Monday, July 21, 2014
Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 2.
This is the second part of a much longer article published in the journal Science Progress, and which may be found here: http://stl.publisher.ingentaconnect.com/content/stl/sciprg/2014/00000097/00000002/art00001
3. A Preliminary Overview of Land Degradation and Soil Degradation.
Land degradation is a phenomenon that is becoming more severe in various regions of the world. Remote sensing measurements (http://www.isric.org/projects/land-degradation-assessment-drylands-glada) indicate that more than 20% of all cultivated areas, 30% of forests and 10% of grasslands are undergoing degradation3,4. Land degradation and desertification are thought to affect 2.6 billion people in more than one hundred nations, spanning in excess of one third of the land surface of the Earth5. The global scale of these issues was reinforced at the United Nations Convention to Combat Desertification, the Convention on Biodiversity, the Kyoto protocol on global climate change and the millennium development goal6, all of which are worsened by the activities of humans. Various inappropriate uses of land may cause soil, water and vegetative cover to become degraded, with the loss of both soil and the biological diversity of flora, with impacts on the structure and functions of ecosystems7. Once land has become degraded, it is more vulnerable to the effects of climate change, particularly rising temperatures and droughts of greater severity. The entire regional environment is encompassed by the term land degradation; however, individual aspects of soils, water resources (surface, ground), forests (woodlands), grasslands (rangelands), croplands (rainfed, irrigated) and biodiversity (animals, vegetative cover, soil) are implicit here8. Land degradation is a complex process, and involves a number of interactive amendments in the properties of the soil and vegetation – being physical, chemical and biological, in their nature.
Hence, the definition of land degradation varies from one region to another, according to the emphasis on particular topics, but the effect is most severe in drylands, and thus the 40% of the earth’s surface that contains them9. It has been estimated that around 73% of rangelands in dryland areas, 47% of marginal rain-fed croplands and a significant percentage of irrigated croplands10 have been degraded. 20% of the world’s pastures and rangelands are degraded through overgrazing, and it is estimated that, through erosion and both chemical and physical damage, some 65% of agricultural land in Africa is degraded, along with 31% of the continent’s pasture lands and 19% of its forests and woodlands10. Overgrazing has primarily been brought culpable for land degradation in Africa, i.e. human impacts, but more recent thinking is that climatic factors are those most important - particularly rainfall variability and long-term drought10. It is in Sub-Saharan Africa that land degradation is most extensive, where it impacts on some 20-50% of the land and therefore affects the daily lives of well above 200 million people7.
The definition of land degradation is the reduction in the capacity of the land to provide ecosystem goods and services and assure its functions over a period of time for the beneficiaries of these11. Land degradation is particularly significant in dryland regions, where large areas and populations are impacted upon. An expanding population, and the migration of large numbers of people into drylands during long wet periods tend to maroon significant numbers there in dry periods. Alternative uses of land, e.g. the introduction of irrigated and non-irrigated cash crops, and the use of water for industrial and urban purposes, at the expense of rural agricultural producers, tend to disrupt traditional production chains in dryland regions. Indeed, entire production systems may break down if these effects are not mitigated or compensated for. By removing protective cover, deep ploughing, heavy grazing and deforestation, the soil is left particularly vulnerable to wind erosion when droughts are severe and protracted.
Overgrazing prevents or delays the regrowth of vegetation, or favours only unpalatable shrubs, especially during long droughts or close to water points. Land degradation may cause a reduction in the productivity of land with associated socio-economic difficulties, e.g. lack of certainty over food security, migration of populations, and that ecosystems may be incompletely developed and damaged. It has been estimated that, worldwide, around $40 billion is lost annually to land degradation - if the embodied costs of increased fertilizer use, and the loss of biodiversity and of unique landscapes were accounted for, this figure would undoubtedly be far greater. To reclaim degraded land is very costly, and if severely degraded, it may no longer provide an essential range of ecosystem functions and services, resulting in a loss of the goods and manifold additional potential environmental, social, economic and non-material bestowals that are necessary for the maintenance and development of society12. There is, however, a confusing use of terminology, caused in part by the jargon of different disciplines, but which obfuscates the compilation of an overview of this broad subject, which aims to compare “like with like”, not “chalk with cheese”.
