An ancient agricultural technique from the Amazon rainforest is attracting the interest of scientists in the US and elsewhere for its ability to restore soil fertility, sequester carbon and provide carbon-neutral or even carbon-negative energy. The technique, being explored by scientists at Cornell University, is called Terra Preta. It involves slowly burning some of the unwanted plant matter in an area and adding the charred remains, termed "biochar", back into the soil.

The investigations of Johannes Lehmann and his team at Cornell have revealed some interesting characteristics of Terra Preta soils. "The knowledge that we can gain from studying the Amazonian dark earths…not only teaches us how to restore degraded soils, triple crop yields and support a wide array of crops in regions with agriculturally poor soils, but can also lead to technologies to sequester carbon in soil and prevent critical changes in world climate," he explained.

The researchers have discovered that the black soil resulting from biochar application not only fixes up to 50% of the carbon that was present in the plant matter but also contains significantly higher levels of minerals and nutrients including nitrogen, phosphorus, calcium, sulphur and organic matter. The soil does not get depleted, as other soils do, after repeated use, and boasts significantly higher fertility levels – so much so that some areas of the Amazon rainforest can attribute their fertility and biodiversity to the intervention of early indigenous peoples using this approach.

The soil is also better able to retain these minerals, meaning that it emits less methane and nitrous oxide. What's more, because the biochar helps retain nitrogen in the soil, less fertilizer is needed, so if Terra Preta is applied to contemporary soil management it can also reduce environmental pollution.

The technique is different from slash-and-burn methods, which use open fires, reduce biomass to ash and release carbon dioxide into the atmosphere. Producing biochar involves low-intensity smoldering fires in which oxygen is excluded, and sequesters rather than releases carbon.

Lehmann is excited by the potential of biochar to contribute to solutions to climate change. "This technique constitutes a much longer and significant sink for atmospheric carbon dioxide than most other sequestration options," he said. "We have calculated that up to 12% of the carbon emissions produced by human activity could be offset annually if slash-and-burn were replaced by slash-and-char."

Biochar fuels
There's another dimension. Making biochar releases heat, meaning that biochar production can constitute a fuel source too. The Cornell team has also explored the technology for this and there are trials underway by various organizations.

"Units to heat a poultry house powered by the poultry litter are being used," explained Lehmann. "One appeal of the technology is that it is scaleable," he added, also emphasizing the importance of establishing a sustainable and economically viable feedstock. "Deforestation is clearly not the way to go," he confirmed. "But there are numerous waste streams or crop residue streams that can be tapped into, especially those that will never have competing uses."

One company that is gearing itself to these opportunities is Eprida in the US, which makes equipment that uses agricultural wastes to produce carbon-fixing soil improvers and energy. Eprida's units, which use a pyrolytic process in an oxygen-free reactor, are relatively small compared with industrial biomass processing plants. They can handle the equivalent of 1 tonne of wood chips per hour – although the biomass could be anything from peanut shells to rice hulls. This size is geared to minimizing the transportation of the biomass and the resulting biochar product.

A spokesperson for the company explained: "Our designers and engineers have set out to develop a unit that…[is] simple and self-sustaining. We feel that rural, small-time farmers are the biggest beneficiary of our machine." If a small farm has its own unit, it can use its own agricultural waste and keep the soil improver for future crops.

Eprida is moving from the development phase towards production and expects to have commercial units up and running by autumn 2008.

The surge in interest in biochar is also reflected in an annual conference (see related links) and the decision of the New Zealand government this month to fund two new professorships in biochar. The positions will be based at Massey University, which has stated that they represent "a first step in building a world-class centre of excellence in biochar research and use".