## 4 Discussion  
### Ammonia Emissions  
Biochar application during digestate storage reduced NH3 emission differently depending on biochar application methods and doses in agreement with the literature, where  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Tests** & **M** & **Bc** \\ \hline Ammoniacal N potentially adsorbed (mg g\({}_{\text{biochar}}\)\({}^{-1}\)) & 5.1 & 3.4 \\ \hline N-NH3 (mg g\({}_{\text{biochar}}\)\({}^{-1}\)) & n.d. & n.d. \\ \hline N-NH4+ (mg g\({}_{\text{biochar}}\)\({}^{-1}\)) & 0.1 & 1.2 \\ \hline \hline \end{tabular}
\end{table}
Table 4: N-NH3 and NH4 adsorbed in test 3.  
Figure 4: Biochar equilibrium adsorption capacity of NH4+ (Qe) at increasing concentrations of N-NH4+ (C0) (T = 25 \({}^{\circ}\)C).  
a wide variation range of NH\({}_{3}\) reduction, from 0 to 96%, was reported for different application methods, doses, biochar characteristics and manure/digestate type. A summary of the literature data is reported in Table 5.  
In some studies, mixing biochar with manure [5; 28] or using it as a floating cover [14] had no positive effect on reducing NH\({}_{3}\) volatilisation, suggesting that biochar did not significantly alter the pH and N dynamics in the stored manure [14; 28], or if an increase in emission was observed, this may be due to the low adsorption capacity of biochar and the high TAN content of the manure [5]. In this study, lower NH\({}_{3}\) emissions were observed for the 2B test (2 cm biochar layer), while in the 1B (1 cm biochar layer) and Bm (biochar mixed with digestate) tests, digestate emitted 44% and 74% of ammonia more than in the 2B case. These results confirm the findings reported by [13], which showed lower ammonia emissions when biochar was applied on the surface, with a maximum reduction with respect to the control of 96% obtained with wood biochar. On the other hand, opposite results were obtained by [15], reporting a greater emission mitigation (52%) in the case of biochar incorporated into digestate with respect to that in which biochar was applied on the surface. However, in this case, a low temperature biochar was used, which is, presumably, richer in oxygen containing functional groups typically involved in ammonia adsorption. In this study, the difference between the tests 2B and Bm, where the amount of biochar applied is the same, can be mainly attributed to the formation of a physical barrier in the test 2B. This hypothesis is confirmed by the final values of ammoniaical N concentrations in the digestate in the two tests which are in good agreement with the ammonia emissions reduction. The effect of the physical barrier is evident also in the highest water evaporation corresponding to the highest dry matter increase in absence of biochar surface layer. The effect of biochar cover in reducing ammonia emissions is more efficient when the mechanical resistance to surface cracks is higher due to the greater thickness, as results when comparing the emissions and the ammoniacal N in the digestate after the storage test in cases 2B and 1B. This observation was also supported by the different ammonia emissions observed in the NH\({}_{4}\)Cl aqueous solution resembling the digestate storage tests. In that case, NH\({}_{3}\) devolatilization in Bc is significantly reduced by the presence of the biochar cover, probably due to the achievement of the equilibrium pressure in the headspace at the interface between the free NH\({}_{3}\) dissolved in the solution and NH\({}_{3}\) in air, according to [PERSON]'s law [36]. On the contrary, in the M test, ammoniaical N is mostly released due to NH\({}_{3}\) devolatilization, as confirmed by the reduction in pH of the solution from 8.8 to 7.6 observed during the test.