“Land” may be understood11 to refer to an ecosystem, and to include land, landscape, terrain, vegetation, water, and climate, while “soil” is a specific entity and a component of land. Degradation or desertification of land refers to an irreversible decline in its “biological potential”: a term which, in itself, resists definition due to its dependence upon a multitude of interacting factors. Land degradation has no single and simply measured marker, but rather the term points to the fact that one of the land resources (soil, water, vegetation, rocks, air, climate, relief) has altered disadvantageously. As an example, a landslide may be seen as a visible process of land degradation, but the land may eventually recover its productivity. Indeed, the “scars” from old landslides may prove more productive than the neighbouring land. According to the UN/FAO definition13, the term “land degradation” generally signifies the temporary or permanent decline in the productive capacity of the land. Another definition of land degradation is: "The aggregate diminution of the productive potential of the land, including its major uses (rain-fed, arable, irrigated, rangeland, forest), its farming systems (e.g. smallholder subsistence) and its value as an economic resource." The broader connection between degradation (usually a result of land use practices) and the consequences of it in terms of land use is key to most published definitions of land degradation. The use of the term land degradation, in contrast to soil degradation, allows the bigger picture to be seen: to incorporate natural resources, such as climate, water, landforms and vegetation. This definition comprises the productivity of grassland and forest resources, and also the productivity of cropland. Since under different circumstances, land degradation may be reversible or irreversible, some definitions draw a distinction between the two cases.
Given sufficient time, all degradation is reversible, and hence the definition is a matter both of the particular focus and the timescale over which the effect is being considered. Soil erosion is a principal cause of land degradation, but it must be acknowledged that there are additional or simultaneous influences - e.g. lowering of the water table and deforestation - that may impair the productive capacity of cropland, rangeland and forests. Hence, the "productive capacity of land" cannot be determined from any one measure, and instead land degradation is estimated11 using “indicators”, which are potential signals that land degradation has taken place, rather than observations of degradation per se. For example, an “indicator” that land degradation is occurring further upslope may be provided by the accumulation of sediment further down. An indication of a reduction in the quality of soil might be falling crop yields, which may be a result of soil degradation and land degradation. Since the soil mediates collectively (holistically) many essential processes involving vegetation growth, overland flow of water, infiltration, land use and land management, its quality is a prime indicator of land degradation – therefore, when soil is degraded the land is too. Evidence from the soil (mainly soil degradation) and from plants growing on the soil (soil productivity) are prime indicators of land degradation.
In addition, the definitions of “dryland” are variable, which serves further to confound the situation. In regard to the severity of land degradation, two basic schools of thought have emerged. Economists take the view that if the situation were really as serious as others claim, market forces would have resolved it by now, and in support of this, it is argued that land managers (e.g. farmers) would not let their land degrade to a degree that a loss of their profits would ensue. However, this does not take account of an imminent failing supply of plentiful cheap oil14 and potentially one of phosphorus-based fertilizer15 too: commodities without which the current agricultural situation could not exist. In many instances, it is only through such inputs that accepted yields can be maintained, and if a farmer decides to turn his industrial farm into an organic farm, he must initially suffer reduced crop yields. Those “others”, tending to come from backgrounds of ecology, soil science and agronomy, believe that there is a serious threat to feeding the growing global population posed by land degradation, in terms of reduced biomass yields and by a compromised environment.
As a consequence of the variation in definitions and terminologies employed by different workers, the statistics pertaining to both the extent of land degradation and its rate of advance, vary considerably. In addition, statements are often made, particularly in the media, such as, “Globally, we are losing 10 million hectares of fertile soil each year. That is 30 soccer fields per minute...”, or “75 billion tonnes of soil, the equivalent of nearly 10 million hectares of arable land, is lost to erosion, waterlogging and salination; another 20 million hectares is abandoned because its soil quality has been degraded.” It is hard to know what precisely is being described here. A direct translation of a mass of soil to a land area implies that there is a given, average depth of soil physically removed, waterlogged or salinized, but as we shall see, the actual and global situation is less straightforward. If “soil” is being used as a synonym for “arable land”, the loss of 75 billion tonnes would accord with an average soil depth of, 75 x 109 t/10 x 106 ha = 7,500 t/ha; assuming an average soil density of 1.4 g/cm3, this accords with a volume of 5,357 m3. This is distributed over one ha = 10,000 m2 = 1 x 108 cm2, and so we have 5,357 m3 x 1 x 106 (cm3/m3)/1 x 108 cm2/ha = 53.6 cm (i.e. half a metre), which does not seem realistic.
