What is biochar?
Biochar is an interesting concept based on a fine-grained and porous form of charcoal. It is a practice that converts agricultural waste (biomass) into a soil enhancer that can hold carbon, boost food security and discourage deforestation [1]. It is by no means a new phenomenon. It has been used for the last 2000 years as a way to boost the fertility of soils but it has recently been championed by green pioneers as a way to combat climate change. However, there are some negative aspects to biochar which will be discussed in this article.
How is biochar made?
Biochar is produced through pyrolysis or gasification — these are processes that heat biomass in the absence or reduction of oxygen [1]. Biochar practices produce two main things:
- oil and gas by-products which can be used as an alternative fuel to fossil fuels.
- A ‘charcoal’ soil enhancer.
It can be argued that when biochar is buried in the ground as a soil enhancer, the system can become “carbon negative” [1].
Carbon sequestration and climate change
Biochar can help to offset climate change by acting as a stable long-term store for carbon in soils (also known as sequestration). The carbon in biochar resists degradation and can hold carbon in soils for hundreds to thousands of years [1]. Because it takes carbon out of circulation, it therefore reduces atmospheric concentrations [2]. There is additional carbon offsetting arising from the avoided emissions from fossil fuel combustion, as biochar oil and gas can be used instead of fossil fuels. Carbon offsetting also occurs with fertiliser application as artificial fertilisers would no longer be required, and it is the production of these that produces copious amounts of greenhouse gas emissions. A recent study suggests that biochar could offset carbon emissions by 1.8 gigatons per year without endangering food security, habitat or soil conservation [3].
Waste products
There is a wide range of potential waste products which could produce biochar. These include wood waste, timber, agricultural wastes, manure, leaves, food wastes, straw, paper sludge, green waste and distillers grain [4]. However, those that have pioneered biochar have suggested the use of fast-growing trees as a biochar ingredient. This brings questions to the fore such as where are these trees being grown and could it damage areas of cleared rainforest by replacing with fast growing species instead of slow growing native trees?
Soil and crop improvement
Biochar raises crop yields by acting as a fertiliser. This soil and crop improvement is arguable, as in some cases charcoal in the soil improves plant growth, in others it suppresses it [3]. Its use as a ‘green’ fertiliser is also arguable as in some cases charcoal stimulates bacterial growth, causing carbon emissions from soils to rise [3]. Charcoal can also conserve and purify water and prevent runoff of chemicals from farm lands [5]. Defra questions the improvements made to soils, including: over how many years does an increase in crop productivity occur; and whilst biochar might improve productivity, is this effect really understood well enough that we can factor-in a long-term enhancement of the carbon sink in vegetation and soils? [6]
The future of biochar
The future for biochar, seems for now to be best suited on a small scale, with farmers turning their waste agri-products into biochar and re-investing it back into their soils, thus allowing them to grow more food crops. The threat of losing millions of hectares of farmland and forest to biochar production seems ill-advised but as the issue is researched more, a way of bringing biochar to the masses could be on the cards in the next few years. The box below suggests the criteria required for sustainable biochar production.
Woolf et al (2010) suggest criteria for sustainable biochar production as follows:
- biomass acquired from agricultural and silvicultural residues should be extracted at a rate and in a manner that does not cause soil erosion or soil degradation;
- crop residues currently in use as animal fodder should not be used as biochar feedstock;
- minimal carbon debt can be incurred from land-use change or use of feedstocks with a long life expectancy;
- no new lands should be converted into biomass production and no agricultural should land be taken out of food production;
- no biomass wastes that have a high probability of contamination, which would be detrimental to agricultural soils, should be used;
- biomass crop production should be limited to production on abandoned agricultural land that has not subsequently been converted to pasture, forest or other uses.
- Biochar should be manufactured using modern technology that eliminates soot, CH4 and N2O emissions while recovering some of the energy released during the pyrolysis process for subsequent use.
Taken from “Sustainable biochar to mitigate global climate change” by Woolf, Amonette, Street-Perrott, Lehman and Joseph (2010) [4]
References
[1] Biochar International http://www.biochar-international.org/
[2] Guardian http://www.guardian.co.uk/environment/2009/mar/24/george-monbiot-climate-change-biochar
[3] Sustainable Biochar to Mitigate Climate Change http://www.nature.com/ncomms/journal/v1/n5/pdf/ncomms1053.pdf
[4] UK Biochar Research Centre http://www.geos.ed.ac.uk/sccs/biochar/Productionsources.html
[5] Biofuel Watch http://www.biofuelwatch.org.uk/docs/biocharbriefing.pdf
