12 April 2001

Biodiversity Key To Absorbing CO2

by Kate Melville

Biodiversity is an important factor regulating how ecosystems will respond to increasing atmospheric carbon dioxide, say researchers from the U.S. Department of Energy's Brookhaven National Laboratory and their collaborators from several universities. The team of investigators, led by Peter Reich of the University of Minnesota, just released results from a major field study that will appear in the April 12, 2001 issue of Nature. The scientists found that more diverse plant ecosystems were better able to absorb carbon dioxide (CO2) and nitrogen (N), both of which are on the rise due to human activities and industrial processes.

"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.

"These findings suggest that protecting biodiversity worldwide will contribute to safeguarding the capacity of ecosystems to capture a larger fraction of additional carbon and nitrogen entering our environment due to industrial processes," says Brookhaven ecologist George Hendrey, who led development of the FACE system and is another coauthor on the current study.

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.