The annual ammonia budget of fertilised cut grassland – Part 2: Seasonal variations and compensation point modeling

The net annual NH ₃ exchange budget of a fertilised, cut grassland in Central Switzerland is presented. The observation-based budget was computed from semi-continuous micrometeorological fluxes over a time period of 16 months and using a process-based gap-filling procedure. The data for emission peak events following the application of cattle slurry and for background exchange were analysed separately to distinguish short-term perturbations from longer-term ecosystem functioning. A canopy compensation point model of background exchange is parameterised on the basis of measured data and applied for the purposes of gap-filling. The data show that, outside fertilisation events, grassland behaves as a net sink for atmospheric NH ₃ with an annual dry deposition flux of −3.0 kg N ha ⁻¹ yr ⁻¹, although small NH ₃ emissions by the canopy were measured in dry daytime conditions. The median Γ _s ratio in the apoplast (=[NH ₄⁺]/[H ⁺]) estimated from micrometeorological measurements was 620, equivalent to a stomatal compensation point of 1.3 μg NH ₃ m ⁻³ at 15 °C. Non-stomatal resistance to deposition R_w was shown to increase with temperature and decrease with surface relative humidity, and R_w values were among the highest published for European grasslands, consistent with a relatively high ratio of NH ₃ to acid gases in the boundary layer at this site. Since the gross annual NH ₃ emission by slurry spreading was of the order of +20 kg N ha ⁻¹ yr ⁻¹, the fertilised grassland was a net NH ₃ source of +17 kg N ha ⁻¹ yr ⁻¹. A comparison with the few other measurement-based budget values from the literature reveals considerable variability, demonstrating both the influence of soil, climate, management and grassland type on the NH ₃ budget and the difficulty of scaling up to the national level.