Chemical vulnerability of red soils in La Mancha (Central Spain)
Introduction
Soil constitutes a key system as it works as a complex reactor, capable of undertaking the functions of filtering, decomposing, neutralizing, inactivating, storing, etc. Therefore depending on the fragility and susceptibility level of each specific profile, soil will be degraded more or less drastically, after undergoing eventual impacts and environmental unbalance due to unsuitable use (Keeney, 1986, Rodvang and Simpkins, 2001). Red soils represent perhaps one of the most important soil resources for the agricultural production of the Mediterranean areas. The term red soil as used in this article covers all red soils sequences found in La Mancha (central region of Spain, Fig. 1), including the Terra Rossas. The great majority of these soils are classified as Alfisols under the US System of Soil Taxonomy, and are rarely related to the orders of Mollisols or Ultisols. The La Mancha red soils are similar to the red soils in the other Mediterranean areas by having:
- a.
Munsell colour of 2.5 YR or redder in the Bt horizon with high clay content,
- b.
basic or acidic pH,
- c.
low organic matter contents,
- d.
medium to high cation exchange capacities,
- e.
textures varying from clayey to sandy loam.
The dominant clay minerals are illite, kaolinite and smectite along with high contents of sesquioxides, hematite and/or amorphous iron. They are moderately weathered soils in good physical and chemical condition, and sustain arable cropping systems with careful management, but are susceptible to erosion and contamination. In fact, nutrients are added to many of these soils by fertilizers and residues, which are not entirely absorbed by the plants allowing excess percolation and contamination to underground and superficial waters, as it is in many developed countries (McLaughlin et al., 1985, Zhang et al., 1996). A reduced supply of organic C and nutrients caused by deforestation, soil erosion and intensive agriculture, and especially by environmental stress conditions (heavy metal contamination, for example) reduces biodiversity in the soils (Brown et al., 1994). So, being aware of soil vulnerability, a term used to express the intrinsic properties which determine the capacity of soil to buffer contamination or allowing it to pass on to another environmental echelon, is of prime importance.
Our primary objective is to suggest a series of intrinsic properties that can help control the red soils vulnerability to chemical degradation. The second objective is to combine these variables, and suggest an index to predict the chemical vulnerability of La Mancha's red soils.
Section snippets
Materials and methods
Forty-seven representative pedons comprising one hundred twenty five soil samples were examined from different locations in La Mancha. Tests were conducted to obtain the most representative parameters on soil vulnerability, by multivariate analysis according to sketch in Fig 2. The soils studied together with five selected soil profiles were developed on basalts, limestones, shales, granites and quartzose sandstones. A general description of the investigated soils is given in Table 1 as well as
Results and discussion
Soils with low vulnerability levels are those which have one or more of the following properties: high levels of organic carbon, of clay (especially smectite), and of carbonate compounds and a deep profile. On the other hand, soils with high vulnerability levels contain low amounts of carbonate compounds, of clay, and of organic carbon and are not deep. These are the relatively resilient levels of vulnerability used by Eswaran (1994) in determining the quality of soils against risks for
Conclusions
The work that is presented evaluates the natural vulnerability of La Mancha's (central region of Spain) red soils, derived from several geological materials. Generally, these soils withstand intensive agriculture (cereal, vineyard, etc.) yet maintain essential soil qualities. The soils analyzed, (with the exception of profile number VII) keep their essential properties even though they undergo intense agricultural production, therefore, one of the attributes is the possession of a low
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