Emission and uptake of ammonia by a fertilized corn field: eddy covariance fluxes over the growing season.

Ammonia (NH3) volatilization from urea application in cultivated fields impacts the nitrogen use efficiency of crops and is a major environmental concern. As NH3 is a main precursor of secondary aerosols in the atmosphere, it contributes to long range transport of reactive nitrogen and affects air quality, climate, and biodiversity. To better understand the processes and temporal dynamics of agricultural NH3 exchange, we measured direct eddy covariance (EC) NH3 fluxes above a corn field in Ottawa (ON). The flux tower was equipped with a 3-D sonic anemometer (CSAT3; Campbell Scientific, UT) and fast time-response Quantum Cascade Tunable Infrared Differential Absorption Spectrometer (QC-TILDAS; Aerodyne Research, MA) for NH3 measurements . Here, we present NH3 fluxes from the fertilizer application in May to the start of the leaf senescence in October 2017. It is the first EC flux dataset for NH3 from field crops over an entire growing season. The emissions of NH3 reached up to 500 ng m 2 s-1 within a few days of the fertilizer application date, suggesting that the hydrolysis of the urea fertilizer occurred fast during this rainy period. When the corn canopy was fully developed, both NH3 emissions of up to 100 ng m-2 s-1 and NH3 deposition towards the end of the growing season of up to -250 ng m-2 s-1 were observed. This highlights the importance (1) of the stomatal compensation point and non-stomatal deposition for the net NH3 exchange and (2) of quantifying the NH3 fluxes over the long periods of time to capture the bi-directional exchange, allowing to report more accurate budgets. Finally, we use a resistance model approach to improve our understanding of leaf and soil NH3 exchange processes, which is important for improving parameterizations used in NH3 emission models and inventories.