Sustainable Management of Buffalo Manure Digestate: Environmental and Economic Assessment of Biochar-Based Cover for Ammonia Emission Mitigation

Storage of livestock effluents represents a major source of ammonia (NH 3 ) emissions into the atmosphere. Reducing ammonia volatilisation is essential to improve the efficiency of nutrient use, mitigate air pollution, and enhance the overall sustainability of livestock production systems aligned with the principles of the circular economy. Therefore, identifying efficient and sustainable mitigation strategies is crucial. Conventional floating covers are commonly used to reduce emissions; however, they present limitations in terms of management, durability, and cost. This study proposes a novel approach by comparing traditional floating cover materials like straw and light expanded clay with biochar as an innovative and sustainable mitigation strategy to reduce ammonia volatilisation from the liquid fraction of buffalo digestate obtained from an anaerobic digestion plant in southern Italy. All cover materials were applied at a uniform thickness of 2 cm under laboratory conditions using a dynamic chamber technique. Additionally, a cost analysis was performed considering the material purchase cost for an average storage tank of 700 m 2 and two hypothetical reduction efficiencies (50% and 70%). Results indicated that biochar was the most effective cover, achieving a 67% reduction in ammonia emissions compared with the uncovered control. Light expanded clay exhibited the lowest efficiency, likely due to its insufficient sealing capacity at the applied thickness. From an economic perspective, biochar becomes increasingly competitive when emission reduction efficiency is accounted for, owing to its favourable physical–chemical properties. The results highlight the potential of using biochar as a sustainable, circular strategy for mitigating atmospheric emissions and improving nitrogen conservation, while also creating opportunities for its subsequent reuse in agriculture as a soil amendment.