Modelling nitrous oxide emissions from mown-grass and grain-cropping systems: Testing and sensitivity analysis of...

The DailyDayCent biogeochemical model was used to simulate nitrous oxide (N₂O) emissions from two contrasting agro-ecosystems viz. a mown-grassland and a grain-cropping system in France. Model performance was tested using high frequency measurements over three years; additionally a local sensitivity analysis was performed. Annual N₂O emissions of 1.97 and 1.24kgNha^-1year^-1 were simulated from mown-grassland and grain-cropland, respectively. Measured and simulated water filled pore space (r=0.86, ME=-2.5%) and soil temperature (r=0.96, ME=-0.63°C) at 10cm soil depth matched well in mown-grassland. The model predicted cumulative hay and crop production effectively. The model simulated soil mineral nitrogen (N) concentrations, particularly ammonium (NH₄⁺), reasonably, but the model significantly underestimated soil nitrate (NO₃^-) concentration under both systems. In general, the model effectively simulated the dynamics and the magnitude of daily N₂O flux over the whole experimental period in grain-cropland (r=0.16, ME=-0.81gNha^-1day^-1), with reasonable agreement between measured and modelled N₂O fluxes for the mown-grassland (r=0.63, ME=-0.65gNha^-1day^-1). Our results indicate that DailyDayCent has potential for use as a tool for predicting overall N₂O emissions in the study region. However, in-depth analysis shows some systematic discrepancies between measured and simulated N₂O fluxes on a daily basis. The current exercise suggests that the DailyDayCent may need improvement, particularly the sub-module responsible for N transformations, for better simulating soil mineral N, especially soil NO₃^- concentration, and N₂O flux on a daily basis. The sensitivity analysis shows that many factors such as climate change, N-fertilizer use, input uncertainty and parameter value could influence the simulation of N₂O emissions. Sensitivity estimation also helped to identify critical parameters, which need careful estimation or site-specific calibration for successful modelling of N₂O emissions in the study region.