Investigating sources and sinks for ammonia exchanges between the atmosphere and a wheat canopy following slurry application with trailing hose

Ammonia exchanges between the atmosphere and terrestrial ecosystems are composed of several pathways including exchange with the soil, the litter, the plant surfaces (cuticle) and through the stomata. In this study, the fate of nitrogen in the different pools (soil and plant) was analyzed with the aim of determining the sources and sink of atmospheric ammonia after slurry application on a wheat canopy. To do this, we measured ammonia exchanges between a winter wheat canopy and the atmosphere following cattle slurry application with a trailing hose. From 12 March to 8 April in Grignon near Paris, France, the ammonia fluxes ranged from an emission peak of 54,300 NH3 ng m−2 s−1 on the day of slurry application (with a median during the first 24 h of 5990 NH3 ng m−2 s−1) to a deposition flux of −600 NH3 ng m−2 s−1 (with a median during the last period of −16 NH3 ng m−2 s−1). The ammonia compensation points were evaluated for apoplasm, foliar bulk, root bulk and litter bulk tissue, as well as for soil surface. Ammonia emission potentials defined by the ratios between the concentration in [NH4+] and [H+] for each N ecosystem pool were in the same order of magnitude for the plant decomposed in apoplastic liquid, green leaf bulk tissue and cuticle, respectively, averaging at 73, 160 and 120; in green leaf bulk tissues, the emission potential decreased gradually from 230 to 78 during the period after slurry application, while in the dead leaf bulk tissues considered as litter, the emission potential reached a maximum of 50,200 after application stabilized at around 20000. The dynamic of the emission potential for roots was similar to the ammonium concentration in the first two centimeters of the soil, with a maximum of 820 reached two days after application and a minimum of 44 reached three weeks later. The surfatm-NH3 model interpreted the emission and deposition fluxes by testing soil surface resistance. We conclude that emission of the first day application was driven by climatic conditions and ammonia concentration at the soil surface, with no surface resistance and with only soil surface emission potential. On the next three days, the ammonia emission originated from the soil surface with the growth of a dry surface layer inducing surface resistance and regulated by slurry infiltration. The following days need a more detailed description of soil surface processes and the integration of vegetation exchanges (stomatal and cuticle pathways), particularly in the last period, in order to explain the ammonia deposition.