What was done
The authors constructed open-top chambers about portions of an annual grassland located in a Mediterranean-type climate in California, USA, after which they fumigated them with air of either 360 or 720 ppm CO2 to study the effects of elevated CO2 on root production and turnover.

What was learned
By the end of the growing season, the plants in the elevated-CO2 chambers had increased their production of new root length by nearly 60%, but their root turnover rates were no different from those of the plants in the ambient-treatment chambers.  There was also an 18% increase in soil moisture content in the CO2-enriched chambers.

What it means
As the CO2 content of the air increases, belowground biomass production should increase in this particular type of annual grassland, either directly from CO2-induced increases in photosynthesis or indirectly from CO2-induced reductions in water use, which tend to increase soil moisture content.  Hence, overall productivity in such grasslands will likely increase as the atmosphere's CO2 concentration continues to rise.