land degradation & development
Land Degrad. Develop. 12: 131±142 (2001)
DOI: 10.1002/ldr.444
OPTIONS FOR INCREASING CARBON SEQUESTRATION
IN WEST AFRICAN SOILS: AN EXPLORATORY STUDY
WITH SPECIAL FOCUS ON SENEGAL
N. H. BATJES*
International Soil Reference and Information Centre (ISRIC) / World Data Centre for Soils, Wageningen, The Netherlands
Received 13 October 2000; Accepted 28 December 2000
ABSTRACT
The organic matter content of many soils in West Africa has been depleted due to overgrazing, agricultural mismanagement,
deforestation and overexploitation of the natural resources. Degraded agro(eco)systems can be managed to increase carbon
sinks in vegetation and soil, and to reduce carbon emissions to the atmosphere. The capacity for sequestering carbon will
increase as annual precipitation increases, and generally as mean temperature decreases, provided the soil and terrain conditions
are not limiting for crop (biomass) growth. The agroecological suitability of three pilot sites, proposed for soil carbon
sequestration projects in Senegal, is assessed and the feasibility of various management options to increase organic carbon
levels in the soil is discussed. For the future, a Land Resources Information System should be developed to consider detailed
data on climate, soil and terrain conditions, status of soil degradation, and land-use systems for West Africa. Upon its linkage to
a dynamic soil carbon model and a socio-economic module, such an integrated system can be used to assess the
ecotechnological and socio-economic potential for carbon sequestration projects in the context of the Clean Development
Mechanism (CDM) proposed under article 12 of the Kyoto Protocol to the United Nations Framework Convention on Climate
Change. If adopted, this mechanism could confer funds to West African countries for the sustainable use and conservation of
their natural resources, thereby providing economic, environmental and societal bene®ts for local populations, while
simultaneously contributing to climate change mitigation. Copyright # 2001 John Wiley & Sons, Ltd.
key words: soil organic carbon (SOC); carbon sequestration; land management options; carbon trading; West Africa; Senegal; Land
Resources Information System
INTRODUCTION
Soil organic matter favourably affects the physical, chemical and thermal properties of the soil as well as its
biological activity. Thus its maintenance is important in sustaining soil productivity and biodiversity. In addition,
due to the relatively large size and long residence time of organic carbon in the soil, soils are a potentially
important, natural sink for carbon released to the atmosphere by fossil fuel combustion. The soil-forming factors,
notably climate and the local biological activity in which man is often a predominating factor, control the amount of
soil organic matter that corresponds with steady state conditions in a certain natural ecosystem or agroecosystem.
This paper ®rst presents data on soil carbon stocks for Africa and West Africa. The latter has been de®ned as
comprising 15 countries: Mauritania, Senegal, Gambia, Guinea, Guinea Bissau, Sierra Leone, Liberia, CoÃte
d'Ivoire, Burkina Faso, Ghana, Togo, Benin, Niger, Nigeria and Mali (see inset in Figure 1). Subsequently, the
technical feasibility of various management options to increase the organic carbon content in soils of the `wet' and
`dry' tropics is brie¯y reviewed. The agroecological potential of three pilot sites, proposed for implementing `soil
carbon trading' projects in Senegal, is assessed. Finally, the recommendation is made to develop a Land Resources
Correspondence to: N. H. Batjes, International Soil Reference and Information Centre (ISRIC) / World Data Centre for Soils, PO Box 353,
6700 AJ Wageningen, The Netherlands. E-mail: [email protected]
Copyright # 2001 John Wiley & Sons, Ltd.
Figure 1. Main soil associations in the three pilot areas for Senegal (from north to south: Podor, Bambey and Velingara) (FAO, 1995); inset shows location of Senegal in West Africa).
132
Copyright # 2001 John Wiley & Sons, Ltd.
N. H. BATJES
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
133
INCREASING CARBON SEQUESTRATION IN WEST AFRICAN SOILS
Information System for West Africa. Such a system would allow identi®cation of areas considered most suited for
carbon sequestration projects in the context of the Clean Development Mechanism (CDM), proposed under article
12 of the Kyoto Protocol (see Brown et al., 2000), in terms of their agroecological conditions and socio-economic
setting.
SOIL ORGANIC CARBON RESERVES
There is a great variation in the amount and vertical distribution of organic and inorganic carbon in boreal,
temperate and tropical soils (Batjes, 1996). Soil organic carbon (SOC) density generally increases with increasing
precipitation, and there is an increase in SOC density with decreasing temperature for any particular level of
precipitation. Other important environmental controls of organic matter behaviour in soil are moisture status, soil
temperature, oxygen supply (drainage), soil acidity, soil nutrient supply, clay content and mineralogy. Humi®cation, aggregation and translocation to the subsoil are the main processes of carbon (C) sequestration in soils.
