Soil Carbon Sequestration and the CDM: Opportunities ... - CiteSeerX

SOIL CARBON SEQUESTRATION AND THE CDM: OPPORTUNITIES AND CHALLENGES FOR AFRICA LASSE RINGIUS UNEP Collaborating Centre on Energy and Environment, P.O. Box 49, DK-4000 Roskilde, Denmark

Abstract. This paper examines soil carbon sequestration in developing countries in sub-Saharan Africa as part of regional and global attempts to mitigate greenhouse gas emissions and the possibility that the development of greenhouse gas mitigation projects will offer local ancillary benefits. The paper documents the improvements in agricultural practices and land-use management in subSaharan Africa that could increase agricultural productivity and sequester soil carbon. During the first five-year commitment period of the Kyoto Protocol, only afforestation and reforestation projects will be eligible for crediting under the Clean Development Mechanism, but soil carbon sequestration and broader sink activities could become eligible during subsequent commitment periods. However, very few cost estimates of soil carbon sequestration strategies exist, and available data are not readily comparable. It is uncertain how large amounts of carbon could be sequestered, and it is unclear how well site-specific studies represent wider areas. It is concluded that there presently is a need to launch long-term (>10 years) field experiments and demonstration and pilot projects for soil carbon sequestration in Africa. It will be important to monitor all environmental effects and carbon ‘costs’ as well as estimate all economic benefits and costs of projects.

1. Introduction Carbon (C) sequestration through improved land-use management is an important and complex issue. This paper examines sequestration of C in soils of developing countries in sub-Saharan Africa as part of regional and global attempts to reduce greenhouse gas (GHG) emissions and the possibility that the development of GHG mitigation projects will offer local benefits. The paper identifies the key political issues at stake and provides an assessment of the ecological and economic possibilities, and constraints, for development of soil C sequestration projects in sub-Saharan Africa. Historically, neither industrialized nor developing countries have managed soils in order to sequester C or to reduce GHG emissions. But international expert groups and North American and European scientists have recently recommended soil C sequestration as a climate policy measure (e.g., Lal et al., 1998; Dumanski et al., 1998; Soil and Water Conservation Society, 1998; Smith et al., 1998; IGBP Terrestrial Carbon Working Group (TCWG), 1998). Terrestrial C sinks are not permanent offsets to fossil fuel emissions (IGBP TCWG, 1998). By reducing the rate Climatic Change 54: 471–495, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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of carbon dioxide (CO2 ) accumulation in the atmosphere, C sequestration would buy time to reduce fossil fuel emissions. Desertification has its greatest impact in Africa and the benefits of promoting and maintaining better soil conditions may be more significant to controlling the spread of deserts than to stemming the rise of CO2 in Earth’s atmosphere. Although this paper primarily examines soil C sequestration in the light of the United Nations Framework Convention on Climate Change (UNFCCC), the significance of soil C sequestration should also be understood in the light of the Convention to Combat Desertification (CCD), a second treaty stemming from the UN Conference on Environment and Development (UNCED) in Rio de Janeiro in 1992. Greater convergence between research into desertification and global climate change will hopefully develop in the future (Grainger et al., 2000).

2. The Clean Development Mechanism Article 12 of the 1997 Kyoto Protocol, which has yet to be ratified by a sufficient number of countries so that it can take effect, establishes the Clean Development Mechanism (CDM), an institutional framework for direct foreign investments in GHG mitigation projects in developing countries. The objective of the CDM is to stimulate sustainable development in the developing countries where the CDM projects will be implemented, the so-called host countries, and to give industrialized countries with high mitigation costs access to low-cost GHG offsets (formally Certified Emission Reductions, or CERs, in the Kyoto Protocol) in developing countries. With the CDM the industrialized countries could count emission reductions and C-sink enhancement in developing countries against their commitments to reduce their GHG emissions. Recently political pressure to include soil activities under the Kyoto Protocol has been growing (New Scientist, 1998, p. 17), even though the issue is a contentious one for the Parties to the UNFCCC (Nature, 2000). Article 3.3 of the Protocol explicitly mentions emissions from sources and removals by sinks as a direct consequence of human intervention affecting land-use change and deforestation, reforestation, and afforestation undertaken since 1990. Article 3.4 identifies agricultural land as a possible C source, and that agricultural land should be included in the emission inventories that are prepared regularly by the UNFCCC Parties. However, the Protocol does not include provisions for national crediting for C sequestration in agricultural soils. During the first five-year commitment period (2008–2012) of the Kyoto Protocol, afforestation and reforestation projects will be eligible for crediting under the CDM. Other sink activities, such as forest conservation and soil C sequestration, will not be eligible. Still, soil C sequestration could become eligible for crediting under the CDM during subsequent commitment periods (Ringius, 2001).

