## 2 Materials and Methods
### Biochar Adsorption Mechanisms Study
#### 2.3.2 Adsorption Tests Using Different Biochar Application Methods  
Two adsorption tests were conducted using an NH4 Cl aqueous solution simulating the conditions established in the manure storage experiments in terms of temperature, pH, NH4\({}^{+}\) concentration and biochar dosage. Two different biochar application methods were used to assess separately the biochar ability to adsorb NH4\({}^{+}\) (trial M) and NH3 (trial Bc). A schematic representation of the two application methods is reported in Figure 1.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Aim** & **Biochar Condition** & **Solution** & **Adsorbed Nitrogen Assessment** & **pH** & **T °C** & **Duration** & **Test** \\ \hline To evaluate the maximum NH4\({}^{+}\) & \begin{tabular}{c} Biochar mixed with \\ a NH4 Cl solution \\ in a closed \\ batch experiment \\ \end{tabular} & \begin{tabular}{c} 0, 5, 10, 50, and \\ 100 mg/L of \\ N-NH4\({}^{+}\) \\ \end{tabular} & \begin{tabular}{c} Differences in NH4\({}^{+}\) \\ concentrations before and \\ after the trial \\ \end{tabular} & unadjusted & \begin{tabular}{c} room \\ temperature \\ \end{tabular} & \(16\) h & n.a. \\ \hline To discriminate N adsorption as NH3\({}^{+}\) or NH4\({}^{+}\) & \begin{tabular}{c} Biochar mixed with \\ a NH4 Cl solution \\ experiment \\ \end{tabular} & \begin{tabular}{c} \(\bullet\) \\ 1 g/L N-NH4\({}^{+}\)Cl \\ aqueous solution \\ \end{tabular} & \begin{tabular}{c} \(\bullet\) \\ 1 g/L N-NH4\({}^{+}\)Cl \\ aqueous solution \\ \end{tabular} & \
\begin{tabular}{c} Differences in NH4\({}^{+}\) \\ concentrations before and \\ after the trial; \\ \end{tabular} &
\begin{tabular}{c} \(\bullet\) \\ 1 g/L N-NH4\({}^{+}\)Cl \\Two open beakers were filled with 40 mL of an NH4 Cl aqueous solution with an initial ammoniacal N concentration of 1 g L\({}^{-1}\) and maintained ad 20 \({}^{\circ}\)C using a thermostatic bath for 48 h. The initial pH was adjusted to 8.8 using NaOH, to simulate the digestate conditions. Finally, a biochar load of 4% wt was applied in both experiments. In the first beaker, biochar was added to the liquid solution and continuously mixed on a magnetic stirrer at 400 rpm in order to maintain the biochar in contact with the liquid solution for the duration of the experiment. Under these conditions biochar could adsorb ammoniacal N present in the liquid solution mainly in form of NH4\({}^{+}\), according to the NH3/NH4\({}^{+}\) equilibrium established at 20 \({}^{\circ}\)C and pH = 8.8. In the second beaker, biochar was placed on a Teflon grid at the top of the vessel at a distance of 1 cm from the free surface of the solution, which was continuously stirred at 400 rpm. Under this configuration, any contact between the biochar cover and the liquid solution was avoided and only the volatilized NH3 could be adsorbed by the biochar cover during the experiment. After 48 h the pH was measured in both cases and the experiments were stopped. In the trial M, the biochar was separated from the suspension through vacuum filtration and dried for 48 h at ambient temperature to remove most of the moisture, avoiding possible ammoniacal N desorption. Then, it was subjected to NH4\({}^{+}\) desorption experiments using the same procedure described in 2.2. The biochar used for the trial Bc was subjected to two subsequent desorption procedures. The first procedure was applied to detect possible physiosorbed NH3 [21]. This consisted in purging a biochar column with a 3 NL min\({}^{-1}\) of N\({}_{2}\) flow at 25 \({}^{\circ}\)C as described in [22] and detecting the desorbed NH3 through an FTIR analyser (Sick GME700, detection limits 0-4000 ppm) placed at the end of the biochar column. Then, the purified biochar was retrieved and subjected to the NH4\({}^{+}\) desorption procedure described in Section 2.2 to detect NH3 adsorbed in NH4\({}^{+}\) form on acidic functional groups present on the biochar surface [35].