Erosion, decomposition and leaching are important soil processes causing C concentrations to decrease in the soil
(Lal, 1998). In addition, as will be discussed later, land management practices are an important determinant of
potential levels of soil organic matter in the soil.
Table I shows average SOC stocks, to 1 m depth, for the world, Africa and West Africa. About 12 per cent of the
global total of 1462±1548 Pg C (1 Pg ˆ 1015 g) is to be found in Africa. The average SOC density for West Africa
(4.2 ± 4.5 kg C 2) is lower than the average found for the whole of Africa (6.4 ± 6.7 kg C m 2), mainly as a result
of the less favourable agroecological conditions.
With comparatively small stores of biomass and soil organic matter (SOM) and limited net primary productivity
(NPP), plants occurring on `drylands' make only a small contribution to the global carbon cycle through
photosynthesis. The possibility of enhancing carbon sequestration through improved management of drylands has
been the topic of several international workshops (Squires et al., 1995; Squires, 1998), while aspects of
sustainability of agroecosystems in semiarid regions have been discussed elsewhere (Stewart and Robinson,
1997). In general terms, the potential for sequestering carbon in soils of drylands decreases as annual precipitation
decreases, and generally as mean temperatures increase (Rasmussen and Oartob, 1994; Stewart, 1995; Grace et al.,
1998).
Table II lists data on SOC densities for selected types of soil from Senegal. In case of well-drained mineral soils,
the values for the ®rst 1 m range from 1.0 kg C m 2 for a cambic Arenosol under sparse vegetation (Pro®le SN013)
to 7.2 kg C m 2 for a plinthic Ferralsol under forest (Pro®le S104). The highest value of 30 kg C m 2 occurs in a
dystric Fluvisol under rice and short grassland vegetation (Pro®le SN002); this higher value seems a direct
re¯ection of the more favourable soil moisture and nutrient conditions in the river ¯ood plain. Up to 80 per cent of
the total SOC reserve in the upper 1 m of the soils under consideration is found in the topsoil (0±0.3 m). This layer
is most prone to change upon modi®cations in cultivation practices and land-use policies. Within each
agroecological zone the SOC density can vary greatly within a soil unit.
Our data on SOC densities can be linked to the information on soil composition for the Podor, Bambey and
Velingara area, the three proposed pilot sites for `soil carbon sequestration' projects (see Figure 1; Table II). Mean
annual rainfall is about 300 mm yr 1 at Podor, 600 mm yr 1 at Bambey, and 1000 mm yr 1 at Velingara, with a
high interannual and within the rainy season variability (Le Borgne, 1988). The length of the single growing period
Table I. Average stock and density of soil organic carbon (SOC) for the world, Africa and
West Africa (in ®rst 1 m of soil)
Soil organic carbon
SOC stocks (Pg C)
SOC density (kg C m
World
2
)
1462±1548
10 9±11 6
Africa
West Africa
170±180
6 4±6 7
25±27
4 2±4 5
Source: computed using the ISRIC-WISE soil database (Batjes, 1996). See text for 15 countries
comprising West Africa.
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
134
N. H. BATJES
Table II. Soil carbon densities for selected soil units from Senegal
Soil unit1
Location
Vegetation/land use
Bg: Gleyic Cambisol (SN001)2
Fo: Orthic Ferralsol (S101)
Fp: Plinthic Ferralsol (S104)
Gd: Dystric Gleysol (SN011)
Gd: Dystric Gleysol (SN003)
Kamobeul-Bolog
E of Velingara
N of Badion
Kamobeul area
Oussoye-Nianbalan
Jd: Dystric Fluvisol (SN002)
Jt: Thionic Fluvisol (SN004)
Jt: Thionic Fluvisol (SN005)
Jt: Thionic Fluvisol (SN006)
Jt: Thionic Fluvisol (SN010)
Jt: Thionic Fluvisol (SN008)
Jt: Thionic Fluvisol (SN009)
Jt: Thionic Fluvisol (SN012)
SW of Oussoye
SE of Oussoye
SE of Oussoye
E of Nianbalan
Niassa valley, NE of Ba®kane
Medina Exp. Polder
Niassa valley, N of Ba®kane
Kamobeul area
Qc: Cambic Arenosol (SN013)
Ql: Luvic Arenosol (H524)
Ql: Luvic Arenosol (H071)
Rd: Dystric Regosol (S102)
Re: Eutric Regosol (H525)
Re: Eutric Regosol (H047)
Vc: Chromic Vertisol (H253)
Tyle-Boucabar Niama
E of Louga
N of Tivaoune
E of Velingara
Diama
NW of Thies
W of Thies
1
2
Rice
Forest
Forest
Rice
Grass fallow, formerly
under rice
Rice; short grassland
Rice (old mangrove area)
Formerly under rice
Not used/managed
Rice and Eleocharis spp.
