Soil–atmosphere exchange of ammonia in a non-fertilized grassland: measured emission potentials and inferred fluxes

Abstract. A 50-day field study was carried out in a semi-natural, non-fertilized grassland in south-western Ontario, Canada during the late summer and early autumn of 2012. The purpose was to explore surface–atmosphere exchange processes of ammonia (NH₃) with a focus on bi-directional fluxes between the soil and atmosphere. Measurements of soil pH and ammonium concentration ([NH₄⁺]) yielded the first direct quantification of soil emission potential (Γ_soil = [NH₄⁺]/[H⁺]) for this land type, with values ranging from 35 to 1850 (an average of 290). The soil compensation point, the atmospheric NH₃ mixing ratio below which net emission from the soil will occur, exhibited both a seasonal trend and diurnal trend. Higher daytime and August compensation points were attributed to higher soil temperature. Soil–atmosphere fluxes were estimated using NH₃ measurements from the Ambient Ion Monitor Ion Chromatograph (AIM-IC) and a simple resistance model. Vegetative effects were ignored due to the short canopy height and significant Γ_soil. Inferred fluxes were, on average, 2.6 ± 4.5 ng m⁻² s⁻¹ in August (i.e. net emission) and −5.8 ± 3.0 ng m⁻² s⁻¹ in September (i.e. net deposition). These results are in good agreement with the only other bi-directional exchange study in a semi-natural, non-fertilized grassland. A Lagrangian dispersion model (Hybrid Single-Particle Lagrangian Integrated Trajectory – HYSPLIT) was used to calculate air parcel back-trajectories throughout the campaign and revealed that NH₃ mixing ratios had no directional bias throughout the campaign, unlike the other atmospheric constituents measured. This implies that soil–atmosphere exchange over a non-fertilized grassland can significantly moderate near-surface NH₃ concentrations. In addition, we provide indirect evidence that dew and fog evaporation can cause a morning increase of [NH₃]_g. Implications of our findings on current NH₃ bi-directional exchange modelling efforts are also discussed.