Ploughed field, Adinkerke, Belgium
Source: J Herbauts
INTRODUCTION |
Soil is often seen as an inert medium, merely a support for human activities. However, soil is more than that: it is a dynamic, living system which comprises a matrix of organic and mineral constituents enclosing a network of voids and pores which contain liquids and gases. The structural arrangements of these components determine the main soil types in Europe; 320 different major soil types can be identified in the EU (CEC, 1985). In addition, soils contain populations of biota ranging from bacteria and fungi to worms and rodents (Figure 7.1); the chemical, physical and biological properties of soils vary both vertically and horizontally at a variety of scales. Soil is formed from the combined effects of climate, vegetation, soil organisms and time on rocks and parent materials (Jenny, 1941). So, any alteration in one of these components may result in changes in the soils. Soil genesis is a long process the formation of a layer of 30 cm of soil takes from 1000 to 10 000 years (Häberli et al, 1991). It is formed so slowly that soil can be considered as a non-renewable resource.
Soil is a complex system where crucial biogeochemical processes occur. In the top 30 cm of one hectare of soil, there are on average 25 tonnes of soil organisms, that is: 10 tonnes of bacteria and actinomycetes, 10 tonnes of fungi, 4 tonnes of earthworms and 1 tonne of other soil organisms such as springtails, mites, isopods, spiders, coleoptera, snails, mice, etc. (Blum, 1988). Earthworms, alone, can represent from 50 to 75 per cent of the total weight of animals in arable soils; in one hectare, 18 to 40 tonnes (that is a layer 1 to 5 mm deep) of soil is ingested each year by the earthworms and passed onto the surface. The number of worms varies with the soil and its management. The soil fauna and flora recycle organic matter to form humus and mix it with the mineral material; they also create and maintain the airways within the soil that are essential to plant roots; some species found in soil control others that are pests to crops.
The carbon stored in soils is nearly three times that in the above-ground biomass and approximately double that in the atmosphere (Eswaran et al, 1993). Replenishing carbon is a slow process not easily achieved; soil protection should therefore, apart from its other benefits, assist in maintaining this reservoir of soil carbon (see also Chapter 27).
In contrast with the earlier concern for the atmosphere and hydrosphere, the need to protect the soil has been appreciated only more recently. Soil is static and thus acts as an enormous receptacle for any type of pollutant which can be mobilised under different triggers (such as acidification) and finally released to the environment. Since the residence time of these substances is far longer in the soil than in air or water, effects are often hidden for a long time. Soil-related problems are also site-specific, which makes any attempt at generalisation very difficult. The unique capability of the soil for recycling biodegradable waste, and the delay until effects are usually detected, has led to complacency, and concern for the well-being of soils has been slight. But soil is necessary for the growth of crops of food, fibre and timber, and it is an essential component of all terrestrial ecosystems. Consequently, the role of the soil is of vital importance to humankind and the maintenance of a healthy natural environment. The 1972 European Soil Charter (Council of Europe, 1972) recognised for the first time that any biological, physical or chemical degradation of soil should therefore be of primary concern and that appropriate measures to protect soils should be implemented without delay. Unlike air and water, soil can be owned as personal property, which renders soil conservation or protection policies difficult to enforce and requires acceptance by landowners and managers. The data and information sources used in this chapter are described in Box 7A.
Soil functions |
Soil protection policies currently being discussed in Europe emphasise consideration of the functions which soils perform, rather than the uses of soils. This represents a fundamental change in approach since it seeks to identify any possible conflicts between uses and forms a sound basis for the assessment of the impact of a given use on particular soils. Soils have six basic functions: basis for biomass production; filtering; buffering and transformation medium; habitat and gene reservoir; foundation; raw material and a historical medium (Blum, 1990). The first three are primarily ecological while the last three highlight the technical/industrial, socio-economic and cultural aspects.
Most production of food, fodder or renewable raw materials is soil-related, with soil providing nutrients, air, water, and a medium in which plant roots can penetrate. Serious degradation of the soil by human activities may cause a loss of production of food and wood. Erosion and compaction are the most important threats to a sustainable agriculture and in (1989­90) affected 15 to 20 per cent of the European land area (Oldeman et al, 1991). Production losses due to these degradation processes may amount to 30 per cent or more yield reduction (as detailed below in this chapter). In Northern Europe, soil acidification may be the main factor impacting on timber production.
It is only in the last few decades that the full significance of these soil functions has properly been recognised. They enable soils to deal with harmful substances, preventing them from reaching the groundwater or the food-chain. Substances can be mechanically filtered, adsorbed or precipitated, and, for organic substances in particular, even decomposed and transformed. This is crucial since about 65 per cent of the inhabitants of Europe depend on groundwater for their drinking water (see Chapter 5). Soil organic matter is an important factor in determining the buffer capacity of soil.
The soil buffers chemical substances as well as temperature; external inputs of chemicals, such as acidifying compounds, are buffered by the basic cations (of sodium, calcium, potassium and magnesium) present in the soil and derived from the weathering of clay minerals. The soil thus acts as a sink in which pollutants accumulate until the buffer capacity is depleted. In optimal conditions, more than 99 per cent of pesticides are transformed into non-toxic compounds within the plough layer of arable soils (Canton et al, 1991; Boesten and van der Linden, 1991). Nevertheless, although small, the remaining part which is not decomposed is sufficient to threaten drinking water in areas with high inputs of pesticides (see Chapters 5 and 22). The presence of other substances, such as copper, may, by reducing soil biological activity, indirectly hinder pesticide degradation; this is an important problem in areas of vine cultivation. Moreover, the buffer function changes with time and, when the buffer capacity is depleted, the soil can turn into a source of chemicals, and pollutants can begin to leach into the groundwater. The metaphor 'chemical time bomb' is being used for a chain of events resulting in the delayed and sudden occurrence of harmful effects due to the mobilisation of chemicals stored in soils and sediments in response to slow alterations of the environment (Stigliani et al, 1991). Soil microorganisms are responsible for the decomposition of organic matter and the transformation of other substances, such as sulphates and nitrates. Environmental changes may reduce substantially the capacity of the soil to withhold pollutants; consequently, soil is a central factor in determining critical loads for natural ecosystems.
Soil provides habitats for numerous organisms and microorganisms (see Figure 7.1); it is also a gene reserve. Relatively little is known about the biology of soils, and many soils under agriculture are currently being subjected to new and alien forms of management. Decline of the quality of soils will generally contribute to the decline in biodiversity (Blum and Prieur in Council of Europe, 1990). Loss of biological activity and of animal species may be caused by removal or burning of vegetation, by excessive application of fertilisers and/or biocides or by acidification from atmospheric pollution. Frequently, biological soil degradation is associated with physical and chemical soil degradation. For example, compaction leading to reduced aeration of the soil may result in a decrease in activity of earthworms, which in turn will cause further degradation of soil structure. Acidification can lead to a loss of cast-forming worms and mycorrhizae. Unfortunately, the degree of biological soil degradation is difficult to estimate. In view of its strong links with other types of degradation it may be assumed that biological degradation is occurring whenever other types of soil degradation are significant, especially erosion, compaction and chemical pollution.
Soil is also a physical medium for the development of infrastructure houses, industrial premises, roads, recreational and leisure facilities and for waste disposal. Today, built-up areas cover about 2 per cent of the total land area in Europe (see Table 5.5), ranging from 0.5 per cent in Iceland (1990) up to 12 per cent in Hungary, 13 per cent in Italy and 14 per cent in The Netherlands (Statistical Compendium), urban areas alone covering 1 per cent of the total land surface of Europe. In many European countries the built-up area has increased by 25 to 75 per cent during the period 195080. Soil sealing through urbanisation dominates in the more densely populated regions and major industrial areas of Western Europe; in West Germany, for instance, about 120 hectares of soil are lost every day (Statistical Compendium). This results in an increase of impermeable surface and a decrease of water infiltration into the soil. As soil sealing is bound to increase in the next century in Europe, a number of countries have taken the view that the best agricultural soils should not be used for building or infrastructure developments and there are several examples of new roads being re-routed to avoid fertile soils.
The ground is a source of raw materials such as clays, gravels, sands and minerals, as well as fuels such as peat. The land used for opencast mining is estimated at 0.05 to 0.1 per cent of the whole of Europe. However, mining activities can have an important impact locally. Lignite mines of the West German border region (North Rhine Westphalia) are the largest opencast mining operations in Western Europe. By 1990, about 250 km2 of land had been excavated for lignite extraction (see Chapter 20). In the Czech Republic, the opencast mining of brown coal is concentrated in a few areas and covers almost 10 000 hectares.
