soil carbon sequestration in sub-saharan africa: a review

land degradation & development

Land Degrad. Develop. 16: 53–71 (2005)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ldr.644

SOIL CARBON SEQUESTRATION IN SUB-SAHARANAFRICA: A REVIEW

T.-G. VAGEN,1* R. LAL2 AND B. R. SINGH3

1Norwegian Centre for Soil and Environmental Research, Jordforsk, Frederik A. Dahls vei 20, N-1432 As, Norway2Ohio State University, School of Natural Resources, 2021 Coffey Road, 210 Kottman Hall, Columbus, OH 43210, USA

3Agricultural University of Norway, Department of Plant and Environmental Sciences, PO Box 5003, N-1432 As, Norway

Received 19 April 2004; Revised 1 June 2004; Accepted 12 June 2004

ABSTRACT

Restoration of degraded soils is a development strategy to reduce desertification, soil erosion and environmental degradation,and alleviate chronic food shortages with great potential in sub-Saharan Africa (SSA). Further, it has the potential to provideterrestrial sinks of carbon (C) and reduce the rate of enrichment of atmospheric CO2. Soil organic carbon (SOC) contentsdecrease by 0 to 63 per cent following deforestation. There exists a high potential for increasing SOC through establishment ofnatural or improved fallow systems (agroforestry) with attainable rates of C sequestration in the range of 0�1 to5�3MgCha�1 yr�1. Biomass burning significantly reduces SOC in the upper few centimeters of soil, but has little impactbelow 10 to 20 cm depth. The timing of burning is also important, and periods with large amounts of biomass availablegenerally have the largest losses of SOC. In cultivated areas, the addition of manure in combination with crop residues and no-till show similar rates of attainable C sequestration (0 to 0�36MgCha�1 yr�1). Attainable rates of SOC sequestration onpermanent cropland in SSA under improved cultivation systems (e.g. no-till) range from 0�2 to 1�5 TgC yr�1, while attainablerates under fallow systems are 0�4 to 18�5TgC yr�1. Fallow systems generally have the highest potential for SOC sequestrationin SSA with rates up to 28�5TgC yr�1. Copyright # 2005 John Wiley & Sons, Ltd.

key words: soil organic matter; soil degradation; global warming; soil restoration; agroforestry; farming systems; ecoregions; sub-Saharan

Africa; carbon sequestration in soil

INTRODUCTION

Recent studies suggest that cumulative pre-industrial anthropogenic CO2 emissions may have been far more

significant than previously assumed, mainly as an effect of forest clearance, and may explain the rise in

atmospheric CO2 between 8000 years BP and 1800 AD (Ruddiman, 2003). In sub-Saharan Africa (SSA), land

use change (biomass burning) was the primary source of CO2 emissions prior to 1950, but its relative contribution

to total CO2 emissions has decreased from 100 per cent in 1950 to under 40 per cent in 2000, while combustion of

liquid fuel (i.e., petrol, diesel, oil) has increased substantially during the same period (Figure 1). Despite these

increases, emissions of CO2 from combustion of fossil fuel in Africa are low (Figure 1). A small number of

countries actually contribute the bulk of these emissions including South Africa (�40 per cent), Egypt, Algeria

and Libya (combined �35 per cent) (Marland et al., 2003). Per capita emissions have increased by approximately

260 per cent between 1950 and 2000 (Figure 1), but the majority of SSA countries have per capita emissions well

below the global average.

Soil carbon (C) sequestration represents a development strategy with high potential in semiarid and sub-humid

Africa where severe soil degradation and desertification are related to perpetual food crises, and overall

impoverishment. The C balance of terrestrial ecosystems is uncertain, in part due to uncertainties and errors in

Copyright # 2005 John Wiley & Sons, Ltd.

�Correspondence to: T.-G. Vagen, Norwegian Centre for Soil and Environmental Research, Jordforsk, Frederik A. Dahls vei 20, N-1432 As,Norway.E-mail: [email protected]

measurements, but perhaps most importantly due to methodological problems resulting in incomplete accounting

(Houghton, 2003). Because only a fraction of the estimated CO2 emissions remain in the atmosphere, there is

uncertainty regarding the various sinks of CO2 in natural ecosystems (Malhi et al., 1999). In much of SSA,

ecosystems undergo regular perturbation making it difficult to assess C sequestration (Goudriaan, 1993). This is

the case in many savanna areas as well as in tropical forests, which poses extra challenges in the quantification of C

dynamics. One of the most important ways of increasing sequestration of atmospheric C in SSA is through

restoration of degraded soils and ecosystems (Lal, 2000). This article is a synthesis of the available information on

various forms of soil degradation and their extents in SSA, and assesses the potential for C sequestration through

soil restoration. The natural processes involved in C sequestration are highly complex, and research results from

SSA are relatively scarce. Therefore, a specific objective of this review is to highlight some of these complexities

and identify future research needs.

