How reducing synthetic nitrogen in Europe affects ecosystem carbon and biodiversity: two perspectives of the same policy[Comment la réduction de l'azote synthétique en Europe affecte le carbone et la biodiversité des écosystèmes : deux perspectives d'une même politique]

In this study, we investigate the impacts of a public policy scenario that aims to halve nitrogen (N) fertilizer application across European Union (EU) agriculture on both carbon (C) sequestration and biodiversity changes. We quantify the impacts on ecosystem C and biodiversity by integrating economic models (supply-side model AROPAj and partial equilibrium model NLU) with an agricultural land surface model (ORCHIDEE-CROP) and a biodiversity model (PREDICTS). The two economic models simulate contrasting ways of implementing a 50% nitrogen reduction policy: a massive land abandonment with a large reduction in agricultural production (AROPAj); an extensification of crop production with a smaller reduction in agricultural production (NLU). Here, we show that the two economic scenarios lead to different outcomes in terms of C sequestration potential and biodiversity. Land abandonment associated with increased fertilizer price in the supply-side model facilitates higher C sequestration in soils (+1,014 MtC) and similar species richness levels (+1.9%) at the EU scale. On the other hand, more extensive crop production is associated with lower C sequestration potential in soils (-97 MtC) and similar species richness levels (-0.4%) because of a lower area of grazing land. Our results therefore highlight the complexity of the environmental consequences of a nitrogen reduction policy, which will depend fundamentally on how it is implemented.