SOIL DEGRADATION
Dr. Victor A. Kavvadias
Soil Science Institute of Athens-National Agricultural Research Foundation
SOIL PROTECTION
The functions provided by soils are manifold because are related to processes
(filtering, storage, buffer functions), while serving human activities such as plant
production, recreation and leisure (Graeber and Blankenburg, 2009). In the past,
research efforts were mostly focused on water and water quality, while this was
inherently limited with regards to soils. Thus soil protection had been addressed
indirectly through measures aimed at the protection of air and water or developed
within sectoral policies.
Now soil protection refers not simply to the physical soil itself but to the soil as
part of a functioning and living ecosystem that provides all the eco-services. An
important advance has been the inclusion of plans for a thematic strategy on soil
protection in the sixth environment action programme in 2001 and the adoption of a
Commission Communication on soil protection, endorsed by the European Council in
2002 (EC, COM 2002) (Houskova and Montanarella 2006). European Union
analyzed and described the threats being faced by the soils of Europe in document
known as “Towards a Thematic Strategy of Soil Protection” or more simply the “Soil
Communication”. EU proposed a series of environmental measures, designed to
prevent soil degradation, including legislation related to mining, waste, sewage sludge
and compost and integrating soil-protection concerns in major EU policies (Blum et
al, 2004).The adoption of Thematic Strategy by EC (EC COM 2006) has given formal
recognition of the severity of the soil and degradation processes in Europe
(Montanarella and Toth 2008). The main threats addressed in the European Thematic
Strategy for Soil Protection, were sealing, compaction, erosion, contamination, loss of
organic matter, loss of biodiversity, salinisation, flooding and landslides (Scape, 2006;
Blum 2009).
SOIL DEGRADATION
Soil degradation is a serious threat for an increasing number of areas all over the
world (Lal et al., 1989). It generally refers to the decline in soil's inherent capacity to
produce economic goods and perform ecologic functions (Lal, 1993). Soil degradation
is defined as a process that causes deterioration of soil productivity and low soil utility
as a result of natural or anthropogenic factors (Ayoub, 1991; Mashali 1991; UNEP
Staff, 1992; Wim and El Hadji, 2002). It can be the result of one or more of several
factors which are potential threats for soil. Globally, it has been estimated that nearly
2 billion hectares of land are affected by human-induced soil degradation (UN,
2000).Soil is subject to a series of human-induced degradation processes, which
namely are displacement of soil material, and internal soil deterioration (Dwivedi,
2002).
The main impact of agriculture on soil degradation is erosion, salinization,
compaction, reduction of organic matter, and non-point source pollution. Loss of
organic matter and soil biodiversity and consequently reducing soil fertility are often
driven by unsustainable agricultural practices such as overgrazing of pasturelands,
over intensive annual cropping, deep ploughing on fragile soils, cultivation of erosionfacilitating crops (e.g. maize), continuous use of heavy machinery destroying soil
structure through compaction, unsustainable irrigation systems contributing to the
salinisation and erosion of cultivated lands (German Advisory Council on Global
Change (WBGU), 1994; EEA, 1999; Darwish and Kawy, 2008). In addition,
intensification of agriculture, some of which is linked in the European Union to the
implementation of the common agricultural policy, may accelerate loss of soil through
erosion.
Soil degradation involves physical loss and the reduction in quality of topsoil
associated with nutrient decline and contamination. In physical loss two main soil
degradation processes are: soil erosion by water or wind; In the second category are
chemical, physical, and biological degradation. The chemical degradation mainly
consists of soil pollution and acidification and its consequences as mobilization of
harmful elements/compounds, salinization and/or sodification, unfavourable changes
in the nutrient regime, and decrease of natural buffering capacity. Surface sealing or
crusting of top soil, soil compaction, structure destruction, and extreme moisture
regime comprise physical deterioration. Biological deterioration includes imbalance
of biological activities via loss of soil organic matter and biodiversity. Biodiversity
and organic matter can decline due to erosion or pollution, leading to a reduction in
soil functions such as control of water and gas flows. Reduced above-ground plant
diversity as a result of tillage, overgrazing, pollutants, and pesticides decreases the
microbial diversity in the soil ecosystem and disturbs its normal functioning
(Christensen, 1989; Boddy et al., 1988).
