Effect of Inoculant on VFA Degradation and Greenhouse Gas Emissions from Liquid Dairy Manure.

The storage of livestock manure is a significant contributor to greenhouse gas (GHG) emissions from the agricultural sector. Most GHG emissions research has been conducted on lab-scale manure storages, which are batch filled without inoculant. We measured continuous emissions of methane (CH₄), nitrous oxide (N₂O) as well as ammonia (NH₃) using tunable diode lasers from 6 replicated meso-scale storage tanks (6.6 m² each) from June to October 2016 (139 days). Tanks were enclosed by steady-state chambers to capture emissions. Half of the tanks were batch filled on day 1 (Jun-1) while the remaining were gradually filled over three stages (day 1, 20 and 40). Each tank within the batch and gradually filled groups had different inoculant levels (0%, 10%, and 20%). Volatile fatty acids (VFA) were samples twice during storage, and analysed using a gas chromatograph. From Aug-5 to Sept-24, there was an 88% drop in the total VFAs in all tanks. The 0% inoculant tanks contained 65% of the total VFAs on Aug-5 and 93% on Sept-24. This suggests slowed degradation when inoculant was absent. This is reflected in the CH₄ emissions, where the 0% inoculant tanks had on average 25% fewer total CH₄ emissions (5.0 vs 6.5 Kg m^-3 or 8.3 vs 1.1 Kg m^-2). Our results showed little difference between 10% and 20% inoculant tanks (6.6 and 6.5 Kg m^-3 or 10.8 and 10.7 Kg m^-2, respectively). Additionally, on Aug-5, the tanks with 0% inoculant showed high concentrations of propionic acid (~3.0 g L^-1) which is thought to disrupt methanogenesis, while inoculant containing tanks all had concentrations below detectable levels. When compared on a total CO₂-equivalent basis of CH₄, N₂O, and NH₃, the 0% inoculant tanks still had 24% fewer emissions compared to inoculated tanks, suggesting that slowed degradation effected all GHG production.