What was done
In two separate experiments, intact swards removed from a calcareous grassland and artificially-constructed monoliths with and without legumes present were grown in greenhouses for two years at atmospheric CO2 concentrations of 350 and 600 ppm.  In addition, half of the intact swards and artificial monoliths received phosphorus fertilization, while the other half did not, in order to study the effects of elevated CO2 and soil phosphorus on growth in calcareous grassland ecosystems.  Furthermore, the authors compared the responses of these model ecosystems with those observed in a Swiss field study conducted on calcareous grasslands that were exposed to similar atmospheric CO2 concentrations in modified open-top chambers for four growing seasons.

What was learned
In the field study, no change in community biomass was observable after one year of exposure to differential CO2 concentration.  However, after the second year, progressively more biomass was exhibited by CO2-enriched ecosystems, reaching a value after four years that was 25% greater than that attained by control ecosystems grown at ambient CO2.  Interestingly, the C:N ratios in CO2-enriched biomass tended to increase over this time, while the amount of nitrogen removed from the soil did not differ between CO2 treatments.  Thus, elevated CO2 increased the nitrogen-use efficiency of these plants, which produced significantly more biomass than ambiently-grown plants with the same amount of nitrogen.  Similarly, atmospheric CO2 enrichment did not alter, or slightly increased, total phosphorus in community biomass, which suggests that elevated CO2 also increased the phosphorus-use efficiency of these plants.

In intact grassland swards, biomass nitrogen contents were unaffected by elevated CO2 at the low phosphorus concentration.  However, at the high phosphorus concentration, elevated CO2 caused greater uptake of soil nitrogen, which resulted in 28% more nitrogen in community biomass relative to that in control communities grown at ambient CO2.

In artificial monoliths lacking the presence of legumes, neither elevated CO2 nor phosphorus fertilization alone significantly affected total plant nitrogen contents.  However, when legumes were present in such communities, total plant nitrogen increased by 77% with phosphorus fertilization, primarily in response to greater nitrogen within leguminous (107%) as opposed to non-leguminous (34 and 20% for non-leguminous forbs and grasses, respectively) tissues.  This finding suggests that when phosphorus is not limiting to growth, atmospheric CO2 enrichment stimulates nitrogen fixation in leguminous species, which also increases its availability for neighboring non-leguminous species.

What it means
As the CO2 content of the air increases, it is likely that nutrient-poor calcareous grasslands, which are often characterized as having low levels of soil nitrogen, will exhibit small but steady increases in community biomass.  If the soils contain adequate levels of phosphorus, it is likely that CO2-induced enhancements in biomass production will be even greater, if leguminous species are present.  Indeed, if soil phosphorus, which often limits the activities of nitrogen-fixing legumes, is sufficiently available, then symbiotic nitrogen fixation by such species will likely be even greater with atmospheric CO2 enrichment, thereby enhancing nitrogen availability to other plants growing in these communities and, consequently, their growth.