Deposition of nitrogen-containing compounds to an extensively managed grassland in central Switzerland

During four intensive observation periods in 1992 and 1993, dry deposition of nitrogen dioxide (NO2) and ammonia (NH3), and wet deposition of nitrogen (N) were determined. The measurements were carried out in a small, extensively managed litter meadow surrounded by intensively managed agricultural land. Dry deposition of NH3 was estimated by the gradient method, whereas eddy correlation was used for NO2. Rates of dry deposition of total nitrate (= nitric acid (HNO3) + nitrate (NO3-)), total nitrite (= nitrous acid (HONO) + nitrite (NO2-)) and aerosol-bound ammonium (NH4+) were estimated using deposition velocities from the literature and measured concentrations. Both wet N deposition and the vertical NH3 gradient were measured on a weekly basis during one year. Dry deposition was between 15 and 25 kg N ha(-1) y(-1) and net wet deposition was about 9.0 kg N ha(-1) y(-1). Daily average NO2 deposition velocity varied from 0.11 to 0.24 cm s(-1). Deposition velocity of NH3, was between 0.13 and 1.4 cm s(-1), and a compensation point between 3 and 6 ppb V NH3 (ppb = 10(-9)) was found. Between 60 and 70% of dry deposition originated from NH3 emitted by farms in the neighbourhood. It is concluded that total N deposition is exceeding the critical load for litter meadows, is highly correlated to local NH3 emissions, and that NH3 is of utmost importance with respect to possible strategies to reduce N deposition in rural regions.