Charred Biomass Could Act as Carbon Sink, Researchers Say

A high-carbon char material made from crop waste and other biomass shows promise for both enriching depleted agricultural soils and locking carbon into the Earth for long periods of time, according to new research by the New South Wales Department of Primary Industries' (DPI) Wollongbar Agricultural Institute.

The material, known as agrichar, is made by heating organic matter without oxygen in a process called pyrolosis. The resulting product is a black carbon-based material that resembles "terra preta," a highly fertile dark soil found in the Amazon region. Believed to have been produced by pre-Columbian people through "slash-and-char" (which involves lower temperatures than "slash-and-burn") agriculture, terra preta has remained highly fertile even after many centuries of intensive farming.

When added to carbon-depleted soils in Australia, agrichar has boosted crop production many times over, according to DPI scientists.

"When applied at 10 tons per hectare, the biomass of wheat was tripled and of soybeans was more than doubled," said Lukas Van Zwieten, a senior research scientist with DPI. Agrichar increased soil pH, raised calcium content and reduced the toxicity of aluminum in the test soils, he added.

"Soil biology improved, the need for added fertilizer reduced and water holding capacity was raised," Van Zwieten said.

Agrichar-enriched soils also emitted fewer greenhouse gases than regular soils, researchers found. Unlike mulch, compost and other crop waste, in which the carbon breaks down and is converted into carbon dioxide over two or three years, agrichar locks in carbon for long periods of time.

"(S)table carbon like agrichar will last up to hundreds of years," said Steve Kimber, a DPI environmental scientist. "This is significant for farmer costs because one application of agrichar may be the equivalent of compost applications of the same weight every year for decades. For the environment, it means soil carbon emissions can be reduced because rapidly decomposing carbon forms are being replaced by stable ones in the form of agrichar."

For now, however, agrichar is hard to come by. BEST Energies Australia operates a pilot plant that produces small amounts for research purposes, but there are currently no commercial producers of agrichar.

"We are hoping the technology will take hold and pyrolysis plants will be built where there is a steady stream of green or other biomass waste providing clean energy that is carbon negative," Van Zwieten said. "But until pyrolysis plants are up and running, the availability of agrichar for farmers will be scarce."

New South Wales Department of Primary Industry, "Soils Offer New Hope as Carbon Sink." URL: (http://www.dpi.nsw.gov.au/research/updates/issues/may-2007/soils-offer-new-hope)