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2.1. AFRICA AND THE CDM More than 260 million people, or about 30% of the total population in sub-Saharan Africa, could be without adequate food by the year 2010 (FAO, 1996, p. 271). The agriculture sector dominates the economies of most sub-Saharan countries, contributing about one-third of the region’s GDP, 40% of the export, and two-thirds of the employment (FAO, 1996). For obvious reasons, increasing the productivity and sustainability of agriculture is a key regional priority; reducing GHG emissions is not. In order to feed a rapidly growing population, the cropland area in 2025 may need to be almost 2.5 times that in 1990 (Marrison and Larson, 1996, p. 341). Unless land-uses and economic conditions improve considerably, it seems unlikely that Africa will benefit significantly from the CDM. Soil C sequestration projects, which increase agricultural productivity and bring economic benefits, would present sub-Saharan countries with a significant incentive for participating in the CDM. For their part investors and the international community might find soil C projects attractive because their positive impact on agricultural productivity gives sub-Saharan host countries an incentive for effective project implementation. While Africa contributed around only 3% of the total global CO2 emissions from fossil fuel burning and cement production in 1995 (World Resources Institute 1998, p. 344), it could participate in the management of the global C cycle through C sequestration. Improvements in agricultural techniques and land-use practices could lead to higher agricultural productivity and soil C accumulation. Soil C constitutes a significant part of the total C stock in Africa, and land-use systems and agricultural practices increasing the soil C stock could produce GHG offsets that foreign investors might purchase under the CDM. Sequestration projects with modest or no local economic benefits are termed carbon farming projects. Examples are forestry projects which, apart from C storage, bring few or no local or national benefits. Since host countries would not benefit greatly from carbon farming projects, it is unlikely that such projects would be implemented adequately. African countries presumably would find C sequestration unattractive unless farmers and society at large would receive additional benefits – for instance, reduced labor and more efficient use of production inputs. In 1995, the UNFCCC Parties initiated a pilot phase for so-called activities implemented jointly (AIJ). In June 1999, one hundred and fourteen AIJ projects had been accepted, approved or endorsed by the national Parties involved (UNFCCC, 1999). One of these projects is being carried out in Africa, that in Burkina Faso. This project aims to sequester 1.4 million t CO2 during a 5-year period through: (i) managing 300,000 hectares of community based forest, (ii) promoting efficient charcoal processing technologies, (iii) introducing solar photovoltaic  Prevention of soil erosion could protect C in soils and the savannas could become a C sink if savanna burning in Africa was reduced (Gachene et al., 1997; Scholes, 1995; Scholes and Hall, 1996, pp. 85–86).

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systems for household lighting and water pumping systems, and (iv) introducing efficient kerosene cooking stoves that will displace use of fuelwood (World Bank, 1999). Among the main reasons for the almost complete absence of AIJ-projects in Africa are low emissions reduction potential, deficient institutional capacity, and a weak private sector (UNEP, 1998; Sokona et al., 1998). Nonetheless, African countries are keenly interested in participating in the CDM and include agricultural and forestry projects among the possible candidate projects (UNEP, 1998, p. 8).

3. Soil Carbon Sequestration: An Overview Globally C is distributed, and is being redistributed, among five interconnected C pools – the oceanic pool, the geological pool, the soil, the terrestrial biomass pool, and the atmospheric pool. The argument for soil C sequestration is based upon the assumption that enlargement of the soil C stock reduces the concentration of atmospheric CO2. Essentially, soil C sequestration will redistribute C from the atmospheric pool to the soil. Generally, the conversion of native, undisturbed, or virgin land to agricultural systems results in a degradation of the soil organic matter (SOM), defined as the sum of all organic substances (dead plants and animals) in the soil, leading to a release of soil C to the atmosphere. On average, a new equilibrium or steady-state is established at a lower level after 20–50 years. But exceptions to this general trend do exist. For instance, relatively small changes of soil C are reported during land conversion in Africa (Woomer et al., 1997, pp. 159–160). Conversion of forest to well-managed pastures may result in similar or even enhanced soil C levels compared with the level that is found in native forest (Cerri et al., 1991; Lugo et al., 1986). C losses through erosional processes may represent a significant removal of soil C which may only partially be emitted to the atmosphere (Stallard, 1998). Erosion contributes about 3 × 109 tons of sediment yr−1 within the conterminous United States and maybe 30 × 109 tons globally. Much of this sediment is stored in channels, behind dams, as alluvium, and as colluvium near sites of erosion. If this sediment contains 1.5% C, it would contain about 0.45 × 109 t C yr−1 (Stallard, 1998, p. 232; Harden et al., 1999). Agricultural techniques have a significant influence on the amount of C stored in soil over time. Changes in agricultural practices and inputs – notably changes in crop varieties, application of fertilizer and manure, rotation and tillage practices – influence how much and at what rate C is stored in, or released from, soils. 3.1. FINDINGS FROM TEMPERATE ZONE SOILS Results from long-term experimental fields in the United States indicate that through agricultural management it is possible to sequester a significant amount

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of C in cultivated land (Buyanovsky and Wagner, 1998). Over a period of one hundred years, a direct relationship between the amounts of C returned to the soil and soil organic C content was observed in these U.S. experiments. Crop residues, manure, and in some cases both, supplied the new annual C entering the soil. In addition, management – specifically whether regular tillage was practiced or not – was an important factor for C sequestration. Over a period of one hundred years the 100-cm soil layer of no-till plots stored 15–20 t more C ha−1 compared to similar plots under regular tillage (Buyanovsky and Wagner, 1998). It is possible, under certain circumstances, to rebuild and maintain the C soil content to a level close to that of undisturbed land. A 3-year rotation (corn, wheat, and clover) with manure and nitrogen on soils that lost more than 30% of their content of organic C in the upper 20-cm layer over a 60-year period, managed to store almost 135 t C ha−1 in the 1-meter soil layer, a level comparable to that of undisturbed land (Buyanovsky and Wagner, 1998, p. 137). Over a period of 26 years, the soil accumulated at the rate of 1.5 t C ha−1 yr−1 . Over a 25-year period, continuous corn on no-till soils receiving mineral fertilizers accumulated C at a rate about 0.5 t C ha−1 yr−1 in the upper 20-cm layer. Over 25 years, 11.1 t C ha−1 were accumulated – a total increase in C stored of about 42% (Table I). Numerous other examples of the potential for soil sequestration could be cited (e.g., Drinkwater et al., 1998). For example, experiments with spring barley showed that soils that received farmyard manure increased from about 30 t to about 85 t C ha−1 in the 0–23 cm layer (Johnson, 1995, p. 359). About 0.5 t C ha−1 yr−1 was sequestered in the soil. Degraded temperate agricultural soils can be recovered though better nutrient management, reduced fallow periods, improved cultivars, and retention of crop residues. Most long-term studies have shown increases in soil organic C as a result of reduced- or no-till systems (Cole et al., 1996; Paustian et al., 1998).

