What was done
Six genotypically different aspen (Populus tremuloides) cuttings were grown in open-top chambers for 2.5 growing seasons in Michigan, USA, at atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, the trees were grown on soils containing adequate and inadequate supplies of nitrogen.  In this particular paper, the authors report the effects of elevated CO2 and soil nitrogen on soil microbial composition, biomass, and functioning.

What was learned
Although atmospheric CO2 enrichment had no effect on soil microbial biomass, even after 2.5 years of treatment, high soil nitrogen supply increased it five-fold over that observed in low soil nitrogen plots.  Similarly, elevated CO2 did not significantly impact microbial community composition, whereas high soil nitrogen supply did.  Moreover, atmospheric CO2 did not influence microbial rates of nitrogen mineralization, nor did it alter the microbial demand for inorganic nitrogen.

In totality, these several observations suggest that the increased fine root biomass and turnover, which led to greater carbon inputs to the soils of the CO2-enriched plots, were not significant enough to elicit any responses in microbial community composition, biomass, and functioning, due to the enormous amount of background organic carbon present in the experimental soils, which was approximately 1000-fold greater than that contributed by the aspen roots.

Notwithstanding this observation, the authors conducted an eloquent review of the scientific literature pertaining to this topic; and they concluded that when root-associated soil carbon inputs are sufficiently large, relative to native soil organic carbon contents, they can influence microbial community composition, biomass, and functioning (see our journal reviews under the Subject Index headings of Bacteria, Fungi, and Microorganisms).

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that aspen trees will exhibit significant increases in growth, regardless of soil nitrogen availability.  These growth increases will occur both above- and below-ground, thus stimulating greater carbon inputs to soils.  Because most forest soils are already relatively rich in organic carbon, however, it is likely that the extra carbon inputs, resulting from the increasing CO2 content of the air, will have little impact on soil microbial composition, biomass, and functioning.  Thus, it is likely that future increases in the air's CO2 concentration will continue to maintain soil microbial diversity beneath regenerating aspen stands.