What was done
The perennial ryegrass, Lolium perenne, was grown for 115 days in growth chambers subjected to continuous labeling with ¹⁴CO2 at atmospheric concentrations of 350 and 700 ppm to determine the fate of photosynthetically-fixed carbon in this species and its associated soil components.  In addition, after 115 days, the experimental soil and belowground plant parts were incubated at ambient and elevated (ambient +2°C) air temperatures for an additional 230 days at the same atmospheric CO2 concentrations to determine the effects of elevated CO2 and temperature on root and root-derived material decomposition rates.

What was learned
Elevated CO2 increased the ¹⁴C-carbon content of roots by 41%.  This single observation is extremely noteworthy, as the authors state that "root biomass is the driving parameter for all subsequent below-ground processes in our plant-soil system."  Indeed, root ¹⁴C-carbon contents were positively correlated with ¹⁴C-carbon contents in soil solutions, which increased by 30% with atmospheric CO2 enrichment.  In addition, ¹⁴C-carbon contents in soil solutions were positively correlated with ¹⁴C-carbon microbial biomass, which was 46% greater at the elevated CO2 concentration.  Finally, ¹⁴C-carbon microbial biomass was positively correlated with ¹⁴C-carbon residues in soils, which increased by 53% due to atmospheric CO2 enrichment.

Elevated CO2 also decreased decomposition rates of roots and root-derived materials present in experimental soils by 14%, even after 230 days of incubation.  In addition, increasing the incubation temperature by 2°C had little effect on altering the reduced decomposition rates observed with atmospheric CO2 enrichment, for even at high temperatures, rates were still 12% lower than they were at ambient CO2.  Furthermore, in a short-term experiment, the authors demonstrated that even a 6°C rise in air temperature could not counterbalance the reductions in decomposition rates brought about by a doubling of the atmospheric CO2 concentration.

What it means
As the atmospheric CO2 concentration increases, it is likely that earth's grasslands --and perhaps all of earth's vegetation-- will acquire an increasing capacity to sequester carbon in their roots and associated soils.  Indeed, using simple correlations the authors demonstrated that increases in root biomass, which commonly result from atmospheric CO2 enrichment, lead to increases in carbon fluxes to soil microorganisms and soil residues, thus enhancing the carbon sequestering abilities of these important components of the carbon cycle.  Moreover, reductions in decomposition rates caused by rising levels of atmospheric CO2 will likely further increase the residency of carbon in belowground carbon pools, even if air temperatures rise in the future.  Thus, it is not likely that any rise in air temperature would spur enormous carbon losses from belowground carbon pools.  Quite to the contrary, any small rise in temperature would likely enhance carbon sequestration, due to the enhanced vegetative growth that would likely result.