Assessment of German nitrous oxide emissions using empirical modelling approaches

Direct nitrous oxide (N2O) emissions from agricultural soils contribute considerably to anthropogenic GHG emissions. Albeit a key source of emissions in many countries, direct N2O emissions are still calculated and reported to the United Nations Convention on Climate Change using default emission factors defined in the IPCC guidelines (IPCC 1996, 2006). It is known that processes controlling production and transport of N2O are highly sensitive to environmental conditions defined by weather, soil and management. The accuracy of N2O emission budgets and the efficiency of mitigation can be improved if those dependencies are considered with regionalized emission factors. In this study an empirical method originating from soft computing techniques based on measured data is developed and applied to quantify direct N2O emissions from agricultural soils at field and national level in Germany between 1990 and 2005. The method is used to derive maps of emission factor distribution of direct N2O emissions of agricultural land in Germany. Model results are compared with alternative empirical approaches from literature. Results from developing empirical models show that grassland and cropland have to be differentiated according to the key controls driving N2O emissions. N2O emissions of German croplands are highly influenced by climatic conditions and soil properties. The variability of N2O fluxes on grasslands is mainly driven by the fertilizer N applied. The model comparison using measured European N2O emissions exhibits profound discrepancies between the models used on a regional scale. The nationwide budgets derived span a narrow range of -8 to 28% relative to direct N2O emissions quantified by the German national inventory report. The emission factor of German agriculture estimated by the developed model is 0.91% of fertilizer N applied.