The degree of soil degradation depends on soil's susceptibility to degradative
processes, land use, the duration of degradative land use (UNEP Staff, 1991; Darwish
and Abdel Kawy 2008). The processes of soil degradation have major implications on
the following:
a) global carbon cycle, mainly due to the decrease in soil organic matter and the
release of CO2 to the atmosphere.
b) reduction in soil buffering capacity that is the capacity of soil to adsorb
contaminants
c) water and air quality
d) biodiversity
e) food production, food and feed safety
f) Human health
SOIL DEGRADATION IN EUROPE
Soil degradation differs markedly across Europe due to the fact that the quality of
Europe’s soils is a result of natural factors, (e.g. climate, soil type, vegetation,
topography) (EEA-UNEP, 2000). In Europe, damage to soils from modern human
activities is increasing and leads to irreversible losses mainly due to local and widespread contamination and soil erosion (EEA, 2000).
Mediterranean region
Land degradation in the Mediterranean region is estimated to threaten over 60% of
the land in southern Europe (UNEP, 1991). The degradation of soil resources is a
major threat in the Mediterranean region due to (1) climate conditions and global
warming, (2) topography, (3) soil characteristics, (4) changes in land-use (e.g.
abandonment of marginal land with very low vegetation cover and increases in the
frequency and extension of forest fires), and characteristics of agriculture (EEA, 1995;
Boydak and Dogru, 1997; Cammeraat and Imeson 1998; Zalidis et al., 2002).
Severe erosion and other degradation processes are taking place in Mediterranean
soils (Martınez-Mena et al., 2002). Land degradation within the southern European
Mediterranean is partially due to dramatic land use changes that occurred during the
second half of this century as well as due to climate linked to human influence and its
possible adverse influence on the environment (Jeftic et al., 1993; Perez-Trejo, 1994;
Hill 2003). In particular the Mediterranean islands have been recognized as “hot
spots” for various forms of soil degradation (EEA, 2000). For example, the main
factors of soil degradation in the Mediterranean island of Sardinia (Vacca et al, 2009)
are essentially human related to soil degradation processes which consist mainly in
accelerated erosion, loss of prime farmland, compaction, salinisation, contamination,
decrease of organic matter content, and adverse alteration of biological processes.
Desertification
It is defined by the United Nations Convention to Combat Desertification
(UNCCD) as “the degradation of the land in arid, semi-arid and dry-sub-humid
areas, as a result of several factors, including climatic change and human activities”
(UNCCD, 1994). Soil desertification is specific phenomenon of arid, semi-arid, dry
sub-humid regions. According to the United Nations Environmental Program (UNEP)
report desertification, which is now considered a worldwide phenomenon, affecting
about one-fifth of the world population, 70% of all dry-lands and one-quarter of the
total land area of the world (Tolba et al., 1992).
According to Montanarella and Toth (2008) the thematic strategy (EC-COM 2006,
EC-SEC 2006), emphasizes that soil degradation processes or threats or a
combination of some of the threats will lead to desertification. Desertification in
Europe occurs everywhere, for example central and northern Europe and is not solely
linked to poor land management or poverty. In the most extreme cases, soil erosion,
coupled with other forms of soil degradation, has led to desertification in some areas
of the Mediterranean. The Mediterranean area is identified as sensitive to
desertification due to a combination of climate conditions, soil and terrain
characteristics, agriculture and exploitation of water resources (Castillo et al., 2004b).
Soil erosion one of the most important source of soil degradation particularly in the
most extreme cases (arid and sub-humid climate) together with the destruction of
vegetation cover (mainly due to frequently repeated forest fires) and structure,
increase of desertifcation of marginal lands in the Mediterranean basin (Perez-Treto,
1994). Current rates of erosion in the Mediterranean countries show that irreversible
processes of soil degradation and desertification are already occurring in
Mediterranean region.
Northern and Western Europe
Northern and Western Europe is highly urbanized (built-up areas occupy 15 % of
its territory) and competition for the limited land available is significant. Soil sealing
and in particular at unsustainable rates, results in the loss or degradation of soil
resources (EEA, 2002a).
Soil contamination in Westerner European countries still remains a problem
despite several initiatives that have been taken the last years to reduce air emissions
and to control and manage organic wastes (e.g. the application of sewage sludge and
the use of landfill for waste disposal). Soil erosion is also an increasing concern in
Northern and Western Europe, although to a lesser degree compared to the other parts
of Europe (EEA-UNEP, 2000; EEA, 2002a, b). Rain falling on mainly gentle slopes
of North Europe is evenly distributed throughout the year and consequently, the area
affected by erosion is less extensive than for example in southern Europe.