3.2. FINDINGS FROM AFRICA Compared to industrialized countries, opportunities for soil C sequestration in developing countries are less well known, and the technical potential for soil C sequestration in Africa and other developing regions is more uncertain (Scholes and van der Merwe, 1996; Lal and Kimble, 2000). Despite this, studies from Africa are generally in agreement with those from industrialized countries. Degraded lands and desertification in Africa offer additional opportunities for C sequestration.  Some studies have found that the more active or dynamic part of the C in soil is found in the

upper 20 cm of the soil profile, while the C below that level is considered inactive or stable, with a mean residence time of more than 1000 years. But while depths of 0–20 cm, 0–30 cm and 0–100 cm are used for the sake of comparison, this practice does not seem to be scientifically justified. For a discussion, see Batjes and Sombroek (1997, pp. 163–164). See also Greenland (1995, p. 10).

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Table I Changes in C content in upper 20-cm layer of Sanborn Field soils under different managements, 1963–1988 Crop, treatment

Continuous wheat Manure Miner. fert. None Continuous corn Manure Miner. fert., no till Miner. fert., convent. till None Corn/wheat/clover Miner. fert. Manure + N

C, ton per hectare 1963 1988 Change

32.6 27.2 25.4

42.7 36.0 24.4

+10.1 +8.8 –1.0

32.3 26.7 24.9 21.9

37.7 37.9 32.5 18.2

+5.4 +11.1 +7.6 –3.7

27.8 30.6

35.9 47.0

+8.1 +16.4

Source: Buyanovsky and Wagner (1998, p. 137).

3.2.1. Semiarid Savannas and Dry Forests In West Africa, overgrazing and demand for fuelwood has led to serious land degradation in savannas, resulting in low biomass and soil C levels (Tiessen et al., 1998). Short fallow periods do not restore soil organic matter and, because crop residues are either removed or burned, 50–70% of the land receives minimal C returns to the soil which thus contain very low C levels. Arable agriculture and animal husbandry is practiced in semiarid West Africa, which receives between 500–1100 mm annual precipitation on an area of about 3,000,000 km2 . Because of high rural population density as well as high animal density, some areas, such as central-western Senegal, are increasingly overexploited and overstocked. In this region, where mean annual rainfall ranges from 500–650 mm, three types of land-use with distinct organic matter budgets and cycles can be distinguished (Tiessen et al., 1998). The first type is comprised of rangeland consisting of degraded brushy savanna. Because of degradation of the savanna, the soil stores only about 7.5–9.9 t C ha−1 in the upper 20-cm layer. The non-degraded savanna is characterized by higher plant productivity and organic matter returns which result in higher soil C levels  The author has converted the C content of the soil, in this case 2.5–3.3 g C kg−1 , into t C ha−1 , assuming a bulk density of 1.5 g cm−3 (or 1.5 Mg m−3 ). Christian Feller, personal communication. The bulk densities of these soils ranged from 1.1 to 1.6. Alain Albrecht, personal communication.

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of about 7.5–18.0 t C ha−1 (2.5–6.0 g C kg−1 ), or a 20–30% difference between degraded and non-degraded savanna (Tiessen et al., 1998, p. 113). The second type of land-use management is characterized by land that is continuously cultivated without application of animal manure and left to fallow for about one in five years. It also stores small amounts of C. In this system, residue inputs are low because above-ground residues are used as fuel, construction, fodder or are burnt prior to the next cropping. The soil C levels range from 4.5 to 13.5 t C ha−1 (1.5–4.5 g C kg−1 ). In the third system, where continuously cultivated land receives animal manure, soil C levels are raised approximately 40% on individual sites, corresponding to levels ranging from 6.0 to 14.2 t C ha−1 (2.0–4.7 g C kg−1 ). But the increasing levels of soil C in areas receiving manure may well be balanced by decreasing levels of soil C in areas in which the animals are grazed, reducing the inputs of plant residues to soils in pasture. As a result of cultivation, soil C levels of degraded savanna decrease by 40% in 3–5 years on sandy soils and in 5–10 years on clayey sand soils in the region (Tiessen et al., 1998, p. 113). Application of animal manure and crop rotation improve soil C levels and productivity. Short periods of fallow have little effect on soils but, if extended and managed adequately, may help to maintain quality of soil and sequester C. Fire prevention and animal exclusion could increase productivity as well as sequester an additional 25% C in soils. Although unfertilized cropland is potentially able to store more C by fertilizer application and better management, this option is presently constrained by the limited economic capacity in the region. Generally, only modest amounts of C are stored in the soil in the semi-arid tropics, including in native ecosystems (see Figure 1 on soil carbon stocks in Africa). Although increasing soil fertility, the use of crop residues as surface mulches and fertilizer may not result in significant storage of C in the soil of the semiarid tropics of West Africa (Geiger et al., 1992). The opportunities for soil C storage seem therefore limited.

