Soil degradation in Mediterranean region
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 modem human activities is increasing and leads to irreversible losses mainly due to local and wide-spread contamination and soil erosion (EEA 2000). 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, and, (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 Dogm 1997, Cammeraat and hneson 1998, Zalidis et al. 2002). Severe erosion and other degradation processes are taking place in Mediterranean soils (Marimez-Meua 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 change linked to human intervention and then possible adverse influence on the environment (Jeftic et al. 1993, Perez-Trejo 1994, Hill 2003). More particularly, the Mediterranean islands have been recognized as “hot spots” for various forms of soil degradation (EEA 2000).
Decline of organic matter
With respect to soil organic matter, it is affected mostly by climate, soil parent material, texnrre, hydrology (drainage), topography, land use (tillage) and land cover and/or vegetation (grasslands, forests, agricultural crops) (Smith et al. 2005, Hanegraaf 2009). Soil organic matter, especially the humified fraction (Graham et al. 2002), constitutes an important source of nutrients, while it is a key factor in maintaining or improving soil structure (Ilay et al. 2008). In fact, soil is one of most important natural capital providing ecosystem services pivotal to sustain life. Soil is the second biggest reservoir for carbon after the oceans. There is more carbon stored in soil than in the atmosphere and in vegetation combined (Mokany et al. 2006). Soil therefore plays an important role in C cycle and in turning on the global climate system.
The general threshold of SOC is 2%; however, local conditions such as soil texture introduce large variation (ca. 1-3%), indicating the importance to stratify thresholds according to qualitative data such as soil type. In recent decades. SOC content has been depleted down to approximately 1% mainly because of its overexploitation and inappropriate management practices. The decline of soil organic matter caused by changes in land use, the intensification of agricultural practices (e.g., deep plugging, rapid rotation) at the expense of the naturally forested areas of Europe and, possibly, the climate change are among the mam 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.
Soils in many Mediterranean areas are generally characterized by low organic matter content, fertility and productivity and are subject to erosion (Albaladejo et al. 1994). Mediterranean area is facing such a low level of SOC concentration because of dry and warm seasons. Low level of SOC hampers most of soil functions causing disservices (e.g., soil erosion, reduction of soil water- holding capacity, low crop yield) and triggering increased external inputs. 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 are considered to be in pre-desertification stage. The 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. More particularly, the loss of SOM in arid and semi-arid areas of Europe 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).
Desertification is defined by the United Nations Convention to Combat Desertification (UNCCD1994) as “the degr adation of the land in arid, semi-arid and dry-sub-humid areas, as a result of several factors, including climatic change and human activities”. Once the desertification process has started, physical processes like wind erosion, soil crusting and surface water erosion accelerate the land degradation process. Furthermore, desertification contributes to environmental crises, such as the loss of biodiversity and global wanning. 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). 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 in 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 degr adation, has led to desertification in some areas of the Mediterranean. Soil desertification is a specific phenomenon of arid, semi-arid, and dry sub-humid regions. According to the United Nations Environmental Program’s (UNEP) report, desertification 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). Vacca et al. (2009) noted that the main factors of soil degradation hi the Mediterranean region are essentially human related which consist mainly of accelerated erosion, loss of prime farmland, compaction, salinization, contamination, decrease of organic matter content and adverse alteration of biological processes.