Land Degradation: An overview
H. ESWARAN1, R. LAL2 and P. F. REICH3
Published in: Eswaran, H., R. Lal and P.F. Reich. 2001. Land degradation: an overview. In: Bridges, E.M., I.D. Hannam, L.R. Oldeman, F.W.T. Pening de Vries, S.J. Scherr, and S. Sompatpanit (eds.). Responses to Land Degradation. Proc. 2nd. International Conference on Land Degradation and Desertification, Khon Kaen, Thailand. Oxford Press, New Delhi, India.
Land degradation will remain an important global issue for the 21st century because of its adverse impact on agronomic productivity, the environment, and its effect on food security and the quality of life. Productivity impacts of land degradation are due to a decline in land quality on site where degradation occurs (e.g. erosion) and off site where sediments are deposited. However, the on-site impacts of land degradation on productivity are easily masked due to use of additional inputs and adoption of improved technology and have led some to question the negative effects of desertification. The relative magnitude of economic losses due to productivity decline versus environmental deterioration also has created a debate. Some economists argue that the on-site impact of soil erosion and other degradative processes are not severe enough to warrant implementing any action plan at a national or an international level. Land managers (farmers), they argue, should take care of the restorative inputs needed to enhance productivity. Agronomists and soil scientists, on the other hand, argue that land is a non-renewable resource at a human time-scale and some adverse effects of degradative processes on land quality are irreversible, e.g. reduction in effective rooting depth. The masking effect of improved technology provides a false sense of security.
The productivity of some lands has declined by 50% due to soil erosion and desertification. Yield reduction in Africa due to past soil erosion may range from 2 to 40%, with a mean loss of 8.2% for the continent. In South Asia, annual loss in productivity is estimated at 36 million tons of cereal equivalent valued at US$5,400 million by water erosion, and US$1,800 million due to wind erosion. It is estimated that the total annual cost of erosion from agriculture in the USA is about US$44 billion per year, i.e. about US$247 per ha of cropland and pasture. On a global scale the annual loss of 75 billion tons of soil costs the world about US$400 billion per year, or approximately US$70 per person per year.
Only about 3% of the global land surface can be considered as prime or Class I land and this is not found in the tropics. Another 8% of land is in Classes II and III. This 11% of land must feed the six billion people today and the 7.6 billion expected in 2020. Desertification is experienced on 33% of the global land surface and affects more than one billion people, half of whom live in Africa.
Land degradation, a decline in land quality caused by human activities, has been a major global issue during the 20th century and will remain high on the international agenda in the 21st century. The importance of land degradation among global issues is enhanced because of its impact on world food security and quality of the environment. High population density is not necessarily related to land degradation; it is what a population does to the land that determines the extent of degradation. People can be a major asset in reversing a trend towards degradation. However, they need to be healthy and politically and economically motivated to care for the land, as subsistence agriculture, poverty, and illiteracy can be important causes of land and environmental degradation.
Land degradation can be considered in terms of the loss of actual or potential productivity or utility as a result of natural or anthropic factors; it is the decline in land quality or reduction in its productivity. In the context of productivity, land degradation results from a mismatch between land quality and land use (Beinroth et al., 1994). Mechanisms that initiate land degradation include physical, chemical, and biological processes (Lal, 1994). Important among physical processes are a decline in soil structure leading to crusting, compaction, erosion, desertification, anaerobism, environmental pollution, and unsustainable use of natural resources. Significant chemical processes include acidification, leaching, salinization, decrease in cation retention capacity, and fertility depletion. Biological processes include reduction in total and biomass carbon, and decline in land biodiversity. The latter comprises important concerns related to eutrophication of surface water, contamination of groundwater, and emissions of trace gases (CO2, CH4, N2O, NOx) from terrestrial/aquatic ecosystems to the atmosphere. Soil structure is the important property that affects all three degradative processes. Thus, land degradation is a biophysical process driven by socioeconomic and political causes.
Factors of land degradation are the biophysical processes and attributes that determine the kind of degradative processes, e.g. erosion, salinization, etc. These include land quality (Eswaran et al., 2000) as affected by its intrinsic properties of climate, terrain and landscape position, climax vegetation, and biodiversity, especially soil biodiversity. Causes of land degradation are the agents that determine the rate of degradation. These are biophysical (land use and land management, including deforestation and tillage methods), socioeconomic (e.g. land tenure, marketing, institutional support, income and human health), and political (e.g. incentives, political stability) forces that influence the effectiveness of processes and factors of land degradation.
