Organic wastes as a resource for Mediterranean soils

An overview of the main soil characteristics and threats in the Mediterranean Basin are analysed. Low organic matter content is one of the common features of Mediterranean soils. The re-use of organic wastes through soil system for different land purposes seem to be the best way to maintain or improve organic matter contents in those soils. An extensive characterisation of different sources of organic waste, before and after composting treatment, from Catalonia (Spain) is presented. Several suggestions are done in order to incorporate best management practices for waste management and their agricultural reuse in relation with sustainability of as southern European region. MEDITERRANEAN SOILS AND THE EXTEND OF MAIN THREATS The soils from the south of Europe have a great diversity. The forming-factors which played the most important role for their development and behaviour are the nature of parent material, climate and topography. Numerous parent materials of different composition and origin are present, but calcimagnesic materials are the dominant in many areas. The soil forming processes are often linked with migration and accumulation of clay, calcium carbonates or more soluble salts as gypsum. The climate is characterised by warm to hot dry summers, with high solar radiation and high evaporation rates, and generally rainy winters, often with maximum rainfall in autumn. Frost risk is generally limited to very short periods. The most singular climate characteristics are the low mean annual rainfall and the summer drought. The length and intensity of the dry season is a good indicator of moisture stress, which in turn is reversibly related to the length of the growing season. The landscape topography is among the most varied in Europe. It is marked by relatively young orogenic systems with high, sharp folded and faulted mountains and hills rising close to the coast. The uplands are highly dissected, complex and partially unstable, with many slopes and shallow rocky soils (Zalidis et al, 2002). An important percentage of this area is used for agricultural purposes and agricultural activity is usually combined with livestock and forest exploitations. The agro-silvo-pastoral systems are traditionally employed since ancient times. The main crops are olives, grapes, almonds, figs and cereals. With irrigation, dominant crops are fruit trees and vegetables. The Mediterranean ecosystem is fragile not only for its special climatic conditions and varied topography but also because is intensively affected by many and diverse human activities (Felipó, 1996). Significant soil degradation processes have been recognised in this area (EEA, 2001). Agricultural soils are vulnerable and have been deteriorated over time by the action of different soil degradation processes. Water erosion is a limiting factor for soil development when slopes are pronounced and no conservation practices in agricultural systems or abandoned agricultural land. Generally, the soil organic matter content is low as a result of ancient and frequent cultivation and use of synthetic fertilisers, which induce also to non-point source pollution. Additionally compaction resulting from the repetitive and cumulative effects of heavy machinery is present in some areas. Over the last half-century irrigation had been the most important change of agricultural land use. The total coverage of irrigated land increased since 1965 to 1982 by almost 50%, (SánchezDíaz, 1993). Water resources in general limit the irrigated areas and irrigation is responsible for salt accumulation in soil. (Szabolcs, 1991). Salinisation may occur also as a result of salt mobilisation during summer by evapotranspiration. Desertification, the final stage of the soil degradation process in dry regions, has also been identified in this area and could extend if the main threats are not controlled. The extension of the dominant soil threats -erosion, salinisation and organic matter declinein European Mediterranean region has recently summarised by European Commission (2001a) as follow: In more than one third of the total land of the Mediterranean basin, average yearly soil losses exceed 15 tones per hectare (UNEP 2000). Soil salinisation is affecting an estimated 1 million hectares in the EU, mainly in the Mediterranean countries and is the major cause of desertification. In Spain 3% of the 3.5 million hectares irrigated land is severely affected, severely reducing its agricultural potential, and another 15% is under serious risk. Soil organic matter decline is of particular concern in Mediterranean areas. According to the European Soil Bureau, based on limited data available, nearly 75% of the total area analysed in Southern Europe have a low (3.4%) or very low (1.7%) soil organic matter content. Agronomists consider soils with less than 1.7% organic matter to be in predesertification stage. Nevertheless, in southern of European countries there are some differences between soils and their development and/or degradation stages due to intrinsic and extrinsic attributes of soils, both exert a relevant influence on soil process intensity. Soil intrinsic attributes are those given mainly by the forming-factors as nature of parent material, climate severity, landscape topography and natural vegetation. They determine the soil capacity for a specific land uses. The soil extrinsic attributes are subject to change according to the land use and management practices. The present low organic matter content in arable soils of Mediterranean region is a result of the ancient cultivation and intensively management over the pas century. In this area the decline in organic matter contents has now become a major process of land degradation (Rusco et al. 2001). NEEDS OF ORGANIC MATTER FOR MEDITERRANEAN SOILS It is a fact that the content of soil organic matter in several areas of Mediterranean region is much low than the previous estimated data. Recent regional studies conducted in Spain show the real organic matter content in two areas, Figure 1. One at the central part of the country (Castilla la Mancha) where most of vineyard’s soils has less than 1.5%, MAPA 2000a. The other, at the province of Lleida at the north-west of Catalonia, where the organic matter content for the most agricultural soils and pastures is lower than 2% (MAPA 2000b). Soil organic matter plays an important role in maintaining soil functions and prevents the progress of some soil degradation processes as soil erosion. Also in soils affected-salts, after drainage practices in irrigated land, the salinization can be mitigated when soil organic matter levels are properly managed. The relationship between the amount of organic matter and soil properties is well known. The soil properties most likely affected by low organic matter content are the following: Changes in soil structure; Aggravation of crusting; Accelerating of runoff and erosion; Evolving favourable conditions for soil compaction; Increase of soil temperature; Decreasing soil moisture and nutrient retention; Reducing soil fertility, soil biodiversity and biological activity. The following factors affect the content of organic matter in soils: The climatic conditions, temperature and moisture regimes, which never work separately. The presence of clay minerals in soil what allows forming stable complexes. The kind of agricultural practices and management techniques used, as conservation tillage or non-tillage and crop rotation. The nature of organic material input, the amount of resistant organic matter, being the stability degree a good indicator for waste quality. Notwithstanding, the decomposition of organic matter is relatively constant in a given climate, soil and management system. Intensive agricultural practices have neglected to maintain soil organic matter content, due to low or no input of plant residues and inappropriate application of farmyard manure. In those conditions the re-use of different organic amendments, in addition to manure, through compost applications seems to be the best way to improving soil organic matter status of such soils (Felipó 1996). For that an accurate management practices should be conducted. Organic wastes (OW) have long been recognised as one of the best methods to maintaining fertility, productivity and soil organic matter. Therefore it can be assumed that OW could be a source of organic matter and a resource for soil protection in Mediterranean basin. THE OPTION LAND RECYCLING IN ORGANIC WASTE MANAGEMENT Nowadays there are other reasons for waste re-uses in agricultural soils than to maintaining or improving soil organic matter content. The pressing needs for OW management due to current amount generated. The environmental problems linked with the different waste management options. And the needs to reduce greenhouse gases emission. Those reasons, in general, work always at the same time. While the land application of OW can provide a solution of these 0 1 2 3 4 5 6 7 8 9 OM %