Hydromorphic vegetation
Young mangrove
Bare with some mangrove
spp.
Fallow (?)
Fallow after groundnuts
Fallow after millet
Cultivated valley bottom
Abandoned area
Reserved forest
Pasture land
SOC (kg C m
2
)
0-30
(cm)
0-100
(cm)
Ratio
(%)
28
23
35
36
19
34
47
72
65
34
80
48
49
54
56
14 2
18
45
07
26
80
13 7
10 2
30 1
52
19 4
22
62
26 8
25 7
26 2
47
36
23
32
42
29
53
38
06
12
11
20
11
30
29
10
21
24
34
26
48
42
60
54
47
59
42
62
69
Soil characterized according to FAO-Unesco (1974) Legend.
Source of pro®le data: Brouwer (1980) and Stancioff et al. (1984).
is less than three months at Podor, from three to ®ve months at Bambey, and from six to nine months at Velingara
(see Figure 3). Mean annual air temperature is about 29 C at Podor and it decreases towards 27 C at Velingara.
Dominant soil types at Podor are cambic (Qc) and luvic (Ql) Arenosols, Lithosols (I), dystric Regosols (Rd) and
calcic Cambisols (Bk), with eutric Fluvisols (Je) along the Senegal river. At Bambey, luvic Arenosols (Ql), ferric
Luvisols (Lf), and chromic Vertisols (Vc) predominate. At Velingara, the main soil units are ferric (Lf) and gleyic
(Lg) Luvisols, with eutric Fluvisols (Je) in river valleys. The main associated soil units, as shown on the Soil Map
of the World (FAO, 1995), are listed in Table III. Main chemical and physical properties, clay mineralogy and
major limitations of dominant soil units in the seasonally dry (sub)tropics have been reviewed by Kauffman et al.
(1996). Further details on the agroecological suitability of the arid zone, dry tropics and subtropics may be found
elsewhere (FAO 1993; FAO and IIASA, 2000).
OPTIONS FOR INCREASED CARBON SEQUESTRATION IN THE SOIL
The potential for carbon sequestration in a given soil, and agroecological zone, is proportional to the original
reserves present under undisturbed conditions or steady state. Options for carbon sequestration must be chosen on
the basis of knowledge of the nature and likely magnitude of C pools, whether organic or inorganic, in the soils of a
given biome or major agroecological region and the responses of these soils to different land uses and management
systems (Batjes, 1999).
Many (agro)ecosystems are not in a steady state, but they accumulate dry matter during a number of years after
which they are disturbed by ®res and other drastic events, as a result of which their SOC levels often show `toothlike' cycles. After each disturbance, a period of constant management is required in order to reach a new steady
state. In this (newly) undisturbed soil, the organic matter content will stabilize at an equilibrium level characteristic
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
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INCREASING CARBON SEQUESTRATION IN WEST AFRICAN SOILS
Table III. Main soil units for the Podor, Bambey and Velingara areas in Senegal
Main soil units1
Pilot area2
Podor
Podor
Podor
Podor
Podor
Podor
Podor
Podor
Bambey
Bambey
Bambey
Velingara
Velingara
Velingara
Velingara
Map unit Association
165
1134
1321
1603
1619
1651
1652
1376
1634
1715
1423
1364
1441
1518
1521
Qc1-1a
Bk4-1a
I-a
Qc3-1a
Ql11-1a
Rd3-1/2b
Rd3-1a
Je37-2/3a
Ql6-1a
Vc1
Lf1-1a
Je21-2/3a
Lf26-1a
Lg7-2/3a
Lg8-1a
Phase
Petro-ferric
Petric
Petric
Soil 13 Area 1 Soil 2
Qc
Bk
I
Qc
Ql
Rd
Rd
Je
Ql
Vc
Lf
Je
Lf
Lg
Lg
100
100
100
70
70
50
50
60
70
100
100
70
50
70
50
Area 2
Bk
Re
G
G
Ge
Qc
30
30
20
20
20
30
G
Lp
Vp
Lf
30
30
30
20
Soil 3 Area 3
Soil 4 Area 4
Jd
Jd
Vc
20
20
20
Qa
Qa
10I
Lg
10
Lp
20
I
10
10
10
1
Data from digital Soil Map of the World at scale 1:5 000 000 (FAO, 1995).
Extent of pilot areas: Podor ~39 300 km2; Bambey ~4200 km2; Velingara ~6300 km2.
3
Soil 1 stands for dominant soil, and soil 2 to soil 4 for associated soils. Percentages express the relative extent of each soil within a map unit.
2
of the `permanent' soil characteristics, and land use or vegetation cover and prevailing management practices.
Generally it will take at least 25 to 50 years before a new organic carbon steady state is reached in soils. This new
steady state may be lower, similar or higher than the original one (Figure 2).
