What was done
The authors grew peat moss hydroponically within controlled environment chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm for up to six months.  In addition, peat moss was simultaneously subjected to three different levels of nitrogen deposition.  Thus, the authors studied the effects of atmospheric CO2 enrichment and nitrogen availability on photosynthesis and growth in this important moss species.

What was learned
Elevated CO2 initially stimulated photosynthetic rates by about 30% in this species.  Photosynthetic acclimation, however, was rapidly induced; and after three days, there were no longer any significant differences in photosynthetic rates between mosses exposed to different CO2 treatments.  In contrast, elevated CO2 reduced rates of dark respiration consistently throughout the study by 40 to 60%.  Thus, CO2-enriched moss always had greater amounts of soluble sugars at its disposal than did moss that was grown at the ambient CO2 concentration.

Although elevated CO2 stimulated sugar concentrations within this species, a significant CO2-induced increase in total dry weight (17%) occurred only under the lowest of the three nitrogen regimes.  Thus, elevated CO2 was the most beneficial to the growth of the moss when nitrogen fertility was the poorest.

What it means
As the atmospheric CO2 concentration increases, it is likely that elevated CO2 will reduce rates of carbon loss via dark respiration in this moss species.  Consequently, even if photosynthetic rates are not stimulated, this species will still exhibit greater rates of carbon sequestration, due to the aforementioned reductions in carbon loss.  Hence, in the future, it is likely that Sphagnum species will continue to act as atmospheric carbon sinks, especially under conditions of poor nitrogen fertility, where CO2-induced increases in growth can further stimulate removal of carbon from the atmosphere.