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Interactive Effects of Elevated CO2, Phosphorus, and Legumes on Community Productivity in a Simulated Grassland
Stocklin, J. and Korner, Ch.  1999.  Interactive effects of elevated CO2, P availability and legume presence on calcareous grassland: results of a glasshouse experiment.  Functional Ecology 13: 200-209.

What was done
Two simulated communities, characteristic of low fertility Swiss grasslands, were constructed and placed in glasshouses receiving atmospheric CO2 concentrations of 360 and 600 ppm for two growing seasons.  In addition, half of the communities received a low dose of supplemental phosphorus, while the other half remained unfertilized. Communities were each composed of 14 species with 60 individual plants.  In one community, 60% of the species were grasses, while the remaining 40% were non-legume forbs.  In the other community, 60% of the species were grasses, 20% were non-legume forbs, and 20% were nitrogen-fixing legumes.  Thus, the authors studied the interactive effects of elevated CO2, phosphorus, and legume presence on community responses to atmospheric CO2 enrichment.

What was learned
After two years of differential CO2 treatment, CO2-enriched communities that contained legumes and received supplemental phosphorus produced the greatest total biomass, while the least total biomass was produced by communities grown at ambient CO2 concentrations that lacked phosphorus fertilization and the presence of legumes.  However, on a percentage basis, the greatest biomass response to atmospheric CO2 enrichment (+30%) occurred in communities lacking legumes, regardless of phosphorus status, while the smallest CO2-induced percent stimulation in biomass production (+16%) occurred in communities containing legumes and receiving additional phosphorus.

Although individual species responses to elevated CO2 varied, certain significant trends were observable between plant functional groups.  Grasses, for example, increased their total biomass by 60%, regardless of phosphorus treatment, as long as legumes were absent.  When legumes were present, however, grasses failed to exhibit any growth response to elevated CO2, suggesting that legumes out-competed grasses for limiting resources in these communities with atmospheric CO2 enrichment.  Indeed, a similar trend emerged for non-legume forbs, though not as extreme, in which their CO2-induced growth response declined from 21 to 15% with the addition of legumes.  Legumes, as a group, displayed an average CO2-induced biomass stimulation of 41% across both phosphorus treatments.

What it means
As the CO2 content of the air continues to rise, nutrient poor grasslands of Switzerland will likely exhibit increased biomass both above- and belowground.  Although additional phosphorus fertilization would likely increase the absolute magnitude of this response, biomass should significantly increase on a percentage basis nonetheless.  Because nitrogen-fixing legumes displayed a competitive growth advantage over other plant types in these simulated grasslands, it may appear that they will flourish at the expense of other species in natural grassland communities as the CO2 content of the air increases.  However, with time, a portion of the additional nitrogen fixed by legumes should become available to neighboring species, which will likely utilize it to their own advantage, thereby preserving the species diversity of the grassland ecosystem.

Reviewed 1 December 1999