POZNAN, Poland (Reuters) - An ancient technique of plowing charred plants into the ground to revive soil may also trap greenhouse gases for thousands of years and forestall global warming, scientists said on Friday.

An Indian farmer plough his field in Chuli village, 255 km (158 miles) west of the western Indian city of Ahmedabad, June 22, 2005. REUTERS/Amit Dave

Heating plants such as farm waste or wood in airtight conditions produces a high-carbon substance called biochar, which can store the greenhouse gas carbon dioxide and enhance nutrients in the soil.

Plants absorb carbon dioxide from the atmosphere as they grow. Subsequently storing that carbon in the soil removes the gas from the atmosphere.

“I feel confident that the (carbon storage) time of stable biochar is from high hundreds to a few thousand years,” said Cornell University’s Johannes Lehmann, at an event on the sidelines of U.N. climate talks in the Polish city of Poznan.

Lehmann estimated that under ambitious scenarios biochar could store 1 billion tons of carbon annually -- equivalent to more than 10 percent of global carbon emissions, which amounted to 8.5 billion tons in 2007.

Under a conservative scenario the technique could store 0.2 billion tons of carbon annually, he said. That would still require heating without oxygen -- called pyrolysis -- some 27 percent of global crop waste and plowing this into the soil.

Lehmann cited experiments on 10 farm crops suggesting biochar can also increase yields by up to three times, because the organic matter holds on to nutrients.

The International Energy Agency (IEA) said in November that global greenhouse gas emissions were so out of control that avoiding more dangerous levels of climate change depended on creating negative emissions later this century.

The energy adviser to 28 industrialized countries cited biochar as one way of achieving that.

The technique rings alarm bells among some environmentalists worried it could spur deforestation, but its chief problem may be that it is barely proven on a commercial scale.

“It will remain theoretical without making demonstration plants on the ground,” Lehmann said.

Soils containing biochar made by Amazon people thousands of years ago still contain up to 70 times more black carbon than surrounding soils and are still higher in nutrients, said Debbie Reed, director of the International Biochar Initiative (IBI).

The IBI was in Poznan to lobby for research funding for biochar. In Poznan, 187 countries are meeting in ongoing talks to agree a new climate treaty to replace the Kyoto Protocol after 2012. They hope to finalize a deal next year.

Lehmann emphasized that the technique was not a substitute for fighting climate change by curbing man-made greenhouse gas emissions, especially carbon dioxide from burning fossil fuels.