What was done
The authors grew the perennial C3 shrubs Larrea tridentata and Prosopis glandulosa and the perennial C4 grass Bouteloua eriopoda for two and a half months in environmentally regulated growth chambers of 350 and 700 ppm atmospheric CO2 concentration to determine the effects of elevated CO2 on growth and nutrient uptake in these three desert species.  Uptake rates of NO3^- and PO4^3- by roots were determined by periodically measuring the nutrient contents of hydroponic solutions in which some of the plants were placed at the end of the 2.5-month study.

What was learned
The doubled CO2 content of the air significantly increased total biomass in all three desert species, with the largest responses of 55 and 69% being reported for Prosopis and Larrea, respectively.  The growth response of Bouteloua was much smaller at 25%, as would normally be expected for a C4 plant.  However, whereas elevated CO2 more than doubled the uptake rates of NO3^- and PO4^3- in Bouteloua, it had no effect on PO4^3- uptake in Larrea nor on NO3^- or PO4^3- uptake in Prosopis; and it actually decreased root NO3^- uptake in Larrea by 55%.

What it means
In a future world of higher atmospheric CO2 concentration, all three of these desert species will likely exhibit increased biomass and better growth rates, which may well lead to "reverse desertification."  The greater response of the C3 shrubs would tend to suggest that they might out-compete the C4 grass in this regard; but the CO2-induced enhancement of the ability of Bouteloua to increase its uptake of NO3^- and PO4^3- from the soil may ultimately put it on an equal footing with the C3 shrubs.  Indeed, the authors speculate that "long-term CO2 exposure may favor Bouteloua as opposed to the C3 shrubs due to its greater capacity for nutrient acquisition."  Hence, although historical surveys indicate that woody plants are prevailing somewhat over grasses in common habitat areas, the results of this study suggest that atmospheric CO2 enrichment may enable grasses to coexist with them to a greater degree in the future than they currently do under the present CO2 concentration.