Depending on their inherent characteristics and the climate, lands vary from highly resistant, or stable, to those that are vulnerable and extremely sensitive to degradation. Fragility, extreme sensitivity to degradation processes, may refer to the whole land, a degradation process (e.g. erosion) or a property (e.g. soil structure). Stable or resistant lands do not necessarily resist change. They are in a stable steady state condition with the new environment. Under stress, fragile lands degrade to a new steady state and the altered state is unfavorable to plant growth and less capable of performing environmental regulatory functions.
Effects of land degradation on productivity
Information on the economic impact of land degradation by different processes on a global scale is not available. Some information for local and regional scales is available and has been reviewed by Lal (1998). In Canada, for example, on-farm effects of land degradation were estimated to range from US$700 to US$915 million in 1984 (Girt, 1986). The economic impact of land degradation is extremely severe in densely populated South Asia, and sub-Saharan Africa.
On plot and field scales, erosion can cause yield reductions of 30 to 90% in some root-restrictive shallow lands of West Africa (Mbagwu et al.,1984; Lal, 1987). Yield reductions of 20 to 40% have been measured for row crops in Ohio (Fahnestock et al., 1995) and elsewhere in Midwest USA (Schumacher et al., 1994). In the Andean region of Colombia, workers from the University of Hohenheim, Germany (Ruppenthal, 1995), have observed severe losses due to accelerated erosion on some lands. Few attempts have been made to assess the global economic impact of erosion. The productivity of some lands in Africa (Dregne, 1990) has declined by 50% as a result of soil erosion and desertification. Yield reduction in Africa (Lal, 1995) due to past soil erosion may range from 2 to 40%, with a mean loss of 8.2% for the continent. If accelerated erosion continues unabated, yield reductions by 2020 may be 16.5%. Annual reduction in total production for 1989 due to accelerated erosion was 8.2 million tons for cereals, 9.2 million tons for roots and tubers, and 0.6 million tons for pulses. There are also serious (20%) productivity losses caused by erosion in Asia, especially in India, China, Iran, Israel, Jordan, Lebanon, Nepal, and Pakistan (Dregne, 1992). In South Asia, annual loss in productivity is estimated at 36 million tons of cereal equivalent valued at US$5,400 million by water erosion, and US$1,800 million due to wind erosion (UNEP, 1994). It is estimated that the total annual cost of erosion from agriculture in the USA is about US$44 billion per year, about US$247 per ha of cropland and pasture. On a global scale the annual loss of 75 billion tons of soil costs (at US$3 per ton of soil for nutrients and US$2 per ton of soil, for water) the world about US$400 billion per year, or approximately US$70 per person per year (Lal, 1998).
Soil compaction is a worldwide problem, especially with the adoption of mechanized agriculture. It has caused yield reductions of 25 to 50% in some regions of Europe (Ericksson et al., 1974) and North America, and between 40 and 90% in West African countries (Charreau, 1972; Kayombo and Lal, 1994). In Ohio, reductions in crop yields are 25% in maize, 20% in soybeans, and 30% in oats over a seven-year period (Lal, 1996). On-farm losses through land compaction in the USA have been estimated at US$1.2 billion per year (Gill, 1971).
Nutrient depletion as a form of land degradation has a severe economic impact at the global scale, especially in sub-Saharan Africa. Stoorvogel et al. (1993) have estimated nutrient balances for 38 countries in sub-Saharan Africa. Annual depletion rates of soil fertility were estimated at 22 kg N, 3 kg P, and 15 kg K ha-1. In Zimbabwe, soil erosion results in an annual loss of N and P alone totaling US$1.5 billion. In South Asia, the annual economic loss is estimated at US$600 million for nutrient loss by erosion, and US$1,200 million due to soil fertility depletion (Stocking, 1986; UNEP, 1994).
An estimated 950 million ha of salt-affected lands occur in arid and semi-arid regions, nearly 33% of the potentially arable land area of the world. Productivity of irrigated lands is severely threatened by build up of salt in the root zone. In South Asia, annual economic loss is estimated at US$500 million from waterlogging, and US$1,500 million due to salinization (UNEP, 1994). Potential and actual economic impact globally is not known. It is not known either for soil acidity and the resultant toxicity of high concentrations of Al and Mn in the root zone, a serious problem in sub-humid and humid regions (Eswaran et al., 1997a).
