What was done
A review of the peer-reviewed scientific literature was conducted to determine the effects of elevated CO2 concentrations on plant respiration rates, which return carbon back to the atmosphere.

What was learned
Although the exact mechanisms behind the observed phenomenon remain to be characterized, elevated atmospheric CO2 concentrations were found to decrease respiration rates in mature foliage, stems, and roots of CO2-enriched plants relative to rates measured in ambiently-grown plants.  When normalized on a biomass basis, a doubling of the atmospheric CO2 concentration reduced plant respiration rates by an average of 17%.

To determine the potential effects of reduced plant respiration on annual global carbon cycling, the authors input a 15% respiration reduction into a carbon sequestration model, and determined that an additional 6 to 7 Gt of carbon would remain sequestered within the terrestrial biosphere, thus strengthening the terrestrial carbon sink.

What it means
As the CO2 content of the air continues to rise, plants will likely exhibit decreased rates of respiratory carbon return to the atmosphere, thus increasing the size of earth's terrestrial carbon sink.  This simple observation led the authors to state that reductions in respiratory carbon losses, due to the rising atmospheric CO2 concentration, "will enhance the quantity of carbon stored by forests," which is an important CO2 mitigation strategy endorsed by the infamous Kyoto Protocol.