Human disturbance, induced by inappropriate land use and soil mismanagement, has caused widespread soil
degradation worldwide. As a result, the SOC contents in many agricultural soils are now below their potential
levels. There are about 494 106 ha of degraded soils in Africa. Main causative factors of degradation are
overgrazing (49 per cent), agricultural mismanagement (24 per cent), deforestation (14 per cent), and over-
Figure 2. Conceptual model of soil organic matter decomposition/accumulation following disturbance (after Johnson, 1995). Scenarios: A,
stabilization at above-original level; B, stabilization at original level; C, stabilization at lower than original level. L/D is the ratio of litter
production over decomposition.
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
136
N. H. BATJES
Figure 3. Agroecological zones and dominant land-degradation types in West Africa (Koning et al., 1997).
exploitation of the natural resources (13 per cent of total area degraded) (Oldeman et al., 1991). Figure 3 illustrates
that vast areas are affected by land degradation in West Africa, including water erosion, wind erosion, chemical
deterioration and physical deterioration. In principle, this is indicative of opportunities for increasing present soil
organic carbon stocks through adapted management, provided overall agroecological conditions are favourable
and adequate socio-economic incentives can be put in place.
Recommended management practices to build up carbon stocks in the soil are basically those that increase the
input of organic matter to the soil and/or decrease the rates of soil organic matter decomposition. These practices
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
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INCREASING CARBON SEQUESTRATION IN WEST AFRICAN SOILS
Table IV. Organic carbon pools and estimated ranges in the quantities and turnover times of
different types of organic matter present in agricultural soils (after Jastrow and Miller, 1997)
Organic C-pool
Type of organic matter
Turnover
time (yr)
Labile
Rapid
Moderate
Microbial biomass
Litter
Particulate OM
Light fraction
Within macro-aggregates1
Within micro-aggregates2
Physically sequestered
Chemically sequestered
0 1±0 4
1±3
5±20
1±15
5±50
2±5
±
18±40
10±30
20±35
50±1000
1000±3000
20±40
20±40
Moderate to slow
Passive
1
2
Proportion of
total OM (%)
But external to micro-aggregates, including particulate, light fraction, and microbial C.
Includes sequestered light fraction and microbially derived C.
will generally include a combination of the following: tillage methods and residue/stubble management; soil
fertility and nutrient management; erosion control; water management; and crop selection and rotation.
Sustainable management of forests and introduction of agroforestry can signi®cantly increase the amount of
carbon held in standing biomass, both above and below ground.
With respect to soil carbon sequestration, it is most desirable to ®x atmospheric C (upon photosynthesis) in those
pools having long turnover times. To model the cycling of C in the soil, soil organic matter must be subdivided into
several compartments considered more or less `homogeneous' in terms of residence times. Eswaran et al. (1995)
de®ned four pools based on carbon dynamics. First, an `active or labile pool' of readily oxidized compounds, the
formation of which is largely dictated by plant residue inputs (and hence management), and climate. Second, a
`slowly oxidized pool' associated with soil macroaggregates, the dynamics and pool size of which are affected by
soil physical properties such as mineralogy and aggregation, as well as agronomic practices. Third, a `very slowly
oxidized pool' associated with microaggregates, where the main controlling factor is waterstability of the
aggregates and agronomic practices have only little effect. Fourth, a `passive or recalcitrant pool' where clay
mineralogy is the main controlling factor, and there are probably no effects due to agronomic practices. Indicative
residence times of the various C-pools, now termed `labile', `moderate', `slow' and `passive', are given in Table IV.
Adequate methods to establish experimentally the partitioning of soil organic matter over the different pools
conceptualized in various models are still lacking. Novel fractionation schemes that truly re¯ect the biological
diversity of organic matter in soil thus are needed (Buyanovsky et al., 1994; Hassink, 1995; Skjemstad et al.,
1998).
ECOTECHNOLOGICAL POTENTIAL FOR SOIL CARBON SEQUESTRATION
Feasibility of Various Management Practices to Increase Organic Carbon Content in the Soil
Making inferences about realistic possibilities for increased carbon sequestration in the soil, through improved
appropriate management, is dif®cult because many of the factors and processes that control the ¯ow of carbon
between soils and plants are still poorly understood (Batjes, 1999; Reichle et al., 1999; Watson et al., 2000).
