What was done
A C3 sedge (Scirpus olneyi) and C4 grass (Spartina patens) common to the wetlands of Chesapeake Bay were grown in open-top chambers receiving atmospheric CO2 concentrations of 360 and 660 ppm to study the effects of elevated CO2 on nitrogenase activity and nitrogen fixation in these plants and in the sediments in which they grew.

What was learned
Atmospheric CO2 enrichment increased nitrogenase activity by 35 and 13% in the C3 and C4 species, respectively; and these stimulations led to increases in nitrogen incorporation of 73 and 23%, respectively, in the C3 and C4 species.  These differential responses of C3 and C4 plants to elevated CO2 were explained by the authors as being "in rough proportion to the relative effect of elevated CO2 on canopy photosynthesis measured throughout the day."

On another note, the authors determined that elevated CO2 significantly stimulated nitrogenase activity in nonsymbiotic nitrogen fixing microbes that existed in plant-free soil sediments.

What it means
As the CO2 concentration of the air continues to rise, it is likely that photosynthetic rates will increase in these C3 and C4 wetland species, thus providing greater resources to support enhanced nitrogen-fixation by symbiotic microbial organisms associated with these plants.  Moreover, increases in the air's CO2 content should also cause "an increase in the N2-fixing activity of free-living [microorganisms] in the marsh ecosystem."  Thus, elevated CO2 will likely increase the availability of nitrogen to plants in wetlands and all ecosystems where nitrogen-fixing organisms exist.