Field scale measurements of NH3 emissions after organic fertilizer application: comparison of different methods

The volatilization of ammonia after application of organic fertilizers to agricultural fields represents a major source of reactive atmospheric nitrogen. 5-70% of applied nitrogen can be lost as ammonia, depending on meteorological conditions, the characteristics of the manure, and the technique used for application. But even for similar conditions, a large range of emission factors can be found in the literature, reflecting a considerable uncertainty. Part of the uncertainty is related to the lack of ammonia flux measurements after fertilization at the scale of whole agricultural fields. Such measurements still represent a challenge, mainly because of the sticky nature of the ammonia molecule, which complicates measurements at high time resolution that are needed for observing the highly dynamic character of these emission events. We report on two field experiments performed in August 2009 at the NitroEurope (www.nitroeurope.eu) site in Oensingen, Switzerland. The ammonia emission after application of liquid cattle slurry on a recently harvested grassland and a crop field was investigated simultaneously by 1) an aerodynamic gradient approach, 2) eddy covariance measurements using a fast mass-spectroscopic ammonia analyzer, and 3) inverse-dispersion calculations based on concentration measurements at half-hourly to several hours time resolution. Due to the sequential application of the manure (refilling of slurry tanks) and the fast decrease of ammonia volatilization, detailed footprint calculations and corrections with a high temporal resolution were crucial for obtaining representative emission fluxes. The comparison of all applied methods showed good agreement on the magnitude of observed emission fluxes, confirming earlier measurements at this field site, being rather at the low end of emission factors in the literature.