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Western rangeland plays hidden role as massive carbon storehouse

Date posted: February 13, 2003

Western Canada's vast tracts of rangeland play a crucial part in ranching, oil and gas exploration, wildlife management and recreation. But one of this land's most important roles remains largely hidden - that as a massive storehouse in the global carbon cycle.

Though the region's native grasslands have only modest above-ground vegetation, it's important to remember that underfoot they house billions of tonnes of organic carbon, say scientists at the Agriculture and Agri-Food Canada (AAFC) Lethbridge Research Centre. Managing these reserves effectively can play a key role in controlling global emissions of carbon dioxide - the chief greenhouse gas targeted by the Kyoto Protocol.

"Increasing concentrations of carbon dioxide in the atmosphere continue to focus increasing attention on the global carbon cycle," says Dr. Henry Janzen, research scientist in nutrient cycling. "Grasslands, because of their expanse and high carbon density, are a prominent part of that cycle, both globally and in Canada. Probably their greatest role is as a repository of carbon already stored. And the grassland management practices that have greatest benefit to the atmosphere are those which hold these large reserves in place."

The AAFC Lethbridge Research Centre has several long-term studies in place to examine changes in soil carbon under various types of land use, and has helped co-ordinate AAFC's greenhouse gas initiative since it began in 1992.

Soil organic matter, says Janzen, contains approximately double the amount of carbon currently in the atmosphere. As a result, even a small shift in soil organic matter can have a large impact on atmospheric carbon dioxide.

"The soils in temperate grasslands contain more carbon per unit area than those of most other ecosystems, worldwide. For example, a soil under grassland in Western Canada may contain, to one metre depth, up to 200 tonnes of carbon per hectare in the black soil zone under fescue prairie," he says. "If soil organic matter is depleted through mismanagement, the carbon lost from the soil finds its way into the atmosphere as carbon dioxide. Conversely, practices that build up soil carbon remove carbon dioxide from the air."

Over the past century, cultivation and other soil-depleting practices have resulted in the loss of roughly one-half to 1 billion tonnes of carbon from Canadian prairie soils. "But the uncultivated grasslands of Western Canada still contain large reserves of carbon. Keeping the plow away is by far the best way for rangeland producers and managers to keep this carbon in store and help slow atmospheric carbon dioxide increases."

Grazing regimes can also have a significant effect on soil carbon storage, he says. "The most practical management option for increasing soil carbon storage may be to alter the grazing intensity, timing or frequency. Grazing regimes can alter the plant community, favouring some species over others. They can also affect plant residue, plant vigour and other important factors."

Further research is needed to pinpoint the best grazing strategies that favour soil organic matter, he says. A challenge for researchers is the complexity of native grassland systems compared to monoculture arable systems. "Some studies have even suggested higher soil carbon storage under well-grazed than under non-grazed treatments. More long-term studies on diverse grasslands are still needed to evaluate these and other opportunities."

Clear opportunities do exist, however, on economically or environmentally marginal land, and on over-grazed grassland, he says. Re-establishment of perennial grass in these areas could eventually result in the recovery of carbon previously lost.

Agriculture and Agri-Food Canada's Lethbridge Research Centre has a mandate to promote innovation for growth, maintain security of the food system and protect the health of the environment.