'Dark earth' and its carbon-holding powers
Biochar has been touted as a great hope for mitigating climate change and boosting soil fertility. But critics caution that more research is needed to understand its effects.
Biochar is based on terra preta, or dark earth — a mixture of bone, manure and charcoal that was first used in the pre-Columbian era to enhance the infertile soil of the central Amazon basin. Some of the charcoal has remained in the soil for thousands of years, leading scientists to believe that it could be used elsewhere as a long-term carbon store.
Johannes Lehmann, a soil scientist at Cornell University, New York, and colleagues, calculate that half of the estimated six billion tonnes of carbon in agricultural, forestry and animal waste could be turned into biochar. And for every tonne of biochar, a third of a tonne of biofuel by-product could be produced.
Some advocates are developing industrial-scale microwaves to produce biochar but critics are concerned that developing a market for biochar could encourage the destruction of tropical rainforests — while others question whether the carbon would really remain in the soil for such long periods.
Lehmann admits biochar is "not a silver bullet" and that no technology could compensate for the current level of emissions. But biochar could help, he says.