Central and eastern Europe
Soil degradation problems are similar to those in Western countries although soil
sealing is less compared to Western Europe (Van Lynden, 2000). Soil Erosion is the
most widespread form of soil deterioration which related to agricultural
mismanagement and deforestation. The effects of soil erosion are expected to get
worse, since climate change is expected to influence the characteristics of rainfall in
ways which might increase soil erosion in central Europe (Sauerborn et al., 1999).
Furthermore soil contamination is mainly a result of inefficient technologies and
uncontrolled emissions. According to (DANCEE, 2000) there are approximately 3,
000 problem areas including former military sites, abandoned industrial facilities and
storage sites which may still be releasing pollutants to the environment leading to
groundwater contamination and related health problems. Furthermore large areas in
the Former USSR have experienced salinisation due to unsustainable irrigation
schemes and cultivation practices (e.g. drying-up of the Aral Sea). In addition, past
agricultural policies favored high productivity via incorrect use of mineral fertilizers,
pesticides and heavy machinery results in increased rates of soil loss by erosion,
pollution of surface and groundwater and soil fertility problems.
SOIL COMPACTION
The most common form of soil physical degradation is soil compaction.
Crescimanno et al. (2004) reported that of 32% of soils in Europe are highly
vulnerable and 18% moderately affected. Soil Compaction decrease soil porosity and
soil infiltration rate, can cause redistribution of water and nutrients, decrease root
development and penetration, and the direction and depth of root growth, and
increases surface runoff. This increases the risk of water erosion and its adverse
effects on soil (EEA, 1995). As the degree of compaction increases, cultivation
becomes more difficult demanding more energy as well as decreasing crop yields and
productivity. Compaction alters also the quantity and quality of biochemical and
microbiological activity in the soil. Large-scale, fully-mechanized crop production
may result in the physical degradation of soils. Compaction, structural damage,
surface crusting and sealing occur frequently in areas used for intensive agriculture
(Varallyay, 1981, 1985).
Soil susceptibility to compaction depends on soil properties and a set of external
factors like climate, soil use, etc. Soils can vary from being sufficiently strong to resist
all likely applied loads to being so weak that they are compacted by even light loads.
The natural susceptibility of soils to compaction mainly depends on soil texture, with
sandy soils being least and clayey soils with a high water table being most susceptible
(SoCO project team report, 2009). European soils used for agricultural and pastoral
purposes have a predominantly low or medium natural susceptibility to compaction.
Compaction of topsoil can easily be countered by reworking the soil and can
eventually be reversed if the biological processes in the soil remain undisturbed. Jones
and Montanarella (2002) stated that although reduced tillage results in higher bulk
densities, in most cases no reduction or even an improvement of soil qualities will
occur, compared to conventional tillage.
Subsoil compaction is assumed to be largely a product of the forces applied when
wheeled equipment is driven over the soil surface (Hakansson, 1994). According to
Jones et al., (2003), more than a third of the soils in Europe are highly susceptible to
compaction in the subsurface soil. Deep soils with less than 25 % clay content are the
most sensitive to subsoil compaction (Hebert, 2002). Compared to topsoil, deep
compaction of subsoil is persistent and cannot easily be reversed (EEA, 1995b).
Approximately the 36% of European subsoils face high or very high susceptibility to
compaction (Van-Camp et al. 2004).
SOIL SEALING
Soil sealing is the covering of the soil surface with an impervious material or the
changing of its nature so that the soil becomes impermeable. Although the
Communication on Soil Protection in 2002 does not address the issue of spatial
planning, it recognizes sealing as a threat to soil. The sealing of soils by impervious
materials is detrimental to its ecological functions. Soil degradation by sealing is
increasing at an alarming rate in developed countries. This process constitutes one of
the principal causes of soil loss in Europe and is especially severe near large cities and
in coastal areas (Castillo et al., 2004 a).
Most affected areas are the urban and metropolitan areas due to many
constructions. The development of transport infrastructures is an important cause as
well. For example in already intensively urbanized countries like Holland the rate of
soil loss due to surface sealing is high. Most of the growth will presumably take place
within or on the edge of the suburban areas. The great urban and industrial
development of the towns surrounding Madrid has caused an irreversible and rapid
soil loss (Rodríguez and González 2007). In addition, in the Mediterranean coasts, soil
sealing is a particular problem along the coasts where rapid urbanization is associated
with the expansion of tourism.
