What was done
Circular FACE plots (20-m diameter) receiving 360 and 560 ppm CO2 were established in a secondary successional grassland located in east central Minnesota, USA, to study the short- and long-term effects of atmospheric CO2 enrichment on gas exchange in plants existing within these nutrient-poor prairie communities.  Short-term effects were assessed by measuring leaf gas exchange rates at atmospheric CO2 concentrations that were reciprocal to growth CO2 concentrations, while long-term effects were assessed by measuring gas exchange rates at growth CO2 concentrations.

What was learned
Elevated CO2 enhanced the mean short-term photosynthetic rate of the C3 grassland species by 64 and 44%, respectively, after one and two years of treatment exposure.  The long-term CO2-induced photosynthetic enhancement for C3 species, however, was only 13 and 8%, respectively, for the two study years.  Similarly, the mean short-term CO2-induced photosynthetic enhancement in the C4 species was 13% in both study years, while the long-term response was a negative 2% for the two years.  Thus, elevated CO2 exposure led to photosynthetic acclimation in all species growing in the nutrient-poor prairie.

On another note, at the ecosystem level elevated CO2 reduced stomatal conductance in the conglomerate of species by 24%, thus increasing ecosystem instantaneous water-use efficiency.  Moreover, in the CO2-enriched plots, leaf nitrogen decreased by an average of 13%, thus contributing to an 11% enhancement in ecosystem photosynthetic nitrogen-use efficiency.

What it means
As the atmospheric CO2 content continues to increase, native plants growing in nutrient-poor grassland communities will likely respond by exhibiting increases in photosynthetic carbon uptake and decreases in stomatal conductance.  Consequently, species growing in such areas should display greater water-use efficiencies, thus allowing more moisture to remain in the soil for a longer period of time, which can stimulate the biological activities of organisms residing therein in many ways, including those that facilitate the belowground sequestration of carbon.  Thus, it is likely that prairie grasslands will continue to function as carbon sinks in a high-CO2 world, even under nutrient-poor conditions.