Metal concentrations in agricultural and forestry soils in northwest Spain: implications for disposal of organic wastes on acid soils

In Galicia (northwest Spain) the application of organic wastes to agricultural land is a common practice, which may increase total and bioavailable metal contents in the soil. In this study, total metal concentrations were determined in acid soils under different use (pasture, cropland, woodland) in an agricultural area where agro-industrial sludges are frequently recycled as manure. The aim was to establish baseline metal levels which could be used to determine the capacity of soils to absorb organic wastes. The estimation of baseline metal concentrations was carried out by two methods, one based on the analysis of means and geometric deviations, and another based on a modal analysis. Results suggested that the modal analysis procedure might be preferable when analysing data sets with a heterogeneous frequency distribution. In general, there was no significant difference in total metal concentrations when comparing soils from different land uses. Baseline levels for each metal indicated that all soils were suitable for organic waste application under current European Union (EU) legislation. From 2015, more restrictive metal limit values have been proposed by the EU, potentially preventing the addition of metal-containing wastes to pasture, cropland and woodland soils. The dissolved metal values in each soil were also estimated by empirical equations relating total metal concentrations, pH and organic matter content. Results showed that only the pasture soils would be suitable for organic waste disposal under the proposed EU metal limits for 2015, due to liming and substantial organic matter content. Total metal concentrations were insufficient to discriminate environmental risk in acid soils of different land use. The determination of baseline levels in reference areas and the estimation of soil metal bioavailability are suggested to define permissible values in the developing legislation.