3.2.2. Cultivation of Woody Savannas In general, approximately 50% of the soil organic C is lost in 20 years when tropical woodlands, grassland, or savanna soils are converted to croplands (Scholes and Hall, 1996, p. 92). Losses are generally highest in the tillage horizon or the topsoil (0–20 cm). Converting woodland and broadleaf savannas to agriculture has resulted in losses of soil C in Southern Africa. Results from research plots in Zimbabwe show a decline in soil C of about 9% after 8 years of continuous maize cultivation, and comparisons to similar soils cultivated with maize over long time periods indicate a loss of as much as 18.6 t C ha−1 , or a 67% loss (Woomer, 1993). A Guinean savanna experienced a relatively smaller loss – 5.2 t C, or 22% – as a result of maize-based cultivation.

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Figure 1. Total stocks of organic carbon in the upper 1 m of soils in Africa (kg C m−2 to 1 m depth). Source: Batjes (1997).

A number of reports of soil C loss due to continuous cultivation in Africa are presented in Table II (Woomer et al., 1997, p. 159). They are not readily comparable because soil depths, measurement periods, and regions differ. Longer-term climatic patterns (e.g., the effects of a prolonged drought) may also influence results. Nevertheless, these reports confirm that cultivation commonly results in C loss from soils. As previously mentioned, however, under some circumstances it is possible to restore the soil C content to a level near that of undisturbed forest. For instance, according to experiments conducted in Nigeria, the first seven years after forest clearing the soil organic C content was reduced from approximately 25.5 to 13.5 t C ha−1 (from 17 g to 9 g kg−1 ) in the upper 15-cm layer, but bush fallow for 12–13 years raised the C content to a level similar to the pre-clearing level (Juo

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Table II Reports of soil C loss due to cultivation in sub-Saharan Africa Soil C loss t/ha/yr

Conditions

Soil depth cm

10 8 6

Cultivation following land conversion in Western Kenya Slash-and-burn conversion in coastal Mozambique Comparison of forest and cultivated Nitisol in the Kenyan Central Highlands Moimbo woodland in Zimbabwe converted to maize cultivation in a sandy Alfisol Cultivation following land conversion in Western Kenya Following forest clearing in Southern Cameroon by slash-and-burn Continuous cultivation in Western Kenya

0–37.5 0–20 0–15

2 4 8

0–50

6

2.7 2.4 2.2 0.9

Period years

0–37.5 0–40

30 4

0–37.5

18–30

Sources: P. L. Woomer et al. (1997, p. 159).

et al., 1995). Guinea grass and leucaena fallows also exhibited a 7-year decrease in soil C followed by a restoration of C to around the initial level. Pigeon pea fallow, however, was unable to sequester an equal amount of C, most likely due to lower biomass production and a lower C:N ratio of pigeon pea residues. Chemical properties of the soil under pigeon pea fallow also deteriorated compared to bush fallow and Guinea grass and leucaena fallows. Such experiments shed light on the question whether planted fallow, especially by applying nitrogen-fixing legumes or grasses producing high amounts of biomass, allows a return to cropping sooner compared to natural bush fallow. More fundamentally, these experiments indicate that fallow is important in order to preserve soil productivity and sequester C in the forest/savanna areas of West Africa. Table III shows results from experiments with continuous cropping with and without fertilizers and manure addition in Kenya (Woomer et al., 1997, p. 161). The initial C stocks ranged from 30.2 to 44.1 t C ha−1 in the 0-20 cm top layer. Over a 4–7 year period, the average annual loss due to cultivation was 0.69 t C ha−1 . In most cases fertilizer addition resulted in a small C increase, although some soils showed net losses, especially where phosphorus was added. Addition of farmyard manure consistently resulted in an increase in soil organic C, although the added amount (5–7.5 t ha−1 yr−1 ) was unable to balance the total losses in the case of Acrisols and Luvisols. Results from experiments in West Africa indicate that conservation tillage practices could lead to a significant increase in both soil C content and crop yield (Lal, 1997a,b). Experiments with continuous maize on newly cleared land in western  Author’s calculations.

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Table III C stocks in agricultural soils of Kenya, annual C losses due to cultivation, and changes in annual C fluxes due to chemical fertilization and livestock manure addition FAO soil order

Total soil C (tC/ha)

Soil C flux (tC/ha/yr)

Acrisols Ferralsols Luvisols Nitisols

44.1 38.0 30.2 43.1

–1.24 0.21 –0.90 –0.49

All soils

39.5

–0.69

C flux from applying N P N&P Manure ————— tC/ha/y ————— 0.11 0.01 0.58 0.89 0.46 0 –0.09 – –0.11 –0.55 –0.32 0.45 0 –0.05 0.10 0.92 Total

0.07

–0.14

0.11

0.80

Source: Paul L. Woomer et al. (1997, p. 161).

Nigeria compared eight different tillage systems over a 6-year period. As the most effective strategy, over a 4-year period no-till + mulch increased the soil C content from 15 to 32.3 t C ha−1 in the 0–10 cm surface layer and increased the corn grain yield (for the first season) from 2.5 to 5.1 t ha−1 . Experiments in western Nigeria with three restorative treatments – continuous maize, pigeon pea-maize, and leucaena-maize – also increased the soil C pool (Lal, 2000). Over a 3-year period the average rate of increase was 0.4 t C ha−1 yr−1 for no-till compared with 1.7 t C ha−1 yr−1 for plow-till watershed. The higher increase in the plow-till watershed was achieved because the soil C pool was highly depleted and had a high potential to sequester C.