Management practices for increasing carbon sequestration in the soil, and their inferred feasibility and associated
relative C gains (Table V), have been reviewed by Bruce et al. (1999). These include ®ve broad classes: (a)
reduction in tillage intensity; (b) intensi®cation of cropping systems; (c) adoption of yield-promoting practices,
including improved nutrient amendments; (d) soil/water conservation measures, and (e) reestablishment of
permanent perennial vegetation. While many of the practices listed in Table V are considered to be technically
feasible by agronomists, their implementation so far has often met with limited success in maintaining or
increasing soil nutrient and carbon stocks in semiarid zones of West Africa (e.g., Pierri, 1995; Smaling et al., 1996;
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
138
N. H. BATJES
Table V. Examples of management practices that can increase organic carbon content in
the soil (partial list adapted from Bruce et al. 1999, p. 386, reproduced by permission of
Soil and Water Conservation Society)
Management practice
Feasibility
Cultivated land
Use of reduced- or no-till
Improved crop nutrition and yield enhancement
Use of forages in rotation
Use of improved varieties
Use of organic amendments
Irrigation
Pasture land
Improved grazing regime
Fertilizer application
Use of improved species/varieties
Rangeland
Improved grazing regime
Degraded land
Reversion to native vegetation
Establishment of fast-growing `cover' crops
Application of fertilizers
Application of organic amendments
Drainage/leaching of saline soils
Relative
carbon gain
H
H
M
M
M
L
M
L
M
M
M
H
M
H
L
M
M
M
L
L
M
M
H
L
L
H
H
M
H
L
Technical feasibility of management practice is expressed per unit area (L, low; M, medium; H,
high).
Breman and Sissoko, 1998). Overall, it is essential to improve the productivity and sustainability of existing
agricultural lands to help reduce the rate of new land clearance, from which large amounts of CO2 from the soil and
biomass are released in to the atmosphere (Paustian et al., 1997).
Opportunities for carbon sequestration in the biomass and soils of terrestrial ecosystems in West Africa will vary
with the agro(eco)systems and agroecological zone. With reference to Reichle et al. (1999), these options can be
summarized as follows:
(1) On forest lands, the focus should be on below-ground carbon (in stable pools), and on long-term management
and utilization of standing stocks, understory, ground cover, and litter.
(2) In the case of agricultural lands, i.e. mainly cropland and grasslands, the focus should be on increasing organic
carbon in the stable SOC pools.
(3) For degraded lands, restoration can offer signi®cant bene®ts in terms of carbon sequestration potential, both
above the ground and in the soil.
(4) In the case of wetlands and peatlands, the focus should be on conservation and/or returning reclaimed wetlands
to their natural state, keeping in mind any potential adverse environmental effects (e.g. CH4 emissions).
Carbon Trading
Carbon trading, as proposed under the Kyoto Protocol, is already an active process (Sampson and Scholes, 2000).
So far, contracts are of variable size in terms of amounts of C sequestered (Brown et al., 2000). Generally, however,
contracts will be easier to monitor when the C sequestration potential is large, in the order of 100 000 t C (L.L.
Tieszen, pers. comm.).
Based on these preliminary data, it is assumed that soil C sequestration at an average annual rate of 0.1±0.2 t C
ha 1 should be feasible in Senegal, provided so-called `best management' practices are used and that `adequate'
socio-economic incentives are provided. Under these conditions a new steady state can be reached after 25 years,
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INCREASING CARBON SEQUESTRATION IN WEST AFRICAN SOILS
Table VI. Major limitations of dominant soils in the seasonally dry (sub)tropics (Kauffman et al., 1996)
Soil units1
Land quality
ACRI
2
Soil moisture
Rootable volume
Oxygen availability
Nutrient availability
Nutrient retention
Aluminium toxicity
Workability
Mechanization
AREN
CAMB
FERR
LUVI
NITO
x
xxx
x
x
x
x
xxx
xx
xx
xxx
xxx
x
xx
x
xx
xxx
x
xx
x
x
x
x
xxx
PLAN
VERT
x
x
xx
xx
x
x
x
1
ACRI, Acrisols; AREN, Arenosols; CAMB, Cambisols; FERR, Ferralsols; LUVI, Luvisols; NITO, Nitosols; PLAN, Planosols (classi®cation
according to FAO-Unesco, 1974).
2
Degree of limitation: x, moderate; xx, severe; xxx, very severe.
corresponding with a total sequestration of 2.5±5 t C ha 1. In order to arrive at the sequestration target of 100 000 t
C indicated above, about 20 000±40 000 ha of ecologically suitable land would be needed to implement a carbon
sequestration project. Many soils of the seasonally dry (sub)tropics, however, have signi®cant limitations for
agriculture (Table VI). Similarly, a comprehensive Land Capability Assessment for Senegal indicated that at best
the land has moderate potential for rainfed cultivation at Bambey and Velingara, and low potential at Podor.
Sustained cultivation will thus require investments in addition to current inputs and conservation practices;
fertilizer and organic matter are indispensable at each growing season to assure acceptable yields (Stancioff et al.,
1984, p. 418). In addition, from a social, economic and cultural point of view the implementation of such largescale carbon sequestration projects will be hampered by the fact that most farmers are smallholders, which makes
project implementation, monitoring, and veri®cation dif®cult.