Soil is a historical medium, concealing archaeological artefacts and palaeontological materials which are a unique source of historical information (see Chapter 8).
Functions may interact in space or time, creating conflicts for different uses of the land. Interactions in space may be due to agricultural practices which have an impact not only on agricultural soils but also on adjacent non-agricultural soils and groundwater. Drift of pesticides may affect sensitive species, and the transport of fertilisers by wind or water from agricultural fields to bordering nature reserves can lead to eutrophication of nutrient-poor ecosystems. The supply of nutrients to these ecosystems usually leads to an increase in biomass and a significant decrease in the number of species or the loss of rare species. On the other hand, the deterioration of natural ecosystems may lead to a decrease of their water-holding capacity and an increase of soil erosion. This can threaten agricultural soils by deposition of eroded soil, or through increased runoff. If runoff increases, infiltration decreases and the replenishment of groundwater is not ensured, thus threatening the ultimate replenishment of the drinking water reservoir. Soil functions may also change in time; for instance, agricultural land may be abandoned or deliberately changed to semi-natural land as a result of set-aside policy.
For centuries, all soil functions were maintained without much difficulty. Problems arose at the beginning of this century, when increasing development started to conflict with the ecological functions of the soil. Expansion of settlements and infrastructure, especially for industries and transport, waste dumping, raw materials mining and intensive agriculture, exert pressure on soil. Sometimes these modifications are positive and enhance certain soil functions (the construction of terraces, for example, has led to reductions in the natural rates of erosion in Southern Europe), but more often a deterioration of soil characteristics takes place as a result of human activities, leading to a degradation of one or more soil functions.
Impairment of any soil function diminishes soil quality and value, and the capacity to provide the basic requirements to support ecosystems. Since degradation of soil functions results from a pattern of competition and overutilisation, there is a need to strike a balance among all the interests concerned and to harmonise soil use at regional level, to enable the essential soil functions outlined above to be performed concurrently. Thus, soil suitability/vulnerability assessment is the logical forerunner to landuse policy making.
Soils of Europe |
Click here to view photos of the following soil types.
A glance at any landscape in Europe shows that even a small area may have a wide variety of soils, depending on the local climate, geology, relief, altitude, exposure, slope, vegetation, hydrology and human influence. As a result, landuse, soil management and degradation hazards differ considerably within the European context. Thousands of years of human activity have marked the soils throughout Europe. Yet these soils can be distinguished by differences in their most important features: their organic and inorganic components, their biological activity and their structure. In Map 7.1, which shows the four most important soil classes in Europe, soil types have been grouped in classes of broadly similar vulnerability to different degradation processes (Fraters, 1994).
The main soil groupings are briefly described below. The soil names used here do not belong to any formal nomenclature, and therefore, to allow for easier comparison, corresponding names from the FAO classification are given in italics (FAO, 1974; 1981).
Well-drained soils cover 44 per cent of the European land area. They occur generally in an undulating relief. Their formation was conditioned by climate and natural vegetation, hence they show a 'zonal' distribution from north to south in relation to rainfall and temperature variations. The well-drained soils differ from each other by their texture, organic matter content and acidity (pH) from which five soil categories can be identified.
From an agricultural viewpoint, the 'black earths', or chernozems, are of major importance.They are characterised by a thick humus-rich topsoil (50 to 100 cm deep), overlaying a lighter horizon containing calcium carbonate. They occur in the continental climate zone of the Central European plain, the steppe, in a belt extending from Poland and Hungary to the Urals, and cover about 9 per cent of Europe's land area. They are very fertile and suitable for arable farming. However, erratic rainfall often limits yields. In the Russian Federation and Ukraine, black earths are notable for extensive cereal production.
The sandy soils are mainly podzols.They develop under the influence of a slowly decomposing acid humus which moves through the soil with percolating rainwater and precipitates in the subsurface in combination with iron and aluminium oxides. This accumulation layer limits rooting depth. In Europe, sandy soils cover over 9 per cent of the land, mainly in Northern Europe where they occur under forest. In Western Europe they are currently used intensively for agriculture with the application of fertilisers to overcome the inherent low fertility. The sandy soils also comprise arenosols, which are confined to limited areas, mainly in Poland, Spain and the UK.
Acid loamy soils are represented mainly by the podzoluvisols. They occur most extensively under the cold continental conditions of the central taiga of the north and the centre of the Russian Federation, and cover about 14 per cent of the land area. In the northern latitudes forestry is the dominant landuse. In the more central areas, grassland and crops occupy a larger portion of the land. Low fertility and a short growing season are limitations to high production.
Non-acid loamy soils
Non-acid loamy soils are widespread in Western and Central Europe, and cover about 8 per cent of the land. They comprise the orthic luvisols on loessic parent materials, which are among the most productive agricultural areas. In more northern latitudes, in Ireland and Sweden, they also include cambisols, which are used for livestock production, and the haplic and luvic phaeozems.
In the Mediterranean countries, red and reddish-brown soils of heavy texture have developed over limestone, the chromic cambisols and the chromic luvisols. Despite their high clay content, these soils are well drained because of their texture and the presence of a permeable subsoil. They are associated in level areas and in depressions with heavy dark clay soils, the vertisols, which, although fertile, present management problems because of their unfavourable physical properties. The luvisols are sensitive to erosion. They are used for vineyards, olives and citrus. The vertisols are preferably used for the production of cereals. Clayey soils cover about 4 per cent of Europe's land area.
These soils or lithosols, cambisols and rendzinas cover 30 per cent of the total land area of Europe, and usually occur on steep slopes. They are predominant in the major mountain and hill ranges of Europe, such as the Pyrenees, the French Massif Central, the Alps, the Apennines, the Carpathians, the Scandinavian mountain ranges and the Caucasus. These soils are not suitable for intensive arable farming and use of heavy machinery. They are used mainly for non-intensive grazing and wood production. Among them, the acid, shallow and stony soils which occur on non-calcareous parent material (such as in Scandinavia and Germany) are more vulnerable to acidification than the non-acid shallow and stony soils on calcareous bedrock, for example in France, Greece, Italy and Spain.
In specific areas, such as the eastern part of Southern Europe, high evaporation results in soil salinisation, which leads to the formation of solonchaks, with excess of salts, or solonetz, with excess of sodium. The semi-arid and salt-affected soils cover 9 per cent of Europe, mostly in the southern part of the Russian Federation, Ukraine and Romania. The lack of rainfall combined with the excess of salt or sodium permits only non-intensive agriculture. The semi-arid soils (xerosols and kastanozems) do have potential for more intensive agriculture, with irrigation.
These soils represent 17 per cent of the land surface of Europe. The imperfect drainage can be due to surface water stagnation (for example, with gleyic luvisols and planosols) or to fluctuating groundwater (for example, with gleysols and fluvisols). Wet soils occur in level relief and in areas with high groundwater tables. They are very extensive in the northern part of Europe (the Russian Federation, Scotland and Ireland), where they occur in association with organic soils. Imperfectly drained soils due to surface waterlogging are extensive in England and in Germany. Organic soils peat and muck are confined mainly to the boreal parts of the Russian Federation, Finland, Ireland and the uplands of Britain. Organic soils or histosols ­ are developed under the influence of high water tables and low temperatures. Remnants of former extensive peat areas occur elsewhere, for instance in The Netherlands and northern Germany.
The dominant feature of these soils is their prolonged waterlogging, which strongly influences their use and management. Depending on the climate under which they occur, they are used either for extensive wood production and/or grazing (such as in the boreal parts of the Russian Federation and Finland), or, after reclamation (drainage), for arable cropping, dairy farming or horticulture (in Western Europe).
Click here to view photos of the above soil types.
Soil vulnerability |
As explained in the introduction to this chapter, the soils in Europe do not all respond similarly to soil degradation processes. The vulnerability of soils to certain contaminants or degradation processes depends upon those characteristics which enable soils to resist alteration and which maintain the biological functions expected of a soil (Bridges, 1991). Soil vulnerability is then the capacity for the soil system to be harmed in one or more of its ecological functions.
To assess the degree of soil vulnerability, the following soil-environmental parameters have been identified as critical for future soil monitoring programmes: pH, clay content, clay mineral type, texture, and organic matter content. These criteria are readily available and are combined with environmental features such as the site relief, total depth of soil, depth of groundwater table, water regime and length of growing season. Vulnerability must be specified with respect to agents, causes and effects (Desaules, 1991). The classification adopted in Map 7.1 and described in the preceding section can facilitate a general understanding of the areas of Europe which are most vulnerable to a certain type of soil degradation (Fraters, 1994).