PRINCIPAL ECOSYSTEMS OF SSA

Tropical forests in humid and sub-humid regions of SSA cover about 366 million hectares (Mha) (Table I), and

play a major role in the global C cycle (Bowman, 2000). Estimated annual rates of change in forest area

(deforestation minus afforestation) for humid and sub-humid SSA ranges from 0�7 (Achard et al., 2002) to 1�2(FAO, 2001) Mha yr�1. These rates range from 0�376 (DeFries et al., 2002) to 5�2 (FAO, 2001) Mha yr�1 when all

forest ecosystems (humid, sub-humid and semiarid) are taken into consideration. The wide range indicates the

high level of uncertainty associated with estimates of land-use change in SSA. Tiessen et al. (1998) reported that C

storage in vegetation, litter and soils of dry tropical forests is approximately 150, 3 and 70MgC ha�1, respectively.

Savannas are widespread in SSA, stretching across the continent from the Sahel to the tropical rainforest zone,

south and east of the Congo basin, and along southwestern and western parts of Madagascar, and cover an area of

about 422 Mha (Loveland et al., 2001). There is a wide range of classification systems for tropical savannas, which

Figure 1. Emissions of CO2 (total) for the African continent from 1885 to 2003. Estimates of total population between 1960 and 2003 areshown as bars. (Sources: Marland et al., 2003, and UNEP.)

54 T.-G. VAGEN ET AL.

Copyright # 2005 John Wiley & Sons, Ltd. LAND DEGRADATION & DEVELOPMENT, 16: 53–71 (2005)

reflects their extreme heterogeneity (Bourliere and Hadley, 1970). Broadly, these areas can be grouped into

woodlands, bushland (Table II) and grasslands (Figure 2; White, 1983). In western Africa, such as in western and

eastern regions of Nigeria and the Accra plains in Ghana, deforestation has lead to increases in derived savannas

(Badejo, 1998) because of decline in soil fertility as a consequence of intensive cultivation, annual bush burning

and overgrazing (Van der Werf, 1983; Adepetu, 1994). However, there are also studies that suggest the estimates of

deforestation in West Africa and degradation of woodlands are highly exaggerated, and to some extent built on

myths and misconceptions about historical land use and land-use change (Roe, 1995; Ribot, 1999). Certain regions

of SSA also experience widespread bush encroachment (Brown and Lugo, 1982), and estimates of C inputs from

biomass in grassland-dominated savannas vary significantly from 0MgCha�1 yr�1 in dry periods to between 5

and 15MgC ha�1 yr�1 in the rainy season (Bourliere and Hadley, 1970; Tiessen et al., 1998).

SOIL DEGRADATION

Soil degradation is a widespread phenomenon in SSA, and is attributed to both socioeconomic and biophysical

factors. Since the processes leading to soil degradation are complex and differ between regions, these must be

assessed and addressed in a spatially targeted manner. Principal processes of soil degradation include erosion by

water and wind, salinization, chemical degradation, loss of nutrients, and soil organic matter depletion. The

total extent of severely degraded soils in SSA is approximately 3�5 million km2, or between 20 and 25 per cent of

the total land area, of which 1�1 million km2 is estimated to be severely degraded due to agricultural activities

(Figure 3).

Accelerated erosion by water is the primary mechanism of soil degradation, especially on arable land. The

problem is particularly acute in the semi-humid and humid tropics (Kirkby and Morgan, 1980; Lal, 1985; Morgan,

1995). About 360 Mha are subject to serious soil erosion risk in SSA. Land-use and land-cover change

Table I. Distribution of humid and sub-humid forests in SSA

Region Evergreen Deciduous Mixed Totalbroadleaf broadleaf

(million ha)

West Africa 50�6 — — 50�6Central Africa 180�3 — 0�4 180�7East Africa 28�6 — 7�8 36�4Southern Africa 79�7 — 6�1 85�8Madagascar 11�1 1�4 — 12�5Total 350�3 1�4 14�3 366�0Modified from Loveland et al., 2001.

Table II. Distribution of savanna shrubland in SSA

Region Shrublands savanna

Closed Open Woody Non-woody Total

(million ha)

West Africa 0�2 43�1 142�0 139�0 324�3Central Africa 0�7 0�1 89�2 16�5 106�5East Africa 45�3 83�3 92�3 95�1 316�0Southern Africa 0�8 77�0 27�0 140�2 245�0Madagascar — 0�1 2�0 31�3 33�4Total 47�0 203�6 352�5 422�1 1025�2Modified from Loveland et al., 2001.

SOIL CARBON SEQUESTRATION 55


Figure

2.Distributionofbushland,woodlandandgrassland(savannaecosystem

s)in

SSA.(A

daptedfrom

White,1983.)



significantly influence runoff and soil loss (Charreau and Nicou, 1971; Roose, 1977; Lal, 1981). Studies conducted

in humid zones of central and eastern Africa indicate soil losses ranging from 300 to 700Mg ha�1 yr�1 on bare soil

and 20 to 300Mg ha�1 yr�1 in croplands (Konig, 1991; Roose and Ndayizigiye, 1997). Estimates of C loss through

soil erosion by water in SSA are few. Roose (1980) reported losses ranging from 0�09 to 1�9MgCha�1 yr�1 in a

humid zone of the Ivory Coast on a sandy-loam soil under subequatorial forest and cereal cultivation, and 0�01 to

0�12MgC ha�1 yr�1 under native dry Sudano-sahelian savanna and cereals. Other studies from west, east and

southern Africa estimated losses of C through soil erosion from 0�2 to 0�7MgCha�1 yr�1 (Roose, 1978; Gachene

et al., 1997; Moyo, 1998). These studies show net losses of total C from slopes only, and do not reflect landscape

level losses of C as this depends on a number of factors such as topography, soil types and management practices.