SOIL CARBON MODELS
Soil carbon models primarily allow the study of possible consequences of a range of management options on soil
carbon stocks/pools with respect to a given cropping system and a particular geographic location. Within the scope
of this exploratory study, preliminary runs were made with Soil Carbon Manager (CRC, 1999), a decision support
tool to assist in developing strategies for sustainable agroecosystems. On the whole, these simulations pointed to
limited possibilities for increasing soil carbon stocks through improved management practices for the Podor and
Bambey area where many soils are sandy. The potential is somewhat better at Velingara, according to this decision
tool. More elaborate models need to be tested in a follow-up study to fully explore possibilities for increasing soil
carbon sequestration in complex mixed cropping, intercropping, and agroforestry systems of West Africa (see
Smith et al., 1997).
CONCLUSIONS
Compared with Podor and Bambey, Velingara has the most suitable rainfall conditions for allowing increases in
biomass production. As such, Velingara appears to be the most appropriate site to evaluate the viability of soil
carbon sequestration projects in the context of the Clean Development Mechanism (CDM), proposed under the
Kyoto Protocol. Soil conditions at Velingara, however, will often be limiting for rainfed cultivation unless
signi®cant amendments can be made (Stancioff et al., 1984).
Integrated soil management is an essential pre-condition for agricultural development in Senegal and
West Africa. Such an approach should combine improved soil hydraulic measures, organic fertility maintenance
and inorganic fertilizer and soil amendments. The synergetic effects which result from this combination of
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
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N. H. BATJES
practices are a prerequisite for achieving the productivity increases needed to cope with the increasing population
pressure, as well as to increase possible return of organic residues into the soil pool (IFDC, 1997; Koning et al.,
1997; Smaling et al., 1996).
The CDM, if adopted, will allow developing countries to sell or trade project-based carbon credits to or with
industrialized countries. Such credits could provide an incentive for participation in climate change mitigation
activities and cover costs that African participants will encounter when engaging in carbon sequestration
projects. An important challenge for West African countries lies in the fact that they need to be more `attractive'
than the other African and developing countries in order to draw and hold CDM project investments. In this
context, researchers and policy makers in West Africa would bene®t from having access to an integrated
Information System for Sustainable Management of Land Resources (LRIS). Proposals to this avail have already
been endorsed by national representatives during a workshop held in Cotonou (WALRIS, 1999). One of the
possible applications of such an LRIS would be a geographic assessment of areas considered suitable for
implementing `carbon sequestration projects' both at the regional and national level. The areas most suited
biophysically for increasing overall agricultural productivity and increasing soil carbon sequestration can be
identi®ed ®rst, using physical land evaluation (FAO, 1976; Rossiter, 1996). Subsequently, the projected carbon
sequestration potential for these areas can be quanti®ed using dynamic simulation models, such as RothC and
CENTURY (e.g., Falloon et al., 1998; Paustian et al., 1997; Smith et al., 1997). These models will need to be
adapted to account for the complex crop assortment of agroecosystems in West Africa. In addition, a socioeconomic module will be necessary to assess all potential costs and bene®ts associated with the various
management options. Farmers acceptance of the recommended ecotechnological options will be dependent on
the availability of adequate socio-economic incentives. Conversely, it is likely that, in accordance with market
principles, CDM projects will be implemented where investors see the best opportunities and least risk to their
investments (Sokona and Nanasta, 2000). The technical, socio-economic and operational issues associated with
soil carbon sequestration projects are very complex (see Brown et al., 2000), and this paper does not pretend to
have those answers.
Reduction of atmospheric emissions of carbon dioxide and other trace gases from fossil fuel use, chemical
industry, and land-use changes and system conversion remains a key issue in mitigating climate change. Reduction
of atmospheric CO2 concentrations by increased carbon sequestration in the soils of West Africa appears to be
mainly useful when addressed in combination with other pressing regional challenges, such as combatting land
degradation, improving soil quality and productivity and preserving biodiversity. Provided the right socioeconomic incentives can be created, for example through CDM projects, this would create `win±win' scenarios
ensuring the farmers' livelihoods. Adapted policy approaches will be essential to realize the transition towards the
necessary integrated soil management technologies (see Koning et al., 1997; Barbier, 2000).
acknowledgements
The foundation for this paper has been prepared at the occasion of the international workshop on `Carbon
Sequestration in Soil' held in Dakar, Senegal (25±27 September 2000). Special thanks are expressed to the USAID
and EROS Data Centre (EDC) at Sioux Falls, SD, and Dr L.L. Tieszen in particular, for the invitation to the Dakar
workshop. I thank my colleagues Otto Spaargaren and Vincent van Engelen for their useful comments, based on
their ®eld experience in West Africa, and Jacqueline Resink for preparing the GIS maps.
references
Batjes NH. 1996. Total carbon and nitrogen in the soils of the world. European Journal of Soil Science 47: 151±163.
