Biodiversity Increases Ecosystems' Ability To Absorb CO₂ And Nitrogen

"The key implication of this research is that, in response to elevated levels of CO2 and N, ecosystems with high biodiversity will take up and sequester more carbon and nitrogen than do ecosystems with reduced biodiversity," says Brookhaven plant physiologist David Ellsworth, one of the study authors.

The experiment, called BioCON (Biodiversity, CO2 and N), is the first field study to test the hypothesis that plant species diversity influences ecosystem-scale responses to elevated CO2 and N levels. It was performed in a scientifically controlled grassland environment at the Cedar Creek Natural History area of the University of Minnesota, using free-air CO2 enrichment, or FACE, technology. This experimental technology was developed by Brookhaven National Laboratory to study the effects of enhanced CO2 on plants in their natural environment, rather than in greenhouses or other enclosures. Each FACE facility consists of six 20-meter diameter experimental plots, each encircled by a ring of five-foot tall vertical pipes capable of releasing varying concentrations of CO2. Computers monitor the wind speed, wind direction, and CO2 level within each ring, and adjust the release of CO2 to achieve an atmospheric concentration at a level that is expected to occur fifty years from now.

In the BioCON study, the six rings were each subdivided into experimental plots measuring 2 x 2 meters. In 1997, these subplots were each planted with either 1, 4, 9, or 16 perennial grassland plant species, randomly chosen from among 16 species, including four nitrogen fixers. The experimental plots within three of the rings received no additional CO2, while the other three rings were bathed in CO2 that was about fifty percent above the present ambient concentrations. Beginning in 1998, half the plots received additional N, comparable to the high rates of N deposition observed as a result of atmospheric emissions in industrialized regions.

At the end of both the 1998 and 1999 growing seasons, the scientists measured the total amount of plant matter (biomass) per square meter in each plot. Biomass is an indicator of carbon accumulated via photosynthesis, the process by which green plants use CO2, water, and sunlight to grow. Nitrogen, an important plant nutrient, is absorbed from the soil to become part of the biomass.

The findings: Elevated levels of CO2 and N resulted in increased biomass when compared with plots exposed to ambient levels of CO2 and N. This effect, however, was greatest in plots with high biodiversity as compared to those with fewer species.

The scientists say the greater uptake of CO2 and N in biodiverse plots may be due to positive interactions among the plant species. For example, with greater diversity, species bloom and absorb CO2 and N over the entire growing season, rather than just part of it.

The other collaborators on this study were from the University of California, Berkeley, and the University of Nebraska. The work was funded primarily by the U.S. Department of Energy with additional support from the U.S. National Science Foundation.

The U.S. Department of Energy's Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies. Brookhaven also builds and operates major facilities available to university, industrial, and government scientists. The Laboratory is managed by Brookhaven Science Associates, a corporation founded by Stony Brook University and Battelle, a nonprofit applied science and technology organization.