LIMITS AND CONSTRAINTS

There are important limits and constraints to current estimates of Soil organic carbon (SOC) stocks in SSA,

particularly related to lack of, or uncertain estimates of, soil bulk density due to methodological problems. The

methods used by different authors are not the same (i.e., core method, clod method, etc.), which contributes to

the uncertainty in estimates. Further, results from different studies are reported for different depths (e.g., 0–10,

0–20, 0–30 cm), which complicates the assessment of changes in SOC stocks and necessitates an adjustment to

enable comparisons of different studies. The majority of results reported in this paper are from the top 10 cm depth.

Results reported for 0–5, 0–15, 0–20 and 0–30 cm depths have been adjusted to represent 0 to 10 cm depth (i.e.,

divided by their sampling depth and multiplied by 10) in the tables.

Differences in sampling techniques and soil analytical procedures for SOC are other confounding effects adding

to the uncertainty of current estimates. Finally, time of sampling has been shown to influence SOC content,

Figure 3. Major soil quality constraints in sub-Saharan Africa and their aerial distribution. Total area of severely degraded soils¼ 3�5 millionkm2. (Source: FAO, 2001.)



especially in savanna regions (Bourliere and Hadley, 1970; Tiessen et al., 1998). Many studies do not indicate the

time of sampling and seasonal variation is therefore an important constraint to the interpretation of research results

on SOC trends and processes in savanna ecosystems.

CARBON DYNAMICS IN DIFFERENT ECOSYSTEMS

Tropical Forests

1. Above-ground biomass. Estimates of above-ground dry biomass range from 120 to 400Mg ha�1 in tropical

humid forests (Brown, 1997), 20Mg ha�1 in degraded tree savannas (Figure 4), and 11Mg ha�1 in agricultural

land. Dry matter production (inputs) in tropical forests differs among soil types and fertility status (Palm et al.,

1996; Clark and Clark, 2000). Other factors include landscape position, and frequency and distribution of tree-fall

gaps, but not slope gradient (Clark and Clark, 2000).

The impacts of deforestation on above-ground biomass C can be dramatic especially in slash-and-burn

cultivation systems (Fassbender, 1974; van Noordwijk et al., 1997). Fassbender (1974) reported biomass losses

of approximately 400Mg ha�1 through the burning process alone, but the amount of biomass varies greatly

depending on forest type, and vegetation structure and density. Kotto-Same et al. (1997) reported losses of 220 Mg

biomass C ha�1 upon conversion to agriculture under slash-and-burn systems in a chronosequence study in

Cameroon. Further, the rate of recovery in fallow systems depends on biomass at the start of the fallow period

(Figure 5). Afforestation (reforestation) is likely to accumulate organic C in biomass rapidly, especially during the

primary stages of the afforestation process, serving as a net sink for atmospheric CO2. Brown and Lugo (1990)

reported that 5MgCha�1 yr�1 were accumulated during the first decade of regrowth of a forest in Puerto Rico,

Figure 4. Biomass (dry matter) estimates (bars¼means; sticks¼ ranges) for different forest/woodland types in sub-Saharan Africa. (Adaptedfrom Brown, 1997.)



while estimates based on a compilation of several studies indicate an average rate of 2�6MgC ha�1 yr�1 over

80 years (Silver et al., 2000). The rate of accumulation is influenced by previous land use and varies little among

life-zones.

2. Below-ground carbon. Malhi et al. (1999) estimated that below-ground C stocks comprise about 60 per cent

(230MgC ha�1) of total C stocks (447MgC ha�1) in tropical forest ecosystems, and total ecosystem C residence

time is 29 years. The SOC storage of natural undisturbed forest is at near steady state with annual inputs of detritus

material from above- and below-ground sources equal to annual respiration. The highest concentrations of SOC in

tropical forest soils are observed in the top 0–20 cm depth. Veldkamp et al. (2003) reported that although SOC

concentrations are low in highly weathered tropical forest subsoils, the total stock is significant due to the soils’

large volume. The SOC in topsoil is more labile than that in the subsoil and is more prone to change upon

deforestation and other perturbations (Siband, 1974; Veldkamp, 1994; Veldkamp et al., 2003). Data from long-

term studies in Senegal (Figure 6) show changes in SOC below 10–20 cm depth to be minor after conversion. The

rate at which SOC is lost after conversion depends on a number of factors including the method of clearing, terrain,

soil types and land use after clearing. In case of burning, fire intensity and ambient weather conditions affect the

amount of SOC lost (Bruijnzeel, 1998).

