# Application of Biochar for the Restoration of Metal(loid)s Contaminated Soils
## 4 Discussion
### Effectiveness of biochar Treatments to Modify Metal Toxicity to Plants
#### 4.2.3 Plant Uptake  
Biochar addition reduces the metal(loid)s concentration in the shoot as compared to plants grown in RA, but increase the concentration in the root, except for Zn and Cu in RA8B. The increase of As and Pb concentration in roots can be explained by increasing bioavailable fractions of these metal(oid)s in the amended soils. In addition, the presence of organic matter from biochar may increase the accumulation in vegetation of these metals due to the soluble organic ligands' formation for the root exudates that increase the uptake of the trace elements ([PERSON] et al., 2018).  
[PERSON] et al. (2018) found that, in general, biochar consistently reduced the average concentrations of heavy metals in plant tissue. However, our results suggest that biochar may also reduce the translocation of metal(loid)s from root to shoot as indicated lower values of SC/RC ratio calculated in RA4B and RA8B than that in RA (Table 4).  
Biochars from different feedstocks varied significantly in their capacity to reduce plant heavy metal concentrations. Thus, it is possible that nutrient enriched manure biochar enhanced plant growth, thereby diluting metal concentrations in the plant biomass ([PERSON] et al., 2018). Biochars produced at 450-500 \({}^{\circ}\)C are preferred for effectively reducing plant uptake of Cd, Pb, Cu, and Zn, since oxygen containing functional groups can effectively complex with heavy metals ([PERSON] et al., 2018). The biochar pH and the application rate of biochar significantly affected the percentage reductions in plant uptake of the heavy metals.  
In the present study, bioaccumulation in _Trifolium pratense_ seems not to be significant since the total metal content in the samples before the experiment (t0) did not differ significantly from the values found in the samples at the end of the experiment (tf) (Table A1). _Trifolium sp._ is herbaceous, with a limited biomass production. This plant is not considered as hyperaccumulator, since it does not remove significant amounts of metals in a highly contaminated soil by itself. Overall, _T. pratense_ demonstrated its ability to survive, so it could represent a potential species to be used in phytostabilization both in the study area and in other similar contaminated areas. According to results in [PERSON] et al. (2018), _Trifolium_ sp., present in the studied area, showed a good capacity to cope with high levels of heavy metal contamination. These results agree with those of [PERSON] et al. (2018), who reported that this species could grow in soils highly contaminated by Cd, Pb, and Zn, by accumulating them in its roots. It is likely that the bioaccumulation of metals could be an adaptive mechanism that allows plants to live under these stress conditions. In any case, the importance of using tolerant species for restoration trials of highly contaminatedsoils is highlighted [80]. More studies are needed to monitor the concentration of metals accumulated in the plant tissues to assess the potential transfer risk to the food chain under these polluted areas.