CABI Blog

We have all heard in the news that to tackle climate change, greenhouse-gas emissions must be reduced on a global scale. Fossil fuel emissions still remain the largest contributor to the anthropogenic greenhouse effect, therefore reducing their contribution to global energy is key¹. However, as this is a process that can not be done overnight, an additional strategy to withdraw CO₂ from the atmosphere is also required. One suggestion has been to plant trees around the world. However if afforestation is done in the wrong areas (e.g. away from the tropics, in tundra areas) then it does more harm than good. As one of the screeners for the Soils section of CAB Abstracts, I have seen a dramatic increase in the discussion of the potential of biochar (agrichar) to alleviate this problem.

What is biochar? It is a black carbon byproduct of a process called pyrolysis, which involves heating green waste or other biomass without oxygen to generate renewable energy. Compared to afforestation (left), when biochar (right) is used as a soil amendment, it has a net 20% gain in carbon sequestration (click on image for larger version).

^{Copyright Nature magazine}

The precise duration of biochar’s carbon storage is under debate, with ranges from hundreds of years to millennia. Whatever the length it can be considered a useful long-term CO₂ sink. However, biochar is not limited in the same way as biomass sequestration through afforestation, grassland conversion or no-tillage agriculture. No-tillage, for example, has been reported to cease carbon capture after 15-20 years, while forests eventually mature over decades of centuries and start to release as much CO₂ as they initially sequestered.

Trials of biochar are being undertaken at the Wollongbar Agricultural Institute in New South Wales, Australia. Already off the initial findings, biochar is being hailed as a saviour of the Australian soils and environment. In addition to locking away carbon in the soil it has decreased the emission of soil nitrous oxide and increased crop growth at a rate of up to 10 tonnes per hectare, while decreasing the need for fertilisers and manure applications.

All these details are great, but the most important question is can biochar sequestration and the associated bioenergy production make a difference to national and global carbon budgets?

Within Lehmann’s Nature commentary² he calculates 3 approaches for the use of biochar within USA, that each could sequester about 10% of the amount of the US fossil-fuel emissions (1.6 billion tonnes of carbon in 2005) with long-term storage. This would be ever greater if the emissions produced by pyrolysis are captured and used in further bioenergy production. This figure compares favourable with no-tillage only accounting for 3.6% of US emissions, with only a short time span of 10-15 years³.

¹ IPCC Climate Change (2007): The Physical Science Basis (pdf document) www.ipcc.ch/SPM2feb07/pdf

² Lehmann, J. (2007) Nature 447 (10 May), 143-144 (available from) http://www.iaiconference.org/images/Lehman_Handful_of_C_-_Nature_5.9.07.pdf

³ Jackson, R.B. & Schlesinger, W.H. (2001) PNAS 45. 15827-15829