4. Management Options for Soil Carbon Sequestration in Africa A number of specific agricultural management and land-use options are available for C sequestration in tropical and subtropical soils (Batjes and Sombroek, 1997, p. 168): 1. conservation tillage (no-till/minimum-till) in combination with planting of cover crops, green manure and hedgerows 2. organic residue management  (1) No-till + crop residue mulch, (2) no-till + crop residue mulch and chiselling in the row zone

to about 50 cm depth once a year, (3) mouldboard ploughing and two harrowings, (4) disc ploughing and rotovation, (5) no-till – crop residue mulch, (6) mouldboard ploughing in the dry season and two harrowings just before seeding, (7) mouldboard ploughing and two harrowings + mulch, and (8) mouldboard ploughing and two harrowings with contour ridging.  Note that both the level of C concentration and the corn grain yield subsequently decreased to around the original level. The author has converted the C content of the soil, in this case 12 and 23.1 g C kg−1 , into t C ha−1 , assuming bulk densities of 1.25 and 1.4 g cm−3 , respectively.

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3. mulch farming, particularly in dry areas 4. water management, including in-situ water conservation in the root-zone, irrigation, and drainage to avoid potential risk of salinization and water-logging 5. soil fertility management, including use of chemical fertilizers and organic wastes, rhizobium inoculation, liming and acidity management in order to take advantage of the CO2 -fertilization effect 6. introduction of agroecologically and physiologically adapted crop/plant species, including agroforestry 7. adapting crop rotations and cropping/farming systems, with avoidance of bare fallow 8. controlling of grazing to sustainable levels 9. stabilizing slopes and terraces. The effectiveness of these agricultural management and land-use options will vary according to local ecological and climate conditions. With respect to option (1), one South African study found no consistent trends in the effect of reduced tillage or no-till practices on soil organic C (van der Watt, 1987). However, another South African study found that conservation tillage reduced emissions of C from soils, especially where wheat was rotated with an annual legume pasture (Agenbag and Maree, 1989). It will also be important to take into account the characteristics of various soil types and the land-use history of sites. With regard to option (6), with over 2000 mm annual rainfall, an increase of 25–70 t C ha−1 was reported 5–10 years after establishing pastures of deep-rooted grasses in Columbia (Fisher et al., 1994). These technologies and land-use practices also vary significantly with respect to external input. In general, options (1) and (2) (depending on the availability of organic residue) or (3), (7), and (8) require a change in the way in which existing resources are applied, whereas options (4), (5), and (9) depend on additional resource input. Options (5) and (6) either might not be available today, or have yet to applied on a broad scale. 4.1. LAND - USE CONVERSIONS IN AFRICA Another option is to reduce or prevent the conversion of land to agriculture in tropical Africa. I discuss this option by ecological zone – the arid-semiarid, subhumid, and humid zones (Table IV). The three zones are broadly defined according to climate and soil fertility characteristics. Soil and biomass C stocks are quite small in the semiarid zone and there is in general little opportunity to mitigate emissions through changing land use. Agricultural lands are mostly concentrated in the sub-humid zone. By improving management methods and increasing productivity in ways such as outlined above, it may be possible to limit the expansion of agriculture. Increased productivity and sustainability could, moreover, reduce the demand for clearing forests (deforestation) and savannas to meet local agricultural and economic needs. A major potential

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Table IV Main land-use related mitigation options by ecological zone in Africa Rainfall zone

Typical soil fertility

Critical issues in land use

C mitigation options

Arid-semiarid 2500 mm

Poor, strongly leached soils

Deforestation, intensification of agriculture

Large above-ground C stocks can be maintained with reduced deforestation, improved forest management.

Wetlands

Organic soils

Conversion for rice and upland crops

Large below-ground C stocks can be maintained if native wetlands are left intact.

Source: Paustian et al. (1998, p. 139).

for CO2 mitigation from converting to agricultural land exists in the humid zone and in tropical wetlands. Degraded lands offer a large potential C sink (Greenland, 1995; Scurlock and Hall, 1998). Large portions of agricultural lands in Africa are degraded or desertified (Table V). Restoration of degraded soils through improved farming systems could lead to a considerable increase in soil C. Eroded soils in the tropics and the subtropics may sequester around 0.2–0.5 and 0.1–0.2 t C ha−1 yr−1 in humid and semi-arid ecoregions, respectively (Lal, 1999, p. 323). Prevention of deforestation, afforestation, and reduced burning of savannas would reduce the release of C into the atmosphere. Degraded grasslands and pastures can be improved by controlling grazing, using improved pasture species, and using chemicals and soil amendments. Although these estimates are useful on the regional and global levels, they are less so in assessing local and project-level opportunities for soil C sequestration. Thus it is necessary to look more carefully at the amount of land that is socioeconomically rather than technically available.

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Table V Areas in millions of hectares of agricultural land, permanent pasture, and forest and woodland of Africa, and the portion of these areas affected by human-induced soil degradation

Africa

Agricultural land Total Degraded

%

Permanent pasture Total Degraded

%

Forest and woodland Total Degraded %

187

65

793

31

683

121

243

130

19

Source: Greenland (1995, p. 16).

