Biases in discrete CH₄ and N₂O sampling protocols associated with temporal variation of gas fluxes from manure storage systems

Manure slurry storage systems are possible sources of methane (CH₄) and nitrous oxide (N₂O), both of which are strong greenhouse gases (GHG). The most commonly used methods to measure these emissions rely on chamber techniques deployed at discrete intervals (i.e. non-continuously). Due to long-term and diurnal variations in GHG emissions, discrete sampling may yield biased estimates when integrating fluxes over time. This research quantified the effect of sampling interval and the 'time-of-day' of sample collection on total emission estimates from discrete sampling compared to continuous monitoring, and characterized temporal flux variations in relation to environmental conditions. Methane and N₂O emissions were measured continuously over the 6mo warm storage season in 2010 from 6 pilot-scale dairy manure slurry storage tanks using flow-through steady state chambers. Discrete sampling was simulated by extracting data from hourly flux time series at 13 sampling intervals ranging from 1 to 21d. For each sampling interval, 24 datasets were generated, one for each hour of the day. When there were high rates of CH₄ ebullition, and crusts did not completely cover the surface, diurnal flux variations were linked with the diurnal surface temperature (T₀) cycle, whereby bubbles that accumulated at the surface overnight burst after sunrise. With complete manure surface crusting, episodic and unpredictable flux events were more common. Diurnal variation in N₂O emissions was strongly linked with variation in T₀ when crusts >2mo old were present. For CH₄, sampling between 1800 and 0800h at intervals ≤7d yielded ±10% deviation between discrete and continuous monitoring with a frequency of 96%. The frequency of achieving ±10% deviation for N₂O was 50% when sampling at ∼2000h. If sampling during these times is not possible, manual measurements should be made in early morning (before 0900h) and late afternoon (after 1700h).