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Soil Carbon Sequestration under Agroforestry Systems in the West African Sahel
Asako Takimoto1, Ramachandran P. K. Nair1, and Vimala D. Nair2
1School
Introduction
Carbon (C) sequestration potential of agroforestry
systems has attracted attention from both industrialized
and developing countries, but very little information
about it is available on the systems in the semiarid and
arid regions of West Africa. In addition to the traditional
agroforestry systems, improved practices and
technologies are now being expanded into these dry
regions for perceived benefits such as arresting
desertification, reducing water and wind erosion
hazards, and improving biodiversity.
Hypothesis: Tree-based systems will retain more C in
soil as well as in biomass than tree-less cultivated
systems.
Objectives: To determine (i) C stored in major
agroforestry systems including biomass and whole and
fractionated soils in Mali, and (ii) trace the plant sources
of C using stable isotope signatures.
of Forest Resources and Conservation; 2 Soil and Water Science Department, UF/ IFAS, Gainesville, FL
Materials and Methods
Results and Discussion
Study Area: The study region consists of six villages in Ségou region, Mali, located between the Niger and Bani rivers (13o 20’
to 13o 25’ N and 6o 10’ to 6o 25’ W ).
Mg C/ha
Selected Land-Use Systems (Treatments):
• Parkland systems : Scattered multipurpose trees on farmlands
 Faidherbia albida (N2 fixing tree used for fodder, medicine, soil amelioration, etc.)
 Vitellaria paradoxa (oil-rich nuts are source of cooking oil for local people and shea butter, an
internationally valuable commodity in cosmetic industry)
• Live fence : Planting relatively fast-growing trees in single or multiple rows with close in-row and
between-row spacing around farmers’plots and fields.
• Fodder bank : Planting exotic and/or indigenous species suitable for animal fodder in relatively
high density
• Abandoned land : Recently abandoned degraded land
Biomass Estimation:
Based on field inventory and general allometric equations.
Soil C 10 – 40 cm
Soil C 40 – 100 cm
• Biomass is the major part of C stock in traditional agroforestry system (parklands) but not
significant portion in improved agroforestry systems (live fence and fodder bank) (Fig. 1)
60
V.paradoxa
parkland
40
20
a
b
c
Fodder
bank
c
Abandoned
land
c
0
20
40
F.albida
parkland
Live fence
Fodder bank
Elemental Analyses: Total soil C was determined by
dry combustion on an automated FLASH EA 1112 N C
elemental analyzer and stable C isotope ratio was
measured in a VG602 micromass spectrometer.
Abandoned land
Calculations:
Relative proportions of soil carbon derived from the major
crops in the area, millet and sorghum, C4 plants vs. trees,
C3 plants, was estimated by mass balance (Balesdent
and Mariotti, 1996) :
C4 contribution = (δ - δWL) / (δG - δWL)
Where δ is the δ13C value of a given sample, δG is the
average δ13C value of C4 plants tissue (-13 ‰), and δWLis
the average δ13C value of C3 plants (-27 ‰).
Figure 1. Aboveground and belowground C stock
a
b
c
• In parkland systems, the largest C storage was associated with the most stable fraction
(<0.53 µm) particularly at the lower depths (Fig. 2a). This is possibly because the trees in
parkland systems are much older (at least 35 years old) than those in the live-fence system
(less than 10 years).
• For the live fence system, the largest C storage was associated with the largest particle size
where most C was newer and less stable than C in smaller fractions (Fig. 3a).
• In the fodder bank system, C content was low in surface soil (Fig. 4a, 4b) probably because
surface soil C was lost during tillage for fodder bank establishment, and C accumulation
through litter fall after system establishment was very low because of frequent biomass
harvest for fodder.
• In all agroforestry systems, contribution of soil C from
the C3 plant was higher closer to the tree (Fig. 2b, 3b)
compared to outside the crown (Fig. 2c, 3c).
a
b
• Total C content within 1 m soil profile was highest in
abandoned land (Fig. 1; p<0.05), indicating the
significant influence of previous land-use (annual
cropping, in this case) on soil C storage (Fig. 5b).
Conclusion
Physical Fractionation: Wet sieving through 250 and 53
µm sieves; fraction sizes 250 – 2000 µm, 53 – 250 µm
and <53 µm.
Live fence
Soil C 0 – 10 cm
60
Soil Sampling:
•
Sampling points: Near the tree trunk and outside the
crown area.
•
Three soil depths: 0 – 10, 10 – 40, and 40 – 100 cm.
Faidherbia albida parkland
Biomass C
Near tree
Figure 2. F.albida parkland
a
Figure 4. Fodder bank
b
Near tree
Figure 3. Live fence
Ag r of or est r y syst ems cont r ibut e t o bet ter C
sequestration in both biomass and soil compared with
tree-less system. However, the extent, especially in
soil, depends on soil management and age of the
system: previous land-use could have significant
influence on soil C storage in young systems.
Outside crown
c
a
b
References: Balesdent, J., and A. Mariotti. 1996. Measurement
of soil organic matter turnover using 13C natural abundance. In:
T.W. Boutton and S. I. Yamasaki (ed.) Mass spectrometry of
soils. Marcel Dekker, New York pp. 83-111
Acknowledgment: The research was conducted in cooperation
with ICRAF (World Agroforestry Centre) Sahelian Program and
financially supported by the Fulbright and World Bank
Fellowships to the first author.
Outside crown
Figure 5. Abandoned land