## 2 Materials and Methods
### Ammonia Emissions Assessment from Digestate Storage  
Digestate storage was reproduced under laboratory conditions, employing cylindrical glass containers of 5 L and 16 cm diameter. To reduce the parameters influencing the emissions, the experiments were conducted inside a climate-controlled room at 20 \({}^{\circ}\)C. Temperature and humidity were checked once a day for the whole experimental period. Glass containers were filled with 3.5 L of digestate. Three different biochar application methods were adopted and compared. In the first test, 140 g of biochar, representing 4 wt% of the liquid digestate sample and about 7 kg m\({}^{-2}\), were applied on the surface of the digestate to form a cover 2 cm thick; in the second test a similar configuration was adopted using 70 g of biochar forming a cover 1 cm thick; finally, 140 g of biochar was mixed with the liquid digestate and placed in the jar without any cover. Table 1 summarizes the three different conditions considered and their abbreviations. Three replicates were made for each treatment for a total of 9 jars. Ammonia emissions and pH values were monitored for the whole storing period of 84 days. To measure the pH, an access tube was placed in each jar in order to place the pH meter probe (MT51302523, Mettler Toledo ([[https://www.mt.com/it/it/home/products/Laboratory_Analytics_Browse/pH-meter/pH-meters/SG23-Meter.html](https://www.mt.com/it/it/home/products/Laboratory_Analytics_Browse/pH-meter/pH-meters/SG23-Meter.html)]([https://www.mt.com/it/it/home/products/Laboratory_Analytics_Browse/pH-meter/pH-meters/SG23-Meter.html](https://www.mt.com/it/it/home/products/Laboratory_Analytics_Browse/pH-meter/pH-meters/SG23-Meter.html)), accessed on 24 June 2024)) below the manure surface without damaging the superficial layer of the biochar. Ammonia emissions were measured with the dynamic chamber technique, as described by [9]. Specifically, the containers were closed only during the measurement activity to ventilate the headspace of the container once per minute. The airflow rate was adjusted by a flow meter. The containers were closed using a screw lid designed to let air in and to connect the container to an expansion chamber. The air exchange of 1.5 L min\({}^{-1}\) was performed with a vacuum pump. Before the ammonia sampling, the pump was turned on for 20 min to create a steady condition. Real-time ammonia detection was run for 15 min with a gas-sensitive semiconductor and electrochemical sensors (Aeroqual, [[https://www.aeroqual.com/s-series-portable-air-monitors/series-500-portable-air-pollution-monitor](https://www.aeroqual.com/s-series-portable-air-monitors/series-500-portable-air-pollution-monitor)]([https://www.aeroqual.com/s-series-portable-air-monitors/series-500-portable-air-pollution-monitor](https://www.aeroqual.com/s-series-portable-air-monitors/series-500-portable-air-pollution-monitor)), accessed on 24 June 2024). The fluxes were evaluated as follows:  
\[\text{F}=\text{Q}\ (\text{C}_{\text{out}}-\text{C}_{\text{in}})\ \text{A}^{-1}, \tag{1}\]  
where \(\text{C}_{\text{in}}\) is the gas concentration of air inlet into the chamber in mg m\({}^{-3}\); \(\text{C}_{\text{out}}\) is the gas concentration of air outlet from the chamber in mg m\({}^{-3}\); \(\text{Q}\) is the airflow rate through the chamber in m\({}^{3}\) h\({}^{-1}\); and \(\text{A}\) the area of the emitting surface in m\({}^{2}\). Simultaneously, as suggested by [14], visual surface monitoring was performed every day, to macroscopically observe the cover performance during the storage period. NH\({}_{4}\)\({}^{+}\) adsorbed by the biochar after each storage trial was indirectly estimated using the method described in [33]. For this purpose, the biochar collected from each jar was mixed for homogenization. About 10 g of biochar were collected and washed with 11 of distilled water to remove digestate residues from the biochar surface and then dried for 48 h at ambient temperature, avoiding possible ammoniacal nitrogen desorption.