5. Costs, Benefits, and Net Costs 5.1. CARBON SEQUESTRATION EFFICIENCY AND LOCAL ECONOMIC RETURNS Depending on soil and environmental conditions as well as resource availability, different land-use changes and agricultural strategies could accumulate soil C. The CDM investor will attempt to maximize the amount of C sequestered relative to cost, or to minimize the cost relative to amount of sequestered C, whereas the farmer will always strive for the highest agricultural yield, or maximize other benefits. Very few cost estimates of soil C sequestration strategies and their local economic effects exist for Africa. A pioneering study examines six alternative strategies mixing application of maize stover, manure, and fertilizer in smallholdings in Kenya. The C sequestration efficiency (i.e., the change in soil organic C due to C inputs expressed as a percentage of the C inputs), the input cost, and effects on agricultural yield are examined (Table VI) (Woomer et al., 1997, p. 162). The C efficiency of the options differs greatly. Positive economic returns vary from 4.1 to 1.3. The only exception is stover management alone (option 2) with a net loss of –1.3, although it has a high C sequestration efficiency. Option 2 is clearly unattractive economically to farmers. Apart from the economic rent from GHG offsets sales, it brings only costs to the farmer. Option 2, which is more C efficient than options 1, 5, and 6, exemplifies a carbon farming project. Option 4 is the most attractive choice from the standpoint of C sequestration, but it results in less economic return than options 3 and 5. If assuming, for a moment, that input costs are identical, and that investors are indifferent about local economic benefits and costs of various C sequestration options, then investors would rank the six options in the following way: 4 > 3 > 2 > 6 > 5 > 1. Assuming that farmers are indifferent about the impact on soil C stocks, they would rank the options in the following way: 3 > 5 > 4 > 6 >  The reason why stover management had a negative effect on yield is not apparent from the study.

A possible explanation is that the maize stover was low in percent N and immobilized N temporarily. For instance, maize grain yields were reduced by 3–30% in the first three seasons in Kenya as a result of application of maize stover causing N immobilization (Snapp et al., 1998, p. 188).

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Table VI C sequestration strategies, C sequestration efficiency, and economic return C sequestration strategy

C seq. efficiency

Rank

Economic return

Rank

1. Fertilizer and stover 2. Stover 3. Manure 4. Fertilizer and manure 5. Stover and manure 6. Fertilizer, stover and manure

1.4 5.4 5.5 6.9 3.6 3.8

6 3 2 1 5 4

1.3 –1.3 4.1 1.6 1.8 1.4

5 6 1 3 2 4

1 > 2. It is not possible to maximize both C efficiency and economic return in this example. Instead, a trade-off between these two goals must be made. Nevertheless, both farmers and investors would rank options 3 and 4 among their most attractive options, and focus on these in exploring project possibilities for C sequestration. A study of the East African Highlands estimates that it would cost $153 t−1 C when maize stover is used in sequestering C, and maize yields would be reduced by a value of $200 (Woomer et al., 1997, p. 162 and p. 169). In this project, the net cost to the farmer of sequestration is $353 t−1 C (Table VII). However, when livestock manure is used, the input cost of sequestration would be $260 t−1 C, and the yield of maize increases to a value of $1066. Thus, the net profit of this strategy is U.S.$806! Moreover, GHG offsets revenue could further increase the farmer benefits from the manure-based strategy. Finally, the study estimates that agroforestry systems could raise the sequestered amount per hectare by 66 t C at an input cost of $87 t−1 C. But because the impact on yield is not estimated, the net costs in this case cannot be calculated. The high costs reported in the study of the East African Highlands compare unfavorably with preliminary studies of C sequestration in degraded drylands in the tropics. According to a UNEP workshop about 1.0 Gt C y−1 could be sequestered through dryland restoration, at a cost of $10–$20 t−1 C (Squires et al., 1997). An earlier study estimates cost of restoring rangelands of about $10 t−1 C (Ojima et al., 1993a). Although these studies indicate that significant low-cost options with considerable ancillary benefits exist, detailed cost assessments of these options are required before their practical potential in terms of actual CDM projects can be determined.

 The study confuses input costs and benefits and ignores net costs.

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Table VII Input cost, effect on yields and net cost of C sequestration in soils of smallholder settings of the East African Highlands Type of C input, land use system

Input cost/t C $

Effect on yields $

Stover, smallholder agriculture

153

–200

353

Labor costs excluded.

Manure, smallholder agriculture

260

1066

–806

Labor costs excluded.

87

n.a.

n.a.

No costing of erosion control, tree planting, and fertilizer input. A full-time farmer managing the project, at a cost of $40/ha is included in estimate.

Smallholder agroforestry

Net cost $

Comments on cost estimates

Source: Based on Paul L. Woomer et al. (1997), n.a. = not available.

5.2. COMPARING SOCIAL PROFITABILITY OF SEQUESTRATION IN AGRICULTURE AND FORESTRY

The net costs of the stover-based and manure-based strategies cannot be compared with the net costs of other forestry and agroforestry strategies because most studies ignore the local costs and benefits. For instance, in their study of C sequestration in agroforestry systems, Dixon and co-workers found that the project costs ranged from $1 to $69 t−1 C (Dixon et al., 1994, p. 86). Local benefits were expected to accrue, but quantitative estimates were not made. Local benefits and costs of forestry and agroforestry projects sequestering C are seldom quantified but it is assumed that projects provide a stream of economic, social and environmental benefits over time at the local level (e.g., Faeth et al., 1994; but see Sampson et al., 2000, pp. 208–210). It is important to count the economic value of all the goods and services produced when comparing the profitability of different sequestration options. The private farmer will select the option that produces the greatest total income. But because the prices of forest and agricultural products and GHG offsets will vary over time there is no system that always will be most profitable. To illustrate, forest systems store in general more C than agricultural systems (Watson et al., 2000,

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Figure 2. Carbon stocks (t C ha−1 ) in and local economic income (US$ ha−1 ) from forest and agricultural systems.

p. 31), but agriculture is in general more profitable than forestry (see Figure 2). When the GHG offsets price is sufficiently high, a forest system will represent the best choice even though forest products have less economic value. But when the GHG offsets price is low, then the agriculture system will be the best option. Changes in agricultural and forest product prices will similarly influence the profitability of the systems. Figure 2 illustrates the tradeoff between private farmer revenue and C sequestration under a forestry and an agricultural system. As mentioned above, deforestation rates might be lowered if increased agricultural productivity could reduce the demand for new agricultural land. Preserving forest C could therefore be a significant side effect of C sequestration in soils in developing countries.

