Soil Degradation in Mediterranean Region and Olive Mill Wastes

Introduction

Soils in the Mediterranean region present an enormous variability according to different soil taxonomy systems (USDA, FAO, etc.)- This soil diversity reflects differences hi climate, geological origin, vegetation, land use and historical development of Mediterranean landscapes. In general, soils have medium to poor fertility, with low organic matter contents due to low natural vegetation developed on them and because the active human activity from more than 2000 years of cultivation. In Southern Europe high rates of soil loss are recorded due to surface sealing deriving from the rabid urbanization in the coastal zones and transport infrastructure, hi most of the coastal areas, there is high competition for the usage of land, with important consequences to the soil resources and in general to the environment. In fact, competition between different uses of soil, leading to soil contamination and consumption of the soil resource, is becoming more severe in the Mediterranean region, mainly as the result of increasing urbanization and tourism. Contamination is high in restricted areas, in urban areas and in hazardous industrial compounds, due to both diffuse and localized sources. In addition, salinization and soil degradation through forest fires play an important role as well. Furthermore, soil has a determining role in maintaining slope stability. However, soil degradation, soil erosion and soil sealing in combination with intense precipitation can be the cause of various catastrophic events.

Above all, the mam environmental problems that the Mediterranean area encounters are scarce water availability for human activity and the soil degradation by soil erosion especially in areas where high quality and easily erodible soils are subject to more intensive agriculture. Soil erosion is mainly due to low vegetation cover, intensity of rainfall, and intensive cropping and over-grazing management. In terms of Mediterranean agricultural systems, fanners practice both extensive rain fed agriculture and intensive irrigated agriculture. Extensive rain-fed agriculture deteriorates the initial soil properties and quality and increases desertification problems, especially when practiced in soils with low organic matter contents and poor quality and where stubble burning and over-grazing practices and lack of organic residue incorporation reduce fertility and biodiversity. Intensive irrigated agriculture with excessive and inefficient water use and chemical rate applications can increase negative environmental impacts associated to soil degradation, by chemical pollution and salinization.

Soil degradation in the form of soil carbon loss has increased rapidly during the last 200 years. Agronomic measures related to managing the vegetation cover, soil management (tillage and nutrient supply) and mechanical methods resulting in durable changes to the landscape are among the measures that are taken so as to alleviate soil degradation problems. The application of organic wastes could be a way of solving two problems, the waste disposal and the collection of the low organic matter content of many agricultural soils. Olive Mill Wastes could be applied as an amendment on the agricultural soils under specific conditions. In this context we review the soil degradation in Mediterranean region and how the sustainable use of Olive Mill Wastes in particular liquid wastes can reduce soil degradation.

Soil protection

The functions provided by soils are manifold because they are related to processes such as filtering, storage and buffer functions, serving human activities such as plant production, recreation and leisure (Graeber and Blankenburg 2009). 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. In accordance with the European thematic strategy for soil protection (EC 2006a, b), the following soil threats were identified: erosion, organic matter decline, contamination, salinization, compaction, soil biodiversity loss, sealing, landslides and flooding (Paya Perez and Rodriguez Eugenio 2018).

Soil degradation

Soil degradation is a serious threat for an increasing number of areas all over the world (Lai et al. 1989). Soil degradation is defined as a process that causes deterioration of soil productivity and lowers 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 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 are, namely, 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 are often driven by unsustainable agricultural practices such as overgrazing of pasturelauds, over intensive annual cropping, deep ploughing on fragile soils, continuous ttse of heavy machinery destroying soil structure through compaction and unsustainable irrigation systems contributing to the salinization of cultivated lands (German Advisory Council on Global Change (WBGU) 1994, EEA1999, Darwislr and Kawy 2008). Soil degr adation involves physical loss (soil erosion by water or wind) and the reduction in quality of topsoil associated with nutrient decline and contamination (chemical, physical, and biological degradation). Soil erosion is one of the most important sources 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 stmcmre and the increase of desertification of marginal lauds 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. The chemical degradation mainly consists of soil pollution and acidification. Its consequences are mobilization of harmful elements/compounds, salinization and/or sodification. unfavorable changes hr the nutrient regime and decrease of natural buffering capacity. Physical deterioration is comprised of surface sealing or crusting of top soil, soil compaction, stmcmre destruction and extreme moisture regime. 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 aboveground 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 degradation processes, land use and the duration of degradative land use (UNEP Staff 1991. Danvish and Abdel Kawy 2008). The processes of soil degradation have major implications at the following: (a) global carbon cycle, mainly due to the decrease in soil organic matter and the release of C02 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, and (f) human health.

 
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