Magic biochar: recycles, fertilises and sequesters

Tom Miles

Magic biochar: recycles, fertilises and sequesters
From the September 2006 edition of Agriculture Today.
New South Wales Department of Primary Industries, Australia

Thousands of years ago, Amazonian Indians burned their waste organic matter in low intensity fires covered with dirt and straw.

The smouldering heat charred the organic matter, and the Indians added the charred material to their soils.

Today these man-made soils are still cultivated intensively by local farmers yet remain fertile and productive, mainly because they contain a very stable form of carbon resistant to decomposition.

The soils, known as Amazonian dark earths, or terra preta, are now attracting attention from scientists who see them as a longterm method of storing excess carbon and reducing greenhouse gas levels.

The Department of Primary Industries at Wollongbar is doing pot trials with biochar made from industrial waste. Researcher Lukas Van Zwieten has found the biochar has the greatest impact on acid soils that have low organic matter and high aluminium levels.

He is now planning field trials and organising an international biochar workshop at Wollongbar in April 2007.

He says there is a lot of interest in biochar because the process recycles urban and rural waste, improves soil fertility and sequesters carbon into the long term carbon pool. As well, the charring process produces gases that can be used as biofuels.

US researcher Johannes Lehmann estimates that by the end of this century, terra preta schemes and biofuel programs could store up to 9.5 billion tonnes of carbon a year.

At the recent World Soils Congress in the US, scientists reported that terra preta soils retain nutrients in plant available forms even after repeated cropping, and retain moisture.

Compared to the surrounding soil, terra preta can contain three times as much phosphorus and nitrogen and up to nine per cent carbon.

Some research suggests that a hectare of metre-deep terra preta can contain 250 tonnes of carbon, as opposed to 100 tonnes in unimproved soils from similar parent material. The extra carbon is in the char, the organic carbon and bacterial biomass.

When researchers charred their own organic matter and spread the resulting biochar on tropical soils in Colombia they found that even small quantities increased maize biomass and improved pasture, particularly in the second and third years of the trial.

The bio-char increased soil pH, cation exchange capacity, potassium availability and moisture retention. There were net increases in soil carbon and reduced emissions of methane and nitrous oxide from the soil.

Scientists have also found that freshly made biochar initially loses carbon, and carbon quality differs according to the biomass used to make it and conditions under which it is made.

The August 10 issue of Nature magazine has a fascinating account of the recent US terra preta workshop and some of the issues that still need to be addressed by researchers. It is available online to subscribers at
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