Soil sealing is almost irreversible. The sealing of land leads to the direct run-off of
precipitation into rivers (EEA, 2001) which subsequently enhance the risk of flooding
at the regional level. In addition sealed areas may have a great impact on surrounding
soils by changing water flow patterns and by increasing the fragmentation of
biodiversity.
Regarding surface seal formation, it is an important factor of soil structural
degradation leading to reduction of infiltration and enhancement of surface runoff (Li
et al, 2009). Soils mostly vulnerable to soil sealing processes are degraded soils
lacking of cover vegetation. Degraded bare clay rich soils are prone to rapid surface
sealing and crusting due to cumulative rainfall and rainfall energy raindrop impact
(Foley 1991; Molina et al, 2007). This is due to rapid dispersion of clay minerals,
thereby reducing aggregate stability, accelerating surface sealing and crusting,
limiting surface water storage and surface infiltration, and enhancing surface runoff
and soil loss (Tolmie, 2000; Janeau et al., 2003; Lado and Ben-Hur, 2004; Li et al,
2009).
SOIL CONTAMINATION
Soil contamination can be divided in point source contamination and diffuse
contamination. Soil contamination from diffuse and localized sources can result in the
damage of several soil functions and the contamination of surface water and
groundwater. The soil functions most affected by contamination are its buffering,
filtering and transforming capacities.
Diffuse Sources
Diffuse pollution is contamination that comes from many individually minor,
dispersed sources. Diffuse soil contamination is in general associated with
atmospheric deposition, certain agricultural practices and inadequate waste and
wastewater recycling and treatment. Pollutants can be washed by rainfall both into the
soil and from soil into surface and groundwaters (POSTnote, 2006). Currently, the
most important soil contamination problems from diffuse sources are atmospheric
deposition of acidifying and eutrophying compounds or potentially harmful
chemicals, deposition of contaminants from flowing water or eroded soil itself, and
the direct application of substances such as pesticides, sewage sludge, fertilizers and
manure which may contain heavy metals.
Acidification is the most widespread type of soil contamination in western and
central Europe where vast areas have been affected, especially in Poland (10 million
ha including natural acidification) and Ukraine (about 11 million ha of agricultural
land). Atmospheric deposition releases into soils acidifying contaminants (e.g. SO2,
NOx), heavy metals (e.g. cadmium, lead arsenic, mercury), and several organic
compounds (e.g. dioxins, PCBs, PAHs). Acidification favours the leaching out of
nutrients leading to loss of soil fertility and possible eutrophication problems in water
and may decrease biological activity. In addition, acidifying components reduce the
buffer capacity of the soil and the pH will gradually decrease. Acidification in
Mediterranean soils, although not frequent, is caused by land use through the removal
of base cations from the soil by harvesting, careless use of nitrogen fertilizers and soil
drainage (Zalidis et al., 2002). Major effect of acidification is the mobilization of
aluminum from clay minerals which the soil might have accumulated (Logan, 1990).
Heavy metal input in agriculture may be caused by human activities, such as
fertilisation and amendment practices, used to increase soil productivity. Heavy
metals together with excessive nitrogen inputs are regarded as the main sources of
contamination in agricultural soils. Metals like Hg, Cd, As, Pb can contaminate the
soil gradually and damage soil and ecosystem functioning. These contaminating
elements will become part of the nutrient cycling resulting in biodiversity decline,
water pollution and consequently a potential danger for human health. (SCAPE 2006)
(SOCO project team report, 2009). High content of heavy metals in soils is reported in
Central-Estern Europe (Van Lynden, 2000). The contaminated sites in Ukraine there
are about 5 million ha, mostly in human settlements and around the industrial
factories and in Lithuania nearly 3 million ha. In addition, the excessive application of
fertilizers usually exceeds the functional ability of the soil to retain and transform the
nutrients. The saturation of the soil with nitrogen or phosphate, have led to losses of
nitrates and saturation of the soil with phosphate, which move into groundwater
(Breeuwsma and Silva, 1992).
