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African Grasses May Slow Global Warming while Boosting Food Production

May 1998

CIAT scientists call it a "win-win" situation. First, when sown to graze cattle, the African grasses boost the production of meat and milk, because they are more nutritious than native savanna grasses. As much as 10 times more cattle can be raised in a given area, and they typically grow over twice as fast. In Brazil some 350,000 square kilometers are now planted to the African grasses.

Second, the ability of these grasses to store large amounts of carbon in the soil means they can slow the global warming that has been linked to atmospheric buildup of CO2.

Through photosynthesis plants capture CO2 from the atmosphere. By storing large amounts of carbon in the underlying soil, African grasses serve as powerful reservoirs or "sinks" for carbon, according to CIAT researchers Myles Fisher and Richard Thomas. "One hectare of introduced pasture may store as much as 15 tons of carbon each year," says Fisher, adding that it’s still unclear how long a planted pasture can maintain high rates of carbon accumulation. "That’s the amount produced by six gas-guzzling cars in a year."

Studies by CIAT scientists and Brazilian colleagues at Embrapa Agrobiology, a center of the Brazilian Agricultural Research Enterprise, provide growing evidence that the amount of carbon "sequestered," or stored, by tropical grasses in the soil is much larger than previously thought. Embrapa’s data on introduced pastures of Brachiaria humidicola in Brazil’s Bahia State "are essentially similar to ours," says Fisher.

"These findings about sequestration of carbon in South American savanna soils are of global importance," says Fisher. Since tropical grasslands cover nearly 9 percent of the world’s land, they hold vast potential as carbon sinks and therefore as a biological brake on global warming (see background document). In Brazil, Colombia, and Venezuela, savannas occupy some 2.5 million square kilometers, an area about one-quarter the size of Canada.

Another environmental advantage of planted and managed pastures is that farmers usually protect them from burning. In contrast, natural savanna in many parts of the tropics is set afire every year to stimulate the growth of tender new shoots that livestock prefer. Unfortunately, much of the carbon in the plants goes up in smoke, back into the atmosphere as CO2, undermining the potential of these savannas to store carbon.

CIAT scientists see deep-rooted grasses and improved pasture management as potentially powerful incentives for governments to protect the earth’s atmosphere. They could, for example, help national governments meet their commitments under the United Nations Framework Convention on Climate Change.

The convention is an international agreement aimed at stabilizing greenhouse gas concentrations in the atmosphere at a level that would prevent human-induced interference with the climate system. It went into effect in 1994 and has been ratified by over 160 countries. Last December in Japan, the convention took a major step forward with the Kyoto Protocol, under which industrialized countries agreed to targets for reducing greenhouse gas emissions, including CO2.

With impetus from Kyoto, trade in ecological services, such as the use of carbon permits (whereby one country buys emission rights from another), may eventually become standard international practice. As giant carbon sinks, tropical savanna pastures could be a valuable bargaining chip for many developing countries.

CIAT and Embrapa’s work raises the question of what else can be done to boost the ability of introduced tropical pastures to sequester carbon.

Mixing forage legumes with the grasses, for example, is a remarkable catalyst for carbon buildup in the soil. CIAT research shows that grass-legume combinations can boost carbon storage in the soil by two and a half to five times the rate found in pure grass pastures. In Colombia, the addition of the forage legume Arachis pintoi increased carbon storage by 7.8 tons per hectare per year, compared with the grass-only pasture. This finding is important since a combination of grasses and legumes gives higher weight gains by animals and keeps the soil productive.

The role of nitrogen in the soil, both through the use of forage legumes and fertilizers, is just one of several topics needing further research, say CIAT scientists. Embrapa Agrobiology, together with the University of the Andes in Venezuela and the MacAulay Land Use Research Institute in Scotland, are working with CIAT on a 4-year project to examine the role of nitrogen in carbon storage in the savannas of Colombian, Brazil, and Venezuela. The research is supported by a grant of 320,000 pounds sterling from the UK’s Department for International Development.

CIAT is one of 16 international centers sponsored by the Consultative Group on International Agricultural Research (CGIAR), an association of nations and international agencies that funds research for development.

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