What was done
The authors grew from seed the invasive reed canary grass (Phalaris arundinacea L.) for four months in well-watered 36- x 47- x 16-cm mesocosms located within greenhouses maintained at atmospheric CO2 concentrations of either 365 or 600 ppm in soils of either low or high N supply (5 mg N l^-1 or 30 mg N l^-1) under conditions where the invading species was either dominant (high invasion: >90% cover) or sub-dominant (low invasion: <50% cover), and where the remaining surface portions of the mesocosms were covered with native graminoids (grasses, sedges and bulrushes) and native forbs that were also grown from seed. Then, at the end of the four-month growth period, they destructively harvested all above- and below-ground parts of all plants, and they collected and analyzed rhizosphere soil to assess the characteristics and sizes of the various soil microbial communities.

What was learned
With respect to soil microbes, Kao-Kniffin and Balser found that under elevated CO2, the structure of the soil microbial community was altered in such a way that the relative abundance of lipid biomarkers thought to indicate gram-negative bacteria changed, with all but one of them increasing. With respect to plant growth, they report that elevated CO2 only increased belowground biomass in the plant communities moderately invaded by reed canary grass; and that the only plants to show a significant increase in aboveground biomass were the native graminoids in the moderately invaded low N treatment.

What it means
The two researchers write that a CO2-induced "decrease in the relative abundance of gram-positive bacteria," such as they observed, "may result in reduced decomposition of complex carbon in the soil, which consequently could increase soil carbon storage (Balser, 2005)," although they also note that "the effect of shifting the composition of the microbial community to one of increasing abundance of fast-growing gram-negative bacteria could lead to more rapid cycling of carbon and nitrogen (Fraterrigo et al., 2006)," leaving the significance of this portion of their findings pretty much up in the air. With respect to their plant observations, however, they clearly conclude that their results suggest that "when CO2 concentrations rise in the future, wetland plant communities comprised of native graminoids may be better able to hinder reed canary grass invasion, particularly under low N environments," which most biologists would likely judge to be a positive outcome.

References
Balser, T.C. 2005. Humification. Encyclopedia of Soils in the Environment, Vol. 2. Elsevier, Oxford, UK, pp. 195-207.

Fraterrigo, J.M., Balser, T.C. and Turner, M.G. 2006. Microbial community variation and its relationship with nitrogen mineralization in historically altered forests. Ecology 87: 570-579.