What was done
The authors grew effectively- and ineffectively-nodulating lucerne (Medicago sativa) plants in 18-meter diameter FACE plots near Zurich, Switzerland, for multiple growing seasons at atmospheric CO2 concentrations of 350 and 600 ppm.  In addition, half of the plants in each CO2 treatment received a high soil nitrogen supply, while the other half only received minimal amounts of soil nitrogen.  Thus, the authors studied the interactive effects of elevated CO2 and soil nitrogen supply on biomass production in lucerne plant lines that are both strongly and weekly adept at facilitating symbiotic N2-fixation.

What was learned
Elevated CO2 increased the yield of effectively-nodulating plants by about 50%, regardless of soil nitrogen supply.  In contrast, atmospheric CO2 enrichment caused a 25% yield reduction in ineffectively-nodulating plants subjected to low soil nitrogen, yet produced an intermediate yield stimulation of 11% for the same plants under conditions of high soil nitrogen.  These results suggest that the ability to symbiotically fix nitrogen is important in eliciting strong positive growth responses to elevated CO2 under conditions of low soil nitrogen supply.

Atmospheric CO2 enrichment also increased the percentage of nitrogen derived from symbiosis in effectively-nodulating plants.  At high soil nitrogen, for example, elevated CO2 doubled the percentage of symbiotically-derived nitrogen in plant tissues from 21 to 41%.  Moreover, at low soil nitrogen, where total plant nitrogen was mostly derived symbiotically, elevated CO2 still increased its symbiotically-derived percentage from 82 to 88%.  These observations thus indicate that symbiotic N2-fixation per se is responsible for facilitating significant CO2-induced growth enhancements in lucerne, particularly under field conditions of inadequate soil nitrogen content.

What it means
As the air's CO2 content rises, it is likely that lucerne species will exhibit significant enhancements in their rates of net photosynthesis.  However, it is also likely that these enhancements in carbon uptake will only cause significant biomass increases if the species are adept at symbiotic N2-fixation.  If they are poor nitrogen-fixers, they may exhibit intermediate to negative growth responses, depending upon available soil nitrogen content.