Does overwintering change the inoculum effect on methane emissions from stored liquid manure?

Greenhouse gas (GHG) emissions, especially methane (CH₄ ), from manure storage facilities can be substantial. Methane production requires adapted microbial communities ("inoculum") to be present in the manure. Complete removal of liquid dairy manure (thus removing all inoculum) from storage tanks in the spring has been shown to significantly reduce CH₄ emissions over the following warm season. This study examined whether the same mitigation effect would occur after fall removal of liquid dairy manure. Emissions of CH₄ , nitrous oxide (N₂ O), ammonia (NH₃ ), and CO₂ were measured from six 11.88-m³ tanks equipped with flow-through chambers. There were three inoculated controls (20% inoculum) and three uninoculated treatments, where inoculum was completely removed in the fall/winter (0% inoculum). Direct N₂ O and NH₃ (indirect N₂ O) were minor contributors to the total GHG budget, contributing <2% on a CO₂ equivalent (CO₂ e) basis. Removal of inoculum led to a 34% decrease in total emissions on a CO₂ e basis and to a 29% decrease in the CH₄ conversion factor compared with the inoculated control (0.37 vs. 0.52; p = .01). Overall, removing inoculum in the fall reduced CH₄ emissions from manure storage tanks; however, fall inoculum removal was less effective than in a previous study where inoculum was removed in the spring. The timing of inoculum removal may affect the efficiency of this CH₄ mitigation strategy. However, this method may be impractical for larger manure storage tanks. Further study is required to overcome challenges of time-sensitive, complete inoculum removal from farm-scale storage tanks.