A qualitative appraisal of the vulnerability of the main soil types to the main soil threats erosion, soil acidification, soil pollution, compaction, organic matter losses, salinisation and waterlogging has been developed and is presented in Table 7.1 (Fraters, 1994). A rough estimation of the area in Europe affected by the main soil threats is given in Table 7.2. The vulnerability is given with respect to soil characteristics assuming that, unless explicitly stated, there are no differences in land characteristics. Land comprises the total physical environment, including climate, relief, soils, hydrology and vegetation. All these characteristics determine whether or not degradation occurs.
Table 7.1 shows that sandy soils are much more vulnerable to soil pollution than 'black earths'. Pesticides are adsorbed by the organic matters of the 'black earths' and degradation is usually fast in the fertile topsoil. Heavy metals too are adsorbed strongly by the soil, which prevents their uptake by plants or soil fauna. In contrast to 'black earths', sandy soils have mostly acid, humus-poor topsoil with limited capacity for adsorption of pesticides and heavy metals. These contaminants are therefore more available for uptake by soil fauna and flora and for leaching from the topsoil than 'black earths' (see Chapter 5). Consequently in sandy soils heavy metals and pesticides threaten the groundwater as well as the soil flora and fauna. Loamy and clayey soils have an intermediate position between the sandy soils and the 'black earths'. The increase in adsorption capacity from sandy to clayey soils, due to the increase in clay content, often accompanied by increasing organic matter content, means increasing buffer capacity and decreasing vulnerability. However, soils with a relatively high concentration of heavy metals may become a potential threat if soil characteristics are changed, for instance if soils are acidified. In that case, the buffer capacity of the soil decreases and heavy metals may be released.
PROBLEMS AND THREATS |
The soil functions described above are at risk because of the misuse of soil by humans. Soil degradation occurs when human-induced phenomena lower the current and/or future capacity of the soil to support human life (Van Lynden, 1994). Soils are generally resilient within certain limits, but outside those limits the soil will not recover naturally even if the load or stress is removed. In this section, emphasis is placed on the most severe soil degradation processes in Europe in terms of their irreversibility: soil erosion, acidification, pollution (by heavy metals, pesticides and other organic contaminants, excess of nitrates and phosphates, artificial radionuclides) and soil compaction. The particular problems of sites contaminated by waste disposal and of derelict industrial land are also developed. Other important but less extensive threats to soil functions are organic matter loss, salinisation and waterlogging. For each of these, the causes, magnitude, impact on the soil functions and remedies are discussed.
Soil degradation processes are not mutually exclusive, operating in strictly separated compartments; interactions will often occur with one type of degradation leading to another. The combined effect of pollution and erosion can illustrate this where pollutants from upstream areas can be transported with eroded sediment, accumulating downstream where the sediment is deposited.
Soil erosion |
Badlands in Abruzzo, Italy |
Soil erosion consists of the removal of soil material by water or, to a lesser extent, by wind. It is a natural phenomenon but it has been accelerated by human activities. It is one of the most widespread types of soil degradation worldwide. Unlike other forms of degradation, erosion removes soil from one area and takes it to another (this may imply important clean-up costs in deposition areas). It may be caused by any human activities that expose the soil to the impact of raindrops or wind, or that increase the amount and speed of surface runoff water. Some farming practices, such as ploughing up-and-down slopes, removal of vegetative soil cover and/or hedgerows, increased field size (open fields), abandonment of terraces, late sowing of winter cereals, overstocking, poor crop management, untimely farming operations and compaction by heavy machinery, may have these effects. The same holds true for improper forestry practices. In the Mediterranean area, deforestation and overgrazing in the past, on well-drained loamy and clayey soils (see Map 7.1), have led to severe soil erosion and degraded vegetation forms such as maquis and garrigue. The occurrence of frequent fires and summer droughts on these systems has led to irreversible desertification (UNEP, 1987 and Box 7B). Tourism, sport and recreational activities (walking, skiing, mountain bikes, off-road vehicles, etc) can also be important causes of erosion in areas which are already fragile and need protection, such as mountain areas, forests and parks (see Chapter 25).
Soil erosion occurs mainly where land is used over-intensively, and the resulting losses are irreversible over time-scales of tens or hundreds of years. Soil erosion is increasing in Europe (Blum, 1990), but figures are not available on the amount of erosion which takes place each year in different areas. However, models are being developed to predict areas at risk (Box 7C and Map 7.2). Maps 7.3 and 7.4 show the occurrence of water and wind erosion, respectively. An estimated 115 million ha, or 12 per cent of Europe's total land area, is affected by water erosion. Forty-two million ha are affected by wind erosion, of which 2 per cent is severely affected. Soil erosion occurs almost anywhere in Europe on sloping land, but particularly in the Mediterranean region (Box 7D), in parts of Central and Eastern Europe and in Iceland, due to a combination of climatic factors, soil properties and grazing practices over many centuries. Iceland is subject to wind erosion, as is the southeastern European part of the Russian Federation in spite of the very different soils and climate (Jansson, 1988).
There is a variety of impacts of soil erosion, and the economicconsequences are considerable. Erosion by water usually affects crop production through a decrease in plant rooting depth, removal of plant nutrients and organic matter, and, sometimes, uprooting of plants and/or trees together with dissection of the terrain by rills and gullies. Erosion may cause serious loss of fertile topsoil at rates of several millimetres per year (Morgan, 1986). In the Russian Federation, it is estimated that the humus content of agricultural soils decreases each year by about 1 per cent, while on irrigated land, far higher rates of humus losses occur; long-term land productivity is endangered since the annual erosion rates exceed the rate of humus formation (Karavayeva et al, 1991). The water-holding capacity of the soil can also be lowered through erosion. Agricultural production losses may add up to 30 per cent of normal yields (Follet and Stewart, 1985). Costs in terms of the quantity of fertilisers and manure required to replace the nutrients and organic matter lost with the removed topsoil are difficult to assess and deserve more attention. Decreases in crop yields due to erosion are not always clearly visible, but the costs of amelioration required to maintain yield levels could give a good indication of the damage done (Morgan, 1980). Arable fields may also be damaged by erosion upstream through deposition of excessive stream sediment loads originating in eroding areas.
In the mountainous Alpine regions, soils are predominantly shallow and usually covered with forest or non-intensive grassland. In the 1970s, large areas were developed into ski-runs. Intensive use of these slopes in the 1970s and 1980s (the Alps receive some 100 million tourists every year see Chapter 25) has led to physical deterioration of the soils, principally by compaction. In addition, the loss of protective soil cover has resulted in erosion, and sometimes, with heavy rainfall, in landslides, causing deaths and devastation of buildings and houses.For example, in Val Pola (Italy) in 1987, a huge rock landslide occurred following a period of heavy rainfall killing 28 people (Agostoni et al, 1991).
Soil erosion may also threaten an ecosystem through the removal of the fertile topsoil, especially in areas where the vegetative cover of the soil has been damaged or removed. Soil material eroded from agricultural land can physically disturb natural ecosystems. The indirect (off-site) negative effects of erosion on the ecosystem include deposition of excessive amounts of sediments, possibly including pollutants (pesticides, fertilisers, heavy metals, etc) transported from upstream areas. In Southern Europe erosion is the major pathway for pollutant transport, while in Northern Europe leaching prevails on sandy and loamy soils. Deposition of sediments in waterbodies can have severe implications for aquatic life and human health.
Practically all soils on sloping land are vulnerable to erosion, but in particular sandy or silty soils are at risk (acid and non-acid loams on Map 7.1). Other factors like organic matter, infiltration rate, structure and surface roughness also play an important role, as well as external factors such as topography, climate, vegetation and management practices.
Old terraces in Peloponnisos, |
Since technological solutions to curb soil erosion abound, it is surprising that this type of soil degradation is still so widespread. One reason for this could be that other aspects of soil conservation socio-economic and ecological factors have been ignored for too long. An integrated approach is required to remedy this in which non-technological factors (population pressure, social structures, economy, ecological factors, etc) determine the most appropriate technical solution. A wide variety of possible technical solutions can be chosen, such as strip and alley cropping, rotation farming, contour ploughing, agro-forestry practices, adjusted stocking levels, mulching, use of cover crops, or construction of mechanical barriers such as terraces, banks and ditches. Though severe soil erosion is nearly always entirely irreversible, in less severe cases damage can still be prevented.
