What was done
The Hurley Pasture Model was used to simulate growth responses of perennial ryegrass (Lolium perenne L.) to a step increase in atmospheric CO2 concentration (350 to 700 ppm) under conditions of low and high soil nitrogen.

What was learned
Under high soil nitrogen conditions, elevated CO2 stimulated net primary productivity and plant biomass by approximately 30% within a few years.  Under low soil nitrogen conditions, however, elevated CO2 caused a much greater stimulation of approximately 114%, although it took longer (5 to 10 years) to attain this greater degree of growth enhancement.

What it means
As the CO2 content of the atmosphere increases, perennial ryegrass will likely respond by exhibiting enhanced rates of photosynthesis and biomass production, even in nitrogen-poor soils, as has been empirically demonstrated by Hartwig et al., (2002).  Over the long haul, in fact, low-nitrogen soils should see much greater CO2-induced productivity increases than high-nitrogen soils.

Reference
Hartwig, U.A., Wittmann, P., Braun, R., Hartwig-Raz, B., Jansa, J., Mozafar, A., Luscher, A., Leuann, A., Frossard, E. and Nosberger, J.  2002.  Arbuscular mycorrhiza infection enhances the growth response of Lolium perenne to elevated atmospheric pCO2.  Journal of Experimental Botany 53: 1207-1213.