What was done
For three months, ryegrass was continuously labeled with ¹⁴CO2 at atmospheric CO2 concentrations of 350 and 700 ppm in order to trace the flow of carbon into plants and subsequently into the soil.  In addition, ryegrass was exposed to either low or high soil nitrogen treatments to study the effects of elevated CO2 and nitrogen fertilization on soil carbon input, decomposition rates, and microbial biomass.  For the decomposition aspect of this project, disturbed and undisturbed root-soil systems were allowed to decompose in sealed containers for about eight months prior to analysis.

What was learned
Elevated CO2 significantly increased shoot biomass by an average of 28% at both low and high soil nitrogen concentrations.  Similarly, atmospheric CO2 enrichment increased root biomass by an average of 42%.  The amount of soluble ¹⁴C-labeled soil carbon was as much as 36% greater in CO2-enriched systems than it was in ambient CO2 systems, regardless of soil nitrogen.  Moreover, the amount of ¹⁴C-labeled soil microbial biomass was 42% higher in elevated CO2 at both nitrogen levels.

Elevated CO2 reduced the average amount of disturbed grass root decomposition that occurred by 13%, relative to that observed at ambient CO2, in both nitrogen treatments.  Similar observations were made for undisturbed root systems, although decomposition rates were slightly higher.

What it means
As the CO2 content of the air rises, ryegrass will likely increase its biomass both above- and belowground.  This phenomenon should then increase soil carbon inputs from, among other things, enhanced root growth and nutrient exudation.  As more carbon enters the soil, a greater resource base for supporting microbes is produced; and enhanced microbial biomass should result.  Interestingly, greater rates, or amounts, of organic matter decomposition do not always occur when total microbial biomass increases, as decomposition rates in this study were clearly reduced by elevated CO2.  Thus, as the atmospheric CO2 concentration increases, it is likely that temperate grasslands will sequester greater amounts of carbon for longer periods of time.  This phenomenon should significantly increase the carbon sink strength of these ecosystems.  Indeed, the authors concluded that "the observed retarded decomposition of this material indicates that?temperate grasslands may counteract the increased decomposition of soil organic matter that is expected to occur due to higher temperatures."