Soil organic carbon dynamics, functions and management in West African agro-ecosystems

https://doi.org/10.1016/j.agsy.2005.08.011Get rights and content

Abstract

Soil fertility depletion has been described as the single most important constraint to food security in West Africa. Over half of the African population is rural and directly dependent on locally grown crops. Further, 28% of the population is chronically hungry and over half of people are living on less than US$ 1 per day as a result of soil fertility depletion.

Soil organic carbon (SOC) is simultaneously a source and sink for nutrients and plays a vital role in soil fertility maintenance. In most parts of West Africa agro-ecosystems (except the forest zone), the soils are inherently low in SOC. The low SOC content is due to the low shoot and root growth of crops and natural vegetation, the rapid turnover rates of organic material as a result of high soil temperatures and fauna activity particularly termites and the low soil clay content. With kaolinite as the main clay type, the cation exchange capacity of the soils in this region, often less that 1 cmol kg−1, depends heavily on the SOC. There is a rapid decline of SOC levels with continuous cultivation. For the sandy soils, average annual losses may be as high as 4.7% whereas with sandy loam soils, losses are lower, with an average of 2%. To maintain food production for a rapidly growing population, application of mineral fertilizers and the effective recycling of organic amendments such as crop residues and manures are essential especially in the smallholder farming systems that rely predominantly on organic residues to maintain soil fertility. There is need to increase crop biomass at farm level and future research should focus on improvement of nutrient use efficiency in order to increase crop biomass. Research should also focus on ways of alleviating socio-economic constraints in order to increase the legume component in the cropping systems. This will produce higher quality fodder for the livestock and also increase biomass at farm-level. This paper reviews various strategies and lessons learnt in improving soil organic carbon status in West Africa soils.

Introduction

Over half of the African population is rural, and directly dependent on locally grown crops or foods harvested from the immediate environment. The growth rate for cereals grain yield is about 1% while population growth is about 3% (UN, 2001). During the last 35 years, per capita cereals production has decreased from 150 to 130 kg/person, whereas in Asia and Latin America an increase from about 200–250 kg/person has been observed (FAO, 2001). Labor and land productivity in Africa are among the lowest in the world. Per capita food production in Africa has been declining over the past two decades, contrary to the global trend. Annual cereal deficit in sub-Saharan Africa amounts to 100 million tons and the food gap (requirements minus production) is widening. Food imports increased by about 185% between 1974 and 1990 while food aid increased by 295% (ECA, 2002). The average African consumes only about 87% of the calories needed for a healthy and productive life. Sixteen percent (16%) of Africa’s current arable land base is so eroded that agriculturally it cannot be useful any longer. In addition to this, 70% of deforestation is caused by farmers who in their quest for food have no incentive to ponder about long-term environmental consequences (ECA, 2002, FAO, 2001).

The Sudano-Sahelian zone of West Africa is the home of the world’s poorest people, 90% of whom live in villages and gain their livelihood from subsistence agriculture (Bationo and Buerkert, 2001). Per capita food production has declined significantly over the past three decades. According to FAO (2003), total food production in Sahelian countries grew by an impressive 70% from 1961 to 1996, but it lagged behind as the population doubled causing per capita food production to decline by approximately 30% over the same period.

Increasing human population pressure has decreased the availability of arable land and it is no longer feasible to use extended fallow periods to restore soil fertility. The fallow period which would have restored soil fertility and organic carbon is reduced to lengths that cannot regenerate soil productivity leading to the non-sustainability of the farming systems (Nandwa, 2001). High population densities have necessitated the cultivation of marginal lands that are prone to erosion hence enhancing environmental degradation through soil erosion and nutrient mining. As a result, the increase in yield has been more due to land expansion than to crop improvement potential (FAO, 2003). For example, the 7.6% yield increase of yam in West Africa was mainly due to an area increase of 7.2% and only 0.4% due to improvement in crop productivity itself (Table 1).

In West Africa as the rest of the continent, removal of crop residues from the fields, coupled with a lower rate of macronutrient application compared to losses, has contributed to negative nutrient balances (Stoorvogel and Smaling, 1990). For nitrogen as an example, whereas 4.4 million tons are lost per year, only 0.8 million tons are applied (Bationo et al., 2004a) (Fig. 1). Additionally, low and erratic rainfall, high ambient soil and air temperatures, inherent poor soil fertility, low water holding capacities and degraded soil structure lead to low crop productivity in this environment. Consequently, the present farming systems are not sustainable (Bationo and Buerkert, 2001).