6. Possibilities and Constraints for Africa This section briefly discusses a number of important issues in assessing the practical feasibility of soil C sequestration projects in sub-Saharan Africa. 6.1. POTENTIAL AND TIME SCALE A simple way to estimate the potential for C sequestration in soils is to assume that the soil C content can be rebuilt to the original, pre-cultivation level (Paustian et al., 1998, p. 140). The further away a soil is from the usual C concentration of  The high rate at which forests are currently being converted to agriculture in Africa (Gaston

et al., 1998) indicates that economic return from agriculture is higher than from forests, at least in the short term, and that the land is more valuable deforested than forested. Deforestation may also be driven by the one-time economic return from the harvest of wood from primary forest, without regard to land-use thereafter.

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a region, the greater the amount of C that can be stored in the soil (Ojima et al., 1993a,b). The time path of C sequestration would be an important factor in investment decisions as it would determine the benefits over time from CDM investment in sequestration projects. Assuming that the recoverable level is between one-half and two-thirds of historic C losses (Cole et al., 1996, p. 751) and that approximately 25–30% of the C originally present in the soil has been lost due to cultivation (Paustian et al., 1998, p. 146), about 13–20% of the initial soil C stock could be regained over a 50–100 year time period. CDM investors are likely to select those projects that achieve significant mitigation gains within a relatively short period of time.

6.2. MEASURING AND MONITORING SOIL CARBON The issue of measurability of stocks of soil C and GHG fluxes relates closely to issues regarding monitoring and verification of GHG offsets. This issue is essential for the functioning, reliability, and effectiveness of soil C sequestration projects, and project baselines would be established on the basis of soil C measurements. As a rule, measurement and monitoring should be reliable, cost-effective, technically sound, readily verifiable, independent and objective, and use internationally peerreviewed methods (MacDicken, 1997). It is possible to produce reliable soil C inventories by using currently available measuring methods and techniques. But certain economic limitations of monitoring soil C should be acknowledged (Winrock International, 1997, pp. 18–19). First, projects that fix small amounts of C are less likely to be monitored cost-effectively. Second, the size of the project area is also a limiting factor on cost-effective monitoring of C. As a rule, because fixed costs constitute a large part of the total monitoring costs, the larger the project area, the lower the unit costs of monitoring. In comparison to estimates of forest biomass, there is less opportunity to measure soil C remotely, and ground-based information will be necessary for estimating soil C stocks and fluxes. Perhaps it will be prohibitively expensive to establish a field monitoring program based on on-site measurements that track soil C fluxes with sufficient precision to be considered satisfactory under the Kyoto Protocol (Lal et al., 1999, p. 58; Subak, 2000). Few studies address this critical issue. Studies and experiments that examine the accuracy, feasibility, and cost-effectiveness of measurement and monitoring are needed.  According to the IPCC, about 20–50% of the soil C in tropical agricultural systems has been

emitted as a result of permanent cultivation (Cole et al., 1996, p. 751).  For the IPCC’s most recent assessment of methods and models, see Watson et al., 2000, pp. 90– 104.

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6.3. PERMANENCE AND LEAKAGE It is evident that soil C re-accumulation schemes would need to be in place over long time-scales, raising the issue of whether C stocks are permanent or potentially reversible. How could stocks be protected against subsequent destructive interference resulting in losses? In this context, it should be realized that below-ground C normally is more protected than above-ground C during fire and other destructive events. Moreover, forests might be felled at a later point, but it is unlikely that agriculture will be reverted back to forests in sub-Saharan Africa. Neither is it likely that farmers who benefit economically from conservation tillage will switch back to intensive tillage practices. More work is needed on the question of permanence of soil C sequestration. For instance, certain types of contracts may help to reduce the risk of reversal of C sequestration (Marland et al., 2001; Ellis, 2001). C sequestration in soils might avoid problems of leakage because of its potential local benefits. The term leakage refers to the situation where a project unintentionally shifts an undesirable activity from the project site to another site, for instance a forest conservation project that prevents deforestation within the project area, and instead increases deforestation outside this area. However, soil C sequestration systems are less likely to create leakage effects because they will frequently be more desirable than alternative land-use systems. 6.4. CLIMATE CHANGE It will be important to know how much C is sequestered by projects and how much is caused by factors outside the projects. Significant changes in exogenous project variables, such as climate change, should be taken into account. Most assessments that consider both rising CO2 and rising temperature suggest that the latter, through its tendency to stimulate decomposers, will cause larger losses of SOM than the former, which may stimulate greater inputs of plant residue (Kirschbaum, 1995; Kirschbaum, 2000; Falkowski et al., 2000). But land-use practices may significantly influence soil C sequestration in Africa even if the global climate undergoes a significant change. One study indicates that sustainable management and land-use practices will potentially have a much larger effect on C storage in grassland and dryland soils than climate change or CO2 enhancement (Ojima et al., 1993a). By comparing changes in C stocks at project sites with observations from reference plots it may be possible to separate and measure the impacts of projects and climate change. Establishing control plots would however increase the expense of  Based on author’s discussion with Brazilian farmers.

