Savanna Grasses -- The Missing Carbon Sink?

The deep-rooted grasses may remove as much as 2 billion tons of carbon dioxide-- a "greenhouse gas"--from the atmosphere yearly, says Dr. Myles Fisher, CIAT ecophysiologist.

"Green plants are small factories that use carbon dioxide, or CO2, and sunlight to produce organic matter. The perennial grasses Andropogon gayanus and Brachiaria humidicola convert as much as 53 tons of CO2 per hectare yearly to organic matter," Fisher says. "That's as much CO2 as a car emits in 133,000 miles, or 213,000 kilometers."

The storage of organic matter was not noticed earlier because the extensive roots of these grasses deposit it as deep as a meter in the savanna soil, Fisher explains.

CIAT and national program scientists introduced Andropogon and Brachiaria, originally from Africa, to the grassy savannas of South America in the 1970s.

South American Savannas

CO2 emissions contribute to global warming because the CO2 acts as a "blanket" around the earth, Fisher explains. The sun's rays penetrate the CO2 blanket, which traps their heat within the atmosphere. The resulting "greenhouse effect" is predicted to raise average global temperatures by at least 2 degrees centigrade or 3.6 degrees Fahrenheit in the next century.

The burning of fossil fuels pumps about 20.5 billion tons of CO2 into the atmosphere yearly, scientists estimate. Burning of tropical forests releases another 5.5 to 11 billion tons. But scientists have measured a yearly increase of only 18 to 25 billion tons of CO2 in the atmosphere.

The ocean, tropical wetlands, and green plants absorb some atmospheric CO2. But scientists cannot resolve the fate of several billion tons.

"Improved savanna grasses must explain part of this difference," Fisher says. "Brazil alone has at least 35 million hectares of introduced pastures--enough to fix 2 billion tons of CO2 per year."

The pasture grasses fix more CO2 when planted with the legumes Arachis pintoi or Stylosanthes capitata, both from South America.

Because Andropogon and Brachiaria adapt well to acid soils, national programs have released one or both of the grasses to farmers in at least 12 Latin American countries.

The South American savannas, almost five times the size of France, are the world's last frontier for agriculture. They produce more than US$15 billion worth of meat, milk, and grains annually, despite fragile, infertile soils. But less than a quarter of their area is farmed.

Cattle grazed on the improved grass-legume associations gain up to 500 kg of weight per hectare yearly, versus 7-20 kg/ha on native savanna. So the cattle business prospers as CO2 levels drop.

"Planting improved pastures on the savannas slows the chainsaw invasion of the rain forests, preserving precious biodiversity," Fisher says.

Fisher, an Australian, joined CIAT in 1985. He was previously a research physiologist with Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO). He holds a doctorate from the University of Queensland.

The paper in Nature magazine is titled "Carbon storage by introduced deep-rooted grasses in the South American savannas." Coauthors, all CIAT scientists, are: Drs. I.M. Rao, M.A. Ayarza, C.E. Lascano, J.I. Sanz, R.J. Thomas, and R.R. Vera.

(CIAT news release)