## 3 Results
### Ce in Leadates  
Figure S1 reports the leachate volume measured after each irrigation. The initial difference in the leachates of the three treatments was due to the difference in the biochar quenching procedure. Despite that the columns were initially saturated, a certain fraction of the pores of the dry biochar were still not wetted after 15 days of equilibration. With the following first irrigation, these pores most likely retained part of the water, thereby resulting in a lower volume of the leachates from the four columns with the soil amended with BCdryQ. This can be explained by considering what was reported by the authors in a \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Substrates**} & **pH** & **EC** & **Salinity** \\  & & **\(\mu\)S cm\({}^{-1}\)** & **mg L\({}^{-1}\)** \\ \hline biochar (dry quenching) a ** & 9.69 \(\pm\) 0.05 & nd & nd \\ \hline biochar (wet quenching) a  & 9.87 \(\pm\) 0.05 & nd & nd \\ \hline soil (CTRL) b  & 7.78 \(\pm\) 0.01 & 160 \(\pm\) 12 & 76 \(\pm\) 6 \\ \hline soil + biochar (dry quenching) (DRY Q) b  & 7.99 \(\pm\) 0.06 & 119 \(\pm\) 4 & 56 \(\pm\) 2 \\ \hline soil + biochar (wet quenching) (WET Q) b  & 7.88 \(\pm\) 0.03 & 100 \(\pm\) 2 & 47 \(\pm\) 1 \\ \hline \end{tabular} a: soil:water ratio: 1:20; b: soil:water ratio: 1:4 (mean \(\pm\) SD; n = 3). nd = not detected.[ENDFOOTNOTE]  
\end{table} 
Table 1: PH, electric conductivity (EC) and salinity of the two biochars (dry quenching and wet quenching) and the three soils used in the lysimeters (CTRL, DRY Q and WET Q). recent study [27]. They studied the changes of BCdryQ and BCwetQ after an experiment on an addition to the soil by using the fast field cycling NMR relaxometry technique. The analyses revealed that the biochars underwent different modifications that altered their initial porosity and hydrophobicity. In particular, BCdryQ was chemically modified so that its hydrophobicity was reduced; BCwetQ changed both chemically and physically, so both the porosity degree and surficial chemistry changed. Due to these transformations, BCdryQ and BCwetQ changed differently in their attitudes to retain water. BCdryQ became less hydrophobic than BCwetQ, which might explain the higher reduction in terms of the leachate volume of the substrate DRY at the first irrigation compared to the leachate of the substrate WET (Figure S1). Moreover, this proves that the water-holding capacity (WHC) of the soil amended with the two biochars changed in time due to the biochar aging. The general decreasing trend of the leachate volumes with the following irrigations is explained by the fact that the evapotranspiration of the plants on top of the columns became more and more significant as the plants were growing. The ANCOVA ('biochars type' as fixed factor and'sampling' as covariate) showed that there was no difference in statistical terms when the CeO\({}_{2}\)NP total content in the leachates was considered (Figure S1). A different outcome resulted when the cumulative CeO\({}_{2}\)NPs leached from the lysimeter were analyzed (Figure 2). This parameter was calculated with the following formula:  
\[CeO_{2}NPscumul_{i}=[CeO_{2}NPs]_{i}V_{leachate\ i}+\sum_{n=1}^{i}(CeO_{2}NPs \ cumul_{i-1}) \tag{1}\]  
where  
[CeO\({}_{2}\)NPs]i = Ce concentration (mg L\({}^{-1}\)) of the ith leachate;  
V leachate i = volume (L) of the leachate at the ith sampling;  
i= sampling number \(e\)[1,6]. The ANCOVA ran on the cumulative CeO\({}_{2}\)NPs showed that both the sampling and the biochar-type factors were significant in determining the amount of CeO\({}_{2}\)NPs leached from the lysimeters (\(p\)-values \(<\) 0.000 and 0.017, respectively). Figure 2 shows the polynomial  
Figure 2: Cumulative Ce leached from the 12 lysimeters after the 6 irrigations for the 3 treatments (CTRL, DRY and WET). Different letters indicate that the means are statistically different (ANCOVA, [PERSON]’s test, \(p\) < 0.005). Curves are polynomial regression fitted to the data points (second grade).