Batjes NH. 1999. Management options for reducing CO2 concentrations in the atmosphere by increasing carbon sequestration in the soil. Dutch
National Research Programme on Global Air Pollution and Climate Change, Bilthoven.
Barbier, EB. 2000. The economic linkages between rural poverty and land degradation: some evidence from Africa. Agriculture, Ecosystems
and Environment 82: 355±370.
Breman H, Sissoko K. 1998. L' Intensi®cation Agricole au Sahel. Editions Karthala: Paris.
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
INCREASING CARBON SEQUESTRATION IN WEST AFRICAN SOILS
141
Brouwer M. 1980. CharacteÁres hydro-morphologiques et possibiliteÂs rizicoles des sols de la valleÂe de Kamobeul-Bolon. BCEOM-IRAT,
Direction de l' Equipement: Dakar (Senegal).
Brown S, Masera O, Sathaye J. 2000. Project-based activities. In Land Use, Land-Use Change, and Forestry, Watson RT, Noble IR, Bolin B,
Ravindranath NH, Verardo DJ, Dokken DJ (eds). Published for the Intergovernmental Panel on Climate Change by Cambridge University
Press: Cambridge; 285±338.
Bruce JP, Frome M, Haites E, Janzen H, Lal R, Paustian K. 1999. Carbon sequestration in soils. Journal of Soil and Water Conservation 54:
382±389.
Buyanovsky GA, Aslam M, Wagner GH. 1994. Carbon turnover in soil physical fractions. Soil Science Society of America Journal 58:
1167±1173.
CRC. 1999. Soil Carbon Manager: a decision support tool to assist in developing strategies for sustainable agro-ecosystems (CD-Rom, version
1.0). Cooperative Research Centre for Soil and Land Management: Glen Osmond (Australia).
Eswaran H, van den Berg E, Reich P, Kimble J. 1995. Global soil carbon resources. In Soils and Global Change, Lal R, Kimble J, Levine E,
Stewart BA (eds). Lewis Publishers: Boca Raton, FL; 27±43.
Falloon PD, Smith P, Smith J U, Szab J, Coleman K, Marshall S. 1998. Regional estimates of carbon sequestration potential: linking the
Rothamsted carbon model to GIS databases. Biology and Fertility of Soils 27: 236±241.
FAO. 1976. A Framework for Land Evaluation. Food and Agriculture Organization of the United Nations, Rome.
FAO. 1988. FAO-Unesco Soil Map of the World, Revised Legend (with corrections and updates; reprint of FAO World Soil Resources Report
60). ISRIC: Wageningen.
FAO. 1993. World soil resources: an explanatory note on the FAO World Soil Resources map at 1:25000000 scale. FAO, Rome.
FAO. 1995. Digital Soil Map of the World and Derived Soil Properties, Land and Water Digital Media Series 1. FAO: Rome.
FAO and IIASA. 2000. Global Agro-Ecological Zones (Global-AEZ). International Institute for Applied Systems Analysis: Laxenburg, and
FAO: Rome.
FAO-Unesco.1974. Soil Map of the World, 1:5000000, vol. 1, Legend. UNESCO: Paris.
Grace PR, Post WM, Godwin DC, Bryceson KP, Truscott MA, Hennessy J. 1998. Soil carbon dynamics in relation to soil surface management
and cropping systems in Australian agroecosystems. In Management of Carbon Sequestration in Soil, Lal R, Kimble JM, Follet RF, Stewart
BA (eds). CRC Press: Boca Raton; 175±193.
Hassink J. 1995. Decomposition rate constants of size and density fractions of soil organic matter. Soil Science Society of America Journal 59:
1631±1635.
IFDC. 1997. Framework for national soil fertility improvement action plans. International Fertilizer Development Centre, Lome
Jastrow JD, Miller RM. 1997. Soil aggregate stabilization and carbon sequestration: feedbacks through organo-mineral associations.
In Soil Processes and the Carbon Cycle, Lal R, Kimble JM, Follet RF, Stewart BA (eds). CRC Press: Boca Raton, FL; 207±223.
Johnson MG. 1995. The role of soil management in sequestering soil carbon. In Soil Management and the Greenhouse Effect, Lal R, Kimble JM,
Follet RF, Stewart BA (eds). Lewis Publishers: Boca Raton, FL; 351±363.
Kauffman JH, Mantel S, Spaargaren OC. 1996. Soils of the humid and seasonally dry (sub)tropics. In National Soil Reference Collections and
Databases (NASREC), vol. 2, Use of ISRIC's Databases for the Characterization of Soils of Major Agro-ecological Zones, Kauffman JH
(ed.). ISRIC: Wageningen; 1±41.
Koning N, Heerink N, Kauffman JH. 1997. Integrated soil improvement and agricultural development in West Africa: why current policy
approaches fail? Department of Economics and Management, Wageningen Agricultural University, and International Soil Reference and
Information Centre: Wageningen.
