What was done
Open-top chambers were installed in a semi-natural grassland in central Sweden and the air within half of them continuously enriched with an extra 350 ppm CO2 throughout five consecutive summer growing seasons.  At the end of each season, aboveground shoot biomass was directly measured, while belowground root biomass was determined via a combination of minirhizotron measurements and assessments of root ingrowth cores.

What was learned
Over the course of the experiment, the CO2-induced increase in aboveground biomass dropped from an initial stimulation of +50% in year 1 of the study to a final stimulation of +5% in year 5.  Conversely, the CO2-induced increase in belowground biomass rose from an initial stimulation of +25% in year 1 to a final stimulation of +80% in year 5.  The authors remark that these trends "resulted in a dramatic change in root to shoot ratios in elevated CO2 compared with the ambient treatment, which increased from -15% in [year 1] to +70% in [year 5]," indicative of "a clear shift in plant biomass partitioning from above to below ground for plants exposed to elevated CO2."

What it means
With respect to the dramatic reallocation of biomass from above- to below-ground with continued atmospheric CO2 enrichment, Sindhoj et al. note that soil "nitrogen limitation was the most probable explanation."  Hence, we see how plants may radically shift their growth patterns in low-fertility soil as they seek out the extra nutrients they need to take full advantage of the huge potential for augmented growth provided by atmospheric CO2 enrichment.