It is in the context of these global economic and environmental impacts of land degradation, and numerous functions of value to humans, that land degradation, desertification, and resilience concepts are relevant (Eswaran, 1993). They are also important in developing technologies for reversing land degradation trends and mitigating the greenhouse effect through land and ecosystem restoration. As land resources are essentially non-renewable, it is necessary to adopt a positive approach to sustainable management of these finite resources.
Views on land degradation
Land degradation has received widespread debate at the global level as evidenced by the literature: UNEP, 1992; Johnson and Lewis, 1995; Oldeman et al., 1992; Middleton and Thomas, 1997; Dregne, 1992; Maingnet, 1994; Lal and Stewart, 1994; Lal et al., 1997. At least two distinct schools have emerged regarding the prediction, severity, and impact of land degradation. One school believes that it is a serious global threat posing a major challenge to humans in terms of its adverse impact on biomass productivity and environment quality (Pimentel et al., 1995; Dregne and Chou, 1994). Ecologists, soil scientists, and agronomists primarily support this argument. The second school, comprising primarily economists, believes that if land degradation is a severe issue, why market forces have not taken care of it. Supporters argue that land managers (e.g. farmers) have vested interest in their land and will not let it degrade to the point that it is detrimental to their profits (Crosson, 1997). There are a number of factors that perpetuate the debate on land degradation:
Issues and challenges
There are sufficient studies and reviews (e.g. Barrow, 1991; Blaikie and Brookfield, 1987; Johnson and Lewis, 1995) that clearly demonstrate the fact that land degradation affects all facets of life. Many issues that confront those working in the domain of land resources include technologies to reduce degradation and also the techniques to assess and monitor land degradation. A number of questions remain unanswered and these include:
There are three steps involved in the process of addressing the problem: assessment, monitoring, and application of mitigating technologies. All three steps are in the purview of agriculturists and specifically, soil scientists. The latter clearly have the responsibility for soil science, and over the past decade substantial progress has been made in communicating the dangers of land degradation. However, much remains to be done.
Soil science has made significant contributions to the task of soil resource assessment but its practitioners have shown little or no interest in the additional task of monitoring the resource base (Mermut and Eswaran, 1997). This still remains a new area of investigation requiring guidelines, standards, and procedures. The challenge is to adopt an internationally acceptable procedure for this task. Soil scientists have an obligation not only to show the spatial distribution of stressed systems but also to provide reasonable estimates of their rates of degradation. They should develop early warning indicators of degradation to enable them to collaborate with others, such as social scientists, to develop and implement mitigating technologies. Soil scientists also have a role in assisting national decision-makers to develop appropriate land use policies.
There are many, usually confounding, reasons why land users permit their land to degrade. Many of the reasons are related to societal perceptions of land and the values they place on land. Degradation is also a slow imperceptible process and so many people are not aware that their land is degrading. Creating awareness and building up a sense of stewardship are important steps in the challenge of reducing degradation. Consequently, appropriate technology is only a partial answer. The main solution lies in the behaviour of the farmer who is subject to economic and social pressures of the community/country in which he/she lives. Food security, environmental balance, and land degradation are strongly inter-linked and each must be addressed in the context of the other to have measurable impact. This is the challenge of the 21st century for which we must be prepared.
Desertification is a form of land degradation occurring particularly, but not exclusively, in semi-arid areas. Figure 1 indicates the areas of the world vulnerable to desertification. The semi-arid to weakly arid areas of Africa are particularly vulnerable, as they have fragile soils, localized high population densities, and generally a low-input form of agriculture. About 33% of the global land surface (42 million km2) is subject to desertification. Table 8 shows the vulnerability of land to desertification in some Asian countries. Twenty-five percent of the region is affected and if not addressed the quality of life of large sections of the population will be affected. Many of these countries cannot afford losses in agricultural productivity. There are no good estimates of the number of persons affected by desertification nor of the number who directly or indirectly contribute to the process. A recent study by Reich et al. (this volume) provides some estimates for Africa.
