Towards reconciliation of top-down and bottom-up national-scale N2O emission estimates in Europe

Nitrous oxide (N₂O) is the third most important anthropogenic greenhouse gas (GHG). In Europe, N₂O contributes about 6 % to total GHG emissions and about 75 % of these emissions are from the agricultural sector. More than half of agricultural emissions arise from microbial production in managed soils with the amount of added fertilizer nitrogen, soil properties, and soil environmental conditions controlling the emissions. These drivers lead to large spatio-temporal variability in N₂O fluxes, which is only poorly accounted for by simple bottom-up methods relying on emission factor approaches (IPCC Tier 1 and Tier 2 methods), and which are commonly used in national GHG inventory estimates.

The Horizon Europe project Process Attribution of Regional emISsions (PARIS) strives to improve national-scale flux estimates by employing regional-scale inverse modelling to atmospheric observations of N₂O (top-down) and biogeochemical soil models. In recent years (2018 onwards), the density and quality of atmospheric observations in Western and Central Europe have improved to the point where inverse modelling at the temporal and spatial scales required for the comparison to nationally reported emissions (UNFCCC) and biogeochemical model output becomes feasible. Here, we report inverse modelling results for the period 2018-2023 for Western and Central Europe derived from three inverse modelling systemsnTEM, UK MetOffice; RHIME, University of Bristol; ELRIS, Empa. These were operated with two different atmospheric transport models: NAME-UM and FLEXPART-ECMWF. Overall, the total N₂O fluxes agreed well, but were larger than in the national reporting to UNFCCC for several countries in Western and Central Europe. Results confirmed strong seasonality in N₂O fluxes for the UK, Benelux, and Germany. In comparison, fluxes from France exhibited less pronounced seasonality. The variability in N₂O fluxes was analysed with respect to environmental drivers, corroborating the important role of soil temperature and soil water content. Finally, the results allow a first comparison of the inversely obtained N₂O fluxes and fluxes simulated by two biogeochemistry models for agricultural soils in Switzerland (DayCent, Agroscope) and Germany (LandscapeDNDC, KIT).