What was done
Four perennial species, two grasses (Danthonia richardsonii and Phalaris aquatica) and two legumes (Lotus pedunculatus and Trifolium repens), were grown across a nitrogen gradient (ranging from 0.25 to 16 mM NO3) in glasshouses receiving atmospheric CO2 concentrations of 357 and 712 ppm for 60 days as individual plants, monocultures, and mixed-species communities to determine the effects of nitrogen and competition on their CO2 growth responses.

What was learned
Biomass of isolated species, monocultures, and mixed communities increased much more over the 64-fold increase in nitrogen than it did over the 2-fold increase in atmospheric CO2, with legumes typically exhibiting greater growth responses across the nitrogen gradient than grasses.  Nonetheless, elevated CO2 tended to increase species biomass in all three of the investigated plant growth regimes.  Of particular interest was the observation that in mixed communities the biomass proportions of species at ambient CO2 were amplified at elevated CO2.  Lotus pedunculatus, for example, contributed the greatest proportional amount of biomass in mixed species communities grown at ambient CO2 and contributed an even greater proportional amount when subjected to elevated CO2 concentrations.  This observation supports the idea that the rising CO2 content of the air will favor the growth of species already best adapted to their present environments.

When grown in isolation, the CO2-induced growth responses of individual plants did not correlate well with their observed responses in mixed communities.  For three of the four species, however, their observed CO2-induced growth responses in mixed community arrangements were consistent with those exhibited in monoculture arrangements, suggesting that regardless of nitrogen supply, plant responses to atmospheric CO2 enrichment are more dependent upon neighboring plant density than they are upon neighboring plant identity.  Thus, plant-plant interactions need to be considered when predicting community responses to rising atmospheric CO2 levels.

What it means
Because the air's CO2 content will likely double within the next century, it is ecologically important to determine how natural plant communities will respond to this change.  In the past, many CO2 enrichment studies addressing this issue were conducted on single species, and their individual results were scaled-up to predict community responses to atmospheric CO2 enrichment.  However, this paper demonstrates that community responses to elevated CO2 can not be reliably predicted from experiments performed on individual plants, and should be determined from studies on monocultures or actual mixed species assemblages where plants are grown in competition with one another.

Based on this study, it is likely that the rising CO2 content of the air will increase biomass production in mixed species grasslands, regardless of nitrogen concentration.  However, biomass production will likely be stimulated even more, if adequate nitrogen is available in the soil.