What was done
A five-year open-top chamber study (ambient CO2 = 360 ppm, enriched CO2 = 720 ppm) was conducted in semiarid shortgrass steppe grassland at the USDA-ARS Central Plains Experimental Range in north central Colorado, USA, where 88% of the ecosystem's biomass was due to three co-dominant species - Bouteloua gracilis (H.B.K.) Lag., Stipa comata (Trin and Rupr.) and Pascopyrum smithii (Rybd.) - and where a subfrutescent shrub (Artemesia frigida Willd.) was also abundant.  Video-image data pertaining to root growth and decay in each treatment-replicate were obtained from 18 minirhizotron tubes, where video images to a depth of 40 cm were obtained four to five times a year, while root biomass data were obtained from two 20-cm-diameter cylinders driven into the ground within each chamber and collected yearly in the autumn.

What was learned
At the conclusion of the five-year study, it was determined that root-length growth had been 52% greater in the CO2-enriched chambers than in the ambient-air chambers, while root-length losses had been 37% greater in the elevated-CO2 air.  The difference between the CO2-induced growth and decay stimulations was largely attributable to the 41% longer life span of the CO2-enriched roots, which resulted in a CO2-induced root-length pool size increase of 41%.  In addition, in the upper part of the soil profile, root diameters were typically observed to be larger in the CO2-enriched chambers, leading to an ultimate CO2-induced root biomass increase of 59%.

What it means
In the words of the authors, "the lower nutrient quality of roots (Milchunas et al., 2005), slower turnover of new soil carbon, and increased life span of roots suggest an increased storage of carbon under elevated CO2."

Reference
Milchunas, D.G., Mosier, A.R., Morgan, J.A. et al.  2005.  Root production and tissue quality in a shortgrass steppe exposed to elevated CO2.  Plant and Soil 268: 111-122.