Abstract
There is a small sample of edaphic geochemistry studies over large geographic areas, especially studies that consider major reference soil groups (RSG) that evaluate both native concentrations of elements and anthropogenically contaminated soils in agricultural settings, considering the long-term effect of agricultural practices on landscape sustainability. In this study, four RSGs were analyzed for the available trace elements Ni, Cr, Cd, Pb, Cu, Mn, and Zn, including other edaphic properties from 2002 to 2012. The main objectives were to assess the range of concentrations of the selected elements in the four typical Mediterranean soils, Cambisols, Luvisols, Calcisols, and Fluvisols, with heavy anthropogenic input (HAI) and compare them to minimal anthropogenic input (MAI). For MAI, the background levels of Pb, Ni, Cd, and Cr were highest in Calcisols, differing from those of Cambisols, Luvisols, and Fluvisols (p < 0.01), Cu is highest both in Calcisols and Luvisols while Mn is higher in Cambisols and Fluvisols (p < 0.05). The background concentration of Zn was the same in all RSGs (p > 0.05). For HAI, the reference levels of Pb, Ni, Cd, Mn, and Cr were highest in the Calcisols, and Cu was high in all RSGs except Fluvisols, while Zn presented the lowest concentrations in the Luvisol RSG, with all these results considering a confidence interval of 95%. Predictive maps for the sampled elements, as well as the edaphic bioavailability, are provided. This environmental impact assessment suggests that the land use is departing from sustainable ecosystem service development and that territorial management practices, with conservation goals in mind, should be adopted.
Highlights
Cadmium (Cd) and chromium (Cr) are both toxic heavy metals, and copper (Cu), lead (Pb), manganese (Mn), nickel (Ni), and zinc (Zn) are plant nutrients, when they are added in excess through organic amendments, inorganic fertilizers, sludges, etc., they can be detrimental to crop growth [1,2,3]
According to Alloway [6], the most important factor in determining the toxicity or deficiency of trace elements and heavy metals is the amount available in the soil for crops. This amount depends on the total content of the element, the soil texture, the adsorption capacity of the soil, and physicochemical factors such as pH and redox potential, which control the balance between the adsorbed fractions and those of the soil solution, presenting the concept of ‘critical load’, which represents the maximum concentration of heavy metals semi-metals in soils and the environment that can be maintained without harmful effects on them; metal toxicity to biological life, including human life, depends on their total concentrations and on their mobility and relationship with other components of the ecosystem, with their bioavailability being the most important parameter to analyze [7,8,9]
The average available Cd content remained constant in the study area (p > 0.05), but we registered a significant decrease of 69% (p < 0.01) in the available Cd background values (MAI) in the same period and maintenance of the reference values (HAI) circa
Summary
Cadmium (Cd) and chromium (Cr) are both toxic heavy metals, and copper (Cu), lead (Pb), manganese (Mn), nickel (Ni), and zinc (Zn) are plant nutrients (i.e., cation micronutrients), when they are added in excess through organic amendments, inorganic fertilizers, sludges, etc., they can be detrimental to crop growth [1,2,3]. The composition of heavy metals and semimetals of agricultural soils tends to be closely governed by the source material According to authors such as Jebreen et al [4] and Rambeau et al [5], carbonate rocks are generally composed of three main minerals: calcite, with common constituents of Al, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sr, V, and Zn; dolomite, with common constituents of Cd, Co, Fe, Mn, Pb, and Zn; and aragonite, with common constituents of Ba, Pb, Sr, and Zn. In general, it is accepted that determining the total content of heavy metals in a soil is not sufficient to understand their relative mobility and ecological availability as pollutants, nor is it a useful tool to estimate potential risks, it is useful to determine whether the concentration of a given element is either increasing or decreasing [6]. A study by Nunes et al [10] determined the background levels in 2002 in the study area, but with an assumption that could lead to error that the bioavailability of metals and semimetals was identical to the availability of the elements measured in the laboratory at a pH of 7.3
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