Soil acidification |
Soil acidification is a natural process, but it has been enhanced recently by human action through the emission of sulphur and nitrogen compounds from the combustion of fossil fuels and from industrial processes, subject to long-range transboundary dispersion (see Chapter 31). Acidification may also be caused by fertilisers and soil drainage. In Central and Western Europe, acidic deposition is by far the most important cause of soil acidification (de Vries and Breeuwsma, 1986). The Scandinavian countries are among the most severely affected areas in Europe, while the main sources of acidic emissions are the Northwestern and Central European countries. This well known discrepancy arises from the preferred track of low pressure systems over Europe giving rise to prevailing southwesterly winds over the north of the continent. Soil acidification is an important trigger for releasing cations such as iron, aluminium, calcium, magnesium and heavy metals (which are present in the soil in significant amounts but usually have little mobility). This has the effect of depleting the soil buffering capacity. Soils vary greatly in their ability to neutralise acidity; sandy soils have a low buffering capacity and thus only a small change in pH may be sufficient to change the soil from a sink to a source of pollutants. Six mechanisms contribute to soil acidification:
Soil acidification is not a visually obvious feature, but it is an important, irreversible threat to the vitality of ecosystems. In agriculture, the effects of natural soil acidification, acidic deposition and acidification through landuse are hidden by liming; in the Russian Federation, however, it is estimated that 5 million hectares of arable land are strongly acidified despite liming (National Report of Russia, 1992). In most non-agricultural areas (forests, extensive grasslands, semi-natural areas), fertilisers are rarely used; in forests, tree canopies filter and concentrate larger quantities of pollutants from the atmosphere than do other vegetation types.
Important indicators of the acidification status of a soil are the aliuminium (Al) concentration and the molar ratio of Al to base cations (Al/BC ratio) in soil solution, since the Al ion is toxic to roots. For both parameters, critical values, related to effects on fine roots, have been derived. Model predictions of the Al concentration and Al/BC ratio in current conditions, and in the steady-state situation that will be reached if present deposition rates of acidifying compounds continue, show that the area currently at risk may increase by 50 per cent in the future (see eg Table 7.3). Critical values of Al or molar Al/BC ratios are estimated (by numerical modelling) to be exceeded in 30 per cent of the forested area of Europe, or about 75 million ha. If acidic deposition does not change in future, up to 45 per cent, or about 110 million ha, of the forested area in Europe may come under risk (de Vries et al, 1992, 1994). The scientific threshold concentration for adverse effects on the environment is called the critical load (see Chapters 4, 5 and 31) and depends on the receptor (air, water or soil).
The most important impact of acidification on the environment is the leaching of acidifying compounds from the soil to the surface water and groundwater (see Chapter 5). Soil acidification is the prerequisite to water acidification, direct atmospheric deposition only contributing to the process. Water draining from acidified soils contains increased levels of aluminium (Van Breemen and Verstraten, 1991) which can have serious impacts on the quality of surface waters (especially aquatic life) and on groundwater (and possibly therefore on the drinking water supply). Indeed, a major effect of acidification is the mobilisation of aluminium from clay minerals which the soil might have accumulated.
Acidification may also cause a depletion of the soil's buffering capacity through the breakdown of clay minerals. The most vulnerable soils are the ones with a low buffering capacity, that is acid and sandy soils (Map 7.1). Acidification considerably reduces the fertility of the soil, mainly by affecting its biology, by breaking up organic matter, and by causing loss of plant nutrients. There is also a risk of heavy metals being released where pH values are low (see eg Box 7E). On non-agricultural land where no chemical countermeasures are applied, acidification may affect plant vitality by causing loss and discolouration of leaves or needles (see Chapter 34); this can lead ultimately to forest die-back (Roberts et al, 1989). Furthermore, it may cause a decrease in the species diversity of the vegetation, followed by changes in plant and soil organisms, eventually favouring the more acid-tolerant and acidophilous species. However, certain acid soil habitats may be worth protecting for their biodiversity.
Apart from emission reductions through coordinated abatement strategies (soil acidification can be slowed down if acidic inputs are reduced), soil acidification and its effects on forest vitality can be counteracted by fertilisation. However, buffering capacity of the soil cannot be restored. This makes soil acidification one of the most severe environmental threats in Europe, the full effects of which cannot be reversed. Liming will increase soil pH, but will also have an impact on soil biota and groundflora which is not necessarily desirable. In any case, liming should be done in such a way as to induce only a gradual increase in pH. This can be achieved by using slowly dissolving limestone, for example dolomite. A rapid pH increase may cause increased mineralisation of the humus layer, thus causing increased leaching of nitrates and base cations to groundwater.
Soil pollution by heavy metals |
Soil contamination by heavy metals, such as cadmium (Cd), lead (Pb), chromium (Cr), copper (Cu), zinc (Zn), mercury (Hg) and arsenic (As), is a problem of concern. Although heavy metals are present naturally in soils, contamination comes from local sources (see Table 7.4), mostly industry (mainly non-ferrous industries, but also power plants, iron and steel and chemical industries), agriculture (irrigation with polluted water, use of mineral fertilisers, especially phosphates, contaminated manure, sewage sludge and pesticides containing heavy metals), waste incineration, combustion of fossil fuels and road traffic (see Chapter 14). Long-range transport of atmospheric pollutants adds to the metal load and is the main source of heavy metals in natural areas.
Heavy metals accumulate in the soil where they are fixed on mineral particles. From there they can be mobilised by 'triggers' such as acidification, and released to soil solution from which they can be taken up by soil organisms and plant roots, or leached into groundwater, thus polluting the food-chain or affecting drinking water quality.
It is difficult to give an evaluation of heavy metal pollution on a European scale since appropriate data are not available. Survey results which are available include those from Northeastern Europe where mosses here have been used to make a survey of heavy metal deposition (Box 7F, Maps 7.5 and 7.6). From the results of a study reviewing the environmental fate and effects of cadmium in Europe over the last 10 years (Jensen and Rasmussen, 1992), the average cadmium concentrations in European soils are estimated to range from 0.06 to 0.5 mg/kg with a tendency towards lower concentrations in Scandinavian soils. Unfortunately, this study refers essentially to Northern European countries due to the lack of information in Southern Europe. In contaminated soils (essentially mining areas), concentrations are 10 to 100 times greater than the average found in surface soils. In Europe, fertilisers produced from rock phosphate give the most important but highly variable contribution to cadmium in soils (from 0.3 to 38 g/ha/year); farmyard manure can give a similar and higher input than commercial fertilisers; atmospheric deposition adds to the cadmium load which over the past 50 years has brought about 2 to 7 g/ha/year to soils. For Europe as a whole, increasing cadmium concentrations have been found in European soils, especially during the last 20 to 30 years. However, only in a few cases has a significant long-term increase of cadmium concentration in harvested crops also been found.
Pollution of agricultural soils by heavy metals may lead to reduced yields (growth inhibition of leafy vegetables is reported at cadmium concentrations of 0.9 to 1.5 mg/kg in sandy soils and about 3 mg/kg in clay soils). Pollution can also lead to elevated levels of these elements in agricultural products, and thus to their introduction into the food-chain (Alloway, 1990). Heavy metals deposited on grassland soils remain predominantly in the top few centimetres and are directly ingested with soil by grazing animals. Soil ingestion is a major pathway of heavy metals to livestock grazing contaminated land (Thornton and Abrahams, 1983). Heavy metals are toxic and it cannot be discounted that low levels may have long-term effects which may not yet have become apparent. They may have negative impacts through the inhibition of soil microorganism activity; for example, surface accumulation of heavy metals in acid forest soils can inhibit litter decomposition by influencing fungal and faunal communities. Cadmium concentration in soil exceeding 4 mg/kg dry weight can inhibit soil microbial processes, growth and reproduction of living organisms. Heavy metals may accumulate in soil, especially in soil with a large binding capacity for heavy metals (for instance in non-acid soils and 'black earths'). The most important factors governing plant uptake of heavy metals from soil are the soil pH, the clay content (texture), the organic matter content, and the metal concentration. This is illustrated in Figure 7.3. In soils with a low binding capacity (the acid and sandy soils of Map 7.1), there will be no accumulation of heavy metals even under high input load, which implies a relatively high concentration of heavy metals in the groundwater, important leaching, and high plant uptake.
