What was done
Swards of perennial ryegrass (Lolium perenne L.) and the N2-fixing white clover (Trifolium repens L.) were grown in boxes placed within FACE plots receiving atmospheric CO2 concentrations of 350 and 600 ppm in combination with low and high soil nitrogen fertilization for four years to study the interactive effects of these variables on biomass production and nitrogen retention in these two contrasting ecosystems.

What was learned
Elevated CO2 increased average aboveground biomass in the white clover ecosystem by 80%, but it had no effect on aboveground biomass production in the perennial ryegrass ecosystem.  Below the surface of the soil, however, just the opposite occurred: the extra CO2 increased root biomass in the perennial ryegrass ecosystem by 94%, while having no effect on the root biomass of white clover.

The total amount of nitrogen taken into the white clover ecosystem was significantly greater than that taken into the perennial ryegrass ecosystem.  Much of this increase could be explained by elevated CO2 exposure, which roughly doubled the amount of nitrogen input through symbiotic N2-fixation in the white clover.  Nonetheless, all combinations of variables led to ecosystem nitrogen gains after four years of growth, with greater gains under higher, rather than lower, nitrogen fertilization.  In addition, elevated CO2 increased ecosystem nitrogen gains in all cases except in perennial ryegrass under low nitrogen fertilization.

What it means
As the air's CO2 concentration increases, swards of perennial ryegrass and white clover will likely display increased rates of photosynthesis and greater biomass production.  However, CO2-induced increases in biomass will most likely be manifested aboveground in white clover and belowground in perennial ryegrass.  In addition, both ecosystems should exhibit increased gains in nitrogen (except, perhaps, in the case of perennial ryegrass growing on low-nitrogen soils), with greater nitrogen gains occurring in white clover, due to CO2-induced increases in symbiotic N2-fixation.