Simulating the fluxes of CO2 and N2O in European grasslands with the Pasture Simulation Model (PaSim)

A mechanistic model of grassland dynamics is used to simulate fluxes of carbon dioxide (CO2) and nitrous oxide (N2O) in European grasslands. The simulations refer to five sites belonging to a monitoring network setup within the framework of the European Union project GREENGRASS. Simulated gross primary productivity ranges from 0.4 to 1.9 kg C m−2 year−1 depending on environmental conditions and management. Ecosystem respiration is calculated in the order of 0.7–1.5 kg C m−2 year−1, resulting in a net CO2 uptake of about 0.3 kg C m−2 year−1, in reasonable agreement with observations. Linear relationships between ecosystem respiration and gross primary productivity, as well as between net primary productivity and annual precipitation are indicated by the simulations. Annual emissions of N2O are predicted in the range of 1–5 kg N ha−1 year−1, a factor of 2–10 higher than observed. This is caused by an overestimation of the background fluxes. On the other hand, the model fails to faithfully reproduce timing, duration and magnitude of peak emissions triggered by the application of mineral and organic fertilizers and by rain events.