Modelling of N2O emissions from grasslands in Denmark

Summary

A dynamic algorithm to simulate the N2O production and emission from nitrification and denitrification has been included in the field component of the FASSET whole-farm model. Firstly, the model was tested on experimental data of N2O emissions from typical North European grasslands with mineral soils (Denmark, Finland, UK), differing in climatic conditions, soil properties and management. A good agreement was obtained between simulated and measured seasonal N2O emissions grazed and mown grasslands. The predicted annual N2O emissions were generally in accordance with the emission factor of 1.25% suggested by IPCC. Secondly, the model was tested under different climatic and management scenarios applicable for Denmark. The reasonable calculations of potent N2O emissions were done for a 4-year rotation with one year of spring barley undersown with grass, two years of grass and a forth year of barley undersown with grass. The grasslands were considered to be either grazed with heifers and treated as a cut based system (4 cuts per year), with removal of the grass production. The level of applied mineral N (ammonium nitrate, 50:50) ranged from 0 to 500 kg ha-1. For estimation of average results, 30 independent calculations with the original climatic and soil datasets were done for 3 sites in Denmark.
The results showed that denitrification was always an important source for N2O emissions, increasing with increasing of denitrification rates for soils with higher clay content and N input. The emissions increased for a climate with a higher rainfall. Nitrification became a significant source for N2O emissions only in very sandy soils, losing importance for the grazed treatment faster than for cut treatments.
There was almost no available N source for N2O emission during the period from late autumn to early spring. For the annual N2O emissions there was no clear climate effect between different sites. Nevertheless, for daily dynamics the higher N2O emission peaks were clearly determined by the weather pattern.
For the grazed treatments N2O emissions were higher than for cut treatments with the same mineral N input. Emissions increased in a non-linear way for both the grazed and the cut treatments with relatively higher emissions at higher N inputs. There were also considerable differences between different soil types. These effects suggest that the simple IPCC methodology is not directly applicable to evaluate N2O emissions from grasslands.