Sequestration of carbon in soil organic matter in Senegal: an overview

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Abstract

Sequestration of Carbon in Soil Organic Matter (SOCSOM) in Senegal is a multi-disciplinary development project planned and refined through two international workshops. The project was implemented by integrating a core of international experts in remote sensing, biogeochemical modeling, community socio-economic assessments, and carbon measurements in a fully collaborative manner with Senegal organizations, national scientists, and local knowledge and expertise. The study addresses the potential role developing countries in semi-arid areas can play in climate mitigation activities. Multiple benefits to smallholders could accrue as a result of management practices to re-establish soil carbon content lost because of land use changes or management practices that are not sustainable. The specific importance for the Sahel is because of the high vulnerability to climate change in already impoverished rural societies.

The project focuses on four objectives in specific locations across the agroecological zones of Senegal. These objectives are: use of soil sampling and biogeochemical modeling to quantify the biophysical potential for carbon sequestration and to determine the sensitivity of the carbon stocks to various management and climate scenarios, to evaluate the socio-economic and cultural requirements necessary for successful project implementation directed toward an aggregation of smallholders to sequester around 100,000 t carbon (C), to support capacity building to develop a Carbon Specialist Team, and to initiate extrapolation from site-specific project areas to the Sahel region and the national level.

Introduction

The collection of papers in this special publication results from an international project: the Sequestration of Carbon in Soil Organic Matter (SOCSOM) in Senegal. The SOCSOM Goal is to:

…provide quantitative analyses of the environmental, ecological, and economic potential for the sequestration of carbon in soil organic matter in specific study areas and to define the necessary socio-economic enabling conditions and policies to implement successful projects.

The project was proposed to emphasize site-specific studies that would quantify historical changes in both carbon stocks and soil fertility and to evaluate potential local mitigation and adaptation responses in Senegal to the global increase in atmospheric CO2. Thus, the project attempts to provide quantitative and site-specific information to evaluate the biophysical, social, economic, cultural, and political feasibility of climate change contributions from these lands in developing countries and to guide further project implementation that would also support agricultural sustainability. SOCSOM, as a project, evolved after two major international workshops (Tieszen, 1999; Tieszen and Tschakert, 2000) to identify the importance and potential role of semi-arid and arid regions and soil organic carbon in climate change studies.

Carbon sequestration is defined as the net removal of CO2 from the atmosphere into long-lived pools of carbon. In SOCSOM, the pools of interest are those that also support agricultural livelihoods. These pools are above-ground biomass, e.g. woody or herbaceous biomass, roots and micro-organisms in soils, and organic and inorganic carbon in soils. Sequestration implies an increase in the sizes of long-lived pools, not simply increasing the fluxes from atmosphere. This project emphasizes the multiple value of re-establishing depleted soil organic matter (SOM) pools by processes that may either increase fluxes to the soil (photosynthesis) or decrease fluxes from the soil (decomposition). All carbon pools are globally important in the livelihood strategies of local communities and individual farmers, but we emphasize soil carbon because of its relationship to soil fertility and agricultural sustainability, especially in semi-arid lands.

It is now well established that the various pools of carbon on Earth are changing, and that the fluxes (the rates of these transfers from one pool to another) have recently increased substantially. Although the geological record illustrates the dynamic nature of these carbon pools, it is only in historical times that the atmospheric pool has increased this rapidly. Atmospheric CO2 has increased about 80 p.p.m.v. since the 1850s, and is presently accumulating in the atmosphere at a rate of about 1.5 p.p.m.v. or 3.3–3.5 gigatons (Gt=Pg) year−1.

The scientific and political consensus now seems to conform with the opinions documented by the UN Framework Convention on Climate Change (UNFCCC) that this degradation represents dangerous interference with the climate. The continuing increase and projections of atmospheric CO2 concentrations and the potential threats to the climate system and global economy require a thorough understanding of carbon cycling on local and national scales.

The increased net fluxes to the atmosphere are attributed to two principal human activities—the burning of fossil fuels and land use changes including deforestation, grassland conversion, and land management, with estimates of about 5.5±0.5 and 1.6±1.0 Pg C year−1, respectively, during the decade of the 80s (Schimel et al., 1995). The differences between fluxes to the atmosphere and net accumulation reflect an increased “sink” not yet defined with respect to location or magnitude. Although precise accounting of amounts and sources remains difficult and changes from decade to decade, over the last 100 years activities associated with industrialization account for around 78 percent of this total atmospheric increase. Since the UNFCCC identified differing roles of developed and developing countries with respect to greenhouse gasses emissions, we will examine these roles in more detail.

