Background
An increase of labile carbon below ground, such as is typically provided by atmospheric CO2 enrichment, in the words of the authors, "could induce two mechanisms acting in opposite ways: (1) an enhanced soil organic matter decomposition due to the stimulation of microbial activity through the so-called priming effect (Kuzyakov et al., 2000); and (2) a retarded mineralization of native soil organic carbon due to the preference of microbes for easily decomposable substrates (Cardon et al., 2001)." So which mechanism is the stronger of the two?

What was done
In a study designed to broach this question at the POPFACE experimental plantation and FACE facility in central Italy, where clones of Populus alba, Populus nigra and Populus x euramericana had been grown since 1999 with nitrogen fertilization throughout the 2002-2004 growing seasons, Lagomarsino et al. conducted a number of physical and chemical analyses of soils that they sampled in June and October of 2004.

What was learned
The researchers report that Hoosbeek et al. (2004) did indeed observe a priming effect of the newly incorporated litter in the first rotation cycle of trees exposed to air containing an approximate 50% increase in atmospheric CO2 concentration, but that in the second rotation cycle Hoosbeek et al. (2006) observed an accumulation of carbon in the soil of that treatment. In harmony with this latter observation, Lagomarsino et al.'s 2004 data revealed no increase in carbon mineralization activity under elevated CO2, but rather a decrease of microbial basal respiration in the non-rhizopheric soil of the CO2-enriched treatment.

What it means
Noting that "microbial carbon immobilization was the dominant process under elevated CO2, limiting the carbon losses from soil," Lagomarsino et al. concluded that their results suggest "a possible positive trend for carbon storage on the long term, independent of soil nitrogen availability."

References
Cardon, Z.G., Hungate, B.A., Cambardella, C.A., Chapin III, F.A., Field, C.B., Holland, E.A. and Mooney, H.A. 2001. Contrasting effects of elevated CO2 on old and new soil carbon pools. Soil Biology and Biochemistry 33: 365-373.

Hoosbeek, M.R., Lukac, M., van Dam, D., Godbold, D.L., Velthorst, E.J., Biondi, F., Peressotti, A., Cotrufo, M.F., De Angelis, P. and Scarascia-Mugnozza, G. 2004. More new carbon in the mineral soil of a poplar plantation under Free Air Carbon Enrichment (POPFACE): Cause of increased priming effect? Global Biogeochemical Cycles 18: 10.1029/2003GB002127.

Hoosbeek, M.R., Li, Y., Scarascia Mugnozza, G. 2006. Free atmospheric CO2 enrichment (FACE) increased labile and total carbon in the mineral soil of a short rotation Poplar plantation. Plant and Soil 281: 247-254.

Kuzyakov, Y., Friedel, J.K. and Stahr, K. 2000. Review of mechanisms and quantification of priming effects. Soil Biology and Biochemistry 32: 1485-1498.