NICCCE - A MODEL FOR CYCLING OF NITROGEN AND CARBON ISOTOPES IN CONIFEROUS FOREST ECOSYSTEMS

A process-oriented dynamic simulation model for turnover of N and C is otopes in coniferous forest ecosystems has been developed. The model i s used to interpret results of experiments on nitrogen saturation and reversibility of nitrogen saturation conducted within a gradient of N- deposition in Europe. Observed and simulated nitrate concentrations in the soil solution both responded quickly to reduced input in an exper iment, with N-input in throughfall excluded. Foliar N-concentrations d ecreased more slowly, and were observed to be unchanged over a 2-year period. Due to isotope fractionation processes delta(15)N values of ec osystem compartments are predicted to increase for systems approaching N-saturation, due to output of nitrate and denitrification products b eing relatively is depleted in N. Sensitivity analysis of the model re vealed a strong influence of the microbial substrate-use efficiency fo r organic carbon on input/output budgets of N. Factors causing an incr ease of primary production such as increasing CO2 concentration are pr edicted to result in decreased nitrate concentrations. The model has h itherto been calibrated with conventional N-14 data. Labeling with N-1 5 On the scale of forest stands, and small head-water catchments, is c urrently being carried out to follow the fate of atmospheric nitrogen input, and to compare model behavior with turnover of N-15 in conifero us forest ecosystems.