Biochar is charcoal added to the earth as a soil conditioner to improve crop yields. It is a carbonrich material made by heating plant material in the absence or near absence of oxygen; the organic matter is degraded by heat, but not combusted, and aromatic carbons are produced. These carbons are diffi cult for microbes to break down and so the carbon in biochar is not simply recycled back into the atmosphere as it would be in other organic matter. Bury this biochar in soils and billions of tonnes of carbon may be sequestered annually, proponents say, reducing greenhouse gas emissions. It also improves plant growth: a win-win scenario.
Biochar advocates, especially the International
Biochar Initiative, want biochar included in any
new agreement that replaces the Kyoto Protocol,
so that it can earn carbon credits under the Clean
Development Mechanism (CDM).
Deciding how much biochar improves plant
growth in soil and reduces carbon emissions through
the entire system is critical to such calculations;
char may improve plant growth and thereby reduce the need for fertiliser, delivering a carbon saving
because fertiliser manufacturer is energy hungry.
The logic: reduce fertiliser usage, save energy, emit
less carbon dioxide. Also, rotting biomass gives off
methane, a potent greenhouse gas, so rather than
let plant material decay, pyrolysing it and burying it
as biochar reduces methane emissions. It may also
reduce nitrous oxide emissions, another greenhouse
gas. But quantifying all these emission reductions is a
work in progress, and research has only really begun.
Simon Shackley, lecturer in social policy for biochar
at the UK Biochar Research Centre in Edinburgh,
believes a formal review by the scientific community
is warranted before biochar enters the CDM or EU
emissions trading scheme.
‘I think it will end up there because it’s got
tremendous potential,’ he says, but a case-by-case analysis will be necessary for every biochar project if
carbon credits were to be made available.
The contribution of biochar to increasing crop
yield is also open to debate and is influenced by the
soil, climate and crops. Studies must also account for
ash, notes Saran Sohi, soil scientist at the UK Biochar
Research Centre, who says he is sceptical of shortterm
gains in biochar addition experiments, since
charred organic material includes, not just biochar,
but nutrient-rich ash that gives an immediate boost
to plant growth.
Biochar advocates frequently promote their
case by referring to terra preta – rich dark soils in
the Amazon created when indigenous people added
wastes and charcoal to soil, between 500 and 7000
years ago. Terra pretta soils are incredibly fertile,
though some of the fertility may owe to the mix of
material with the charcoal. Nonetheless, the tiny
pores in charcoal improve soil’s retention of water
and the valuable nutrients therein, explains Sohi.
Indeed, the charge density and ability to retain
cations like magnesium, potassium, ammonium and
calcium is greater in biochars than any other organic
matter. So biochar helps soils to retain nutrients and
thus increases plants’ supply of valuable nutrients.
Carbon in decaying vegetation is returned to the
atmosphere fairly rapidly by microbes, adding to
greenhouse gas emissions. But studies indicate that
the carbon in biochar, regardless of the source crop,
remains in the soil for between 200 and 1000 years.
Soil scientist Johannes Lehmann has tested charcoal
that was dumped around blast furnaces in the 1870s
and found that the black carbon had remained
stable in the soil. Indeed, if it was a matter of just stabilising biochar, it would be easy, according to
Sohi. But factoring in other costs and benefits is
essential. Carbon debt must be accounted for: if
you cut down a forest to produce biochar, you loose
the benefits of that forest and those benefits must
now be subtracted from any emission gains obtained
by future land use. This issue arose when scientists
calculated how long it would take an energy crop,
such as sugar cane, to produce enough energy to pay
back emission costs incurred in converting the land.
A study in Science last year showed this was over 400
years in some cases (319(5867), 1235).
Accounting for added benefits in terms of crop
yield is also fraught with difficulties. ‘Biochar is a very
complicated proposition mainly because of the soil,’
says Sohi. He is confident of the benefits in coarsegrained
sandy soil, for example, where biochar
would allow it to keep more water and therefore
more nutrients. But performance will vary depending
on soil type and local conditions.
Laboratory studies will not account for the
practicalities of getting biochar into soil at an
agricultural level and moving it around, so farm-scale
experiments are needed. To this end, the UK Biochar
Research Centre has begun testing among farmers
on arable land in Scotland.