Lal R. 1998. Soil processes and the greenhouse effect. In Methods for Assessment of Soil Degradation, Lal R, Blum WH, Valentine C, Stewart
BA (eds). CRC Press: Boca Raton, FL; 199±212.
Le Borgne J. 1988. La pluviomeÂtrie au SeÂnegal et en Gambie. ORSTOM / Universite Cheikh Anta Diop de Dakar: Dakar
Oldeman LR, Hakkeling RTA, Sombroek WG. 1991. World map of the status of human-induced soil degradation: an explanatory Note (revised
edition) UNEP and ISRIC: Wageningen.
Paustian K, Levine E, Post WM, Ryzhova IM. 1997. The use of models to integrate information and understanding of soil C at the regional scale.
Geoderma 79: 227±260.
Pierri C. 1995. Long-term soil management experiments in semiarid Francophone Africa. In Soil Management: Experimental Basis for
Sustainability and Environmental Quality, Lal R, Stewart BA (eds). CRC Press: Boca Raton, FL; 225±266.
Rasmussen PE, Oartob WJ. 1994. Long-term effects of residue management in wheat fallow. I - inputs, yield, and soil organic matter. Soil
Science Society of America Journal 58: 523±530.
Reichle D, Houghton J, Kane B, Ekmann J, Benson S, Clarke J, Dahlman R, Hendrey G, Herzog H., Hunter-Cevera J, Jacons G, Judkins R,
Ogden J, Palmisano A, Socolow R, Stringer J, Sulres T, Wolsky A, Woodward N,York M. 1999. Carbon Sequestration: State of the Science.
US Department of Energy, Of®ce of Science, and Of®ce of Fossil Energy: Washington DC.
Rossiter DG. 1996. A theoretical framework for land evaluation. Geoderma 72: 165±190.
Sampson RN, Scholes RJ. 2000. Additional human-induced activities - Article 3.4. In Land Use, Land-Use Change, and Forestry, Watson RT,
Noble IR, Bolin B, Ravindranath NH, Verardo DJ, Dokken DJ (eds). Published for the Intergovernmental Panel on Climate Change by
Cambridge University Press: Cambridge; 183±281.
Skjemstad JO, Janik LJ, Taylor JA. 1998. Non-living soil organic matter: what do we know about it? Australian Journal of Experimental
Agriculture 38: 667±680.
Smaling EMA, Fresco LO, De Jager A. 1996. Classifying, monitoring and improving soil nutrient stocks and ¯ows in Africa. Ambio xxv:
492±496.
Smith P, Smith JU, Powlson DS, McGill WB, Arah JRM, Chertov OG, Koleman K, Franko U, Frolking S, Jenkinson DS, Jensen LS,
Kelly RH, Klein-Gunnewiek H, Komarov AS, Li C, Molina JAE., Mueller T, Parton WJ, Thornley JHM, Whitmore AP. 1997. A
comparison of the performance of nine soil organic matter models using datasets from seven long-term experiments. Geoderma 81:
153±225.
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)
142
N. H. BATJES
Sokona Y, Nanasta D, 2000. The Clean Development Mechanism: An African Delusion? Change 54: 8±11.
Squires VR. 1998. Dryland soils: their potential as a sink for carbon and as an agent in mitigating climate change. Advances in GeoEcology 31:
209±215.
Squires VR, Glenn E P, Ayoub AT. 1995. Combatting Global Climate Change by Combatting Land Degradation. University of Adelaide,
Adelaide; The University of Arizona, Tucson, AZ, United Nations Environment Programme (UNEP): Nairobi.
Stancioff A, Staljanssens M, Tappan G. 1984. Mapping and remote sensing of the resources of the Republic of Senegal. The Remote Sensing
Unit, South Dakota State University (under contract to the USAID): Brookings, SD.
Stewart BA. 1995. Soil management in semiarid areas. In Soil Management and the Greenhouse Effect. Lal R, Kimble J, Levine E, Stewart BA
(eds). CRC and Lewis Publishers: Boca Raton, FL; 251±258.
Stewart BA, Robinson CA. 1997. Are agroecosystems sustainable in semiarid regions? Advances in Agronomy 60: 191±228.
WALRIS. 1999. Regional implementation workshop: Establishment of an Integrated Land Resources Information System for the conservation
and rehabilitation of land resources in West Africa (WALRIS). Cotonou, Benin, 23±25 February 1999. ISRIC: Wageningen.
Watson RT, Noble IR, Bolin B, Ravindramath NH, Verardo DJ, Dokken DJ (eds) 2000. Land Use, Land-Use Change, and Forestry (A Special
Report of the IPCC). Cambridge University Press: Cambridge.
Copyright # 2001 John Wiley & Sons, Ltd.
LAND DEGRADATION & DEVELOPMENT, 12: 131±142 (2001)