Forest soils seem to accumulate more heavy metals than grasslands because forest crowns constitute receptors with a larger deposition surface, and forest litter is not exported (De Temmerman et al, 1987). Metal concentrations in the soil solution can remain low for decades or even centuries, as long as the binding capacity of the soil is not saturated. This reservoir of metals in the soil may turn into a source if the binding capacity of the soil is lowered. This may occur through a change in pH (pH<4) due, for example, to atmospheric deposition, changes in landuse, or climatic changes. Figure 7.4 illustrates how the behaviour of heavy metal cations adsorbed onto soil undergoes an abrupt change over a narrow range of pH.
Reduction in the emissions of heavy metals is the most direct way to decrease the atmospheric deposition of these elements and their build-up in soil. Despite the great increase in traffic, substantial reductions in lead emissions have already been achieved through incentives to use unleaded petrol. Nevertheless, emissions of heavy metals from industrial plants (especially from smelters of Eastern Europe) are still very important. Many of the reduction measures already being considered and implemented for NOx and SO2 emissions will also reduce heavy metal emissions. Indeed, a decrease in acidification brought about by a reduction in the emission of these compounds can lead to a restoration of the binding capacity of soils.
On agricultural land, the heavy metal load can be decreased by:
Cadmium balance studies in European agricultural soils with moderate to high cadmium contamination confirm that a zero accumulation could be achieved by a combination of measures aimed at reducing the present cadmium inputs to the following rates: 50 to 70 per cent for fertilisers, 65 to 75 per cent for sewage sludge and 50 to 65 per cent for atmospheric deposition.
Soil pollution by pesticides and other organic contaminants |
The development and widespread use of synthetic and natural organic compounds in industry and agriculture has led to concern over the possibility of environmental pollution by these chemicals (see Chapters 5, 17, 22 and 38). Not only are pesticides known to pollute soil directly by affecting soil organisms, but also soil acts as a vector for the pollution of water. Organic pollutants enter soils through atmospheric deposition, direct spreading onto land (sewage sludge), or contamination by wastewaters and waste disposal. Besides pesticides, organic contaminants include many other components, such as oils, tars, chlorinated hydrocarbons, PCBs and dioxins. There is such a wide variety of organic substances that their detection and monitoring in the soil is practically impossible. Long-term studies at Rothamsted Experimental Station in the UK (ITE, 1989) have shown that, since the 1860s, there have been large increases in the soil burden of polynuclear aromatic hydrocarbons (PAHs), which are derived from combustion of fossil fuels. Their effects on soil microbial processes are unknown. PAHs accumulate on or within herbage, where they become part of the dietary intake of herbivores. In the Russian Federation, concentrations of PCBs in soils in industrial areas exceed by 20 to 30 times the national reference value (National Report of Russia, 1992).
Pesticides (mainly fungicides, herbicides and insecticides) are used in agriculture to protect crops and to ensure good quality of the harvest. The use of pesticides is also very cost-efficient compared with mechanical or other alternatives for pest control. Many pesticides, especially the older ones, have a rather broad activity spectrum; that is, not only are the target organisms affected, but also non-target organisms, and side-effects to soil organisms or human beings are likely to occur. Persistence of pesticides in the plough layer and their leaching to groundwater are dependent, for the most part, on their intrinsic properties of transformation (degradation) and mobility in the soil. The most important problems occur for pesticides which are very persistent or very mobile. These problems are not related to the mode of action of a pesticide.
Intensive use of pesticides in agriculture is recent after the Second World War in Western and Northern Europe, the amount used increased enormously up to about 1980 and is still increasing in Southern Europe (see Chapter 22 for the data on consumption and load of pesticides in Europe).
The use of pesticides may unintentionally lead to:
Up to now, only the threat of pesticides to groundwater as a resource for drinking water has received much attention. In the EU, Council Directive 80/778/EEC sets a maximum admissible concentration of 0.5 µg/l for the total amount of pesticides and metabolites in drinking water (see Chapters 5 and 22). The fact that there are about 1000 different compounds (active ingredients) on the market (Promochem, 1993), showing different behaviours in the soil, makes it extremely difficult to identify and evaluate the threats on soil posed by pesticides (see Chapter 17). Effects on non-target organisms in soil are expected to occur very frequently; they may influence not only the soil biotic functions, but also the abiotic functions. Until very recently, only the impact on major soil functions such as mineralisation and nitrification had been studied. Most pesticides only hamper these functions for a relatively short period after application (up to a few months) after which the soil may completely recover from this loss of activity. Side-effects in the soil are very much dependent on the specific pesticide used.
The use of pesticides has been, and still is, widespread. Consequently, the reduction of side-effects can be obtained only through improved application and legislation (UNECE, 1992a). This might include:
Soil pollution by nitrates and phosphorus |
Vineyards in |
Nitrogen and phosphorus are elements essential to all forms of life and are therefore relevant to soil systems and to food-producing crops. They are important plant nutrients, but overapplication may lead to nitrogen or phosphate saturation in the soil, causing losses of nitrates into shallow groundwater, and saturation of the soil with phosphate, which may also move into the groundwater. Nitrate and phosphate pollution originates mainly from diffuse sources such as high manure and fertiliser application levels on agricultural soils and by atmospheric deposition on forests and natural areas. Unlike nitrate, where the major source comes from agriculture, phosphorus losses from sewage water (and animal manure) are the major factors contributing to increased content in water (see Chapter 14).
Nitrogen input levels to agricultural soils differ significantly within Europe. This is illustrated in Chapter 22 (Map 22.3), where the combined contribution of nitrogen from fertiliser and manure is depicted. Fertilisation inputs and related effects on water are usually less important in Southern Europe, with the exception of areas with intensive fruit and vegetable production; in these areas, the interaction of fertilisation and irrigation causes severe soil problems.
Natural areas and forests generally receive nitrogen input only from the atmosphere (deposition, and fixation by vegetation). In parts of Europe, ammonia generated by decomposition of manure-slurry in the soil, or that vented from sheds holding large numbers of livestock, is transported long distances, causing damage to forests and certain types of moorland vegetation over quite wide areas. Some landuse practices, such as drainage of peat soils, can enhance nitrogen leaching.
Unlike nitrogen, phosphorus is strongly fixed in soils on average, only 3 per cent of total phosphorus in soil is extractable and available to plants (Heck and Hanotiaux, 1976) often leading to deficiency problems in agricultural systems. The main soil components responsible for phosphate bonding are organic matter, clay minerals, and hydrous oxides of aluminium and iron. It is only in areas where phosphate is applied in large amounts under wet conditions on soils with low adsorption capacity (eg, sandy soils with low organic matter content under intensive agriculture in Northern Europe) that the load may reach levels where phosphorus is released to the groundwater; on slopes, runoff can contribute to large quantities of phosphorus to surface waters. This is particularly the case for intensive crops (such as maize) on sandy soils with a shallow groundwater table, because the phosphate fixing capacity is much lower under anaerobic conditions. These soils are common in Denmark, The Netherlands, the UK, France and Germany. In The Netherlands, it is estimated that 80 per cent of soils are saturated with phosphorus. The problem with phosphorus is that it can take 15 to 30 years before the soil is saturated and effects are seen, as in the Po valley in northern Italy. Where there are high phosphate concentrations in freshwater, eutrophication may occur. A level of 0.02 mg phosphorus per litre is sufficient to allow eutrophication and many freshwater bodies in Europe have higher concentrations than this (see Chapter 5).
Together with an intensification of landuse in general, the main effects of nitrogen and phosphorus application in excess of plant uptake are the loss of nutrient-poor habitats within agricultural areas throughout Europe, the leaching and runoff to surface water of nitrogen and phosphorus, and the leaching to groundwater mainly of nitrogen in the form of nitrate. The most vulnerable soils are the sandy and loamy soils (see Map 7.1).
The amount of leaching depends on the soil, the local climate and crop management (see Box 7G). Well-drained soils with their plentiful supply of air favour the formation of nitrate so that the risk of leaching is increased. In waterlogged conditions (which are common for peat and muck soils and many wet soils, see Map 7.1), nitrate is denitrified by bacteria into nitrogen or nitrous oxide (N2O) which dissipate to the atmosphere (N2O is a greenhouse gas which contributes to climate change problems as described in Chapter 27); however, the amount of nitrate available for leaching is proportionally reduced. In intensive horticultural systems, interaction between high fertiliser inputs and irrigation enhances nitrate leaching. Most nitrate leaching occurs in the autumn when fields are either fallow, recently sown, or awaiting sowing. The impacts of (over)application of nutrients on surface water caused by runoff and leaching are treated in detail in Chapter 5.
