NICCE: 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 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 δ15N 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 15N. 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 CO2 concentration are predicted to result in decreased nitrate concentrations. The model has hitherto been calibrated with conventional 14N data. Labeling with 15N 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 15N in coniferous forest ecosystems.