## 3 Results and Discussion
### Soil Aluminium Fractions  
#### 3.2.1 Exchangeable Al Extracted by NH\({}_{4}\)Cl and by CaCl\({}_{2}\)  
Figure 1 shows the concentrations of exchangeable Al extracted by NH\({}_{4}\)Cl (Figure 1a) and by CaCl\({}_{2}\) (Figure 1b) in response to application of soil amendments. The concentration of Al-NH\({}_{4}\)Cl was higher in the control than in the other treatments (\(p<0.05\)). On average, the Al-NH\({}_{4}\)Cl contents decreased approximately 4 times after soil amendments addition compared to the control. All soil amendments led to similar concentrations of Al-NH\({}_{4}\)Cl with values ranging from 16.37 mg kg\({}^{-1}\) in PM to 21.1 mg kg\({}^{-1}\) in HPC240. The materials obtained by thermal treatment (biochar and hydrochar), regardless of the temperature, resulted a content of Al-NH\({}_{4}\)Cl in the soil similar to Ca(OH)\({}_{2}\) which is a traditional material used for liming. Exchangeable Al (Al\({}^{3+}\)) is considered the most toxic Al and might have high values in soils with pH \(<\) 5.5. In the present study, it could be assumed that pH changes are driving the reported alterations of NH\({}_{4}\)Cl-extractable Al, as the values shown by the chars were similar to those exhibited by the lime control. While the ability of biochar to reduce extractable Al was already reported [19], this is the first time that a similar fact is witnessed for hydrochar amendment. Al extracted by CaCl\({}_{2}\) is considered as the exchangeable Al in soils with the best correlation with plant available Al for a selected number of plant species [33]. Surprisingly, in the present work, the concentrations of exchangeable Al extracted by CaCl\({}_{2}\) (Al-CaCl\({}_{2}\)) were not reduced after soil amendments application (Figure 1b). When produced in a higher temperature both biochar (BPC600) and hydrochar (HPC240) increased the Al-CaCl\({}_{2}\) contents compared to the other treatments. Therefore, the results show that the temperature adopted in the thermal process is a key factor to determine the effect of biochars and hydrochars on the Al-CaCl\({}_{2}\) concentration in the soil.  
2.2 Al Extracted by Acid Ammonium Oxalate (Al\({}_{0}\)) and by Sodium Pyrophosphate (Al\({}_{\text{p}}\))  
Aluminium extracted by acid ammonium oxalate (Al\({}_{0}\)), by sodium pyrophosphate (Alp), and inorganic non-crystalline Al (Al\({}_{0}\)-Al\({}_{\text{p}}\)) in the soil are shown in Figure 2a-c, respectively. The concentration of Al\({}_{0}\) ranged from 163 to 182 mg kg\({}^{-1}\) (Figure 2a) and it was significantly higher in the soil amended with slaked lime than in soils after chars (biochar and hydrochar) application. Possibly, slaked lime promoted the formation of non-crystalline Al (Al\({}_{0}\)) from exchangeable Al (NH\({}_{4}\)Cl) [36]. On the other hand, char addition transformed the soil non-crystalline Al into organo-aluminium complexes as it will be shown in the following sections. No significant differences were observed among the other treatments (\(p>0.05\)).  
Figure 1: Exchangeable Al extracted by NH\({}_{4}\)Cl (**a**) and CaCl\({}_{2}\) (**b**). Means followed by the same letter are not significantly different (\(p<0.05\)) using [PERSON]’s HSD test.  
In the present study, Al\({}_{\text{p}}\) was not affected by soil amendments (Figure 2b). On average, the concentration of Al\({}_{\text{p}}\) was 25% lower than the Al\({}_{0}\) concentration. As Al\({}_{0}\) represents the total non-crystalline Al and Alp is the total organo-aluminium complexes [37], Al\({}_{0}\)-Al\({}_{\text{p}}\) has been considered an indicator of inorganic non-crystalline Al [31]. In the present study, both biochar and hydrochar, regardless of the temperature of preparation, had similar concentration of inorganic non-crystalline Al (Figure 2c). In addition, PM promoted higher concentration of inorganic non-crystalline Al than the hydrochars (\(p<0.05\)).