 Some propose to use adjustable equilibrium C stocks (tC ha−1 ) for biosphere domains in order

to address the effect of global warming on the size of C stocks and to distinguish effects that are under the control of land managers from those that are not. See Kirschbaum et al., 2001.

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monitoring and estimation. Physiological, biogeochemical models could be used to isolate the CO2 -fertilization effect. Other ways in which climate change may effect projects – such as changing precipitation and moisture patterns – should also be controlled for (for models and discussion, see Parton et al., 1995; King et al., 1997; Cao and Woodward, 1998). 6.5. CARBON ‘ COSTS ’ AND ENVIRONMENTAL SIDE - EFFECTS Under the CDM, careful assessment of the direct and indirect C ‘costs’ of sequestration strategies as well monitoring of all significant GHG fluxes, including methane (CH4 ), would be required of projects. N-fertilizer application, animal manure, crops residues, and nitrogen derived from N-fixing legumes could contribute to an increase in emissions of nitrous oxide (N2 O), a potent greenhouse gas, from soils (Mosier et al., 1998). The GHG emissions associated with the manufacture, transport, and application of fertilizer may significantly reduce the net amount of C sequestered in the soil, and perhaps even increase net GHG emissions from soil C sequestration (Schlesinger, 2000; Izaurralde et al., 2000). Experiments from temperate zone soils indicate that the application of N fertilizer to sequester soil C is unlikely to result in net mitigation (Robertson et al., 2000). Although greater concentrations of SOM in manured fields are achievable, manuring could be associated with declining SOM on a proportionally larger area of off-site lands. Hence, manuring would not contribute to a net sink for C in soils. Irrigation, a third frequently recommended practice to increase C sequestration, is often associated with significant energy use and, when practiced in arid regions, may result in net C loss due to the precipitation of calcium carbonate (CaCO3 ) from irrigation water with dissolved calcium (Ca) (Schlesinger, 2000). 6.6. CAPACITY- BUILDING AND INSTITUTIONAL STRENGTHENING The institutions which currently exist in Africa would be insufficient for participating in a future GHG offsets market and attracting significant foreign investments. It would be important to assist countries in sub-Saharan Africa in developing the institutional capacity that is necessary to fully participate in and benefit from the global GHG offsets market. Capacity-building and institutional strengthening, including stimulation of the private sector, would be important in order to prepare Africa for participation in international offsets under the CDM. 6.7. ‘ADDITIONALITY ’ Only C-sink management and sink enlargement that is additional to that which would occur in the absence of a CDM project would qualify for crediting under the CDM. Put differently, only C sequestration over and above the baseline – the baseline should realistically reflect what would have happened otherwise (see Ellis and Bosi, 1999, for a review of methods for baseline-setting) – would generate

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credits or CERs. The involved parties must convincingly demonstrate that the project would not have been introduced ‘but for’ the CDM. The emerging rules for the CDM assume that all opportunities for achieving economic gain from activities maintaining or enlarging C sinks are, in principle, exhausted in the base case. Consequently, if interpreted narrowly, the ‘additionality’ criterion would exclude revenue-generating projects. But lack of investment capital, high interest rates, and property rights, among other things, often hinder the adoption of agricultural practices and land-use management that could benefit resource-poor, small-scale farmers in Africa and sequester C in soils (Izac, 1997). Analyses therefore should include assessment of the effects of project constraints and barriers, and identify feasible ways to remove them.

7. Conclusions Soil C, which currently is not included among the C sinks that would be regulated under the CDM, could be an effective strategy financed by industrialized countries investing in sequestration projects in developing countries. This option would combine the goals of sustainable development, food security, and desertification control in developing countries with creditable low- and medium-cost GHG mitigation projects. Additionally, increased productivity and sustainability in agriculture could reduce the demand for clearing forests and savannas to meet local agricultural and economic needs. Before undertaking any changes in land uses and agricultural management, it will be important to make detailed assessments of the overall effect in terms of net sequestration, or release, of GHGs into the atmosphere. There is presently an urgent need to launch long-term (>10 years) field experiments and demonstration projects in Africa. Existing data are not readily comparable, it is unclear how well site-specific findings represent wider areas, and it is uncertain how large amounts of C could be sequestered. To develop suitable CDM projects, it will be important to conduct experimental trials designed to generate reliable and comparable data. Estimating all environmental and economic benefits and costs would be important. Also opportunity costs (i.e., the foregone benefits of land-use alternatives) should be estimated. Under some circumstances it is necessary to look beyond soils in order to sequester sufficiently large amounts of C. Some soils in Africa have a very limited potential for C sequestration and only through tree-based systems would it be possible to sequester reasonably large amounts of C. Useful ways of combining agricultural and forestry systems should therefore be identified (Sanchez, 2000). But agroforestry should be regarded as a means to combine agricultural production with forestry rather than promoting trees as a substitute for agricultural crops. Comparing soil management, forestry and agroforestry systems for their economic, social and environmental effects will increasingly be necessary. But sub-Saharan

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Africa will not profit significantly from soil C sequestration under the CDM as long as the problems due to a rapidly growing population, tremendous land use pressures, and poverty remain unsolved.

Acknowledgements The paper has benefited from the comments and suggestions made by three anonymous reviewers and the editor. The author is grateful to N.H. Batjes for providing the soil C map for Africa. The paper is based upon a study commissioned by the African Technical Division of the World Bank, Washington, D.C. The Danish Government sponsored the study.

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