Transforming agriculture in West Africa agro-ecosystems and expanding its production capacity are prerequisites for alleviating rural poverty, household food deficits and environmental exploitation (Bationo et al., 2004a). Reversing the declining trend in agricultural productivity and preserving the environment for present and future generations in West Africa must begin with soil fertility restoration and maintenance (Bationo et al., 1996). Soil fertility is closely linked to soil organic matter, whose status depends on biomass input and management, mineralization, leaching and erosion (Roose and Barthes, 2001, Nandwa, 2001). It is well recognized that soil organic matter increases structure stability, resistance to rainfall impact, rate of infiltration and faunal activities (Roose and Barthes, 2001). Optimum management of the soil resource for provision of goods and services requires the optimum management of organic resources, mineral inputs and the soil organic carbon (SOC) pool (Vanlauwe, 2004). The importance of SOC has increased interest and research on its build up in the soil–plant system with current emphasis on conservation tillage. SOC can play an important role and its maintenance is an effective mechanism to combat land degradation and increase future food production.

Various farm practices have been employed to build SOC stocks in West Africa. Crop (CR) residue application as surface mulch can play an important role in the maintenance of SOC levels and productivity through increasing recycling of mineral nutrients, increasing fertilizer use efficiency, and improving soil physical and chemical properties and decreasing soil erosion. However, organic materials available for mulching are scarce due to low overall production levels of biomass in the region as well as their competitive use as fodder, construction material and cooking fuel (Lamers and Feil, 1993). In a study to determine CR availability at farm level Baidu-Forson (1995) reported that at Diantandou in Niger with a long-term annual rainfall 450 mm, an average of 1200 kg ha−1 of millet stover was produced at the end of the following year barely 250 kg ha−1 remained for mulching. Powel and Mohamed-Sallem (1987) showed that at least 50% of these large on-farm disappearance rates of millet stover could be attributed to livestock grazing.

Animal manure has a similar role as residue mulching for the maintenance of soil productivity but it will require between 10 and 40 ha of dry season grazing and between 3 and 10 ha of rangeland of wet season grazing to maintain yields on 1 ha of cropland (Fernandez-Rivera et al., 1995). The potential of manure to maintain SOC levels and maintain crop production is thus limited by the number of animals and the size and quality of the rangeland. The potential livestock transfer of nutrients in West Africa is 2.5 kg N and 0.6 kg P ha−1 of cropland (de Leeuw et al., 1995).

Scarcity of organic matter calls for alternative options to increase its availability for improvement of SOC stock. Firstly, the application of mineral fertilizer is a prerequisite for more crop residues at the farm level and the maintenance of soil organic carbon in West African agro-ecosystems and therefore most research should focus on the improvement of nutrient use efficiency in order to offer to the smallholder farmers cost-effective mineral fertilizer recommendations. Secondly, recent success stories on increasing crop production and SOC at the farm level is the use of the dual purpose grain legumes having ability to derive a large proportion of their N from biological N fixation, a low N harvest and substantial production of both grain and biomass. Legume residues can be used for improvement of soil organic carbon through litter fall, or for feeding livestock with the resultant manure being returned to the crop fields.

The impact of organic resource quality on SOC is less clear. Low quality organic resources contain substantial amounts of soluble polyphenols and lignins that may affect the longer-term decomposition dynamics and contribute to the build up of SOC (Palm et al., 2001). Future research needs to focus more on whether the organic resource quality concept is also useful for predicting different degrees of stabilization of applied organic C in one or more of the organic matter pools.

The challenge in increasing SOC content is to embrace the holistic strategy of Integrated Soil Fertility Management (ISFM) that puts into consideration the biophysical and socio-economic constraints faced by the farmer community. The implementation of the ISFM strategy will break the vicious cycle responsible for land degradation, food insecurity and poverty in West Africa agro-ecosystems through improved knowledge of soil management and the capacity of farmers to invest in improved soil management technologies (Fig. 2).

This paper will discuss first the status of soil organic carbon at agro-ecosystem and farm level followed by the factors affecting SOC and functions of SOC before discussing the effects of soil and crop management on SOC and concluding on the future research challenges with emphasis on SOC quantity and quality.

Section snippets

Soil organic carbon status at agro-ecosystem and farm level

Soil organic carbon is an index of sustainable land management (Woomer et al., 1994, Nandwa, 2001) and is critical in determining response to N and P fertilization. There is however no clear agreement on the level of SOC below which response to N and P fertilization does not occur. For example, while Berger et al. (1987) reported such level to be 3.5 mg kg−1 in the northern Guinean zone, Bationo et al. (1998) in a study in West Africa found very strong response to mineral fertilizer at SOC levels

Factors affecting SOC

Clay and silt play an important role in the stabilization of organic compounds and small variations in topsoil texture could have large effects on SOC (Bationo and Buerkert, 2001). In this context, a survey of West African soils (Manu et al., 1991) indicated that for the soils investigated cation exchange capacity (CEC) depended directly more on SOC (r = 0.86) than to soil clay content (r = 0.46) (Table 4). de Ridder and van Keulen (1990) found a difference of 1 g kg−1 in SOC to result in a

Functions of soil organic carbon (SOC)

SOC plays an important role in supplying plant nutrients, enhancing cation exchange capacity, improving soil aggregation and water retention and supporting soil biological activity (Dudal and Deckers, 1993). Although it has been difficult to quantify the effects of SOC on crop and ecosystem productivity (Dudal and Deckers, 1993) results from experiments in some African countries already indicate favorable responses due to SOC.

