What was done
Naturally-senesced leaves of two-year-old ash tree (Fraxinus excelsior) seedlings growing in solardomes having atmospheric CO2 concentrations of 350 and 600 ppm were collected, inoculated with various fungal species, and incubated for 42 days to study the effects of elevated CO2 on leaf litter decomposition in this species.

What was learned
Elevated CO2 significantly reduced the amount of nitrogen present within senesced leaves, thus giving CO2-enriched leaf litter a higher carbon to nitrogen ratio than litter collected from ambiently-grown seedlings.  The higher and lower concentrations of carbon and nitrogen, respectively, within CO2-enriched litter likely contributed to reductions in the amount of fungal colonization present on decaying leaves, as visually assessed by the authors.  Of 14 different combinations of fungal organisms in this decomposition study, only one caused a significant decrease in the carbon content of CO2-enriched litter after 42 days of incubation.

What it means
As the atmospheric CO2 concentration increases, it is likely that the leaf litter of ash tree seedlings will be decomposed by fungal organisms at slower rates than those observed under ambient CO2 concentrations.  With reduced rates of decomposition, senesced ash leaves should retain their photosynthetically-sequestered carbon for longer periods of time.  Hence, the carbon sequestering abilities of ash tree seedlings should increase, not only from CO2-induced increases in photosynthesis and growth, but also from CO2-induced reductions in carbon losses via slower decomposition of leaf litter.