Soil C stocks generally decrease by 25 per cent following deforestation and cultivation of previous forest areas

(Houghton et al., 1987). However, studies show significant local variations (i.e., climate, topography, soil type)

and the effect of post-clearing land use. In Nigeria, Van Noordwijk et al. (1997) suggested that it is possible to

Figure 5. Rates of recovery (average dry-matter production) in fallow systems following cultivation. Letters a, b, and c indicate dry-matterproduction in forest on fertile and non-fertile soils and under continuous low-input cultivation (rice–cowpea). (Adapted from Palm et al., 1996,

based on data from Vitousek and Sanford, 1986.)



maintain SOC levels fairly close to that under forest through management systems that do not increase pH and

minimize tillage. Kotto-Same et al. (1997) also reported relatively stable SOC contents during a slash-and-burn

cycle in Cameroon despite significant changes in the above-ground biomass. Solomon et al. (2002) reported

significant reductions in SOC after conversion of humid tropical forests to maize (Zea mays) cultivation in

southeastern Ethiopia, with SOC stocks ranging from 58�3 to 63�9MgCha�1 in forest soils and 33�9 to

39�7MgCha�1 in cultivated soils. These changes represent losses of 55–63 per cent SOC after conversion from

forest to continuous cultivation. Similar trends have been reported by Sombroek et al. (1993) for cultivation of

previously forested soils.

Quantifying changes in SOC upon afforestation is also complex (Paul et al., 2002). Important factors

determining the rate of recovery of SOC during afforestation include climate, soil type and depth, historical

and current land use, severity of soil degradation, selection of species for afforestation, and level of disturbance. As

for regeneration of biomass in fallow systems, the initial size of the SOC stock at the beginning of the fallow period

largely determines the rate of regeneration and long-term trends in SOC dynamics (Nye and Greenland, 1960;

Szott et al., 1999). Szott et al. (1999) reported that revegetation in fallow periods is more related to climatic zone

and rainfall than to soil base status with significant variability within climatic zones due to complex interactions

between soil, vegetation and management. Swaine and Hall (1983) reported substantially higher SOC stocks after

four years of fallow (�83�3MgC ha�1) than in natural forest (�63�5 CMg ha�1) in humid forests in Ghana.

Studies conducted in the humid natural forest zone of Madagascar showed a rapid increase in SOC in the first two

years of fallow (Brand and Pfund, 1998; Vagen et al., unpublished). Kotto-Same et al. (1997) reported re-

accumulation rates in recovering fallows to be approximately 9�4MgC ha�1 yr�1 in a six-year fallow period.

Several studies have shown decreasing SOC contents during the first five to eight years of fallow following clearing

of natural forest with SOC increasing and stabilizing at levels similar to natural forest systems after 12 to 15 years.

Juo et al. (1995) reported a decrease of 12MgC ha�1 (0–15 cm depth) in SOC after seven years of fallow and a

Figure 6. Change in SOC stocks with years under cultivation after conversion from forest. Bulk density is assumed to be 1�4 (0–30 cm) and 1�5(30–60 cm). (After Siband, 1974.)



recovery to levels similar to those at clearing after 12 to 13 years. King et al. (1997) reported rapid revegetation of

nascent (less degraded) savannas in Gabon through fire-protection of the forest–savanna edge, but significantly

slower rates of vegetative regeneration in ancient (severely degraded) savannas. The latter savanna areas had

approximately 4 gC kg�1 lower content than less degraded savanna and significantly lower base cation contents.

On low base status soils, the sustainability of fallows depends strongly on Ca and P replenishment.

Paul et al. (2002) synthesized global data on afforestation and reported highly variable results. Generally, SOC

stocks decreased over the short term but increased over the long term, and reached a steady state after about 30

years. An analysis of a subset (n¼ 36) of the data used by Paul et al. (2002) from experiments conducted in SSA

confirms these highly complex relationships. Areas previously under grassland show no change or a slight

decrease in SOC following afforestation (Eucalyptus, Pinus and P. spatula) (Table III). Areas previously under

cultivation, on the other hand, show an increase in SOC following afforestation (succession) (Table III).

The Savanna Ecosystem

1. West Sudanian savanna. The West Sudanian savanna (WSS) is characterized by flat topography (200–400 m

elevations), a strong seasonal tropical climate and annual rainfall between 600 mm (northern regions) and 1000

mm (southern regions). Walter (1939) reported a strong relationship between grass production and rainfall for

semiarid regions of the WSS, with an increase in dry matter production of 10Mg ha�1 with every 100 mm increase

in rainfall. Soils of this region are mostly Ultisols and Alfisols (south) and Entisols (north). Well-drained savanna

soils of western Africa generally have low SOC contents.

