass production in all six species under conditions of optimal water supply, but only in the presence of high soil nitrogen; and under conditions of water stress, the plants responded even better.  Water-use efficiency also increased with atmospheric CO2 enrichment in all species but one; and this response, on average, was nearly twice as great for plants growing at high nitrogen as opposed to low nitrogen.  In considering the natures of the specific environments in which each species naturally grew, however, the authors noted that "results from the present experiment suggest that elevated CO2 tends to favor species already best adapted to their environments."

What it means
This study suggests that as the CO2 content of the air continues to rise, species adapted to particular environments of low or high nitrogen and low or high water will likely become even better adapted to their environments.  In the authors' own words, "a rise in CO2 would not change the relationships between plant species in the natural environment, but would reinforce existing ones."  Hence, rising CO2 levels should be a positive force for helping to maintain species diversity within natural ecosystems.