Dynamic nitrogen deposition thresholds during forest stand development in a Douglas fir forest analysed with two nitrogen models SMART2 and MERLIN

In contrast to the classical critical load (CL) concept, based on long-term steady-state conditions, a dynamic deposition threshold (DDT) is introduced. This DDT takes into account all relevant dynamic aspects of vegetation development/forest growth, mineralisation, immobilisation and denitrification, depending on the successional stage of the forest. DDT values for nitrogen were determined for a Douglas fir rotation by two process-based nitrogen models SMART2 and MERLIN using three different criteria for critical nitrogen leaching. During most of the rotation time, the predicted DDT values were higher than the corresponding traditional CL. SMART2 and MERLIN predicted a maximum DDT of 4.9 and 4.6 kmol N per ha per year (69 and 64 kg N per ha per year, respectively), when accepting a critical N leaching level of 1.73 kmol N per ha per year related to impacts on ground water quality. This is due mainly to relatively high tree uptake during the first 50 years of a forest rotation, compared to a long-term estima