Traditional farming systems in the West African savanna use few external inputs and have high social cohesion

(Ruthenberg, 1971). Jones (1971) reported SOC contents of 6�0 gC kg�1 (0–15 cm) in native savanna, which

decreased to 1�7 gC kg�1 after 18 years of cultivation. Manlay et al. (2004a,b) reported SOC stocks of 11�7 and

21�3MgC ha�1 for bush systems under various cropping intensities and agricultural plots near village compounds,

respectively. Rice (Oryza sativa) systems had the highest SOC stock of 41�3MgCha�1 compared with

15�4MgC ha�1 in topsoil of fallow areas at 0 to 20 cm sampling depths. Native savannas in northern Nigeria

contain 15�6MgCha�1 in 0 to 15 cm layers, decreasing to about 8�2MgC ha�1 at 15 to 30 cm depth (Agbenin and

Goladi, 1997). Management systems with no fertilizer inputs and those with only inputs of NPK fertilizers had the

lowest topsoil SOC stock of 8�0 and 7�1MgC ha�1, respectively. Treatments receiving cow dung in combination

with fertilizers had 16�1MgC ha�1. In Sadore, Niger, Bationo et al. (2000) reported SOC contents of 1�7 gC kg�1

in soils under traditional cultivation, and 3�0–3�5 gC kg�1 in fallow areas and in fields where a combination of crop

residues and mineral fertilizers had been applied over a 14-year period, compared to 2�0 gC kg�1 in areas with

continuous cultivation. These differences were mostly confined to the topsoil layer, which also corresponds well

with more recent studies from Senegal (Figure 8).

Pieri (1989) reported data of long-term experiments on fertilizer, tillage and cropping systems from Burkina

Faso and Chad (Figure 7). The lowest annual losses of SOC were observed in cotton (Gossipium hirsutum)–cereal

rotations with 2 and 4 year fallows (1�2 and 0�5 gC kg�1 yr�1). Changes in SOC stocks of Alfisols in western

Nigeria were reported by Lal (2000) to be 16 and 20MgC ha�1 under three years of plow-till and no-till

continuous maize, respectively. Leuceana–maize agroforestry systems had 0�2 and 2�8MgCha�1 and pigeon pea

(Cajanus cajan)–maize systems had 1�4 and 8�3MgC ha�1 higher SOC stocks than continuous maize in plow-till

Table III. Changes in SOC after afforestation of previous grassland and cultivated lands in The Congo and Nigeria

Previous land use Tree species Initial SOC stock Change in SOC stock Rate of change in SOC(Mg C ha�1) (Mg C ha�1 yr�1) (Mg C ha�1 yr�1)

Grassland Eucalyptus 5�0 �1�5 to 2�8 �0�15 to 0�28P. patula 42�5 to 115�1 �14�8 to �1�1 �0�57 to �0�08Pinus 5�0 �0�6 to 4�7 �0�12 to 0�21

Cultivated Succession 14�8 to 33�7 �4�2 to 14�8 �1�40 to 1�48Adapted from Paul et al., 2002.



and no-till systems, respectively. The pigeon pea–maize system increased SOC stocks at a rate of

�2�5MgCha�1 yr�1 under plow-till, while continuous maize reduced SOC stock at a rate of 0�23MgCha�1 yr�1.

Pigeon pea is, of course, useful as food source and is widely used in the reclamation of compacted and degraded

soils due to its nitrogen-fixing capacity and deep tap-root system.

2. East Sudanian savanna and highlands of East Africa. Estimates of SOC contents in native (natural) woodlands

of East Africa vary widely. Solomon et al. (2000) reported SOC contents of 18�7 gC kg�1 for savanna areas in

northern Tanzania, which is similar to the 21�3 gC kg�1 (�28�2MgCha�1) reported by Glaser et al. (2001) for the

same area, but significantly lower than the 40�8 gC kg�1 reported by Van de Vijver et al. (1999). The latter found

no changes in SOC content after burning of savanna grasslands. Other studies show intermediate values

(30 gC kg�1) (King et al., 1997) for savanna soils of central Africa. According to Batjes (2004), C stocks in

East Africa range from 37 to 39MgC ha�1 (0–30 cm depth). These estimates do not, however, take into account

the substantial variation between different ecoregions of East Africa. The SOC content is generally higher under

trees in savanna ecosystems (Charreau and Vidal, 1965; Belsky et al., 1993; Kang, 1997; Rao et al., 1998)

probably due to reduced radiation (shading) under tree canopies and nutrient enrichment as a result of increased

litter inputs in these zones compared to surrounding grasslands (Belsky et al., 1993). Studies of long-term

cultivation in East Africa often show trends similar to those reported in West African countries with increases in

SOC content under cultivated land converted from savanna. Hartemink (1997a) reported a reduction in SOC

content from 25 to 15 gC kg�1 (65�5–39�3MgCha�1) between 1966 and 1987 under continuous sisal (Agave

sisalana) cropping on land converted from forest in 1956 in northern Tanzania (0–20 cm depth) where changes

were highly dependent on soil type (Figure 9).

