Sheet n°296 - May 2008
w
Reforestation using exotic plants
can disturb the fertility of tropical soils
ned effects of overgrazing and deforestation
caused fertility loss in
12.8 million km2 of the
Earth’s land surface in
the space of 50 years, by
disturbing the functionality of the biogeochemical cycles regulating
soil fertility. The world’s
high rate of population
growth and expansion
of crop cultivation over
greater areas contribute to an acceleration
of such degradation. In
attempts to check the
spread of this soil degradation, many reforestation programmes
have been undertaken
in tropical and mediterranean zones, taking advantage of fast-growing
tree species. Such initiatives do succeed in
bringing improvements
to an ecosystem’s biomass production. However, there is little information on their impact
on essential soil microbial functionalities
that ensure its fertility.
A research programme
recently conducted by
an IRD team and its partners1 found new clues
for understanding the
influence of these exotic
species on the biodiversity of communities of
mycorrhizal fungi and
rhizobial soil bacteria.
Exotic tree plantations
appear to generate,
under certain climatic
conditions, an abrupt
decrease in soil microbial diversity which
lead to significant degradation of its fertility.
©IRD / Esther Katz
orld Soil InformaW
tion (ISRIC) figures
suggest that the combi-
In Africa, advantage is taken of eucalyptus’ rapid growth in order to produce paper pulp.
In many regions of the world, the impact of human activity on the environment intensified considerably over the
past century. The high world population
growth rate and the expansion of areas
given over to crop production associated
with climatic changes (longer periods of
drought, irregular rainfall patterns) induced by global warming, have contributed
to the acceleration of desertification. According to World Soil Information (ISRIC)
rate, in the space of 50 years, 12.8 million km2 of soils have thus experienced
diminished fertility. With the aim of limiting such land impoverishment, which
is hitting the intertropical and mediterranean zones particularly harshly, a range
of reforestation programmes using rapidgrowing forest species (such as eucalyptus, exotic pine or Australian acacias)
was undertaken from the mid 1970s. Establishment of bacterial and mycorrhizal
symbioses provides these trees with the
adaptation ability necessary for growth
on virtually barren, mineral-deficient
soil. Although no proof is needed as to
their effectiveness for producing plant
biomass in harsh environmental conditions and their utility as windbreaks to
control erosion, there is little information
on their potential impact on the genetic
and functional biodiversity of the soil microorganisms. A research programme
run since 2005 in Senegal and Burkina
Faso by an IRD team and its partners1
yielded clues for understanding the influence of exotic plants on the structure
and biodiversity of these communities
of fungi and bacteria. In Burkina Faso,
controlled experiments showed that the
development of E. camaldulensis, the
eucalyptus species most often planted
in the world, outside its area of origin,
significantly reduced the diversity of the
mycorrhizal fungi communities essential
for the healthy functioning of the ecosystem. This negative effect was also found
in the soil of a Senegalese plantation of
Acacia holosericea where, scarcely a few
months after its introduction, the soil’s
microbial characteristics had completely
changed. This quick-growing species
had effectively selected certain species
of mycorrhizal fungi and bacteria of the
genus Rhizobium, ending in a reduction
in the species diversity of these symbiotic communities. The soil sampled from
areas surrounding the A. holosericea
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CONTACT :
ROBIN DUPONNOIS
Directeur de recherche IRD
Laboratoire commun de
Microbiologie IRD-ISRAUCAD
Address : IRD Bel-Air
Route des hydrocarbures
BP 1386
Dakar
Sénégal
Tel : + 221 (0)33 849 33 22
robin.duponnois@ird.sn
REFERENCES :
REMIGI, P., FAYE, A., KANE, A.,
DERUAZ, M., THIOULOUSE, J.,
CISSOKO, M., PRIN, Y., GALIANA,
A., DREYFUS, B. & DUPONNOIS,
R.,The exotic legume tree
species Acacia holosericea alters microbial soil
functionalities and the
structure of the arbuscular
mycorrhizal
community,
Applied and Environmental
Microbiology, 2008, Vol. 74,
N° 5, p.1485-1493.
Doi : 10.1128/AEM.02427-07
KISA, M., SANON, A., THIOUJ., ASSIGBETSE, K.,
SYLLA, S., SPICHIGER, R.,
DIENG, L., BERTHELIN, J., PRIN,
Y., GALIANA, A., LEPAGE, M. &
DUPONNOIS, R., Arbuscular
mycorrhizal
symbiosis
can counterbalance the
negative influence of the
exotic tree species Eucalyptus camaldulensis on
the structure and functioning of soil microbial communities in a sahelian soil,
FEMS Microbiology Ecology,
2007, 62 (1),
p. 32-44.
Doi : 10.1111/j.15746941.2007.00363.x
LOUSE,
plantation had a balanced distribution of
mycorrhizal fungi species, whereas the
breakdown of the fungal spore content
in soil from the plantation showed a predominance of one species and therefore
a strong imbalance in the composition
of the mycorrhizal fungi community. In
the knowledge that a plant ecosystem’s
productivity is closely dependent on a
soil’s mycorrhizal diversity, there is a risk
that the Australian acacia might create
a new ecosystem whose physical, chemical and biological characteristics will
not necessarily be favourable to a recolonization of the habitat by native species. The research also demonstrated
that the environments generated by this
species were less resistant to water and
heat stress. In a context of global climate
change, such habitats could therefore
experience a drastic fall in their microbial activity and thus lose their ability to
be the basis of proper development of
the plant cover.
The conclusions of the study conducted
in Senegal in a precisely defined environment cannot, however, be generalized to tropical soils as a whole. Indeed,
investigations on another A. holosericea
plantation, in Burkina Faso, yielded the
observation of an increase in microbial
functional diversity. The contradictions
between these sets of results should
prompt the organizations involved in natural resources management to plan for
possible introductions of exotic species
case by case, taking account not only
of potential impacts of the plant species
under consideration for introduction, but
also of the nature of the soils they are to
colonize. For although this practice can
yield highly satisfactory results, such as
increases in the species richness of severely degraded environments, such as
old mining areas, it can also upset for
a long time the organization of the microbial communities which guarantee
the fertility of a soil.
Grégory Fléchet - DIC
Translation - Nicholas Flay
1. This research work was conducted with the support of scientists
from the Département de biologie
végétale of the Cheikh Anta Diop
University of Dakar (Senegal) and
from the Laboratoire Sol-Plante-Eau
of the Institut de l’Environnement et
des Recherches Agricoles (Inera) of
Ouagadougou (Burkina Faso).
KEY WORDS :
Soil, tropical environment,
microbial diversity,
exotic species
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© IRD / Robin Duponnois
Sheet n°296 - May 2008
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Experimental acacia plantation 15 km west of Kaolack (Senegal).
Grégory Fléchet, coordinator
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