The concentration levels of cadmium and lead in 250 horizon samples from 125 different soil profiles of undisturbed soils of La Rioja, Spain, were determined. Knowledge of regional contents as well as the variability and background values of trace elements in soils is of critical importance to assess human impact on the soil environment and to determine baseline values and quality standards. Heavy-metal horizontal distribution was mainly affected by the nature of the bedrock and, to a smaller extent, anthropogenic activity. Vertical distributions were mainly of a natural origin depending first on the bedrock and second on the result of soil processes.

The goal of the present work was to increase our knowledge on the behavior of manganese and nickel in soil within a Mediterranean environment. The study assessed the concentration levels of Mn and Ni (heavy metals selected for their essential role in the development of plants) in 250 soil horizon samples within 125 soil profiles of undisturbed soils in La Rioja (Spain). The study was undertaken to investigate and predict Mn and Ni concentrations on a regional scale. The analysis of spatial distribution of the elements was found to be affected by the nature of bedrock and, to a lesser extent, the anthropogenic origin. The variation of vertical distributions can be related, first, to natural sourcesmainly the bed rocks-and, second, to soil processes. The geographical distribution of soil Mn is important to agriculture, nutrition, and health. Soil Mn and Ni maps of the area were elaborated, using geostatistics and geographic information systems. Mapping of geographical distributions will be useful in future research to determine regional patterns of Mn and Ni bioavailability, Mn and Ni deficiencies, and the possible consequences of land disposal of Mn-and Ni-laden wastes.

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Estimating the spatial variability of soil properties is significant for evaluating environmental impacts. For example, many soil properties are directly used in the modelling of environmental processes such as global climate change. These aspects have not previously been studied at this level in La Rioja (a region of Spain with a humid Mediterranean environment). The intention of this study was to provide quantitative information on soil assessment and mapping methods for natural soils in a humid Mediterranean environment. The properties considered included: pH and organic matter, calcium carbonate and clay contents. For testing, samples were selected from several different soil types which, in theory, were only affected by pedogenetic processes and had developed on different parent materials. More than half of the samples did not contain any CaCO₃, while the rest of the samples presented a variety of CaCO₃, forms, with high percentages being present in certain cases (up to 65% on the surface). It was possible to establish two different areas: one predominantly acidic and the other principally basic. The predominately basic samples were due to the high percentage of carbonate in the parent materials. The clay content on the surface was similar to that in the subsurface layers. Finally, the organic matter contents in the uppermost layers presented average values of 3.9%,with a range of from 0.3 to 17.5%. The major variations in soils were determined by soil type (therefore by soil forming processes), parent material composition, and vegetation type. This study reveals that these methods are useful to determine the spatial distribution of some soil properties in undisturbed soils. The contour maps of soil property variability could be used to improve future sampling designs and management decisions.