Ferralsols are highly vulnerable to loss of SOC following conversion to agricultural land use (Nye and

Greenland, 1960; Hartemink, 1997b), and more so than Acrisols, while SOC contents in Cambisols may increase

Figure 7. Rates of SOC loss under different cropping systems in the West African Sahel. (Adapted from Pieri, 1989.)



under permanent cropping relative to bushland (Figure 9). Albrecht et al. (1992) showed that allophanic soils (e.g.,

Andisols) contain more SOC than low activity clay (e.g., Ferralsols and Ultisols) and high activity clay soils (e.g.,

Vertisols). The content of SOC depends on texture (Feller and Beare, 1997), especially clay content in low-activity

clay soils.

Nitrate accumulation in commonly observed in soils with high anion adsorption capacity in subsoil layers (Cahn

et al., 1992; Hartemink et al., 1996; Sprayogo et al., 2002). Trees increase the supply of nutrients to the soil

through recycling from subsoil layers and may enhance soil fertility in situations where nutrient or water stress

occurs in the soil surface (Buresh and Tian, 1998). The use of agroforestry trees to enhance soil fertility is

widespread in SSA, particularly in densely populated areas where land is becoming increasingly limited, such as in

the East African highlands. These improvements in the general fertility of agro-ecosystems may also provide

opportunities for creating C sinks, depending on the type of agroforestry system used and the soil types (Albrecht

and Kandji, 2003). Watson et al. (2000) estimated that agroforestry systems can sequester C at an average rate of

0�2–3�1MgCha�1 yr�1. Studies conducted on Ferralsols in Kenya showed SOC stocks ranging from 2�6–3�7MgC ha�1 (0–20 cm depth) (Impala, 2001) and 3�1–8�3MgCha�1 (0–30 cm depth) (Onim et al., 1990)

under agroforestry systems. Ferric Acrisols under agroforestry in Togo contained SOC stocks of 3�4–12�5MgC ha�1 (Albrecht and Kandji, 2003). Yet, these relationships are complex, especially in dryland areas

and on highly degraded soils (e.g., highly acid Ferralsols). Jackson et al. (2000) observed that wetter areas were

losing SOC with increasing woody vegetation cover while dryland areas were gaining SOC stock. Nandwa (2001)

reported SOC accumulation of �16Mg ha�1 during four years of fallow in Uganda with losses of 19�7Mg ha�1 in

the following four years of cultivation (with tillage) of previously fallowed plots.

Figure 8. SOC stocks (Mg ha�1) under cultivation (groundnut) and fallow in a mixed-farming system in Senegal. Boxes show fourths, pointsare median values. (Adapted from Manlay, 2000.)



3. Savannas of southern Africa. Fire, a common management practice, is used to control bush encroachment,

eliminate dead vegetation, and promote earlier grass growth at the end of the dry season just before the onset of

rains. Bush encroachment occurs over vast areas and is often linked to historic land use (Hudak et al., 2003). In

general, more SOC is found under trees than beneath grasses (Belsky et al., 1993; Hibbard et al., 2001). Pacala

et al. (2001) estimated that over 1MgCha�1 yr�1 may accumulate as a consequence of bush encroachment.

However, bush encroachment is also a management problem in rangelands, as dense woody thickets lead to less

growth of grasses due to increasing competition with trees (Hudak et al., 2003). This is in contrast to the findings

from open savannas reported earlier where herbaceous production is enhanced under tree canopies (Belsky et al.,

1993), which illustrates the complexity of tree–grass interactions. Dominy and Haynes (2002) reported higher or

similar SOC contents in grasslands under Kikuyu grass in KwaZulu-Natal than in native savanna and under rye-

grass pasture.

Understanding the long-term effects of burning on soil nutrient dynamics and SOC content is important to the

development of improved rangeland management systems and in determining its impacts on the global C cycle.

Bird et al. (2000) studied the impacts of fire on SOC contents in savannas of south-western Zimbabwe for different

soil types. Natural savanna areas protected from fire contained 14�4 and 20�0MgCha�1 in clay soil compared with

10�0 and 12�6MgCha�1 (5 cm depth) in sandy soils measured at centre points between trees and approximately 1

m from tree trunks, respectively. On protected plots, the SOC stock increased by approximately 50 per cent

(10�0Mg ha�1) for clay soils during a 50-year period. Annual cycles of fire reduced SOC stock to about 5�5 and

10�0MgCha�1 for sand and clay soils, respectively, and resulted in about 20 per cent higher losses than where the

savanna was burnt every five years. Snyman (2003) reported 13�9 gC kg�1 in an unburnt savanna soil in South

Africa, which decreased to 10�2 gC kg�1 immediately after a wildfire (at 0–5 cm depth). The SOC contents were

similar for burnt and unburnt areas 18 months after burning. Fynn et al. (2003) reported SOC contents of 57 and

Figure 9. SOC dynamics under continuous cultivation to sisal and bushland at different depths and with different soil types. (After Hartemink,1997b.)