In regions with high livestock densities, accumulation of phosphorus in the upper soil layers occurs. This is the case in the central and southern sandy regions of The Netherlands, and in the Po valley in Italy, where some of the highest livestock densities in Europe are found (Breeuwsma and Silva, 1992). In soils saturated with phosphorus, especially those with shallow groundwater, high phosphorus concentrations occur in the upper groundwater layer and surface water causing eutrophication.
The problem of nitrate pollution is recognised internationally, and is usually associated with intensive agricultural practices. In the EU, Council Directive 91/676/EEC on nitrates imposes on Member States the establishment of mandatory codes of good agricultural practice and the designation of vulnerable areas. Codes of good agricultural practice try to narrow the imbalance between fertiliser input and plant uptake, and the following measures are favoured:
In general, nitrogen leaching cannot be completely prevented, especially in areas with precipitation excess and where high production levels are achieved on permeable soils.
Phosphate is more a local problem in Europe. It has been partly limited since the use of phosphates in detergents was controlled. However, unlike nitrate, phosphate binds to soil particles and if a soil becomes phosphate-saturated then phosphate will remain in solution along with nitrate. In The Netherlands, the capacity of the soils to adsorb phosphate is already exhausted at several locations. For these soils saturated with phosphate, rehabilitation would then involve the replacement of topsoil, which is not practicable.
Soil pollution by nitrates and phosphorus |
Contamination of soil with artificial radionuclides has given rise to much public concern since the Chernobyl accident. The radionuclide currently causing most interest is caesium-137 (Cs-137) although others may also be detectable in some areas (eg, strontium-90 and plutonium isotopes). Contamination from atomic bomb tests, nuclear accidents and/or explosions all have a high risk of causing widespread contamination through the transport of artificial radionuclides by wind and water. Radioactive contamination of soils can also arise from leakage of waste dumps or during production, use and transport of radionuclides. Although these may not necessarily lead to effects over extensive areas, impacts may be locally highly hazardous (see Chapter 18).
Radionuclides behave chemically in soil as their stable isotope counterparts. The half-lives of the radionuclides (or the time to decay to half their original activity) and the retention properties of soils vary considerably; for example, clayey soils tend to retain radionuclides (as well as other contaminants) better than sandy soils. Since most sources of contamination come from the surface, most artificial radionuclides are concentrated in the upper few centimetres of mineral soils. In organic soils, they tend to concentrate at greater depths.
Radionuclides in the upper layers of soil may expose plants and animals to radiation, and may in extreme cases present a threat to humans from direct ionising radiation (see Chapter 16). From the soil, radionuclides can be available to enter the food-chain, leading to intake by humans and animals through ingestion. Exceptionally, wind-blown contaminated soil material may lead to exposure through inhalation. Some plants concentrate radionuclides and can act as indicators of soil contamination. At very high levels of exposure, flora can also sustain deleterious effects from radionuclide contamination, as noticed for example after the Chernobyl accident (see Box 18E in Chapter 18). Fauna living on or in the soil can equally be affected; the direct ingestion of soils by ruminants can be a significant pathway for artificial radionuclides to enter the food-chain and ultimately to reach humans.
A full European survey of radionuclides in soils does not exist. The impacts and radioactive contamination of the land surface resulting from fall-out after the Chernobyl nuclear accident in April 1986 are presented in Box 18E. Radioactivity levels of more than 40 GBq/km2 (1 GBq = 109Bq) were still reported in a zone of 55 000 km2 around the Chernobyl power station five years after the accident (National Report of Russia, 1992). In a zone of 10 000 km2 (Mnatsakanian, 1992) radioactivity levels of more than 550 GBq/km2 were found.
In the framework of the CEC/CIS collaborative programme (see Box 18E), a project was started at the end of 1993 with the aim to establish a first European atlas of radioactive contamination and external exposure resulting from the Chernobyl accident (Joint Study Project 6).
Contaminated land |
Past and present economic activities have often resulted in contamination of the soil underlying the place where these activities took place. Contaminants affecting soil may be in the form of solids, liquids or gases. The most common toxic soil pollutants include metallic elements and their compounds, organic chemicals, oils and tars, pesticides, explosive and toxic gases, radioactive materials, biologically active materials, combustible materials, asbestos and other hazardous minerals. These substances commonly arise from the disposal of industrial and domestic waste products in designated landfills or uncontrolled dumps. The existence of unauthorised dumps and the improper disposal of an extremely wide variety of wastes and sludges is one of the environmental problems which deserves the highest priority in Europe (see Chapter 15); in Central and Eastern Europe, it may be a limiting factor for foreign investment. Since the second half of the 1980s, attempts have been made to register contaminated land in the EU. According to a recent study (CEC, 1992), out of 200 000 hectares of derelict industrial land, about 25 per cent was from the coal and steel industry, of which more than 90 per cent was concentrated in five countries only (Belgium, France, Germany, Spain and the UK). In East Germany, 70 000 hectares have been abandoned following lignite mining. The process of site assessment is difficult, but some countries, such as Denmark and The Netherlands, have started a systematic recording, monitoring and clean-up programme of contaminated sites (see Box 7H).
A tentative list of contaminated sites in the EU has been compiled (Table 7.5). More than anything else, this inventory reveals the inconsistency of definitions (in many countries, contaminated sites have been restricted to landfills) rather than the actual number of contaminated sites. Better information can be obtained only if contaminated sites are properly registered and if reference levels for all potential contaminants are established.
The number of polluted sites is already huge and remediation costs are very high, roughly estimated at more than ECU 100 billion (Carrera and Robertiello, 1993). It is expected that the situation in Central and Eastern Europe will be at least as bad, especially in industrial areas. Considering the rehabilitation costs, a compromise needs to be found between economically sustainable remediation policies and potential health effects.
In addition to the diffuse contributions to degradation described in the preceding sections, point sources also have important effects, mainly at a local level. Soil pollution can be created either directly by spillage, leakage, handling, or disposal and leaching of waste. Contamination from such sources often becomes detectable after relatively long periods when pollutants have already begun to be released, contaminating food supplies and drinking water. Uncontrolled dumping can threaten human health and the environment. Leaching processes in landfills can contaminate soil and groundwater. Almost all municipal solid wastes contain hazardous and toxic substances which can be leached to water.
The pollution of soil at both former and present industrial sites represents potentially serious threats to human health and soil biology. These problems have appeared, for example, with the expansion of cities, where industrial sites have been transformed into residential areas. Since there is no registration of such areas, the extent of soil pollution is difficult to assess. When such data do exist, they are often unreliable because many of them are collected unsystematically and are often based on qualitative information. Many sites of former polluting industries and landfill dumps are forgotten until contamination re-emerges to pollute water supplies or redevelopment surveys reveal the hazard. Preliminary inventories in The Netherlands indicate that at 20 per cent of present and former industrial sites, serious soil pollution will occur (Meeder and Soczó, 1992). For Germany it is estimated that 10 to 20 per cent of the 135 000 suspected sites (industrial areas, landfills and military sites) will potentially be confirmed as contaminated (NATO/CCMS, 1992).
Because of the wide variety of soil pollutants and concentrations, impacts are imprecisely known. The effects of waste disposal, past and present, are often not confined to the actual site but may influence a large surrounding area (tens of square kilometres) including agricultural land, dwellings and/or nature reserves. As indicated in Table 5.4, a wide range of pollutants is involved and many are unknown.
Various technologies are available for treatment. These include soil excavation, washing and disposal. However, this type of treatment is very expensive (more than ECU 500 per tonne of soil) and sometimes clean-up of soils is not possible although the actual threat to human health and the environment is high. In case of heavy metal pollution, the metal ions can be immobilised in the soil by increasing the chemical binding capacity. Cost-effective 'soft technologies' exist (ECU 3 to 350 per tonne of soil) which can be used in situ and where soil is treated with immobilising additives such as lime, 'Thomas basic slag' or hydrous iron and manganese oxides (Mench et al, 1993).
Strategies to tackle problems concerned with point sources of pollution require both prevention and remediation of soil and groundwater contamination. The prevention can be focused on:
Soil and groundwater protection includes technical and legal measures, and management to avoid (new) contamination or to reduce further dispersion of contaminants by insulation (sealing). To verify the quality of soil and groundwater and to check the efficacy of insulation measures, monitoring networks need to be systematically set up (see Box 7 I). For the rehabilitation of priority sites, a remediation strategy is required.
