Better farming practices could help agriculture bury about 10 percent of atmospheric carbon from emissions caused by human activity over the next 25 years -- while improving soil, crop and environment quality, slowing erosion and desertification, and enhancing biodiversity.

The key is to build up plant matter in the soil. This would pull more carbon dioxide out of the air and convert it into plant matter, which is largely carbon -- a process called carbon sequestration. It would also improve soil quality, reduce soil erosion and make farming more productive and sustainable, according to a report, Soil carbon sequestration for improved land management, recently published by FAO.

Carbon dioxide is the most prominent "greenhouse" gas. Carbon sequestration can be used to partially offset a country's carbon dioxide emissions, helping it meet its commitments under the Kyoto Protocol, the mechanism for implementing the UN Framework Convention on Climate Change.

The Protocol's Clean Development Mechanism allows developed countries to offset some of their emissions by funding climate-friendly projects in the developing world. This offset process, in its infancy, will eventually "credit" agricultural development that increases plant matter, and thus carbon sequestration.

Reversing the damage

Agriculture also emits carbon dioxide when tillage, often unnecessary, exposes organic matter in the soil. This soil organic matter -- which is mainly carbon -- then rots back into carbon dioxide. In one experiment at Rothamsted Highfield in the United Kingdom -- part of the world's oldest existing agronomic experiment, started in 1843 -- converting grassland to arable land cut the soil carbon content by 55 percent over 20 years due to tillage. That carbon had gone back into the atmosphere as carbon dioxide. Wind or water erosion can do the same. But this can often be reversed.

"Forests store more carbon than farmland, but farmland has the potential to increase its carbon storage, whereas mature forests may do so at lower rates," says Jacques Antoine, an FAO soil scientist who worked on the report.

Increasing soil organic matter in farmland also improves soil structure, allowing water to reach the crop's roots rather than running off the surface, taking precious topsoil with it. Reducing or eliminating tillage can protect soil organic matter. So can leaving crop residues in place after harvest. A technique known as conservation agriculture integrates all of these practices.

But crop residues may be needed for animal feed. Or leaving them on the ground may delay re-sowing. The economics of farming are harsh, especially in the developing world. So the report looks at the big picture for several different farming systems, such as agroforestry, grazed grassland and arable land.

Bury it. Measure it

On paper, the best way to bury carbon productively is agroforestry -- combining trees and crops, together or in sequence. Agroforestry can replace slash-and-burn agriculture, which accounts for much of the deforestation in the developing world, and the trees provide income. Agroforestry could be suitable for up to 300 million hectares of degraded farmland in the tropics. But trees and crops can compete for moisture and nutrients, so agroforestry must be well managed to be profitable.

Grasslands could be as useful in practice. The world's estimated 3.2 billion hectares of grasslands could store as much soil carbon per hectare as forests. But about 70 percent are degraded, often through overgrazing. Stopping overgrazing is the key, followed perhaps by improving degraded grassland by planting it with legumes or grass species with deeper root systems. These activities could increase carbon sequestration -- and provide more sustainable grazing, and thus more income.

Still, arable land remains crucial. Buildup of soil organic matter can be encouraged through better crops and agronomic practices, including minimizingtillage, leaving crop residues in place, mulching and using manure and even sewage sludge as fertilizer.

Before any of this can figure in carbon audits, researchers need better measurement tools. They must assess when, how and where carbon is best sequestered, the success of current afforestation and rehabilitation projects, and how much soil organic matter is built up in plant roots. And they need a mass of other scientific and socio-economic data.

FAO helps gather this information through the Terrestrial Carbon Observation Initiative and the Global Terrestrial Observing System. These systems, which monitor and collect data on environmental phenomena, are operated jointly by various institutions, with FAO hosting the secretariats.

True green revolution

The report concludes that soil carbon projects offer "an opportunity for semi-arid and sub-humid regions to meaningfully participate in climate mitigation, while improving human well being." It calls for cooperation between the United Nations and other organizations and the bodies responsible for implementing UN conventions on climate change, desertification and biodiversity. Such collaboration will result in "a true green revolution," says the report.

"Agriculture in the developing world can do much to mitigate climate change while becoming more sustainable and productive," says Mr Antoine. "Soil carbon sequestration for improved land management is the first route map towards that goal."

28 March 2002