According to Zalidis et al (2002) the main soil functions that degraded due to
extensive use of pesticides are the food web support, the retention and transformation
of toxicants and nutrients, soil resilience, and the ability of soil to protect surface and
ground water. Uncontrolled use of pesticides is linked to alteration of soil properties,
the degradation of soil’s toxicant retention and transformation function, the
destruction of part of the soil flora and fauna (Pimentel and Levitan, 1986), leaching
and drainage of pesticides into the surface and ground water,
Localized sources
Point source contamination is often linked to no operational industrial plants, past
industrial accidents, uncontrolled industrial municipal and agricultural waste
disposals, and mining activities (EEA-UNEP, 2000). Sites contaminated in these ways
can pose serious threats to health and to the local environment as a result of releases
of harmful substances to water resources, uptake by plants and direct contact by
people. Major pollutants include heavy metals, organic contaminants such as
chlorinated hydrocarbons, and mineral oil.
Soil contamination from local sources is widespread across Europe. Estimates of
the number of contaminated sites in the EU-15 (before the enlargement with the
Central and Eastern European Countries) range from 300.000 to 1.500.000 (EEA,
1999). The largest and probably most heavily affected areas are concentrated around
the most industrialized regions in northwest Europe, from Nord-Pas de Calais in
France to the Rhein-Ruhr region in Germany, across Belgium and the Netherlands and
the south of the United Kingdom (EEA-UNEP, 2000).
In the mining industry, which is a major driver of soil degradation in central and
eastern European countries, the risk of contamination is associated with sulfur and
heavy metal-bearing tailings stored on mining sites, and the use of certain chemical
reagents such as cyanide in the refining process. Waste landfilling is an important
potentially contaminating activity as well. On average, 57 % of municipal waste
generated in the EU is landfilled. Application of farm manures, sewage sludge, and
composted green wastes lead to air pollution (odour and ammonia) and to diffuse
water (nitrate and phosphate) pollution, Moreover the potential soil contamination is
greatly increased in landfills that that do not comply with the minimum requirements
set by the landfill directive (Directive 1999/31/EC) (European Commission, 1999).
For Mediterranean countries disposal of Olive Mill Wastes (OMW) are considered a
major environmental problem. OMW contain oil compounds that may result in
increased soil hydrophobicity and decrease water retention and infiltration rate (AbuZreig and Al-Widyan 2002). The use of OMW in agriculture may also affect acidity,
salinity, N immobilization, microbial activity, nutrient leaching, lipids concentration,
organic acids and phenolics (Gioacchini et al, 2007; Mekki et al, 2007; Sierra et al,
2007). These compounds may cause alterations in N cycle, changes in soil microbial
activity as well as contamination of surface- and groundwater.
Land contaminated with naturally occurring radioactivity is considered for uranium
and other mining tails, phosphogypsum dumps, metal industry, etc. In some parts of
Europe, soil is contaminated by artificial radionuclides. After the Chernobyl incident
it was shown that nuclear fallout, especially 137 Cs, is retained in the soil, causing
potential pollution hazards.
SOIL ORGANIC MATTER DECLINE
Soil organic matter (SOM) is affected mostly by climate, soil parent material,
texture, hydrology (drainage), topography, land use (tillage) and land cover and/or
vegetation (Grasslands, Forests, agricultural crops) (Smith et al., 2005; Hanegraaf
2009).
The decline of SOM caused by changes in land use, intensification of agricultural
practices (e.g. deep plugging, rapid rotation), at the expense of the naturally forested
areas of Europe, and, possibly, climate change (accelerating the decay of organic
matter releasing more CO2 and increasing climate change) is among the main threats
to soil fertility and quality (Matson et al., 1997; Vleeshouwers and Verhagen, 2002;
Freibauer et al., 2004; Davidson and Janssens, 2006)
In European Union most soils are out of equilibrium as regards soil organic matter
contents. Land use and climate change have resulted in soil organic carbon loss at a
rate equivalent to 10 % of the total fossil fuel emissions for Europe as a whole. Jones
et al. (2005) calculated that 0.6% of soil carbon in European terrestrial ecosystems is
lost annually. In addition approximately 45% of soils in Europe have a low or very
low SOM (0-2% organic carbon) and 45% have a medium SOM (2-6% organic
carbon).
Almost half of European soils have low organic matter content, principally in
southern Europe but also in areas of France, the United Kingdom and Germany. In the
Mediterranean region the loss of organic matter during last decades is estimated at
around 50% of the original content (Van Camp et al., 2004). Nearly 75% of the total
area in Southern Europe has a low (3.4%) or very low (1.7%) soil organic matter
content (Montanarella 2005). Soils with less than 1.7% organic matter to be in predesertification stage.
