Australia – charred farm waste could gobble up carbon

Soil emits large amounts of carbon dioxide into the atmosphere. But adding burnt agricultural waste may help to lock in the carbon. ABC reports that 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 Australian trial suggests.



Recent glasshouse trials found soils mixed with the charred waste, called agrichar or biochar, were more attractive to worms and helpful microbes. Soils also needed less fertiliser and in some cases had a better capacity to hold water, the researchers say.



“When applied at 10 tonnes per hectare, the biomass of wheat was tripled and soybeans was more than doubled,” says Dr Lukas Van Zwieten from the New South Wales Department of Primary Industries (NSW DPI) at Wollongbar.



A 2005 study in the journal Nature found that carbon levels in UK soils had fallen steadily since the 1970s, and the soils were now releasing some 13 million tonnes 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.



Ancient Amazonian soils inspire new product



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,” says NSW DPI team member Stephen Kimber.



A commentary in Nature last month, by US soil expert Dr Johannes Lehmann of Cornell University, argued in favour 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 says 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. He says a small amount of plastic contamination is acceptable but biomass sources containing heavy metals could be a problem.



Also an energy source



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 says.



NSW DPI currently uses a pilot plant at BEST Energies Australia in Somersby to produce research quantities of agrichar.



The company recently won top honours at the 2007 UN Association of Australia‘‘s World Environment Day Awards for its pyrolysis technology.

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