Temporal Variation of Methane and Nitrous Oxide Emissions From Dairy Slurries and Implications On Discrete Sampling.

Slurry storages are sources of methane (CH₄) and nitrous oxide (N₂O) emissions. Due to temporal flux variations, chamber measurements with discrete sampling may yield biased emission estimates, particularly when integrating over extended periods. This research quantified temporal variations in CH₄ and N₂O emissions from dairy slurries, and examined the potential implications on discrete sampling protocols. Six pilot-scale storages in Truro Nova Scotia, Canada, were filled with dairy slurries of different total solids content in May, 2010. Each storage was permanently fitted with a dynamic chamber and gas fluxes were measured continuously for 6 mo. Discrete sampling was simulated by extracting data from continuous hourly flux datasets at sampling intervals ranging from 1 through 21 d, and for each time-of-day. Long-term flux variation was observed for both gases with lags prior to the onset of increased emissions. When CH₄ production was high, daily flux variations for incompletely encrusted slurries were linked with the dissipation of bubble-scums in the morning as the surface heated. In contrast, CH₄ emissions from completely encrusted slurries were more episodic. Daily variations in N₂O emissions were in phase with the surface temperature cycle when there was a crust >2 mo in age. This flux variation may have been due to decreased N₂O solubility and increased rates of production at elevated temperatures. For discrete sampling intervals of 1-7 d, to achieve ±10% deviation (in total fluxes) from continuous measurements, samples should be taken between 1800-0800 h, and 1900-2000 h for CH₄ (96% probability) and N₂O (50% probability), respectively. Results demonstrated that temporal CH₄ and N₂O flux variations for manure storage systems are affected by surface conditions. When using discrete sampling to quantify emissions, sampling intervals should be ≤7 d. Biases in N₂O emission estimates appear to be more sensitive to the time-of-day of sampling compared to CH₄.