Factors influencing the bioavailability of some selected elements in the agricultural soil of a geologically varied territory: the Campania region (Italy) case study

Bioavailability of some major and trace elements was evaluated in 1,993 agricultural topsoil samples collected across Campania region (Southern Italy). Samples were analysed by an ammonium nitrate leaching prior to ICP-MS and ICP-ES analysis. The study aimed primarily at comparing the distribution of the bioavailable concentrations of selected elements with their pseudo-total concentrations, determined using a modified Aqua Regia digestion. Geochemical maps of the pseudo-total and bioavailable concentrations were generated using a multifractal inverse distance weighted (MIDW) interpolation. In addition, the spatial distribution patterns of the percent bioavailability of elements, based on the ratio among the bioavailable and pseudo-total fractions, were determined. A main focus was made on Al, Ca, K, Mg, Cu, Tl since they are linked, albeit for different reasons, to agriculture. The median value of the percent bioavailability showed the order Ca>K>>Mg≃Tl>>Cu>>Al and that represents a positive finding in terms of both agricultural productivity and environmental quality. A linear regression was applied to data to unveil any dependence between the bioavailable fraction and the pseudo-total content of elements. Furthermore, a multiple linear regression was then applied including in the model also the grain size distribution (i.e., clay, silt, sand) and organic matter content of samples, to evaluate their possible role in promoting the environmental availability of elements. The pseudo-total concentrations of Al, Ca, K, and Mg alone resulted to be poorly able to predict the variability of the bioavailable fraction whereas for Tl and, above all, Cu resulted to be more capable to forecast the patterns of the bioavailable concentrations. The addition of the grain size distribution and organic matter content to the models expanded the predictive capability of Ca, K, and Mg, whereas a marginal improvement was showed by Al, Cu, and Tl suggesting that other unknown factors probably exert a major influence on their mobility. As regards Tl, Cu, and other potentially toxic elements the results obtained suggest that the use of total or pseudo-total concentrations, when dealing with the assessment of risks deriving from consumption of agricultural products, should be of limited significance. It was demonstrated, in fact, that further factors, probably related with physical-chemical characteristics of soils, could influence the real amount of these elements transferable from soil to plants. This study represents a methodological contribution to a better understanding of the processes underlying the spatial variability of chemical elements in soil. Considering the positive outcomes obtained, further researches are planned to include more factors (e.g., soil pH, redox potential, content in Fe and Mn oxides, etc.) in the predictive models.