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 comaprison (control plots). Maximum nitrogen loss by denitrification was observed with incresing soil temperature and soil pH and decreasing soild redox potential. The spatial variation of the denitrifying activity was driven by the micro-topography of the forest. On drained mounds with raw humus layers (redox potential: 659 7 to 738 3m V) the rate of denitrification was low throughout the year. In waterlogged depression (redox potential: 259 ء 15 to 515 7mV) the denitrifying activities were highest in spring and summer and reached up to 6.49 mg N m-2 per day. Simulation of the denitrification rate by increasing temperature was most pronounced on the plots having a low redox potential. Elevated N deposition incrased the annual denitrification rate from 1.7 at ambient deposition to 2.9 kg N ha-1 per year (May 1996-October 1997). Increased denitrifying 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.