58 gC kg�1 (33�1 and 33�6Mg ha�1), respectively, in upper soil layers (0–5 cm) under fertilized pasture and

unfertilized native rangeland. The SOC contents at 5 to 10 cm depth were 33 and 51 gC kg�1 (19�1 and

29�6MgC ha�1), respectively. Long-term experiments with burning of grasslands in South Africa have shown that

seasonal burning is important to SOC loss (Figure 10), with annual or biennial burning in the winter season

resulting in significantly higher SOC losses than triennial burn and no-burn control (Fynn et al., 2003). Burning

during spring had no effect on SOC loss among treatments (Figure 10). Effects of burning on SOC stock were

negligible below 2 cm depth (Figure 10).

Grasslands in the highlands and western regions of Madagascar are subjected to extensive burning annually.

Studies of the impacts of biomass burning on soil quality in general and SOC dynamics in particular are scarce in

Madagascar despite the focus on fire by international development agencies during the past decades (Kull, 2002).

OPPORTUNITIES FOR SOC SEQUESTRATION

Rather than ‘potential,’ Ingram and Fernandes (2001) suggested the term ‘attainable’ when considering SOC

sequestration because it takes into consideration factors that limit input of C to the soil system. The actual SOC

stored is normally lower than the attainable level due to other factors like soil erosion, tillage, residue removal,

drainage and other forms of disturbance that limit C input into the system or reduce SOC content. Cumulative

change and rate of change in SOC for a range of conversions are summarized for different regions of SSA in

Figure 10. Distribution of SOC (Mg ha�1) in soil profile under different grassland burning frequencies and seasons in South Africa. Bulkdensity¼ 1�16. (Adapted from Fynn et al., 2003.)



Tables IV and V. In cultivated areas, sequestration in the range of 0�05–0�36MgCha�1 yr�1 (Table V) may be

attainable through conversion from traditional cultivation techniques (with tillage) to no-till systems or systems

where a combination of animal manure and fertilizers are applied. This translates into sequestration of

1�0–7�5TgCyr�1 in permanent cropland (20�9 million ha), which corresponds to attainable levels of about

0�2–1�5TgCyr�1 if 20 per cent (see Sampson and Scholes, 2000) of this area is subjected to improved crop

management. Conversion from cultivated land to fallow (agroforestry) in semiarid and arid savanna areas increases

SOC stock between 0�1 and 5�3Mgha�1 yr�1. These rates may be achieved through improved management on

degraded cultivated land and establishment of improved fallows (Tables Vand VI). Given the vast areas of tropical

savanna in SSA, there is thus a high potential for sequestering C in soils through improved management of

cultivated land, and introduction of agroforestry systems such as mixed fallows and natural fallow systems.

However, several uncertainties associated with current estimates of SOC and lack of accurate assessments of

land-use dynamics (change) in different biomes makes extrapolation difficult. More research is therefore needed

on both extents and rates of land-cover change (e.g., deforestation) and the impacts on SOC contents and stocks

under various natural- and agro-ecosystems. This requires spatially explicit data on SOC dynamics and trends, and

the development of statistical sampling frameworks at various spatial scales. Given the extents of, for instance, the

Table IV. Changes in SOC stocks for different land-use conversions in SSA

Region Change in land use Mean Min. Max. SOC gains* N

From To Mg C ha�1

HS Natural forest Cultivated �24�30 �24�40 24�20 2Fallow �4�00 �8�00 0�00 2

Cultivated Fallow 1�38 0�24 2�78 100 9Agroforestry 6�00 — — 1Afforestation 1�97 �2�90 9�38 71�4 21

WSS Cultivated Cultivated CR 2�60 — — 1Cultivated F 0�70 — — 1Cultivated NT 1�23 �3�00 3�90 66�7 6Fallow 6�14 2�10 12�46 100 10Afforestation 1�68 �2�93 5�67 66�7 9

Cultivated M Fallow �5�80 — — 1Fallow Cultivated �1�88 �2�12 �1�63 2Savanna Cultivated �1�72 �5�07 �0�30 5

Cultivated C 0�20 0�00 0�60 100 3Cultivated CR �0�83 �0�90 �0�75 2Cultivated F �1�49 �5�67 0�60 20 5Cultivated M 0�96 0�33 1�95 100 3

ESS Savanna Cultivated 0�87 �13�10 19�65 33�3 3Woodland Cultivated �14�26 �18�20 �11�40 4

Cultivated M 5�40 — — 1SA Savanna Cultivated �21�85 �31�60 �12�10 4

Fallow 0�00 — — 1Pasture 5�55 �7�70 18�80 50 2

Pasture Afforestation �3�30 �4�87 �1�73 2Fallow Cultivated �9�00 — — 1

HS¼ humid and sub-humid; WSS¼West Sudanian savanna; ESS¼East Sudanian savanna; SA¼ southern Africa.Cultivated M¼w/manure; Cultivated C¼w/cover crops; Cultivated CR¼w/crop residues; Cultivated F¼NPK fertilizers only; CultivatedNT¼ no till.N¼ number of observations.*Percent of observations with net gain (> 0).Sources: Agbenin and Goladi (1997); Aweto (1981); Bationo et al. (2000); Dominy and Haynes (2002); Drechsel et al. (1991); Feller et al.(1981); Glaser et al. (2001); Hartemink (1995, 1997a); Impala (2001); Juo et al. (1995); Lal (2000); Manlay (2000); Materechera andMkhabela (2001); Morris and Gray (1984); Onim et al. (1990); Pieri (1989); Solomon et al. (2000, 2002); Trouve et al. (1994).



savanna regions in SSA, the costs associated with such assessments using conventional methods are prohibitive.

