1. Results and Discussion
Soil Carbon Sequestration under Agroforestry Systems in the West African Sahel
Asako Takimoto1, Ramachandran P. K. Nair1, and　Vimala D. Nair2　　
　1School of Forest Resources and Conservation; 2 Soil and Water Science Department, UF/ IFAS, Gainesville, FL
Introduction
Materials and Methods
Results and Discussion
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.
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 ).
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)
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
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.
V.paradoxa
parkland
Fodder
bank
Abandoned
land a b c c c
F.albida
parkland
Live fence
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).
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).
Figure 1. Aboveground and belowground C stock
Soil Sampling:
Sampling points: Near the tree trunk and outside the crown area.
Three soil depths: 0 – 10, 10 – 40, and 40 – 100 cm. a b c a b
Faidherbia albida parkland
Live fence
Fodder bank
Abandoned land
Conclusion
Physical Fractionation: Wet sieving through 250 and 53 µm sieves; fraction sizes 250 – 2000 µm, 53 – 250 µm and <53 µm.
Agroforestry systems contribute to better 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.
Near tree
Outside crown
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.
Figure 2. F.albida parkland
Figure 4. Fodder bank
References: Balesdent, J., and A. Mariotti 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
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 ‰). a b c a b
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.
Near tree
Outside crown
Figure 3. Live fence
Figure 5. Abandoned land