What was done
At a field station of the Swiss Federal Institute of Technology near Zurich, three 18-meter-diameter circular plots of white clover (Trifolium repens L.) were maintained at the ambient atmospheric CO2 concentration of 350 ppm, while three similar plots were maintained at a concentration of 600 ppm throughout the daylight period.  In this situation, the authors developed a protocol for using stable isotopes of carbon (C) to determine if carbon sequestration increases following an increase in atmospheric CO2 concentration.

What was learned
The 71% increase in atmospheric CO2 concentration increased the above-ground growth of the clover by 146%.  In addition, there was a 50% increase in the input of newly-fixed C in the soil of the CO2-enriched treatment.  Furthermore, decomposition of L. perenne roots grown at elevated CO2 decreased by 24% compared to roots grown at ambient CO2.

What it means
The authors note that "the occurrence at elevated CO2 of both greater plant material input, through higher yields, and reduced residue decomposition rates would be expected to impact soil C storage significantly."  Indeed, they state that "soil carbon input and storage under elevated CO2 did tend to increase," indicating that "soils may be important sinks for increasing atmospheric CO2."  This negative feedback effect of atmospheric CO2 enrichment on the rate of rise of atmospheric CO2 would thus be expected to provide a significant brake on whatever degree of global warming might possibly occur as a consequence of the ongoing rise in the air's CO2 content.