Sheet n°307 - October 2008
A role for slash and burn farming
in greenhouse effect control
orld estimates
indicate that
between 200 and 500
million farmers practise
slash and burn farming.
This mode of agriculture, mainly established
in the tropical forest
regions, attracts strong
criticism in the international community. Since
the 1992 Earth Summit
in Rio, the United
Nations Food and
Agriculture Organization
(FAO) has considered
that this cultivation
model contributes to
deforestation and global
warming by facilitating
carbon release into the
atmosphere. However, a
study conducted by an
IRD team and its Laotian
partners of the National
Agriculture and Forestry
Research Institute
(NAFRI) in the undulating region of northern
Laos has demonstrated
that slash and burn
cultivation rather
favours storage of
organic carbon. Examination of the first
centimetres of the fertile
layer of a 2 ha slope
proved that the organic
carbon accumulated in
substantial quantities in
the steepest part of the
cultivation plot. These
results, which are about
to be confirmed for the
whole of Laos, showed
that maintaining the
slash and burn methods
could play a key role in
limiting the greenhouse
effect linked to human
activity.
© IRD / Bernard Moizo
W
Typical slash-and-burn farmed landscape in Laos.
Slash and burn farming, also called
swidden cultivation, embraces any agrarian system in which plots of land are
cleared of vegetation cover by cutting
and burning before temporary cultivation
for short periods of time. This method
involves periods of fallow longer than
the time allotted for crop growing, which
rarely exceeds 3 years. Currently, an
estimated 200 to 500 million people
in the world practise this type of agriculture. Because the system is mainly
established in forest environments, it is
accused of contributing actively to deforestation. The regular burning of plots
of land has prompted the notion that it
plays a significant role in amplifying
the anthropogenic greenhouse effect.
Since the Rio de Janeiro Earth Summit of 1992, the FAO (United Nations
Food and Agriculture Organization)
has been advocating the replacement
of this practice by continuous cultivation, judged to be less harmful for the
environment. Now, however, a study
conducted in northern Laos, by a team
combining IRD and NAFRI researchers,
on a plot of upland rice cultivated using
the principles of swidden methods, has
restored the reputation of this farming
practice. Contrary to what previous
research work suggests, slash and burn
farming practised on the tropical forest
slopes would favour soil carbon storage.
This was the conclusion reached after a
survey entailing 581 soil auger drillings
from which measurements were made
of the quantity of organic matter present
in the first 40 cm of soil. On this steep
terrain, characteristic of most slash and
burn cultivation sites, measurement of
the organic carbon stores allowed highly
accurate identification of the areas most
favourable for its accumulation. Analyses indicated that the most effective was
the sector where the relief showed the
most abrupt differences in elevation. In
the tropics, the slopes would act therefore as conveyors transporting organic
matter. During fallow periods, the grasses and shrubby species that gradually
recolonize that environment capture the
carbon from the atmosphere, allowing
storage in its organic form in the soil.
When crops are growing, this carbon
and the charcoal produced by burning
of the plant residues are abundantly
washed down by runoff water then
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CONTACT :
VINCENT CHAPLOT
Unité de recherche Sols,
Usages des terres,
Dégradation, Réhabilitation
(SOLUTIONS)
Address :
IRD/SBEEH
Rabie Sanders Building
Université du KwazuluNatal
Boite X01
Scootsville, 3209
Afrique du Sud
Tel : +(27) 33 260 54 87
vincent.chaplot@ird.fr
REFERENCE :
Chaplot V., Podwojewski
P., Phachomphon K.,
Valentin C., Spatial
variability and
controlling factors of
soil organic carbon
under steep slopes of
the tropics, Soil Science
Society of America
Journal, 2008, sous presse
transported towards the foot of the slopes. This organic matter does not end
up in the valley bottoms but comes to
rest on the sheerest parts of the slopes,
on overhangs. This is probably because
masses of rain water, the vehicle of organic matter transport, infiltrates at such
points. Another determining factor for
organic-matter storage capacity appears
to lie in the slash and burn technique
employed. When farmers have burned
the secondary forest that developed on
the plot after several years of fallow, they
do not pull out the tree stumps. The roots
limit erosion processes and would thus
hold back the soil organic matter. Similar
research applying the investigation technique tested on the slopes of northern
Laos is confirming this trend at national
scale. Examination of 3 000 auger samples taken from all parts of the country is
indicating a correlation between the elevation differences of agricultural terrains
and the organic carbon storage capacity
of soils.
In Laos, 70 % of cultivated land is located
on slopes with strong height variations.
And slash and burn methods involve
30 % of the total surface area of national
territory, representing 70 000 km2. If
these ‘shifting’ cultivation strategies were
to be replaced by uninterrupted crop
growing, as FAO advocates, the soil’s
organic carbon storage capacity would
diminish significantly. At national scale,
that would correspond to a net loss of
26 million t of carbon, released into the
atmosphere or into the rivers through
soil erosion processes. This loss may
seem insignificant in relation to the 1.5
billion t of organic carbon stored in all the
land of the Earth. But extrapolation of a
conversion to continuous cultivation
practices to all plots farmed by slash
and burn methods, points to a 60140 % rise in global carbon emissions
into the atmosphere. This is a scenario
for disaster, highly unlikely certainly, but
which makes a powerful argument for
maintaining slash and burn agriculture
as a means of compensating for global
greenhouse gas emissions attributable
to human activities.
Grégory Fléchet - DIC
Translation - Nicholas Flay
KEY WORDS:
slash and burn agriculture,
erosion, organic carbon,
greenhouse effect
PRESS OFFICE :
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+33 (0)4 91 99 94 87
presse@ird.fr
© IRD / Vincent Chaplot
Sheet n°307 - October 2008
For further information
INDIGO, IRD
PHOTO LIBRARY :
DAINA RECHNER
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indigo@ird.fr
www.ird.fr/indigo
Slope affected by slash and burn agriculture. The arrows indicate sites of rain water
infiltration and runoff favouring accumulation of organic carbon in the steepest zone.
Grégory Fléchet, coordinator
Délégation à l’information et à la communication
Tél. : +33(0)4 91 99 94 90 - fax : +33(0)4 91 99 92 28 - fichesactu@ird.fr