## 3 Results and Discussion
### Soil Aluminium Fractions
#### 3.2.3 Organo-Aluminium Complexes of Different Stabilities  
Organo-aluminium complexes (OAC) of low and medium stability (Al\({}_{\text{Cu}}\)) and OAC of high stability (Al\({}_{\text{p}}\)-Al\({}_{\text{Cu}}\)) in soil amended with slaked lime, manure and its biochar and hydrochar are shown in Table 2.  
In general, the concentrations of OAC of high stability were higher than those of low and medium stability. The concentrations of OAC of low, medium and high stability followed the order HPC240 \(\approx\) HPC200 \(\approx\) Time \(\approx\) BPC600 \(>\) Control \(\approx\) BPC450 \(\approx\) PM. Only the biochars increased the number of organo-aluminium complexes of low and medium stability, when compared to the control soil or to the addition of lime. Slaked lime and hydrochar addition led to an increase in organo-aluminium complexes of high stability. The amendment with HPC240 led to significantly higher amounts of OAC of high stability, compared to any other treatments. The higher amount of OAC of high stability in soils amended with hydrochars could be due to the presence hydroxyl and carboxyl groups, as they are known to complex with Al [17]. The aforementioned functional groups were significantly more abundant in hydrochars compared to biochars [14]. The differences in the type of OAC generated by biochars and hydrochars points to a contrast in the organic matter in biochars and hydrochar, particularly a more condensed aromatic structure \begin{table}
\begin{tabular}{c c c} \hline \hline
**Soil Amendment** &
\begin{tabular}{c} **Organo-Aluminium Complexes** \\ **(OAC) of Low and Medium Stability** \\ \end{tabular} & **OAC of High Stability** \\ \hline Control & 49.00 \(\pm\) 1.74 b & 69.00 \(\pm\) 5.06 cd \\ PM & 32.26 \(\pm\) 1.86 c & 76.81 \(\pm\) 6.93 bc \\ Lime & 48.25 \(\pm\) 1.74 b & 88.36 \(\pm\) 3.46 b \\ BPC450 & 58.92 \(\pm\) 1.74 a & 54.88 \(\pm\) 7.80 d \\ BPC600 & 62.34 \(\pm\) 0.45 a & 66.06 \(\pm\) 5.99 cd \\ HPC200 & 52.03 \(\pm\) 0.05 b & 91.27 \(\pm\) 15.18 b \\ HPC240 & 33.20 \(\pm\) 3.79 c & 108.35 \(\pm\) 6.03 a \\ \hline \hline \end{tabular} Means followed by the same letter in the column are not significantly different (\(p<0.05\)) using [PERSON]’s HSD test.  
\end{table} 
Table 2: Organo-aluminium complexes of low, medium and high stability in response to addition of soil amendments. Figure 2: Al extracted by acid ammonium oxalate (Al\({}_{0}\)) (**a**) and by sodium pyrophosphate (Alp) (**b**); inorganic non-crystalline Al (Al\({}_{0}\)-Al\({}_{\text{p}}\)) (**c**). Means followed by the same letter are not significantly different (\(p<0.05\)) using [PERSON]’s HSD test. in the biochars [14]. Biochars produced at different temperatures promoted similar concentrations of OAC in the soil. However, there seems to be a strong influence of the hydrothermal temperature, controlling the amount of OAC of high stability formed in the soil. Compared to biochar, hydrocarbons had higher easily oxidizable carbon (OC) content and less stable carbon structures [15]. The lower recalcitrance of the carbon in hydrochloric, compared to biochars, seems to impact hydrochars' ability to form OAC in the soil. Interestingly, slaked lime did not alter any OAC in the soil compared to the control. In fact, there is no general agreement about the effect of lining on the forms of non-extractable Al [23]. These results suggest that pH changes are not one of the main mechanisms involved in the formation of OAC upon char addition to the soil. Relationship between Soil Aluminium Fractions and Chemical Properties of Manure and Its Biochars and Hydrochars  
A principal component analysis (PCA) was conducted to determine the relationships between the chemical properties of soil amendments (Table S1) and soil Al fractions. The two main components (PC1 and PC2) of PCA explained 81% of the total variation in the properties evaluated (Figure 3).