Decline of soil organic carbon content is threatening the diversity of organisms in
soils. It can also limit the soil’s ability to provide nutrients to crops leading to lower
yields and affect food security. Loss of soil organic matter reduces the water
infiltration capacity of a soil, leading to increased run-off and erosion and vice versa:
In particular in arid and semi-arid areas of Europe loss of SOM is closely linked to the
process of soil erosion. Erosion reduces the organic matter content by washing away
fertile topsoil which may lead to desertification under semi-arid conditions
(Montanarella et al, 2004).
SOIL EROSION
Often, different types of soil degradation occur at the same time. The most visible
form of soil degradation is expressed in soil erosion. Soil erosion is the detachment
and movement by wind or water of soil particles from their place of origin (Larson et
al, 1985).
Soil erosion is a natural process, accelerated by human activities and unsustainable
agricultural practices like poor water and irrigation management, large-scale farming,
deforestation, overgrazing, and construction activities (Yassoglou et al., 1998), and it
is effectively irreversible. With a very slow rate of soil formation, any soil loss of
more than 1 t ha-1yr-1 can be considered as irreversible within a time span of 50-100
years (Jones et al., 2004). Hillel (1991) concluded that deterioration of soil properties
due to erosion is mainly linked to diminished infiltration and water availability, loss
of organic matter reducing root penetration, soil moisture and permeability, loss of
nutrients and an ultimate loss of production potential (Van Der Knijff et al., 2000).
Probably the most serious loss in long-term productivity from soil erosion is loss in
plant-available soil water capacity can result in significant reductions in crop yields.
(Larson et al, 1985).
Soil erosion can occur by wind or water. Soil erosion in Europe is due mainly to
water (about 92 % of the total affected area) and less to wind. The major causes for
water erosion are intense rainfall (particularly pronounced in clay soils after long
droughts), topography, low soil organic matter content, percentage and type of
vegetation cover and land marginalization or abandonment. The combination of these
major drivers is mainly found in Mediterranean zone where several areas have a high
risk of erosion mainly due the increase in frequency and extent of forest fires and to
unsustainable cultivation practices (SoCo Project Team report, 2009). Almost 50–70
% of agricultural land in Med countries is at moderate to high risk of erosion
(UNECE, 2001). Moreover, in parts of the Mediterranean region, erosion has reached
a stage of irreversibility and in some cases erosion has ceased because there is no
more soil left (Montanarella, 2005).
Although the Mediterranean region is the most severely affected by erosion soil
erosion can be considered, with different levels of severity, an EU-wide problem. Soil
erosion affects large areas of Europe; about 17 % of the total land area in Europe is
affected to some degree, with around 27 million ha in the EU (Oldeman et al., 1991).
These leads to one of the major and most widespread forms of soil degradation in EU.
In particular 26 million hectares in the EU suffer from water erosion and 1 million
hectares from wind erosion (GLASOD, 1990).
Wind erosion is currently recognized as a major source of environmental
degradation (Okin et al., 2001) through its direct influence on particularly vulnerable
land and it is considered a environmental and agricultural problem in many arid and
semiarid agricultural regions of the world (Gomes et al., 2003). A soil’s susceptibility
to wind erosion is determined by its erodibility (mainly soil texture and organic matter
content) and the climate’s erosivity (mainly wind velocity and direction and
precipitation) (SoCo Project Team report, 2009). It is localized in some parts of
Western and central Eastern Europe (EEA, 2002b) and in the marginal drylands of the
Mediterranean region (Massri et al, 2002).
The effects of wind erosion on agricultural productivity are detectable only after
years or decades due to the fact that wind erosion, can result in a loss of the finest soil
particles and soil nutrients, and in degradation of soil structure (Lowery et al., 1995;
Larney et al., 1998), and subsequently in an ongoing decrease in soil fertility. longterm wind erosion significantly change some soil properties, including increased sand
content and pH values, and decreased clay content, organic matter, nutrients, and soil
water (Zhao et al., 2006).
According to Lyles (1988), favorable conditions to wind erosion exist when the
soil surface is loose, dry and finely granulated, when the soil surface is smooth and
generally lack crop coverage in the windy season and when the susceptible area is
sufficiently large. Farm lands with these characterizes in arid and semiarid regions are
vulnerable to wind erosion (Li et al., 2004). In soils consists mainly of coarse sand
and silt, sand accumulation does not result in any loss of original soil clay particles or
nutrients, only in a reduction of clay and nutrient concentration because of additional
sand deposited in the soil (Zhao et al 2006).
