Modelling the effects of nitrogen addition on soil nitrogen status and nitrogen uptake in a Norway spruce stand in Denmark

The long-term effects of application of 35 kg n ha−1 year−1 on the status, uptake and leaching of nitrogen were evaluated by application of the dynamic C and N circulation model SOILN, on experimental field data from a N addition experiment in a Norway spruce forest in Denmark. The model describes N uptake by two different pathways, either nitrogen is taken up as inorganic N after mineralisation or nitrogen is taken up directly from the organic pool as amino acids with a C/N ratio 4 assumed to imitate mycorrhizal uptake. In the control plot the organic N uptake supplies 29% of the total plant N uptake, which by N addition was reduced to 14% of the N uptake within the 3 years of the N addition experiment. The long-term effects of chronic N addition were predicted by running the model for a 30 year period until year 2025. The total N load in the N-addition plot exceeded the critical load significantly. The additional N input caused the N-leaching to increase significantly, accounting for 34% of the extra input. The remaining 66% of the additional input accumulated in the ecosystem, 22% went to increased biomass N concentrations in the plants and 45% accumulated in the soil causing C/N in the top soil layers to decrease. The continuous high input of mineral N in the N addition plot reduced the organic uptake to account for c. 4% of total N uptake.