Charred Farm Waste Gobbles Carbon

June 11, 2007 — Burning agricultural waste without oxygen could provide a way to lock up massive amounts of greenhouse gas, stimulate plant growth and produce renewable energy all at the same time, a new study suggests.

Recent greenhouse trials found soils mixed with the charred waste, called agrichar or biochar, were more attractive to worms and helpful microbes.

Soils also needed less fertilizer and in some cases had a better capacity to hold water, the researchers say.

"When applied at 10 tons per hectare, the biomass of wheat was tripled and soybeans was more than doubled," said Lukas Van Zwieten from the New South Wales Department of Primary Industries (NSW DPI).

A 2005 study in the journal Nature found that carbon levels in U.K. soils had fallen steadily since the 1970s, and the soils were now releasing some 13 million tons of carbon each year.

Since last year a number of researchers around the world have been interested in reducing CO2 emissions from soil using agrichar.

This charred product is the result of burning biomass without oxygen, a process called pyrolysis.

The NSW DPI trial, which is the first Australian trial of agrichar, found pots of soil treated with the product emitted significantly less CO2 and nitrous oxide than control pots.

Nitrous oxide is a greenhouse gas 300 times more potent than CO2.

The original idea of using char in the soil comes from the observation of rich black soils in the Amazonian Basin known as 'terra preta' soils, produced by pre-Columbian agricultural burning.

"These soils are a couple of thousand years old and the people using them are still reaping the [soil fertility] benefits," said NSW DPI team member Stephen Kimber.

A commentary in Nature last month, by soil expert Johannes Lehmann of Cornell University, argued in favor of using agrichar to sequester carbon in soils, saying it would be less likely than trees to release carbon into the atmosphere.

Kimber says agrichar is a form of carbon that's very stable in the soil and doesn't break down like mulch and other organic matter.

He says carbon can stay locked up in agrichar for hundreds of years, although some suggest it could do the job for millennia.

Agrichars trialled by NSW DPI include those from poultry litter, cattle feedlot waste as well as municipal green waste and paper mill sludge.

Each agrichar has its own characteristics and interacts differently with different soil types.

Some agrichars raise soil pH at about one-third the rate of lime, raise calcium and reduce aluminium toxicity.

Kimber said more research needs to be done on working out which agrichars are best for which soils and on the impact of any contamination in biomass.

Biomass burned by pyrolysis can also be used as a renewable energy source, but there is a trade-off.

The more oxygen used to burn it, the more energy can be produced but the less agrichar, says Kimber.

"It's a compromise," he said.