NICCE: a model for cycling of nitrogen and carbon isotopes in coniferous forest ecosystems

Abstract A process-oriented dynamic simulation model for turnover of N and C isotopes in coniferous forest ecosystems has been developed. The model is 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 experiment, with N-input in throughfall excluded. Foliar N-concentrations decreased more slowly, and were observed to be unchanged over a 2-year period. Due to isotope fractionation processes δ 15 N values of ecosystem compartments are predicted to increase for systems approaching N-saturation, due to output of nitrate and denitrification products being relatively depleted in 15 N. Sensitivity analysis of the model revealed a strong influence of the microbial substrate-use efficiency for organic carbon on input/output budgets of N. Factors causing an increase of primary production such as increasing CO 2 concentration are predicted to result in decreased nitrate concentrations. The model has hitherto been calibrated with conventional 14 N data. Labeling with 15 N on the scale of forest stands, and small head-water catchments, is currently being carried out to follow the fate of atmospheric nitrogen input, and to compare model behavior with turnover of 15 N in coniferous forest ecosystems.