Carbon emissions to the atmosphere from land cover change and management are concentrated in the tropics, and influence environmental services and human needs in various ways (Lambin et al., 2003). Human historical and pre-historical contributions, however, are much different than those operating today. Human activities such as land cover change and management during historical and pre-historical times account for 43 percent of total emissions (185 Pg), one third occurring before 1850. This release was concentrated in temperate zones. Although we are not suggesting we should diminish efforts to decrease emissions from fossil fuel consumption, the dubious legacy just identified creates a potential opportunity.

In principle, many of the losses can be reversed by creating and maintaining “sinks” where previous stocks existed, through restoration and appropriate management of carbon pools in soils and forests (see Kessler and Breman, 1991 for West Africa; Lal et al., 1999). Although sinks can help reduce the rate of increased atmospheric CO2 from both combustion and land use, the relative roles of developed and developing countries will continue to change. Houghton (1999) and Houghton et al (1998), Houghton et al (1999) document the importance of land use, suggesting that by 2020 developing countries could account for 60 percent of global energy use and associated carbon emissions. Although the imbalance in the carbon system is largely driven by the combustion of fossil fuels, note that 20 times more carbon is exchanged between the atmosphere and the Earth's vegetation and soils than is released from fossil fuels.

Most scientists believe that rapid CO2 accumulation in the atmosphere will result in climate change, particularly global warming. Although it is not yet possible to accurately predict the explicit impacts of global warming, the consequences might be substantial. The review of the Intergovernmental Panel on Climate Change (IPCC) assessment (Watson et al., 2001) indicates that global mean surface temperatures have increased, these increases have affected agricultural and natural systems in detectable ways, the increases in global warming are largely accounted for by human activities, and the projected increases in this century are substantial, especially over land masses.

The impact of global climate change for Africa was recently presented in a valuable special publication by Desanker (2001) and Desanker and Justice (2001). As Hulme et al. (2001) indicate, the uncertainty in probable climate changes, especially for the Sahel, is high and current agreement among modeled projections is low. The possible combination of increased temperature, little increase in precipitation, and enhanced evapo-transpiration suggests, however, that agriculture is likely to be negatively impacted in semi-arid areas, especially those already near thermal optima for crops.

Africa, the continent arguably most vulnerable to the impacts of predicted changes, is likely to face higher inter-annual variability of rainfall, and more extreme climate events such as droughts and floods, especially in arid and semi-arid areas already severely afflicted by land degradation and irreversible desertification (Glantz, 1994). Caution is advised, however, because regional satellite analyses (Tucker et al., 1994) show the advances of the Sahara desert largely paralleling the fluctuations in rainfall (Nicholson et al., 1998; Tucker and Nicholson, 1999) rather than longer-term environmental shifts. In addition, Prince et al. (1998) show no large regional reduction in productivity. Similar complexities are shown in southern rangelands (Dube and Pickup, 2001).

This research suggests that the two fundamental components in the global effort to combat climate change, mitigation and adaptation, may differ substantially along north–south or developed–developing country perspectives. Seemingly, the opportunity or responsibility to reduce driving forces for climate change through mitigation rests largely with developed countries because of their contributions from fossil fuels related to industrialization (e.g. the USA contributes nearly 25 percent of all emissions derived from fossil fuels). Yet, in addition to direct emissions reductions, developing countries can facilitate reductions in net atmospheric accumulations by the strong development of sinks—a direct activity suggested by the Kyoto Protocol.

Thus, developing countries have the potential to become active partners in mitigation efforts, even though their current rates of release from fossil fuels are low. Larger differences exist with respect to adaptive capacity. Developing countries, especially many in Africa, are highly vulnerable to climate change by virtue of geographic location, both from the anticipated adverse impacts of global warming, and because of the reduced technological, institutional, and financial mechanisms available to them.

The attainment of a global future with a secure and sustainable climate system may be facilitated by an approach which justly recognizes that both developed and developing countries have important, although different, responsibilities and roles. Efforts must be undertaken to reduce emissions from fossil fuels and to introduce energy-efficient technologies. The replacement of high carbon-emitting energy sources must be emphasized in both developed countries and in developing countries, as their energy needs increase. We must also recognize that current use of fossil fuels means that carbon sequestration options (both geological and biological) will remain important features of carbon management as shown, e.g. by the recent Carbon Sequestration Leadership Forum (CSLF, 2003).

Both developed and developing countries must initiate activities in agricultural and natural ecosystems that maintain current stocks and enhance potential sinks. Developing countries, poised to bear disproportionate impacts of climate change, also need technologies and resources to facilitate adaptation. The small project discussed here, SOCSOM, was undertaken to help insure that developing countries had opportunities to play active roles in processes of mitigation and adaptation with respect to climate change.

