What was done
The authors grew plants of perennial ryegrass (Lolium perenne cv. Bastion) from seeds in growth chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm in a hydroponic system for 24 days under three different fertilization regimes, where nitrogen (N) was supplied as either ammonium (NH4⁺), nitrate (NO3^-), or an equimolar mixture of both forms of N.  Plants were harvested 14, 19 and 23 days after the start of the study.  Dry masses of roots, stems and leaves were determined, along with leaf area.  The plant material was also analyzed for total nitrogen, inorganic nitrogen and total non-structural saccharides.

What was learned
With respect to total biomass production, in all three treatments - in the words of the authors - "the beneficial effect of elevated CO2 was similar."  Additionally, the authors found a significant decrease in the content of organic nitrogen "only in leaves of plants supplied with ammonium at elevated CO2."

What it means
Contrary to the highly-hyped claims of Bloom et al. (2002) - which were given massive coverage in the popular press (possibly because they implied a potential negative impact of the ongoing rise in the air's CO2 content) - the results of this study did not reveal any deleterious effects of a doubling of the air's CO2 content on either the uptake of nitrate nitrogen or its impact on plant growth or tissue N content.  In fact, the only indication of any negative effect of extra CO2 in this study was the finding that organic N content was slightly reduced in the leaves of plants that received their nitrogen from ammonia.  These results thus provide no support for any of the claims of Bloom et al. or the popular press.