Great difficulty surrounds determining threshold values for pollutants above which soil may be regarded as seriously contaminated and for which a clean-up procedure is necessary. Indeed, as mentioned in the introduction to the present chapter, soils are composed of mineral and organic constituents which vary in concentration and content geographically, as well as in depth down the soil profile. In some cases, natural concentrations in clean soils and waters can exceed the threshold trigger level for pollutants. For instance, the concentration range in unpolluted sites varies from 0.01 to 1 mg/kg for cadmium, 2 to 200 for lead and 2 to 100 for copper (Bridges, 1991), while in the EC Directive on sewage sludge (86/278/EEC), limit values for concentrations of heavy metals in soil range from 1 to 3 mg/kg for cadmium, 50 to 300 mg/kg for lead and 50 to 140 mg/kg for copper (see Box 7J). In comparison, concentrations of heavy metals found in polluted sites in Poland reach 290 mg/kg for cadmium, 4650 for lead and 1200 for copper. It is therefore difficult to define a minimum concentration above which a soil may be considered contaminated by any particular pollutant.
Only a small number of countries such as The Netherlands and Denmark have elaborated comprehensive legislation with respect to the registering and cleaning up of polluted soils. In most countries, reference has to be made to general rules of law or with other provisions relating to the removal of abandoned waste, or pollution of groundwater.
Whether or not clean-up operations are undertaken, polluted soils raise a number of legal and administrative issues.
To answer this it is necessary to establish a systematic examination of polluted areas. The transfer of property, or the closure of an installation, offer good opportunities for an assessment of soil quality. A legal obligation which imposed on the operators of industrial installations a duty to report spills and other causes of new soil pollution would also be useful. A mandatory soil investigation could also be required before any construction permit is delivered, at least in former industrial areas (Bocken, 1993).
How clean is clean? How dirty is dirty?
Many countries have guidelines or standards related to target or threshold values of certain potentially hazardous substances. However, the values differ from country to country. At EU level, several directives exist with regard to maximum admissible values for nitrates in drinking water, heavy metals in sewage sludge and air pollutants and other chemical substances, for example. However, none of these addresses soil directly.
The use of critical loads of contaminants for soils is the subject of research in many European countries. The idea is to set up acceptable concentrations of contaminants which are permissible in soils, particularly those soils used to produce foodstuffs.
Whatever approach is used (multifunctionality or end use), it is first important to know where seriously polluted soils are found so that control over their end use can be ensured. The potentially severe effects that such control might have on the real estate market cannot be ignored.
Soil clean-up itself presents a number of serious problems. Clean-up costs are often so high that the only practical approach is to share the cost between the owner of the polluted property and society. The waste resulting from the clean-up has itself to be treated and disposed of in some way (see Chapter 15). Since soil pollution can affect areas or people far from where the pollution originated (migration of pollutants to the groundwater), clean-up activities need to cover the whole area affected by pollution even if parts belong to different owners (eg, heavy metal pollution from smelters near residential areas).
Finally, a distinction must be made between past and new pollution. The level of financial resources required means that the decision to clean up past pollution needs to be based on the actual danger to human health and the environment and be subject to a list of established priorities. At the same time, new pollution must be avoided and clean-up required as soon as the soil quality objectives are exceeded.
Soil compaction |
Soil compaction is caused by the repetitive and cumulative effect of (heavy) machinery on the same piece of land, or to a lesser extent by the trampling of cattle when overstocked, in wet conditions or on wet soils. But it is not confined solely to agricultural soils. Building sites and intensively used recreational areas, for instance, are also susceptible. Compaction may occur at shallow soil depths as well as deeper in the subsoil. Shallow soil compaction occurs during seed bed preparation, spreading of fertilisers and pesticides. Subsoil compaction occurs when tractor wheels pass through the open furrow during ploughing, creating a persistent 'plough pan'. Severe subsoil compaction is also caused by heavy machinery used during harvest and the spreading of slurry with high capacity tankers having heavy axle loads. Soil compaction is potentially a major threat to agricultural productivity.
Soil compaction mostly affects highly productive soils and heavily mechanised agricultural land or wet grasslands with high cattle density (see Map 7.7). According to the World map of the status of human-induced soil degradation (Oldeman et al, 1991) about 33 million ha or 4 per cent of European land is at risk for soil compaction. In Map 7.7, it is mainly shallow compaction which is shown. However, due to the increasing weights and axle loads of farm vehicles, subsoil compaction is becoming an urgent soil conservation problem, especially in Central and Eastern Europe (Håkansson and Petelkau, 1994). Deep soils with less than 25 per cent clay are most sensitive to subsoil compaction (Hébert, 1982). Such soils cover large areas in the Russian Federation, Poland, Germany, The Netherlands, Belgium and northwestern France. These correspond on Map 7.1 to the acid and non-acid loamy soils, and to a lesser degree the sandy soils. Because subsoil compaction is not easily detected without specific measurements, the area affected by this type of soil degradation tends to be underrated.
Compaction changes the quantity and quality of soil biochemical and microbiological activities. The major physical impact of soil compaction is to reduce soil porosity, which means less air and water are available to plant roots. At the same time the roots have more difficulty in penetrating the soil and have reduced access to nutrients in the soil. Biological activity is substantially reduced. Another effect of compaction is the increase in surface runoff, since less rainwater is able to percolate. This increases the risk of water erosion and loss of topsoil and nutrients.
Recent calculations (Boels and Van der Akker, in press) show that surface soil compaction may cause yield reductions of up to 13 per cent (average 5 per cent), whereas subsoil compaction may reduce yields by 5 to 35 per cent (average 12 per cent). In the Russian Federation, decreases in yield up to 50 per cent have been recorded on compacted soils (National Report of Russia, 1992).
Surface soil compaction is easily countered by reworking the soil and eventually disappears after a couple of years if biological processes remain undisturbed. Subsoil compaction however is persistent and cannot be restored easily since present techniques do not provide long-term solutions. Since compaction is mostly caused by heavy machinery, it can be prevented by increasing the number of axles and wheels under agricultural machinery, increasing tyre width, and reducing tyre pressures (Håkansson and Petelkau, 1994). The use of smaller and lighter vehicles does not necessarily help since it may require more frequent passages, which may counter the preventive effect. In general the use of large machines is preferable as they require fewer passages on the land, but the ground pressure under wheels or tracks should be below a certain threshold; less than 100 kPa has been proposed as critical value (Soane, 1982). It is the timing of operations more than the design of the equipment which is the crucial factor; for example, using machinery during wet conditions should be avoided.
Other threats |
As mentioned in the first section of this chapter, soil organic matter is important in maintaining soil structure, in retaining water, and as a nutrient reserve and chemical buffer. Certain landuses adversely affect the amount of organic matter in soils. Intensive arable cultivation, especially in Western Europe, has led in certain cases to a decline in organic matter and a loss of biological activity and diversity (in Belgium, France, the UK, etc); in Beauce (France), organic matter has decreased by half in 16 years (Secrétariat d'Etat auprès du Premier Ministre, Chargé de l'Environnement, 1988). The ploughing up of grassland, the abandonment of rotations in the agricultural system and the burning of crop residues all reduce the amount of vegetation matter returning to the soil. Unfortunately, it is not possible to determine an ideal or target level of organic matter required in a healthy soil. Nevertheless, there is a threshold value under which the soil is less able to hold moisture, store nutrients and adsorb pollutants; when its biological activity is reduced, the soil is less stable and more prone to erosion, leaching and runoff. Loss of organic matter is particularly apparent in regions where peat soils have been drained and farmed intensively. It is estimated that 3.2 million hectares suffer from losses of nutrients or organic matter in Europe.
Various measures could be taken to increase the organic content of soils. These include the introduction of grass into agricultural rotations; the ban on straw and stubble burning (Denmark, the UK), which is likely to result in crop residues being ploughed back into soils; and the increase of land left fallow or under grass as a consequence of the EU set-aside policy, which could lead to improvement in organic matter content. Monitoring of the current extent of organic matter loss is also needed.
Salinisation arises in a number of ways: by irrigation with improper drainage, intrusion of salt water from the sea or from saline fossil sources, and through evapotranspiration of saline soil moisture. It has direct negative effects on soil biology and crop productivity, and indirect effects leading to loss of soil stability through changes in soil structure (alkalinisation). In Europe, the surface area affected by salinisation is estimated to be 3.8 million ha (Oldeman et al, 1991; Szabolcs, 1991).
