What was done
Fallen leaves from plants growing in an alpine grassland, a lowland calcareous grassland and a lowland wet tropical rainforest under ambient and elevated CO2 concentrations were collected and analyzed for their initial lignin, nitrogen and carbon concentrations, after which they were weighed, put in 1-mm-mesh nylon bags and placed upon the soil surfaces from which they were taken.  Then, at the conclusion of a full growing season in each ecosystem, the mesh bags were retrieved and the percentage biomass remaining in the bags determined.

What was learned
The quality of the naturally-senesced leaf litter of the CO2-enriched plants in all three ecosystems "was not significantly different from that produced in similar communities maintained at current ambient CO2 concentrations."  In addition, decomposition rates of litter produced under ambient and elevated CO2 did not differ significantly, with one exception.  In the high alpine sedge, Carex curvula, growing in the Swiss Alps, decomposition of litter produced under elevated CO2 "was significantly slower than that of litter produced under ambient CO2."

What it means
In all three ecosystems studied, the equivalence of litter quality and rate of decomposition in ambient and CO2-enriched atmospheres suggests that with more above- and below-ground biomass production under CO2-enriched conditions, more carbon will be sequestered in the soil than is being trapped there now.  This phenomenon should help to mute the ongoing rise in the air's CO2 content and similarly impact any CO2-induced global warming that may be occurring.