Temporal aspects of N-enrichment and emission of N2O to the atmosphere

Human activities result in N enrichment of the biosphere, primarily via the production of fertilizers and emission of NOx from combustion. This anthropogenic N enhance the ecosystems' primary production, hence the accumulation of organic N. As a result, affected ecosystems may take a long time to reach near-steady-state conditions, i.e. when inputs are balanced by losses through denitrification and downstream transport (terrestrial-subsoil-freshwater-sea). This implies a cumulative enhancement of N2O emission by anthropogenic N. The purpose of this paper is to explore the consequences of such cumulative effects for the future emission of N2O, assuming that the present N management does not change. Data for the total dissipation of nitrogen from the Norwegian society were used as inputs in a model of N-transformations and -transport in terrestrial ecosystems, subsoils, and freshwater. Model performance was found to be in general agreement with observed N-retention and process rates in the environment. By concomitant simulation of a pristine and a polluted scenario (i.e. with the present Norwegian N-pollution from time zero), the net impact of the N-pollution on N2O emission could be estimated. This--anthropogenis N2O emission--outside the agricultural system was estimated to be 1.2, 3.7, 4.5, 5.2 and 5.7 Gg N2O-N yr-1 at time = 0, 50, 100, 200 and 300 years, respectively, after onset of the pollution. Emission estimates based on IPCC guidelines were identical with the model's prediction for emission after 50 years of N-pollution. This is a good agreement, since most of the N-pollution in Norway has taken place the last 50 years. However, this suggests that the IPCC calculation routines overestimate present emissions which are due to recent N inputs, and underestimate future emissions due to cumulative effects.