What was done
The authors report the final soil carbon sequestration results of the POP-EuroFACE experiment conducted on a poplar plantation established in early 1999 on former agricultural fields near Biterbo, Italy, where three control plots and three CO2-enriched plots (to a target atmospheric concentration of 560 ppm) were each planted with equal-area sections of three Populus genotypes: P. x euramericana Dode (Guinier) genotype I-214, P. nigra L. (Jean Pourtet), and P. alba L. (genotype 2AS11).

What was learned
Over the course of the six-year study, Hoosbeek and Scarascia-Mugnozza report they detected an additional mean carbon (C) sink of 32 g C m^-2 year^-1 in the forest floor litter layer; but in the case of the soil itself, they found that over the first half of the experiment the increase in soil carbon was suppressed under FACE. Over the second half of the study, however, they measured an additional mean sink of 54 g C m^-2 year^-1 in the top 10 cm of the FACE treatment's mineral soil. This result was not statistically significant, however, due to a combination of high soil spatial variability and the low number of treatment replicates. Nevertheless, it suggests that if the experiment had been allowed to continue for a longer period of time, the difference likely would have become significant. And, last of all, they report that the amount of stabilized soil organic matter did indeed increase under FACE.

What it means
The two researchers conclude that the forest floor, as a whole, "will serve as an additional carbon sink under future increased atmospheric CO2 concentrations in poplar stands," which should help to mitigate further increases in the air's CO2 content and whatever increases in global air temperature they might possibly cause.