It is probably neither accurate nor appropriate to compare the mass of soil that is thought lost to erosion (blown or washed away) with an area directly of land that has become unproductive. It appears most likely that the “lost” 10 million hectares should not be compared directly with the mass of soil that is allegedly “lost”: the latter being a global phenomenon. Thus, the global area of arable land amounts to 1,387 million ha, and if 75 billion tonnes of soil were being lost over this expanse, it would equate with a soil depth of: (75 x 109 t/1.4 g/cm3) x 1 x 106 (cm3/m3)/1,387 x 106 ha x 1 x 108 cm2/ha = 0.39 cm (4 mm), which might appear more reasonable, while another estimate, that the loss over the world’s arable land is 24 billion tonnes would accord with an average loss of a little over 1 mm, which might be imperceptible. However, as we discuss later, the rate of “loss” of soil occurs quite variously according to climate and location (and also timescale), across the globe, and not all the soil that is moved over an area, is actually removed from it.
4. Degradation of soil.
4.1 Mechanisms of Soil Degradation
Soil degradation is a principal cause of land degradation, and soils may be degraded via several identifiable and different mechanisms, as we now outline under the following headings13.
(1) Water erosion.
This is the removal of soil particles by the physical action of water (Figure 5). This is normally manifested as sheet erosion (a more or less uniform removal of a thin layer of topsoil), rill erosion (small channels in the field) or gully erosion (large channels, similar to incised rivers). One highly significant aspect of water erosion is that it is the finer and more fertile fraction of the soil that is removed selectively.
(2) Wind erosion.
This is where soil particles are physically removed by the action of wind, and it is fine to medium sized sand particles that are most affected. This normally is sheet erosion, involving the removal of thin layers of soil, but hollows and other features can also be sculpted.
(3) Loss of soil fertility.
This may be defined as the degradation of the physical, biological and chemical properties of soil, which leads to a loss in its productivity. Additional factors include: (a) the loss of SOM, which reduces the biological activity of soil; (b) a further consequence of declining SOM is the deterioration in the physical properties of soil, e.g. its structure, degree of aeration and ability to hold water and to drain effectively; (c) soil nutrient levels that are vital for the healthy growth of plants may become deficient, or rise to toxic amounts; (d) substances that are toxic substances may accumulate in the soil – e.g. from pollution, or the over-application of fertilisers.
When the level of groundwater close to the soil surface rises, or surface water is not adequately drained, the soil may become waterlogged. In this case, the root zone becomes saturated by water, typically resulting in an oxygen deficiency.
(5) Accumulation of salts.
When there is an increase in the concentration of salt in the soil water solution, the effect is termed salinization. In contrast, when the number of sodium cations (Na+) on the soil particles increases, the effect is called sodication, and the resulting soil is termed to be “sodic”. It is poor irrigation management which, more often than not, is to blame for salinization while sodication tends to be a naturally occurring feature, especially in regions with a fluctuating water table.
(6) “Soil burial”'.
This is also known as sedimentation, and may occur when fertile soil is buried under less fertile sediments (by flooding); or it can be a consequence of winds which blow sand over fertile grazing lands, or catastrophic events, e.g. eruptions from volcanoes.
The above are the primary causes of land degradation, but we may also note:
(7) Water table lowering.
This is often a result of groundwater extraction at a greater rate than the water table can be recharged by natural processes.
(8) Loss of vegetative cover.
Leaving soil bare is a principal route to erosion and degradation. By applying vegetative cover, the soil is protected from erosion both by wind and by water. In addition, organic material is supplied, which assists in maintaining sufficient nutrients to serve healthy plant growth. The roots of plants roots contribute to a good structure of the soil and aid water infiltration.
(9) Increased stoniness and rockiness.
