## 3 Results and Discussion
### Influence of Soil Characteristics on Plant Uptake
This trend is particularly notable in soil A due to the specific characteristics of this Histosol: low pH with a high organic matter content. Humic compounds can greatly influence the distribution of metallic elements, such as tungsten, between the solid and liquid phase of the soil. This is also in relation to the environmental variations in the soil, such as dynamic redox conditions [43], that can modify the interactions between humic compounds and the surfaces of oxides and hydroxides of iron on which the metal can be adsorbed. Similar to what was found in the case of arsenic and antimony in the soil [44; 45; 46], organic materials with a high binding capacity, strongly influence the adsorption of anions on variable charge minerals in soils. The content of tungsten in plants does not seem to be linked to the texture of the soils. For example, Figure 4 reports the uptake trend in relation to the clay content of the three studied soils. The highest clay content in the Vertisol (Soil B) does not influence plant uptake. Similar trends were also obtained considering the plant tungsten concentration in relation to the silt and sand fraction of the three soils (data not reported). Another interesting aspect to tackle is the trend in plant uptake of tungsten from the soil. In the study of plant uptake from soil, there is often an assumption of linearity: the concentration of a substance in plants increases with an increasing of its soil concentrations. However, deviation from linearity has been observed in several contaminant uptake studies [47; 48; 49]. Plants generally take up elements more efficiently at low than at high soil concentrations, thus deviating from linearity with a decrease in uptake with increasing soil concentrations. The use of nonlinear functions to take into account this behavior has been proposed by several authors [48; 50; 51]. Figure 4: Mean values of tungsten concentrations in plants in relation to clay content of the different soils. Bars indicate standard deviation. (**a**) shoots; (**b**) roots. To evaluate tungsten uptake, it may be useful to estimate the increase in concentration in plants with increasing both soil concentrations and soil solution concentrations. The trend of tungsten uptake for shoots and roots was analogous in both cases, and it is reported in Figure 5, as an example, in relation to soil solution concentration. Also, in this experiment, the increase in tungsten in the plants was not linearly correlated with an increase in tungsten concentration in soil. The uptake of tungsten by the plants, in relation to the concentration of the element in the soil, and in the soil solution although not far from linearity,  
Figure 5: Trend of tungsten uptake for shoots and roots at increasing W concentrations in soil solution. Data are expressed as mg kg\({}^{-1}\). (a) Soil A; (b) Soil B; (c) Soil C.  
is described more accurately by a trend that can be modeled using a Freundlich-like equation. The absorption is higher when the concentration in the soil is lower, whereas it is reduced when the concentration of an element in the soil increases. In recent years, models of plant uptake have been proposed for organic [52; 53] and inorganic [54; 55] contaminants at contaminated sites to define the risk to human health. In agricultural soils, where the concentration of potentially toxic elements is much lower, simpler models can be successfully used to provide useful information on the soil to plant transfer process. One of the models is based on the use of a Freundlich-like equation to evaluate plant uptake in the presence of increasing quantities of an element. This approach has been successfully used also in the case of contaminated soils [56]. The Freundlich-like equation (1):  
\[q=K\,C^{\frac{1}{n}} \tag{1}\]  
where q is the tungsten concentration in plants (mg kg\({}^{-1}\)) and \(C\) is the tungsten concentration in the soil or soil solution (mg kg\({}^{-1}\)).