Salinisation is most strongly tied to site-specific soil properties and climatic conditions and therefore its distribution is restricted to Southeastern Europe where semi-arid conditions prevail, that is on semi-arid and salt-affected soils (Map 7.1). The irrigation of the Hungarian plain, for instance, has caused salinisation and alkalinisation of more than 20 per cent of the region. In the Russian Federation, about 7 per cent of agricultural soils are saline, part of them naturally so for example the solonchaks. In Romania, where huge irrigation schemes were introduced 25 years ago (about 3.2 million ha), it is estimated that 200 000 ha have been salinised through irrigation, which represents about 6 per cent of total irrigated land. Salinisation is reversible but reclamation of saline/alkali soils is expensive, as it requires complex amelioration techniques.
Waterlogging due to accidental or deliberate flooding, increased runoff from higher areas resulting from lower infiltration rates, or raising of the water table (as a result for example of irrigation) may also negatively affect crop productivity. Waterlogging, by driving the air from the soil, reduces the redox potential to anoxic conditions. This has an impact on many biochemical processes in the soil and may even trigger the release of hazardous substances previously safely stored in the soil. Waterlogging also increases the risk of compaction and, in drier areas, the risk of salinisation through capillary rise of saline groundwater. The most vulnerable soils are the wet soils and parts of the semi-arid and salt-affected soils (Map 7.1).
Waterlogging occurs mainly in the north of the Russian Federation and at a few scattered sites in the rest of Europe (foremost along the Black Sea coast and the lower Danube Valley). The area involved is estimated at 0.8 million ha (Oldeman et al, 1991).
RESPONSES |
Ecological responses |
The reversibility of the effects of various threats on soils is an important concept in soil protection. Irreversible changes should be avoided if they impair any of the soil functions. In some cases, soil properties may return naturally to their normal range of values once the threat is removed. Then the main question is the time-scale required. In many cases, the soil will not return naturally to its original condition, but would do so under appropriate management. In other cases, the soil will not return to its original condition but could, under management, be converted to some other desirable state. Therefore, to achieve a sustainable landuse policy, those uses leading to effects which can be reversed only by technological means should be avoided or minimised.
It is inevitable that some uses of soils will result in damage to the soil, or in the reduction of the ability of the soil to perform various functions; the most important soil threats have been extensively discussed above. A number of forms of damage can, to a greater or lesser extent, be reversed by a variety of management measures, either by the use of an ecological approach to management, or by application of technology. For example, surface soil compaction can be reversed naturally (the bulk density will decrease once the loading is reduced). Soil compaction can also be reversed by suitable cultivation techniques, but severe subsoil compaction will require deep ripping with large machines and is reversible only technologically. If a soil has been acidified by atmospheric deposition, reversibility would express the ability of the soil to recover (to increase its base saturation) as the inputs were decreased or as basic substances were added. In some cases, rehabilitation may not be practicable.
As developed above, it is possible to rank soils in terms of their vulnerability to a particular stress, such as their sensitivity to acidification, wind or water erosion or nitrate leaching, or their sensitivity to compaction through excessive animal load and agricultural or civil engineering traffic.
The critical-load concept follows the same type of approach. It has been developed in connection with assessments of the impact of acidic deposition (see Chapters 4 and 31) but the approach could be applied more broadly. The concept aims at defining the maximum load of a given stress which will not produce adverse changes in soils. The load, which could be pollutant inputs, physical loading or fertiliser inputs, has much in common with the maximum permissible loads calculated for heavy metal inputs in sewage sludge. The definition of critical load will vary with the particular threat: for instance in relation to compaction it might be a quantitative estimate of the maximum physical loading of a soil which will not produce increases in bulk density, or decreases in permeability resulting in adverse effects on the functioning of the soil system. The critical-load approach provides a means of linking controls on the load of given stresses to soil protection by causeeffects models, but the necessary models are available for only a few stresses at the moment. Nilsson and Grennfelt (1988) have determined critical loads of sulphur and nitrogen for forest soils. Studies by RIVM in The Netherlands have produced models defining the maximum inputs of nitrogen fertiliser, and the timing of these inputs, to limit leaching of nitrate to groundwaters (see above and Chapters 5 and 22). In effect, these models are defining a critical load of nitrogen fertiliser with respect to impacts on water quality. Considerably more research is needed, however, before quantitative models can be developed to allow the determination of rates of recovery (reversibility) for any particular threat. Reference levels for all potential contaminants should be available in future.
Technical responses |
The technical solutions to specific pollutant problems have been described above, together with the magnitude, impacts and causes of the different problems, and preference has been given to prevention methods. However, in cases of severe soil contamination, clean-up methods may be necessary. There is a variety of techniques with very varying divergent costs, and decisions should be taken case by case. It has been estimated that, for the EU in 1988, the total expenses for the implementation of a 15-year remediation programme for those sites requiring immediate intervention would have been ECU 27 billion, an average of ECU 5 per inhabitant and per year (Carrera and Robertiello, 1993).
In any overall risk assessment of any given use, or of the growth of a given crop under a given management regime, it is desirable to balance soil suitability with sensitivity before making any decision. Thus, the combined suitability/vulnerability approach would take into account the risks of erosion, or nitrate leaching, for example, as a consequence of growing a particular crop at a given location and under a specific management scenario. The aim would be to balance suitability against any consequent risks an optimisation procedure.
Policy and legislation |
The term 'soil protection' is used here to indicate a broad protection of soils which is not linked to any particular end use (ITE, 1989). Indeed, the difficulty when dealing with soil protection legislation is that there is a wide variety of soil types, functions and sensitivities to various degradation types. Because of the broad definition of soil used in this chapter (see the Introduction), soil protection implies, therefore, the protection of a complex dynamic, heterogenous system. This renders the setting of realistic targets very difficult. The purpose of this section is not to make a comparative analysis of the rules applied in soil protection in different legal systems but rather to identify the major questions which could be usefully addressed in this context and to indicate a number of policy implications of some of the possible solutions.
Up to now, soil conservation policies and legislation have aimed at ensuring the maintenance of the production potential of soils, mainly for agricultural and forestry uses, to protect watercourses and reservoirs and to prevent, or reverse, soil contamination occurring mainly through human activities (industrial activities and waste disposal).
In a number of countries (Austria, Italy, The Netherlands, Switzerland) laws exist that focus specifically on soils or on causes of soil degradation (such as overgrazing). Statutory protection of soils against erosion is probably the oldest type of legislation (eg, the Act for Resolution on Sand Erosion and Reclamation of 1895 in Iceland). Legal measures to control soil pollution are very recent in most countries, whereas legislation relating to other types of soil degradation is far less common, and is difficult to implement because soil, unlike air and water, is a personal property; its protection can be achieved only through the negotiation of voluntary management agreements between the relevant authorities and the land owners or occupiers.
In a report for the Council of Europe (1990), six types of legal treatments of soil protection are not mutually exclusive, which, have been identified:
CONCLUSIONS |
Soil degradation is a crucial environmental problem today, a problem that is increasing, and one that tends to get too little attention (Barrow, 1991). Soil is a dynamic, living system which is formed so slowly that it can be considered a non-renewable resource. Compared with air and water, soil has the particularity to act as a sink in which pollutants are filtered or transformed until the buffer capacity is depleted. The soil can then turn into a source of chemicals, and pollutants can start leaking into the groundwater. The fact that pollution effects are hidden for a long time has led to complacency, and until more recently concern for the well-being of soils has been slight. Finally, unlike air and water, soil is a personal property which renders any soil protection measure difficult to implement.
In Europe, the three most severe soil degradation processes in terms of their irreversibility are erosion, acidification and pollution by heavy metals, pesticides and other organic contaminants, nitrates and phosphates, and artificial radionuclides. Other important threats are soil compaction, losses of organic matter through improper management practices, salinisation and waterlogging.
To establish the magnitude of soil degradation requires a qualitative and quantitative assessment but there is still little accurate information for all of Europe, measurement is difficult and existing data are inconsistent. Since soil-related problems are site-based, any attempt at generalisation is also very difficult. There is a need for development of models for environmental impact prediction and risk assessment. For each main threat identified, an assessment has been made of the causal factors, the magnitude and severity of the damage, and remedies. Reversibility is the main criterion by which the different problems affecting soils in Europe have been selected and ranked.
The analysis of the processes by which soils are polluted and degraded shows that international as well as national action needs to be implemented to protect soils, as some problems arise across national borders and can be solved only through the joint efforts of all countries particularly where the various substances are taken in from the atmosphere. To implement any soil protection programme, it is essential:
Soil protection requires an integrated approach within the larger context of sustainable development, taking into account socio-economic factors (UNCED, 1992).