Nitrous oxide emission factors from an intensively grazed temperate grassland: A comparison of cumulative emissions determined by eddy covariance and static chamber methods
2022
Murphy, Rachael M. | Saunders, Matthew | Richards, Karl G. | Krol, Dominika J. | Gebremichael, Amanuel W. | Rambaud, James | Cowan, Nicholas | Lanigan, Gary J.
Quantifying nitrous oxide (N₂O) emissions from grazed pastures can be problematic due to the presence of hotspots and hot moments of N₂O from animal excreta and synthetic fertilisers. In this study, we quantified field scale N₂O emissions from a temperate grassland under a rotational grazing management using eddy covariance (EC) and static chamber techniques. Measurements of N₂O by static chambers were made for four out of nine grazing events for a control, calcium ammonium nitrate (CAN), synthetic urine (SU) + CAN and dung + CAN treatments. Static chamber N₂O flux measurements were upscaled to the field scale (FCH FIELD) using site specific emission factors (EF) for CAN, SU+CAN and dung + CAN. Mean N₂O EFs were greatest from the CAN treatment while dung + CAN and SU + CAN emitted similar N₂O-N emissions. Cumulative N₂O-N emissions over the study period measured by FCH FIELD measurements were lower than gap-filled EC measurements. Emission factors of N₂O from grazing calculated by FCH FIELD and gap-filled were 0.72% and 0.96%, respectively. N₂O-N emissions were derived mainly from animal excreta (dung and urine) contributing 50% while N₂O-N losses from CAN and background accounted for 36% and 14%, respectively. The study highlights the advantage of using both the EC and static chamber techniques in tandem to better quantify both total N₂O-N losses from grazed pastures while also constraining the contribution of individual N sources. The EC technique was most accurate in quantifying N₂O emissions, showing a range of uncertainty that was seven times lower relative to that attributed to static chamber measurements, due to the small chamber sample size per treatment and highly variable N₂O flux measurements over space and time.
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