NITROGEN MOBILITY IN A NITROGEN LIMITED FOREST AT KLOSTERHEDE, DENMARK, EXAMINED BY NH4NO3 ADDITION

The fate and effects of increased N deposition were investigated by experimental manipulation in a mature Norway spruce plantation at Klosterhede, western Denmark. Ambient N deposition was 23 kg N ha(-1) year(-1). Addition of 35 kg N ha(-1) year(-1) as ammonium nitrate to a 500 m(2) plot was carried out monthly by handspraying. The soil solution chemistry responded promptly to the nitrogen application. Nitrate concentrations increased at all depths, and nitrate leaching increased from 0.3 to 2.3 kg N ha(-1) year(-1). Nitrate leached during winter and spring when water transport was highest. Despite the increased nitrate leaching, 92% of the nitrate input was retained. The ammonium input was completely retained within the system, and soil water concentrations of ammonium were only slightly changed by the nitrogen addition. Mineralisation was the major source of ammonium in the soil. An increase of exchangeable ammonium could account for 20% of the added ammonium. No changes in concentrations of other major ions due to the nitrogen addition were detectable. There was no detectable growth response or change in the nitrogen concentration of the foliage after the first growing season with nitrogen addition although the foliage was nitrogen deficient. A longer period of treatment is probably necessary to demonstrate the response of biological processes. The implications of the direct increase in nitrate leaching at nitrogen-limited conditions are discussed.