What was done
The authors grew Plantago lanceolata seedlings for 6 weeks in controlled environment growth rooms having atmospheric CO2 concentrations of 400 (ambient) or 800 ppm.  Root exudate was collected from experimental chambers every fourth day and carbon partitioning was determined for plants at the end of 42 days of differential CO2 exposure.

What was learned
Elevated CO2 increased shoot, root and total dry matter production by 159%, 180% and 164%, respectively.  Although there was a tremendous growth stimulation resulting from atmospheric CO2 enrichment, there was no apparent effect on the root:shoot ratios of these plants when compared with plants grown at normal CO2 levels.  The amount of plant carbon recovered as root exudate was not significantly influenced by CO2 concentration.  However, plant carbon recovered in the potting medium (sand) was 3.2 times greater in elevated CO2 than in ambient CO2 concentration, due largely to the CO2-induced stimulation of root growth that occurred. 

What it means
As the CO2 content of the air continues to rise, Plantago lanceolata seedlings should respond by exhibiting large increases in above and belowground biomass.  In the short-term, higher CO2 levels may not increase root exudate into the soil, but it should increase carbon inputs through enhanced root growth, which in the long run should stimulate the activities of certain microbial and fungal components of the soil biota by increasing soil organic carbon content.