Therefore, new approaches are needed, which are both rapid and cost-effective. The use of diffuse reflectance

spectroscopy (DRS) for prediction of SOC is one particularly promising methodology that has been shown to work

well for a range of tropical soil types (Shepherd and Walsh, 2002; Sanchez et al., 2003).

Soil degradation and food security are among major challenges in SSA. There is a strong need to develop

national and regional strategies for restoring degraded soils and ecosystems. The conversion of degraded/marginal

soils to restorative land use and adoption of recommended soil and crop-management practices on favorable soils

also has a significant potential for enhancing the SOC pool (Lal, 2004). Identification of soil-specific techniques of

soil and water conservation and management, quantification of temporal changes in SOC stocks and soil quality,

establishing relationships between soil quality and SOC stocks on the one hand, and soil quality and productivity

on the other, and identification/implementation of policies that encourage farmers to adopt recommended

management practices are important researchable priorities. Commodification of SOC, through trading C credits

either under for instance Kyoto’s CDM or theWorld Bank’s BioCarbon Fund or other marketing mechanisms, may

be an important method of increasing farm income and providing incentives for exchanging soil quality and

restoring degraded soils and ecosystems.

Table V. Changes in SOC stocks with different land-use conversions in SSA, based on selected long-term studies

Region Change in land use Mean Min. Max. SOC gains* N

From To Mg C ha�1 yr�1

HS Natural forest Cultivated �0�90 �1�00 �0�80 2Fallow �0�57 �1�14 0�00 2

Cultivated Fallow 1�06 0�23 2�77 100 9Afforestation 0�12 �0�29 0�56 71�4 21

WSS Cultivated Cultivated CR 0�19 — — 1Cultivated F 0�05 — — 1Cultivated NT 0�33 �1�00 1�30 66�7 6Fallow 1�37 0�10 5�30 100 10Afforestation 0�07 �0�98 0�57 50 9

Cultivated M Fallow �0�14 — — 1Fallow Cultivated �0�11 �0�18 �0–06 2Savanna Cultivated 0�05 0�00 0�15 33�3 5

Cultivated C �0�12 �0�15 �0�09 3Cultivated CR �0�06 �0�13 0�15 20�0 2Cultivated F 0�09 0�00 0�20 66�7 5Cultivated M 0�04 �0�62 0�94 33�3 3

ESS Savanna Cultivated �2�77 �5�30 �0�77 3Woodland Cultivated 0�36 — — 4

Cultivated M 0�55 — — 1SA Savanna Cultivated �0�82 �1�26 �0�40 4

Fallow 0�00 — — 1Pasture 0�05 �0�31 0�40 50 2

Pasture Afforestation �0�16 �0�19 �0�13 2Fallow Cultivated �0�75 — — 1

HS¼Humid and sub-humid; WSS¼West Sudanian savanna; ESS¼East Sudanian savanna; SA¼ southern Africa.Cultivated M¼w/manure; Cultivated C¼w/cover crops; Cultivated CR¼w/crop residues; Cultivated F¼NPK fertilizers only; CultivatedNT¼ no till.N¼ number of observations.*Percent of observations with net gain (> 0).Sources: Agbenin and Goladi (1997); Aweto (1981); Bationo et al. (2000); Dominy and Haynes (2002); Drechsel et al. (1991); Feller et al.(1981); Glaser et al. (2001); Hartemink (1995, 1997b); Impala (2001); Juo et al. (1995); Lal (2000); Manlay (2000); Materechera andMkhabela (2001); Morris and Gray (1984); Onim et al. (1990); Pieri (1989); Solomon et al. (2000, 2002); Trouve et al. (1994).



CONCLUSIONS

The largest potential for increasing SOC is through the establishment of natural or improved fallow systems

(agroforestry), which give attainable rates of C sequestration in the range of 0�1 to 5�3MgC ha�1 yr�1. The

performance of fallow systems depends on soil properties and climatic conditions, and the degree of soil

degradation at the time of fallow establishment and choice of species composition. In croplands, the critical issue

in increasing SOC is the addition of crop residues or manure and the reduction in soil disturbance through

conservation tillage or no-till systems. Additions of manure in combination with crop residues and no-till,

therefore, seem to yield similar rates of attainable C sequestration (0�05–0�36MgC ha�1 yr�1). Soil organic

carbon is a key indicator of soil quality and the dynamics of processes leading to its depletion (soil erosion,

deforestation, cultivation, etc.) or increase (improved management, fallow, etc.) must be better understood to target

interventions and to arrive at more accurate estimates of its potential for sequestering atmospheric C in SSA.

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soil carbon sequestration in sub-saharan africa: a review

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