When the levels of soil erosion are extreme, stones and rock may be left bare of soil and exposed. While it is helpful to identify the different kinds of soil degradation, as we have under the foregoing headings, it is a fact that there is frequently a synergy between them. Thus we may note that at the front of a storm there are strong winds, and so wind erosion and water erosion may occur simultaneously. In a “thin end of the wedge” manner, once a soil has undergone some degree of erosion, by whatever means, it is left more vulnerable to further and more rapid erosion than another soil that has suffered a lesser degree of erosion, although the latter may be similar in all other respects. The level of SOM is a pronounced indicator of erosion susceptibility, and soils in which the concentration has become less than ca 2% are particularly prone to erosion, because the soil aggregates are less tightly bound, meaning a greater likelihood of individual soil particles being removed. Slopes that are steep, regions where the rainfall is heavy and how much SOM there is, are significant determinants as to whether and how fast degradation of the soil will occur. While less severe types of erosion can be reversed through changes in management practices (e.g. growing cover crops), at a more serious level and over a long time, the process is effectively irreversible, because any remedial strategy must be measured against the relatively slow processes by which soils are created, taking perhaps hundreds or thousands of years (particularly in cold dry climates) to form a mere few centimetres of soil.
4.2 Human influences
Human activities can influence profoundly the evolution of soils. The construction of “tarmac” roads increases the area of impermeable surface, causing streaming and ground loss. Soil erosion is accelerated by particular farming practices, including an increase of the size of fields, with the removal of hedges and ditches, while meadows are converted to ploughed fields, and farming spring cultures (e.g. sunflower, corn, beet) is on the increase, leaving the ground naked over the winter, when conditions of rain and wind are at their most forceful. Unsustainable methods of modern agriculture are the single major contributor to global soil erosion. When agricultural lands are tilled, their soil is broken into finer particles, a problem that has been accentuated through the use of mechanized farm machinery that permits deep ploughing16. The latter increases the area of soil that is exposed to water erosion. Mono-cropping, farming on steep slopes, the use of pesticides and artificial fertilizers (which destroy the soil food web and hence those microbes whose exudates bind soil together), row-cropping, and surface irrigation methods all take a further toll on the soil and contribute to its erosion. The loss of nutrients from soils is quite specific and it is in the finer soil that the loss, e.g. of phosphorus, occurs more greatly. Tillage increases wind erosion rates, because the exposed soil becomes dehydrated and breaks up into smaller particles that are easily swept away by the wind. The latter situation is worsened when trees are removed from agricultural fields, so that the wind can travel over greater distances and build-up to higher speeds. The rates of erosion are increased by overgrazing, which both removes vegetative cover and causes the soil to become compacted.
When a forest is undisturbed, its floor soil is covered by layers of leaf litter and humus, which between them form a resisting shield against the impact of raindrops. Both layers are porous and allow rainwater to percolate into the soil beneath them, rather than washing over the surface to form runoff. Before they impact the ground, the raindrops are reduced in their kinetic energy by striking the foliage (canopy) above. Having fallen through around 8 metres (26 feet) the raindrops achieve their terminal velocity, and since forest canopies are usually higher than this, the terminal velocity may again be met lower down, even after striking the canopy. However, the intact forest floor, with its layers of leaf litter and organic matter, is still able to absorb the impact of the rainfall and so it is this, principally, that resists soil erosion more than the overlying foliage17. The rates of erosion are increased when deforestation takes place, because the humus and litter layers are lost from the soil surface, thus exposing it to rainfall. Through the loss of the vegetative cover, and severe soil compaction from logging equipment, the process of erosion is exacerbated. The occurrence of fires can lead to appreciable further erosion, especially if followed by heavy rainfall. The slash and burn method, as applied to tropical forests, is one of the main contributors to global soil loss through erosion, which has rendered complete regions of some counties unproductive. The Madagascar high central plateau has become devoid of vegetation, which amounts to around 10% of the nation’s land area, where there are furrows caused by gully erosion more than 50 metres in depth and one kilometre in length.
4.4 Urbanization and road building.
The process of urbanization denudes the land of vegetative cover, which changes patterns of drainage, and the construction phase itself causes the soil to become compacted. The application of a layer of asphalt or concrete, which is impermeable, both raises the volume of surface runoff water and allows faster wind speeds to be developed over the surface. Both these effects enhance the degree of soil erosion, and as an additional consequence, the sediment in runoff water from urban environments is frequently tainted with fuel, oil, and other toxic materials. Neighbouring watersheds are impacted upon because the volume and rate of water that flows through them is changed, and they become filled with chemically polluted sedimentation. In addition to degrading the land that it flows over, the increased flow of water through local waterways also makes the rate of bank erosion more severe18.