What was done
The authors grew poplar tree cuttings for five months in treatment combinations of ambient (345 ppm) or elevated (693 ppm) atmospheric CO2 concentration and low or high soil nitrogen content to determine the effects of these parameters on soil biota.  Earlier studies using this experimental design had demonstrated that atmospheric CO2 enrichment led to greater coarse-root biomass and increased production and mortality of fine roots.  The current investigation was done, therefore, to determine what role, if any, CO2-induced increases in soil carbon deposition may have on soil biota.

What was learned
Microbial biomass carbon in the rhizosphere was not affected by atmospheric CO2 concentration regardless of soil nitrogen content.  Protozoa numbers, however, were 6.5 and 2.5 times greater with elevated CO2 relative to their ambient counterparts at low and high soil nitrogen contents, respectively.  This suggests that elevated CO2 led to increased bacterial turnover in response to higher rates of predation by a larger protozoan population.  Elevated CO2 had no effect on mycorrhizal growth at low soil nitrogen content, but at high nitrogen content, CO2 enrichment led to the production of mycorrhizal root masses that were twice as great as those produced with ambient CO2.  In addition, at high soil nitrogen content, fine roots in the elevated CO2 treatment had twice as many microarthropods associated with them in comparison to fine roots in the ambient CO2 treatment.

What it means
Elevated CO2-induced carbon increases under poplar tree cuttings elicit mixed responses from the soil biota, depending upon soil nitrogen content and biota type.  Atmospheric CO2 enrichment stimulated the growth of bacteria, as seen by massive increases in the protozoan population, which feeds upon bacteria, regardless of soil nitrogen content.  Fungal soil components, however, only responded to elevated CO2 in the presence of high soil nitrogen.  Although this was a relatively short study for investigating the impact that additional carbon inputs into the soil may have on soil biota, it still suggested that the extra carbon would be metabolized by both bacterial and fungal organisms.