The effect of increased nitrogen deposition (30 kg N ha^-1 per year as NH₄NO₃) on nitrogen losses by denitrification was studied in a Norway spruce (Picea abies) forest in Central Switzerland. Denitrification and potential controlling variables were measured on five replicate areas, treated with a solution of NH₄NO₃ (N-treated plots), or only moistened with rain water for comparison (control plots).
Maximum nitrogen loss by denitrification was observed with increasing soil temperature and soil pH and decreasing soil redox potential. The spatial variation of the denitrifying activity was driven by the micro-topography of the forest. On drained mounds with I aw humus layers (redox potential: 659 ± 7 to 738 ± 3 mV) the rate of denitrification was low throughout the year. In waterlogged depressions (redox potential: 259 ± 15 to 515 ± 7 mV) the denitrifying activities were highest in spring and summer and reached up to 6.49 mg N m^-2 per day. stimulation of the denitrification rate by increasing temperature was most pronounced on the plots having a low redox potential.
Elevated N deposition increased the annual denitrification rate from 1.7 at ambient deposition to 2.9 kg N ha^-1 pet. year (May 1996-October 1997). Increased dentrifying activity due to elevated N inputs occurred particularly after rainfall events with nitrogen additions and reached up to 13.58 mg N m^-2 per day. This suggests that part of the N input is immediately denitrified and partly compensates the elevated nitrogen inputs.