In addition erosion is caused not only by wind and water but also by tillage, mainly
due to the use of heavy powerful tillage machinery. Improperly used, tillage can set in
motion a wide range of degradative processes e.g. deterioration in soil structure,
accelerated erosion, depletion of soil organic matter and fertility, and disruption in
cycles of H2O, C, and major nutrients (Lal, 1993) leading to crop yield reductions. In
Mediterranean region, olive crops and vineyards, when intensively ploughed, are
among the crops more susceptible to erosion because a high percentage of the soil
surface remains uncovered by vegetation all the year round.
FLOODS AND LANDSLIDES
Floods and landslides are not a threat to soils in the same manner as the threats
already listed. Decreased organic matter content and soil compaction result in reduced
water retention capacity of the soil and therefore the capacity of the soil to store and
retain water is decreased, which results in an increased risk to flooding (Jones and
Luca Montanarella, 2002). Landslides are often related to the geological situation,
bank erosion and changes in land us (e.g. deforestation or by abandonment of land).
The threat of landslides is increasing due to population growth, summer and winter
tourism, intensive land use and climate change (Montanarella and Toth, 2008).
.
SALINISATION-SODIFICATION
Salinisation, is the accumulation of water-soluble salts (include sodium, potassium,
magnesium and calcium, chloride, sulphate, carbonate and bicarbonate) on or near the
surface of the soil, whereas sodification concerns an increased content of
exchangeable sodium (Na+) resulting in completely unproductive soils.
The main natural factors influencing soil salinity are climate, the salt contents of
the parent material and groundwater, land cover and topography. Secondary
salinisation is caused by human interventions like land use, farming systems,
excessive groundwater extraction, industrial activities and land management, such as
the use of salt-rich irrigation water and/or insufficient drainage and evaporation of
saline groundwater, (European Commission, 2000b). The salt accumulation in a
certain area is governed first and foremost by the water balance of the area and
particularly by the ratio of evapotranspiration to drainage (Richards, 1989). In
particular in arid areas fresh water scarsity and poor irrigation practices are linked to
salinization. Saline soils have developed in most arid regions, where climate is the
determining driver. Salinisation affects around 3.8 million ha in Europe (EEA 1995b)
Soil salinisation affects approximately 1 million hectares in the EU, mainly in the
Mediterranean countries where the 25 % of irrigated cropland is affected by moderate
to high salinisation and is a major cause of desertification (Programa de Accion
Nacional Contra la Desertificacion 2001). The most affected zones in dry land areas
of Europe by salinization are located in Hungary, Romania, Spain, Italy, Albania,
FYROM and Greece, according to several authors (Szabolcs, 1991; Misopolinos and
Szabolcs, 1996; Yassoglou et al., 1998; EEA, 2000; Montanarella, 2005; Varallyay,
2009).
Most Mediterranean countries are arid or semi-arid with mostly seasonal and
unevenly distributed precipitations. Due to the rapid development of irrigation and
increased demand for domestic water supplies, conventional water resources have
been seriously depleted. As a result, wastewater reclamation and reuse is increasingly
being integrated in the planning and development of water resources in the
Mediterranean region, particularly for irrigation. Ravikovich, (1992) stated that in
clayey soils (Alluvial Vertisol) increasing salinity problems are caused by irrigation
with domestic effluent water and Mirlas et al, (2003) underlined that the main reason
for soil salinization near local water reservoirs is the changes in the quality of the
water, i.e. the transition from seasonal winter flood water to domestic effluents.
Moreover poor irrigation practices using brackish groundwater in some areas lead to a
major problem of soil salinization.
Salinization has a direct negative effect on soil biology and crop productivity, and
an indirect effect. This increases the dispersion and destruction of the soil structure
and causes the formation of crust on the soil surface (Goldshleger et al., 2009). An
increase in the soil salinity is followed by an increase in the soil SAR, (Rhoades,
1990), leading to loss of soil stability through changes in soil structure (Szabolcs,
1996). High levels of salinity in soils provoke the withering of plants and increases
the impermeability of soil layers, eliminating the possibility to use the land for
cultivation. Excess sodium on the exchange complex results in the destruction of the
soil structure that due to a lack of oxygen, cannot sustain either plant growth or
animal life (SoCo project team report, 2009).
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