When it comes to using plant waste to mitigate climate change, most people think of turning it into ethanol or biodiesel for use as a fuel. But a new study suggests we may have more to gain by converting plant material into biochar, a type of charcoal, and burying it in farmers' fields.

Biochar is produced by heating plant waste in an oxygen-free environment, a process known as pyrolysis. This also yields syngas – a mix of carbon monoxide and hydrogen – plus a small amount of oil. Both can be burned as fuels.

Typically, up to 60 per cent of the plant's carbon ends up as biochar. When buried, this can lock the carbon away for thousands of years if necessary. The pyrolysis itself releases no carbon dioxide into the air.

Burning issue

The new study was the work of James Amonette at the Pacific Northwest National Laboratory in Richland, Washington, and colleagues. It centres on a computer model they developed to compare the carbon emissions that would be saved by converting the world's available supplies of plant waste into either biofuel or biochar.

The model showed that converting all the world's available plant waste into biofuels would cut carbon emissions by 10 per cent from today's levels. Turning it into biochar could cut emissions by up to 12 per cent – or 1.8 gigatonnes of the 15.4 gigatonnes emitted each year (Nature Communications, DOI: 10.1038/ncomms1053).

Carbon storage

However, the relative benefits of biochar and biofuel will vary from region to region. "It depends on the fertility of the soil in the region where you are producing the biochar, and whether you are offsetting coal or some other form of energy," Amonette says.

In regions with highly fertile soil and a high proportion of coal in their energy-generation mix, such as the American Midwest, Amonette says it may be better to convert all the available plant waste into biofuel. "But in South America, Africa, south-eastern parts of the US and most of the rest of the world on average, you're better off going with char."

Burying biochar also increases soil fertility. The Biochar Fund, based in Heverlee, Belgium, is carrying out trials of biochar with rural communities in the Democratic Republic of the Congo and southern Cameroon to improve the fertility of soil in these regions.

Midway through the second growing seasons in Cameroon with biochar in the soil, average maize yields have increased from 1.7 tonnes per hectare to 2.5 tonnes per hectare. "In many cases, we saw a spectacular boost in both biomass and grain yield because of the addition of biochar; these extremes are generally found on the poorest soils," says Laurens Rademakers, Biochar Fund's managing director.

Biochar increases the pH of acidic soil, and helps it to retain nutrients such as ammonium, calcium, magnesium, potassium and phosphorus. Some biochars are also highly porous, allowing them to trap moisture and improve the water retention of soils in dry regions, says Amonette.