SOCSOM emphasizes the importance of carbon in soil for several reasons. Soil carbon, especially organic forms (SOC), is a large pool that globally exceeds that of the biomass and that of the atmosphere. It is, however, dispersed over broad areas and dilute, only uncommonly occurring at concentrations greater than 3 percent. The carbon in this pool is dynamic (Parton et al., 2004) because its turnover time is rapid on both human and geologic time-scales. For example, some compartments of SOC in this pool have a mean residence time of days to dozens of years and are therefore considered “active” or “slow.” This SOC can be stabilized by biochemical and physical features of the soil into still more stable or “passive” fractions that may have residence times of hundreds to several thousands of years.

The SOC is an important determinant of soil fertility because of its impact on ion exchange capacities and its near-stoichiometric relationship to nitrogen (C:N ratios often between 8 and 20). The SOC is susceptible to mineralization as organic forms are converted to CO2 and returned to the atmosphere with a proportional release of soil N. The magnitude of the SOC pool and its rate of replenishment are determined in part by the rate that CO2 is incorporated into plant tissues by photosynthesis or net primary production (NPP), followed by the addition of plant and animal residue into the soil, or direct carbon input to soil from plant roots. This biological replenishment is a function of both the NPP and the land cover type or species.

These relationships, driving variables, and algorithms governing exchanges are incorporated in various carbon biogeochemistry models, of which CENTURY (Parton et al., 2004) is perhaps best known and most commonly used to simulate carbon fluxes and pool changes. The researchers explored the applications of this model in different ways across our sites in Senegal, as the SOCSOM project evaluates management opportunities and climate constraints on pool sizes, turnover rates, and the magnitudes of fluxes or exchanges. Because SOM contributes to soil structure and fertility, it is closely associated with those features that make soils valuable for the production of food and fiber. Thus, SOM is intimately integrated with human land use.

The changes in land use and land cover from grasslands, shrublands, woodlands, or forests to agricultural lands have clear and dramatic impacts on the services those ecosystems provide. The loss of biomass carbon and its release to the atmosphere is obvious and dramatic in both its suddenness and its magnitude. Sanchez (2000) summarizes magnitudes of woody biomass losses from the “Alternatives to Slash and Burn” projects as high as 200 t ha−1 along the margin of the humid tropics to lower values, likely from 75 t ha−1 to only a few t ha−1 in semi-arid systems. What is not so obvious is the continuing loss of SOC as these new agricultural uses now exploit the carbon and associated nutrient capital in these soils.

Under most agricultural practices, soils have been depleted of SOC by nearly 50 percent in 20–50 years (Scholes and Hall, 1996) as documented in various experimental (Jenkinson and Rayner, 1977) and natural systems. Woomer's work in East Africa illustrated some of the complexity and mitigating factors when he documented losses of around 0.69 t C ha−1 year−1. These losses were reduced when fertilizer, especially animal manure, was applied (Woomer et al., 1997), even in recently cultivated land. This loss of soil C is associated with losses of N and thus usually results in decreased fertility unless NPP can be sustained with exogenous fertilizer and accompanied by residue returns. If not, the normal spiral leads to decreasing fertility, decreasing yield, and reductions in SOC. The consequences of this are “unintentional” land degradation (Levia, 1999), and desertification which may affect 2.6 billion people worldwide and as much as 20–50 percent of the land in sub-Saharan Africa, containing 200 million people (Nachtergaele, 2002).

Section snippets

Sahel and Senegal land degradation

Although the relationships among poverty and land degradation or depletion of soil carbon are complex, a recent Food and Agricultural Organization (FAO) study (FAO, 2002) concludes that when the natural soil resources are depleted, poverty results. Senegal land has been cultivated and farmed without appropriate management of organic and mineral fertilizer applications (Tiessen et al., 1998), resulting in enhanced mineralization and loss of SOM (Feller, 1977; Kushwaha et al., 2001). Sanchez

Workshops to define the SOCSOM approach

SOCSOM in Senegal was undertaken as a feasibility project to advance understanding on how developing countries could participate in climate mitigation and improve smallholder livelihoods through sustainable management. The project was supported because co-operative development activities at the US Geological Survey/Earth Resources Observation Systems Data Center (EDC) clearly defined the important roles that soil carbon restoration could play in improving agricultural productivity and

Acknowledgements

This special publication and the research and development activities have received core funding from the USAID. We especially thank Paul Bartel who managed this project activity. Initial planning and project development were facilitated by the major international workshops with significant support from the Sand County Foundation and from numerous organizations including the World Bank, various Consultative Group on International Agricultural Research Centers, the United Nations Environment

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