# Application of Biochar for the Restoration of Metal(loid)s Contaminated Soils
## 4 Discussion
### Effectiveness of biochar Treatments to Modify Metal Toxicity to Plants  
#### 4.2.1 Germination  
Germination and seed emergence are the first and crucial stages of the plant cycle [66]. Our results demonstrate the ability of biochar to promote vegetation emergence in soil with high metal(loid)s content.  
The germination rate calculated for RA was very low (17.83 \(\pm\) 3.54) respect to NC, suggesting the existence of phytotoxic concentrations of the metals in the soil that prevent plant emergence. In previous studies conducted in contaminated soils in the area, it was observed that the acidic pH value and the high concentration of metals resulted in a lack of available essential nutrients, a potential toxicity in soil solution, and an absence of vegetation [29; 54]. [PERSON] et al. [67] found that low pH together with high concentrations of Cd, Pb and Zn could have increased the solubility and toxicity of these elements, inhibiting seed germination in _Bouteloua dactyloides_ and _Cynodon dactylol_. In fact, a significant correlation was found in our study between soil properties and germination rate, pH being the most explanatory variable with a positive coefficient r = 0.704, \(p<0.01\) (Table A3).  
In soils amended with biochar, higher emergence rates were observed than in RA, regardless of the doses used. Probably due to the improvement of soil properties, mainly the increase in pH (with values above 4). In fact, according to [PERSON] et al. [38]and [PERSON] [68], the increase in pH and the decrease in EC represent favourable conditions for seed emergence and vegetation establishment. The increase of pH values would have promoted the reduction of metal toxicity by decreasing their bioavailability, being of particular importance the reduction of Cu and Zn bioavailable fractions, as evidenced by PCA (Figure 5) and correlation analyses. According to the correlation analysis, Ge would have risen by increasing the bioavailable fraction of Pb and As (r = 0.723, \(p<0.01\); r = 0.429, \(p=0.002\) respectively), and by decreasing the soluble Zn and Cu (r = -0.72, \(p<0.01\); r = -0.588, \(p<0.01\), respectively) and bioavailable Zn and Cu (r = -0.705, \(p<0.01\); r = -0.636, \(p<0.01\), respectively) (Table A3).  
In addition, for RA, non-significant variations were observed in the rate of the number of plants present at the end of the experiment with respect to the initial germination. This variation determined the value of the survival percentage, which was lower in the contaminated soil than in the other treatments. Probably, the decrease in the number of plants observed in RA over the incubation time was due to the toxicity content of Zn and Cu.  
In the 8% amended treatment the germination rate of _Trifolium pratense_ is similar to that of the uncontaminated soil, although the biomass values differ greatly, which it could be related to the influence of the presence of a high content of metals on the normal growth and development of the plants. The existence of a positive correlation (r = 0.429, \(p=0.002\)) between the bioavailable fraction of As and germination (Figure 5) indicate that the presence of this element in the soil could stimulate plant growth when it is present in low concentrations (\(\mu\)g/day). This is likely due to the fact that As, by interacting with both plant metabolism and nutrients, could act as a stimulator for the essential micronutrients uptake, such as phosphorus (P) [69]. However, controversial results are found in the literature regarding the role of As in plant growth.