## 1 Introduction  
Manure storage is one of the pillars of building systems on modern farms. Proper storage facilities are required to handle the increasing volumes of manure due to intensive livestock farming operations [1], which accounted for an annual European production of 1400 million tonnes of manure in 2013 [2]. Open manure storage is a common practice; however, the continuous N losses, mainly of ammonia (NH\({}_{3}\)), require farmers to take measures to save nutrients and reduce environmental risks [3] in compliance with EU Directive 2016/2284, also known as the National Emission Ceilings (NEC) Directive, such as by applying additives to manure or covering storage tanks. Biochar is the carbon-rich solid product of biomass pyrolysis [4]. In general, biochar has been shown to be effective in reducing gas emissions from manure composting and in recovering nutrients from wastewater due to its sorption capacity [5]. However, it is only recently that biochar has been explored both as a manure additive [6] and a floating cover for storage tanks [7; 8; 9], thanks to its tuneable porosity, pH and surface chemistry [10]. Indeed, by acting on the pyrolysis conditions, depending on the initial feedstock composition, it is possible to direct the process toward conditions producing biochar with favourable characteristics for reducing ammonia emissions [11; 12]. [PERSON] et al. [6] summarized the positive effects of biochar addition to different types of animal manure such as dairy cattle slurry and poultry litter in terms of reduction in greenhouse gases emission and retainment of nutrients. Similarly, many authors showed promising results when biochar is used as storage tank cover. [PERSON] and [PERSON] [13] reported a reduction of 96% of NH\({}_{3}\) emissions covering the 16 L dairy digestate for a storage period of 7 weeks. [PERSON] et al. [14] recorded a reduction ranging between 13 and 23% with biochar application on swine manure surface of 2.28-4.56 kg/m\({}^{2}\), respectively. [PERSON] et al. [8] found that a 2 cm layer of biochar can reduce up to about 80% of the NH\({}_{3}\) emissions from manure storage tank in lab conditions. [PERSON] et al. [15] obtained an NH\({}_{3}\) emissions reduction of 48% with biochar application on digestate surface. Biochar interacts with ammonia nitrogen dynamically through different mechanisms: (i) adsorbing NH\({}_{4}\)\({}^{+}\) in the liquid phase onto the biochar layer partially immersed in the liquid at the top layer of the tank, (ii) adsorbing gaseous NH\({}_{3}\) devolatilizing from the manure tank, (iii) altering the NH\({}_{4}\)\({}^{+}\)/NH\({}_{3}\) equilibrium in the top layer of the liquid due to its typically basic pH, and (iv) providing a physical barrier at the liquid-surface interface, which reduces the gas transport between the manure surface and the atmosphere [16]. This is even more effective in the presence of adverse weather conditions (e.g., high wind speed) [16]. As reported by [17] in the presence of NH\({}_{4}\)\({}^{+}\), electrostatic interaction and cation exchange are the main mechanisms involved. The presence of oxygen-containing functional groups, the negative charge on the biochar surface and the high Cation Exchange Capacity (CEC) [18] are found to be conductive to NH\({}_{4}\)\({}^{+}\) adsorption. A positive correlation between CEC of pig manure derived biochar and NH\({}_{4}\)\({}^{+}\) adsorption was found [19]. Although some authors have indicated that there is no direct correlation between the porosity and surface area of biochar and NH\({}_{4}\)\({}^{+}\) adsorption [19], the microporous structure of biochar increases its selectivity towards NH\({}_{4}\)\({}^{+}\) compared to larger molecules in real wastewater systems [20]. In case of gaseous NH\({}_{3}\) adsorption it was observed that physio-sorption is the prevalent mechanism, with respect to chemio-sorption [21]. Biochar improves its adsorption performance as the presence of oxygen-containing functional groups acidic sites increases [22].