Gas emissions from liquid dairy manure: Complete versus partial storage emptying

When manure slurry is removed from storages for land application, there is often ‘aged’ manure that remains because the storages are not completely emptied. Aged manure may act as an inoculum and alter subsequent methane (CH<inf>4</inf>), nitrous oxide (N<inf>2</inf>O) and ammonia (NH<inf>3</inf>) emissions when fresh manure is added to the system, compared to an empty storage that is filled with fresh manure. Completely emptying manure storages may be a practice to decrease gas emissions, however, little pilot-scale research has been conducted to directly quantify the inoculum effect. Therefore, we compared CH<inf>4</inf>, N<inf>2</inf>O, and NH<inf>3</inf>emissions from three pilot-scale slurry tanks (~10.5 m³each) filled with a mixture of fresh manure and an inoculum of previously stored manure (i.e., partial emptying) to three tanks that contained only fresh manure (i.e., complete emptying). Gas fluxes were continuously measured over 155 d of warm season storage using flow-through steady-state chambers. The absence of an inoculum significantly reduced CH<inf>4</inf>emissions by 56 % compared to partially emptied (inoculated) tanks, while there was no difference in N<inf>2</inf>O emissions. There was a significant 49 % reduction in greenhouse gas (GHG) emissions because the overall budget (as CO<inf>2</inf>-eq) was dominated by CH<inf>4</inf>. Complete manure storage emptying could be an effective GHG mitigation strategy; however, NH<inf>3</inf>emissions were significantly higher from un-inoculated tanks due to slower crust formation. Therefore additional NH<inf>3</inf>abatement should be considered.