## 3 Results  
### Storage Tests: Digestate Characteristics, NH3 Emissions, Surface Monitoring and Biochar Adsorption  
After the storage tests, a representative sample of the digestate in each vessel was analyzed in three replicates (Table 3). Compared to the initial digestate, the dry matter content increased, highlighting a reduction in the water content of the samples, especially for Bm, whose DM content is almost double the initial value. On the other hand, both TAN and TKN contents were found to decrease after the test. 2B showed the best behaviour, reducing its TAN content by half compared to the initial value. In Figure 2 the NH3 emission rates are presented along with pH variations as function of time for the different storage tests performed when applying different biochar application methods. For all the treatments, the ammonia emission rates decreased gradually, tending to zero approximately after 84 days. Initially, for 1B and 2B, NH3 emissions were very close to zero until the day 2 and 5, respectively, when an increase in the emission rates was reported, from 0.8 to 4.9 mg NH3 m\({}^{-2}\) h\({}^{-1}\) and from 0.4 to1.5, respectively. Differently, for Bm the emission rate was high even at the beginning of the test and monotonically decreases over time. The test 2B was characterized by a lower cumulative NH3 emission (10.38 \(\pm\) 0.57 g NH3 m\({}^{-2}\)), \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Treatment** & **DM** & **TAN** & **TKN** \\
**(g\(\cdot\)kg\({}^{-1}\))** & **(g\(\cdot\)N\(\cdot\)kgDM\({}^{-1}\))** & **(g\(\cdot\)N\(\cdot\)kgDM\({}^{-1}\))** \\ \hline No treatment & 46.22 \(\pm\) 1.23 & 20.63 \(\pm\) 0.57 & 47.46 \(\pm\) 1.43 \\
2B & 62.05 \(\pm\) 1.23 & 10.40 \(\pm\) 1.09 & 39.28 \(\pm\) 2.78 \\
1B & 66.78 \(\pm\) 3.94 & 6.53 \(\pm\) 0.53 & 37.75 \(\pm\) 0.86 \\ Bm & 79.84 \(\pm\) 2.85 & 6.34 \(\pm\) 0.22 & 33.57 \(\pm\) 1.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Digestate characteristics before and after the treatments. while the cumulative NH\({}_{3}\) emissions are 14.98 \(\pm\) 0.43 and 18.08 \(\pm\) 1.31 g NH\({}_{3}\) m\({}^{-2}\) for 1B and Bm, respectively. All treatments were significantly different (_p_ < 0.05). No significant difference was found for the pH trend during the experiment, which varied between 7.9-9.2, 7.9-9 and 7.9-8.9 for 2B, 1B and Bm respectively. The surface of the digestate in each type of storage test changed substantially during the study. The 1B cover started breaking in the middle of the vessel and sinking after 21 days (in one of the replicates) consistently with the thinner and less resistant biochar layer (Figure 3a). At the same time, a slight increase in ammonia emissions was observed in one sample, from an average of 9.9 on day 16 to 13.1 g NH\({}_{3}\) m\({}^{-2}\) in the following days. During the following weeks, the other replicates went through the same process, resulting in an average emission rate of 11.8 g NH\({}_{3}\) m\({}^{-2}\). After the first 21 days, Bm showed the formation of a crust made up of biochar previously mixed into the digestate (Figure 3b). The biochar started floating and formed a crust that thickened as time goes by. In this test, a reduction of the digestate volume/level was also noted, consistently with the water evaporation (Figure 3c) and the consequent increase in DM content at the end of the trial (Table 3). The 2B cover remained almost intact until week 9. At the same time, a pick of 6.8 mg NH\({}_{3}\) m\({}^{-2}\) h\({}^{-1}\) in the NH\({}_{3}\) emission rate curve was observed; then the biochar cover started to be increasingly submerged by the liquid. Biochar ammonia adsorption during the storage tests was assessed applying a N-NH\({}_{4}\) desorption procedure. As a matter of fact, biochar from the test 2B desorbed 0.122 mg N-NH\({}_{4}\) g\({}^{-1}\)\({}_{\text{biochar}}\), while biochar from 1B desorbed 0.069 mg N-NH\({}_{4}\) g\({}^{-1}\)\({}_{\text{biochar}}\), consistently with the lower amount of thickness of the latter.