The centre is also involved in a project in Cambodia
where rice husks are converted into energy in a rural
electrification scheme. Biochar is a by product. ‘We’re
looking at its potential in rice paddies and maybe
some other crops. The aim is to get an application
under CDM to get carbon credits,’ says Shackley. The
idea is to offer a financial benefit because decaying
husks have been turned into biochar, a valuable soil
conditioner, which can reduce less fertiliser inputs
and methane emissions, and so lessen the addition
of greenhouse gases to the atmosphere.
Bruno Glaser, who led studies on Amazonian dark
earth soils, predicts that the best biochar technologies
will use regional material flows, in the same way the
pre-Columbian Indians did while creating terra preta.
He has advocated transforming sewage into biochar.
Lehmann agrees that ‘tapping into waste streams’ is
the key to attaining emission reductions and making
biochar economically feasible.
‘Good systems are those where the biochar is
already on a pile somewhere and constitutes a waste
removal issue. Yard waste on a pile, for example
,creates a liability in terms of methane emissions and
a huge cost to municipalities,’ explains Lehmann.
‘Whereas if you have to collect the biomass, or grow
it on a plantation, then under the scenarios we have
looked at, they are not working.’
Lenhman is sceptical that anyone could make
money from dedicated biochar plantations. ‘I can’t
see that this makes any sense from an emissions
point of view or from an economic point of view; I
can’t see it happening.’ It’s not feasible in the US
because it costs maybe $40–50 dollars to produce
a tonne of biomass and transporting biomass or
biochar as a commodity is simply not economically
feasible, he says. ‘You can’t ship biochar very far and
not loose all your emission reductions that you can
claim.’ So growing trees, turning them to charcoal
and burying it as biochar, it seems, is not on the
Many leading soil scientists have also recently
shifted away from the idea of dedicated biochar
crops and say that crop residues and urban and
municipal wastes are the better way to go.
A few examples of pilot units for producing
biochar, such as one in Senegal built by the
NGO Pro-Natura, are in operation, and more are
planned. Best Energies in Australia has developed a
commercial-scale pilot plant, which takes 300kg of
biomass/hour and converts it to biochar. The process
is run on syngas from the pyrolysis – a mixture of
carbon monoxide and hydrogen gases – and about
half the carbon in the original material goes to the
energy producing cycle. The company developed its
technology to deal with low-grade waste like paper
sludge, which currently gets dumped in landfill.
Mantria Industries in the US will open a
community facility in Tennessee in July to produce
biochar. According to Lehmann, a farmer in West
Virginia pyrolyses poultry litter to generate sufficient
energy to offset his entire fossil fuel requirements for
heating barns year round and sells the remaining
biochar to farmers. But there is a need for scientists
to take the next step, ‘to evaluate and test biochar at
scale of implementation,’ says Lehmann.
Lehmann argues that biochar should not be
excluded from carbon credits, if it meets the criteria
for true carbon emission reductions, through rigorous
testing. But environmental groups are wary. Ecologist
Reyes Tirado, honorary research fellow (Greenpeace)
at the University of Exeter, UK, says biochar’s
effectiveness and its long-term sequestration of
carbon are in doubt and that it is dangerous to
presume that it will work worldwide, in every soil
type, climate and crop. She is concerned that farmers
may invest in a still uncertain, hyped technology that
may fail to deliver and that it could encourage the
removal of valuable organic matter from croplands.
‘There are real sustainable alternatives to the
uncertainties of biochar that are feasible, economic
and applicable right now, she says.
Almuth Ernsting of Biofuelwatch agrees
that there is too much uncertainty and says that
proponents are looking to scale-up too fast to the
level of plantations. There is a lack of research to
warrant biochar’s inclusion in the post-2012 climate
agreement, she argues.
But biochar proponents are themselves now
divided. There are those who say we should forget
about biochar plantations and focus on turning
wastes into biochar. It is estimated that this could
allow 1bn t of carbon to be sequestered annually.
Of a more optimistic bent are those proponents who
still believe that degraded land can be turned over
to biochar crops and that this could sequester 9bn
t of carbon annually. Research currently under way
should help decide the outcome of this debate.
The biofuel experience, particularly in the US, has
refocused regulators attention on life cycle analyses,
so that knock-on effects are totted up on both sides
of the balance sheet. But it is almost impossible to
predict every problem that may arise when a new
innovation arrives, so practical applications will have
to be run through and tested. Biochar is unlikely to
be the miracle solution touted by some and scientific
and economic questions remain, but it warrants
attention and rigorous, informed debate.