# Application of Biochar for the Restoration of Metal(loid)s Contaminated Soils
## 4 Discussion
### Effectiveness of biochar Treatments to Modify Metal Toxicity to Plants
#### 4.2.2 Plant Biomass  
The total biomass values inform about the effect of contamination on plant development, determining strong limitations for vegetation establishment. It is worth mentioning that the current experiments were conducted under controlled and fully favorable greenhouse conditions, which promoted the plant growth observed in contaminated soil. However, in real conditions, strong limitations prevent the establishment of vegetation in RA soil with no plant cover.  
A significant increase in plant growth was observed in RA4B and RA8B with respect to RA, suggesting decline effect of metal phytotoxicity. As also observed in other studies [42; 69], the use of biochar as an amendment improves plant performance. Root biomass values varied significantly among treatments depending on the dose of amendment used, but aerial biomass values did not differ significantly among the amended treatments. Probably the lower biomass of amended soil with respect to non-contaminated soil is due to the accumulation of metals in plant tissues, which may have caused limited growth [53; 70]. Thus, it has been observed that the values of soluble and bioavailable fractions of Zn and Cu are negatively related to root (r = -0.937, \(p<0.01\); r = -0.77, \(p<0.01\)) and aerial part biomass (r = -0.884, \(p<0.01\); r = -0.654, \(p<0.01\)), (Figure 5 and Table A3).  
The presence of large amounts of metal(loid)s can affect plant physiology by reducing respiration and growth. In addition, the accumulation of metal(loid)s in high concentrations can interfere with photosynthetic processes and inhibit fundamental enzymatic reactions [71]. When these metals are present in the soil at a low concentration, plants continue to grow uniformly despite accumulating these metals. The ability of plants to accumulate heavy metals in their tissues can therefore be used to control soil pollution and, in particular, the amount of heavy metals [71].  
The bioavailable fractions of As and Pb showed positive correlation with the values of shoots (r = 0.895, \(p<0.01\); r = 0.679, \(p<0.01\)) and roots (r = 0.875, \(p<0.01\); r = 0.505, \(p<0.01\)) (Table A3) biomass. According to the biomass values and correlation data, it is observed that the presence in the soil of a high content of available Zn and Cu mainly affects the development of roots, limiting their growth, but does not influence too much the development of the aerial part. Otherwise, a small increase in the bioavailable fraction of Pb and As seems could promote plant growth, although it is not possible to give a complete answer since the mechanisms they implement are still unknown. (Figure 5).  
The improved soil characteristics provided by the addition of biochar are related to increased plant growth. According to [PERSON] et al. (2017) and [PERSON] et al. (2018), the increased water holding capacity and improved structure caused by the biochar addition represent material conditions conducive to plant growth. But the same properties that in some cases so effectively reduce metal mobility can also immobilize essential macronutrients ([PERSON] et al., 2018), decreasing the fertility of an already compromised soil. Thus, if maintenance of soil quality and good conditions for plant growth is required, biochar alone may be ineffective for sustainable remediation and revegetation ([PERSON] et al., 2018; [PERSON] et al., 2018) and it should be complemented by the application of organic or inorganic fertilizer. For example, under nutrient-limited conditions, its combination with other organic amendments, such as compost, may be necessary ([PERSON] et al., 2018). In any case, the selection of a biochar that manages to maintain the balance between nutrient deficiency/retention and pollutant immobilization could be difficult. However, evidence of reductions in the mobility of different types of organic contaminants allows the same amendment to be implemented on multi-element contaminated sites, considering biochar as a cost-effective and versatile material for use on a wide variety of contaminated soils.