What was done
Aspen (Populus tremuloides) cuttings were grown for two years in open-top chambers receiving atmospheric CO2 concentrations of 367 and 715 ppm.  In addition, cuttings were subjected to low and high levels of soil nitrogen.  Thus, the authors were able to study the interactive effects of elevated CO2 and soil nitrogen availability on plant growth and belowground carbon and nitrogen dynamics.

What was learned
After two years of differential CO2 exposure, elevated CO2 increased the total biomass of aspen cuttings by 26 and 50% at low and high levels of soil nitrogen, respectively.  Of particular interest, atmospheric CO2 enrichment increased coarse root biomass by 24% at low soil nitrogen and by 78% at high soil nitrogen levels.  These belowground growth increases surely led to increases in root turnover and root exudation, as indicated by 50 and 88% increases in microbial ¹⁴C-carbon contents at low and high levels of soil nitrogen, respectively.

Elevated CO2 also increased microbial nitrogen uptake by an average of 265%, while plant nitrogen uptake was enhanced by 32%.  In addition, the absolute amount of nitrogen in cuttings increased by 17% with atmospheric CO2 enrichment, suggesting that the ability to acquire soil nitrogen may increase with root biomass over time.

What it means
As the CO2 content of the air rises, it is likely that regenerating aspen will exhibit enhanced rates of photosynthesis leading to greater aboveground and belowground biomass production.  Increases in belowground biomass will likely lead to larger root systems that can acquire greater absolute amounts of nitrogen from soils to support CO2-induced increases in biomass.  In addition, CO2-induced increases in belowground biomass should enhance microbial growth and activities, which can often lead to greater nutrient availability for plants upon their turnover.  Thus, aspen will likely exhibit greater growth in soils of both low and high soil fertility as the air's CO2 content continues to rise.