Effects of Gradual Filling on Greenhouse Gas Emissions from Liquid Dairy Manure.

Storage of animal manure is a large contributor to greenhouse gas emissions from the agricultural sector. Most emission research has been conducted on lab-scale manure storages These are usually batch filled without inoculant, whereas farm-scale dairy manure storages are loaded gradually, at daily or weekly intervals. We measured methane (CH₄) and nitrous oxide (N₂O) emissions from 6 meso-scale storage tanks (6.6 m² each) from June to October 2016 (139 d). Tanks were enclosed by steady-state chambers and concentrations were continuously measured using tunable diode lasers. 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 continuously filled groups had a different inoculant level (0%, 10%, and 20%). Tanks containing inoculum had more CH₄ emissions in gradual filled tanks (~7.4 Kg m^-3), compared to batch filled tanks (~5.6 Kg m^-3). The 10% inoculant had 31% more CH₄ emissions in gradual filled tanks, while the 20% inoculant tanks had 21% more CH₄ emissions. This is due to higher average temperature in gradual filled tanks (16.8 °C vs 18.9 °C). In 0% inoculant tanks, CH₄ emissions had opposite treatment effects. Gradual tank had 27% less CH₄ emissions (5.8 vs 4.2 Kg m^-3). This is due to a combination of high solids (20% dry matter) and lower average temperature (17.4 °C). N₂O emissions showed no treatment difference between continuous and batch filled tanks (0.95 Kg m^-3 vs 0.90 Kg m^-3, respectively). When compared on a total CO₂-equivalent basis of CH₄, N₂O, and NH₃, the batch fill tanks had 13% less emissions. Overall, the results suggest that batch filled studies may underestimate the CH₄ emissions expected from real dairy farms.