Animal manure contains valuable plant nutrients which need to be stored until field application. A significant proportion of slurry nitrogen is volatilized in the form of ammonia (NH₃) during storage. This impacts human health, biodiversity, air and water quality and thus urgent action is needed to reduce NH₃ emissions. In this experiment, we evaluated the NH₃ emission mitigation potential of biochars derived from miscanthus (MB) and solid separated anaerobic digestate (DB), and orthophosphoric acid activated MB (AMB) and DB (ADB) as well as lightweight expanded clay aggregate (LECA) during four months of liquid manure storage. A slurry without amendment was included as a control (Ctrl). Acid activated and non-activated biochars were applied on top of the slurry maintaining a 7 mm thick surface layer, while LECA was applied in a 2 cm thick layer. NH₃ emissions were measured by photoacoustic analyzer. In comparison to Ctrl, acid activated biochar decreased (p < 0.05) NH₃ emissions during the slurry storage. Activated biochar reduced the emissions by 37-51% within the first month of slurry storage and achieved a 25-28% emissions reduction efficiency throughout the four month period due to the reduction in emission mitigation efficiency as the storage period progressed. LECA reduced NH₃ emissions by 21% during storage. Losses of NH₃ as a percentage of total ammoniacal N were 29-31% for activated biochars, 35-39% for non-activated biochars and 33% for LECA. In conclusion, acid activated biochars and LECA could be good floating-covers to mitigate NH₃ emissions during manure storage, but activated biochars may have better mitigation potential than LECA.