Carbon sequestration has limits

Scientists understand the theory of carbon sequestration, but transferring that information to farmers around the world is the bigger challenge.

Pulling carbon out of the atmosphere and putting it into the soil adds fertility and better water holding capacity, but there are limits, scientists said April 24 during an international carbon sequestration webinar.

“There is a limitation when we are looking at carbon sequestration,” said soil ecologist Keith Paustian of Colorado State University.

The soil carbon balance tends toward saturation, but the practices should not stop because it can become depleted again. It takes about 30 to 50 years to reach carbon equilibrium.

Putting more carbon back into the land requires management changes such as planting more trees, protecting grasslands, im-proving water management and practising minimum tillage, but policy changes are also needed.

Jean-Francois Soussana of the French National Research Institute for Research and Agriculture said farmers need to be rewarded for their efforts, and their management must not be too costly.

Farmers and ranchers need financial incentives to encourage them to try new methods. He said a payment of US$100 per tonne of carbon sequestered is a good start.

“Farmers who change their practices and systems have some costs, and it is not so simple to readjust a system to sequester carbon in soil,” Soussana said.

They also need the capacity and knowledge to make changes and realize that carbon sequestration can renew degraded soils, which will yield more and reduce the threat to world food security.

“Improving the soil and improving the yields comes together,” he said. “

“When you start with a soil that has been degraded which has lost a lot of organic matter, restoration is extremely effective in improving yields. Having more soil organic matter means you have better water retention and also you have better water infiltration usually.”

Carbon stored in the topsoil of croplands and grasslands could reach a depth of 40 centimetres.

A 1.3 percent annual increase in crop yields can be obtained in Asia, Latin America and Africa if farmers worked to add .4 percent more soil carbon each year.

“Some say increasing this rate of soil carbon by .4 percent per year is too high,” he said.

“This rate of carbon sequestration rate in soil, which was measured over many years, was higher than the targets. When you start with soils with low carbon stocks, you usually get higher sequestration rates,” he said.

Sequestration rates vary widely between different farming and agro-forestry practices, said Eric Toensmeier, author of the Carbon Farming Solution and a lecturer on agri-forestry and perennial crops at Yale University.

“The general idea is we want farmers and ranchers and land managers to have a tool kit of practices that they can use where possible to get the highest carbon we can,” he said.

“What tends to be the trend is that when trees are added, those rates go up. Annual cropping practices tend to be below one tonne per hectare (.4 tonne per acre) per year. By adding trees to those practices, we can sometimes get two or three times more carbon.”

Annual cropping and grazing could add 12 to 20 tonnes of carbon per acre, but when trees are added, 60 to 80 tonnes could be sequestered as a maximum potential.

“This does not include carbon in above ground biomass, which can be in some cases very significant in some of these systems,” he said.

However, soil organic carbon could be depleted under bad management or less than ideal conditions.

The Food and Agriculture Organization has publications on soil carbon sequestration, which can be seen at bit.ly/1V3L2qU.

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