As shown in Table 9, a high population density in an area that is highly vulnerable to desertification poses a very high risk for further land degradation. Conversely, a low population density in an area where the vulnerability is also low poses, in principle, a low risk. The Mediterranean countries of North Africa are very highly prone to desertification. In Morocco, for example, erosion is so extensive that the petrocalcic horizon of some Palexeralfs is exposed at the surface. In the Sahel, there are pockets of very high-risk areas. The West African countries, with their dense populations, have a major problem to contain the processes of land degradation. Table 10 provides the area in each of the classes of Table 9.
About 2.5 million km2 of land are under low risk, 3.6 are under moderate risk, 4.6 are under high risk, and 2.9 million km2 are under very high risk. The region that has high propensity is located along the desert margins and occupies about 5% of the landmass. It is estimated that about 22 million people (2.9% of total population) live in this area. The low, moderate, and high vulnerability classes occupy 14, 16, and 11% respectively and together impact about 485 million people. Cumulatively, desertification affects about 500 million Africans and though they have relatively good soil resources (Eswaran et al., 1997b,c) their productivity will be seriously undermined by land degradation and desertification.
A New Agenda
Although soils are an ecologically important component of the environment, the availability of research and development funds does not match their significance in terms of the cost to society if soils become degraded. The perception that enough is already known about soils, so that generalizations can be made for all soils, is incorrect. There is also a failure to recognize that agriculture is one of the major ‘stressors’ of the environment (Virmani et al., 1994), particularly from a soil degradation point of view (Beinroth et al., 1994). If these attitudes prevail, major catastrophes in the future become more probable.
The purpose of such discussions is to emphasize that soil is virtually a non-renewableresource. It is a basic philosophy that society has an obligation to protect soil, conserve it, or even enhance its quality for future generations. The role of society in sustaining agriculture can be demonstrated, and conversely, the role of soil in sustaining society. The paradigms that brought some countries of the world to agricultural affluence must be evaluated, as well as the policies and practices that have contributed to land degradation and decline in productivity in other countries. We need to look at current concerns, and the urgent need to develop new paradigms for managing soil resources, as proposed by Sanchez (1994) for example, that will carry us through the next few decades. Some of the valid arguments of the past now have little validity in the face of contemporary environmental degradation. New concepts must be defined, the research gaps identified, and the needs that will enable our institutions to meet the challenges and requirements of the next 20 years must be indicated.
Land degradation results from mismanagement of land and thus deals with two interlocking, complex systems: the natural ecosystem and the human social system. Interactions between the two systems determine the success or failure of resource management programs. To avert the catastrophe resulting from land degradation, which threatens many parts of the world, the following concepts from Eswaran and Dumanski (1994) are relevant:
The thrust of a new agenda for resource assessment and monitoring with respect to land degradation (including desertification), has several components. It must be stressed that any research and development activity should be in the larger context of the ecosystem as addressed by Sanchez (1994) and Greenland et al. (1994).
Components of a national strategy to address land degradation (and desertification) comprise:
A 'scale-sensitive information system' or database on land degradation must be made available for the vertical network of decision-makers to enable them to make effective policies concerning the use and management of resources. Decision-makers at all levels of society should be able to participate in the design and implementation of any tool that affects the social, economic, and ecological well-being. This ensures successful implementation of the program.
Agenda 21 (UNCED, 1992, Chapter 12) emphasizes land degradation through desertification, and the international community, particularly through UN organizations, has launched several activities to address it. Other aspects of land degradation only receive a casual mention in Agenda 21, and are briefly considered in Chapter 10 under the general heading, "Integrated Approach to the Planning and Management of Land Resources". In this sense the problem of land degradation has been diluted and as such has not received the global attention that it deserves. Though the stated objective in Agenda 21 is, "to strengthen regional and global systematic observation networks linked to the development of national systems for the observation of land degradation and desertification caused both by climate fluctuations and by human impact, and to identify priority areas for action", we believe that we have yet to mobilize the soil science community to develop a proactive programme in this area. Land degradation remains a serious global threat but the science concerning it contains both myths and facts. The debate is perpetuated by confusion, misunderstanding, and misinterpretation of the available information. Important challenges are:
There is an urgent need to address these issues through a multi-disciplinary approach, but the most urgent need is to develop an objective, quantifiable, and precise concept based on scientific principles.
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