EFFECTS OF ENHANCED NITROGEN DEPOSITION IN A SPRUCE FOREST AT KLOSTERHEDE, DENMARK, EXAMINED BY MODERATE NH4NO3 ADDITION

The fate and effects of increased N deposition were investigated by ex perimental manipulation in a mature Norway spruce plantation at Kloste rhede, western Denmark. The ambient N deposition was 15-20 kg Nha(-1)y r(-1). Addition of 35 kg Nha(-1)yr(-1) as ammonium nitrate to a 500-m( 2) plot was carried out by handspraying monthly for 4 years. During th e first year the spray was labeled with N-15. Tree growth was depresse d by 4 successive spring or summer droughts throughout the treatment y ears. The nitrogen concentration in needles increased in the third yea r of treatment and critical low P concentrations (0.6-1 mg Pg(-1)) dev eloped possibly as a combined effect of drought and N stress on roots and mycorrhizae. Reduced retranslocation of N was indicated by increas ed concentration and flux of N in needle litterfall. Increased uptake of N was observed in all parts of the trees and the fraction of the in put N taken up was increased from 32 to 43%, whereas the fraction immo bilized in the soil was decreased from 40 to 33% of the input N. The d ecomposition remained unchanged, whereas the net N mineralization was increased 85% over the control. The effect on net mineralization may h ave resulted from saturation of the immobilization process rather than from an increase in gross mineralization. Soil respiration was not af fected, indicating that microbial activity did not respond. Nitrificat ion and denitrification were not important in either control or treate d soils. Soil solution chemistry responded promptly to nitrogen applic ations. Nitrate concentrations increased at all depths, and nitrate le aching increased from < 0.3 to 4.2 kg Nha(-1)yr(-1) by the third year. Reduced biological control on nitrate leaching was indicated by incre ased nitrate concentration in soil water at the end of the growing sea son. Ammonium concentrations increased down to 15 cm depth, but all am monium was retained in the system. Despite the increased nitrate leach ing, 92% of the total inputs were retained. No changes in concentratio ns of other major ions due to the N additions were detectable. The eff ects of the successive droughts on growth and nutrient cycling caused significant interaction on the response to N addition. Continuation of the N addition treatment will be necessary to observe the full respon se of the simulated deposition increase in this forest. (C) 1998 Elsev ier Science B.V.