What was done
The authors grew one C3 grass (Panicum laxum) and two C4 grass species (Panicum coloratum and Panicum antidotale) for about two months in controlled environment chambers receiving atmospheric CO2 concentrations of 360 and 710 ppm and low and high levels of soil nitrogen to determine the effects of elevated CO2 and soil nitrogen on growth in these differing Panicum grasses.

What was learned
Under conditions of high soil nitrogen, atmospheric CO2 enrichment increased total plant dry mass by 27% for Panicum coloratum and 28% for both Panicum laxum and Panicum antidotale.  However, under conditions of low soil nitrogen, elevated CO2 had no significant effects on the dry mass of Panicum laxum and Panicum antidotale, while dry mass actually decreased by 25% in Panicum coloratum.  Thus, nitrogen deficiency appeared to preclude any CO2-induced growth response in these Panicum grasses, in contrast to what has been observed in other grass species (Navas et al., 1999).

What it means
As the CO2 concentration of the air continues to rise, it is likely that Panicum grasses will respond favorably by initially increasing their biomass, but only under conditions of adequate to high levels of soil nitrogen.  Over a longer time period, it is likely that Panicum species will also respond favorably, even under low soil nitrogen conditions; for even if there is but a small CO2-induced stimulation in photosynthesis, tiny amounts of excess carbohydrates can be exuded belowground to ultimately stimulate a whole host of microbial activities, including fungal mycorrhizal construction and networking, which can increase soil nutrient availability for plant growth and development (see Fungi in our Subject Index).

On another note, it is possible that the initial slow response of certain grasses to atmospheric CO2 enrichment under low soil nutrient conditions may play a role in the ongoing worldwide expansion of woody plants into grass and desert areas, which is described in several of our journal reviews listed under Trees (Range Expansions) in our Subject Index.

Reference
Navas, M.-L., Garnier, E., Austin, M.P. and Gifford, R.M.  1999.  Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along a nitrogen gradient: differences between isolated plants, monocultures and multi-species mixtures.  New Phytologist 143: 323-331.