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

Abstract

Abstract. The net annual NH3 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 NH3 with an annual dry deposition flux of −3.0 kg N ha−1 yr−1, although small NH3 emissions by the canopy were measured in dry daytime conditions. The median Γs ratio in the apoplast (=[NH4+]/[H+]) estimated from micrometeorological measurements was 620, equivalent to a stomatal compensation point of 1.3 μg NH3 m−3 at 15 °C. Non-stomatal resistance to deposition Rw was shown to increase with temperature and decrease with surface relative humidity, and Rw values were among the highest published for European grasslands, consistent with a relatively high ratio of NH3 to acid gases in the boundary layer at this site. Since the gross annual NH3 emission by slurry spreading was of the order of +20 kg N ha−1 yr−1, the fertilised grassland was a net NH3 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 NH3 budget and the difficulty of scaling up to the national level.