Soil organic matter is not only a major regulator of various

Effect of soil and crop management on soil organic carbon

Soil organic carbon is lost through erosion, runoff and leaching (Roose and Barthes, 2001). Erosion and runoff contribute a large portion of carbon losses and these are highly accelerated in cultivated land as compared to undisturbed forest or savanna (Table 7). Topsoil nutrients and organic carbon generally decrease with increasing erosion (Kaihura et al., 1998) with the amount of eroded carbon depending more on the erosion quantity than on the carbon content of the eroded sediments (Roose,

Conclusion

The complementarities of livestock and crop production suggests the need for research on possibilities to increase nutrient use efficiency for higher crop residue production and to improve the production of alternative feed supplies. The aim of such research should be to increase both fodder quantity and quality thus preserving more crop residue for soil application. Research should also focus on ways of alleviating socio-economic constraints in order to increase the legume component in the

References (73)

  • V. Balasubramanian et al.

    Crop residue management and soil productivity in savanna areas of Nigeria

  • A. Bationo et al.

    Role of manures and crop residues in alleviating soil fertility constraints to crop production: with special reference to the Sahelian and Sudanian zones of West Africa

    Fertilizer Research

    (1991)
  • A. Bationo et al.

    Comparison of five soil testing methods to establish phosphorus sufficiency levels in fertilized with water-soluble and sparingly soluble-P sources

    Fertilizer Research

    (1991)
  • A. Bationo et al.

    The effects of crop residue and fertilizer use on pearl millet yield in Niger

    Fertilizer Research

    (1993)
  • Bationo, A., Buerkert, A., Sedogo, M.P., Christianson, B.C., Mokwunye, A.U., 1995. A critical review of crop residue...
  • Bationo, A., Rhodes, E., Smaling, E.M.A., Visker, C., 1996. Technologies for restoring soil fertility. In: Mokwunye,...
  • Bationo, A., Lompo, F., Koala, S., 1998. Research on nutrient flows and balances in West Africa: State-of-the-art. In:...
  • A. Bationo et al.

    Soil organic carbon management for sustainable land use in Sudano-Sahelian West African

    Nutrient Cycling in Agroecosystems

    (2001)
  • A. Bationo et al.

    The African network for soil biology and fertility: new challenges and opportunities

  • Bationo, A., Nandwa, S.M., Kimetu, J.M., Kinyangi, J.M., Bado, B.V., Lompo, F., Kimani, S., Kihanda, F., Koala, S.,...
  • M. Berger et al.

    Le maintien de la fertilite des sols dans l’quest du Burkina Faso et la necessite de l’association agriculture-elevage

    Cotton et Fibres Tropicales

    (1987)
  • Breman, H., Traore, N., (Eds.)., 1986. Analyses des conditions de l’élevage et propositions de politiques et de...
  • R.J. Carsky et al.

    Maintenance of soil organic matter and N supply in the moist savanna zone of West Africa

  • C. Charraeu et al.

    Manuring of millet and sorghum

    Agricultural Soils

    (1964)
  • C. Charreau et al.

    L’amélioration du profil cultural dans les sols sableux et sablo-argileux de la zone tropicale sèche ouest-africaine et ses incidences agronomiques

    Agronomie Tropicale

    (1971)
  • de Leeuw, P.N., Reynolds, L., Rey, B., 1995. Nutrient transfers from livestock in West African Agricultural Systems....
  • N. de Ridder et al.

    Some aspects of the role of organic matter in sustainable intensified arable farming systems in the West African semi-arid tropics

    Fertilizer Research

    (1990)
  • R.K. Dixon et al.

    Forest sector carbon offset projects: near-term opportunities to mitigate greenhouse gas emissions

    Water Air and Soil Pollution

    (1993)
  • R. Dudal et al.

    Soil organic matter in relation to soil productivity

  • ECA (Economic Commission for Africa)

    Harnessing Technologies for Sustainable Development

    (2002)
  • FAO (Food and Agriculture Organization of the United Nations

    Soil Fertility Management in Support of Food Security in Sub-Saharan Africa

    (2001)
  • FAO (Food and Agriculture Organization) 2003. FAOSTAT Agriculture Data,...
  • Feller, C., Brossard, M., Frossard, E., 1992. Caractérisation et dynamique de la matière organique dans quelques sols...
  • Fernandez-Rivera, S., Williams, T.O., Hiernaux, P., Powell, J.M., 1995. Faecal excretion by ruminants and manure...
  • ICRISAT (International Crop Research Institute for the Semi-Arid Tropics), 1993. Annual Report 1992. ICRISAT Sahelian...
  • F. Kaihura et al.

    Impact of soil erosion on soil productivity and crop yield in Tanzania

    Advances in Geoecology

    (1998)
  • Cited by (327)

    View all citing articles on Scopus
    View full text