## 4 Conclusions  
There is still a limited understanding of the behavior of tungsten in the environment [62], both because it is a not essential element and because until recently it was considered completely inert. However, tungsten has placed among the possible emerging pollutants by EPA [63]. The present work, to our knowledge, is the only one dealing with the problem of tungsten in Italian soils. The study of this element in different types of Mediterranean soils and the potential passage in the food chain, following the transfer from the soil to the plant, is very important to focus attention on an environmental problem that is currently very undervalued. Our results highlight the importance of soil properties in determining the transfer of tungsten from the soil to plants. The obtained data show that with decreasing pH and increasing organic matter content the plant uptake decreases, due to lower tungsten concentrations in soil solution in bioavailable forms.  
In this work, we have taken into consideration those characteristics of the soil that are generally considered in environmental regulations. However, knowledge of the behavior of this element in the soil requires further investigation, so as to better understand the processes that regulate its bioavailability. Among these, a role of primary importance can be played by the redox potential because the oxidizing and reducing conditions determine the behavior of trace elements in soil. Redox variations in soil and the influence of these variations on the mineralogy [61; 64] is an essential aspect for future research on the geochemical behavior of tungsten, which is strictly linked to plant bioavailability.  
In the last few years, several studies on tungsten in plants have revealed several toxic effects related to the Mo-enzyme inhibitor and gene expression [30; 36]. However, in our experiment, the addition of increasing amounts of tungsten to the soil did not cause a biomass reduction or toxicity effects in plants. It is possible that considerable amounts of tungsten accumulate in plants, especially in alkaline conditions. Under the worst conditions, plants may be insensitive to tungsten at levels that are already  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Soil**} & \multicolumn{3}{c}{**Shoots**} & \multicolumn{3}{c}{**Roots**} \\ \cline{2-7}  & **Log K** & **1/n** & **R\({}^{2}\)** & **Log K** & **1/n** & **R\({}^{2}\)** \\ \hline A & \(-0.066\) & 0.863 & 0.970 & 0.619 & 0.553 & 0.988 \\ B & 0.260 & 0.670 & 0.961 & 0.874 & 0.528 & 0.959 \\ C & 0.6992 & 0.534 & 0.973 & 1.14 & 0.685 & 0.968 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Relationship between tungsten concentration in shoots and roots and tungsten concentration in soil solution. Parameters of the Freundlich-like model.  
toxic to animals. This phenomenon has also been detected for molybdenum in the case of forage for ruminants [65].  
The low phytotoxicity of tungsten highlights the need to find appropriate models aimed at predicting the passage of tungsten from the soil to plants. We applied a Freundlich-like model for _Zea mays_ uptake at increasing tungsten concentrations in three different Italian soils. The aim of this approach is to respond as simply as possible to the need to assess the potential effects derived from the transfer of W into the food chain, and also to enable the regulatory authorities to better define the hazards of this element in the environment. The transfer mechanism of tungsten from the soil to roots and shoots is of great interest and should be further investigated to understand how the environmental source of this element may impact on human health, which is an urgent issue that must be addressed in the near future.  
Conceptualization, G.P. and F.P.; Writing - original draft, G.P. and F.P.  
This research received no external funding. This study was supported by the Italian National Research Council CNR.  
The authors thank [PERSON] for technical support.  
The authors declare no conflict of interest.