Reactive nitrogen is an indispensable nutrient for agricultural production, since half of the crop production depends on human nitrogen fertilization, but reactive nitrogen also contributes to climate change through nitrous oxide emissions. Legumes fix nitrogen that can be used by subsequent crops, and emit less nitrous oxide than non legume crops. Ruminants use important areas of land, notably pastures and are associated to emissions of methane through enteric fermentation and methane and nitrous oxide during manure management. Introducing legumes to replace livestock could allow for reductions of Green House Gas (GHG) emissions, although this effect depends on how this introduction modifies livestock and cropland intensification and extensification. We evaluate the impacts of legume introduction in Europe on global agricultural emissions and production using a global agricultural intensification model: the NLU (Nexus Land-Use). We decompose effects of a demand side scenario representing a shift of animal protein to legume protein on GHG emissions taking into account indirect effects and characteristics of legumes. We also decompose effects of this scenario on the calorie price of the representative crop. For a 11.4kg/capita/year legumes introduction scenario, the net effect is an emission decrease of 100 million tCO2eq/year. It also decreases price of crops by % in 2050. The reduction of global demand decreases GHG emissions by 600 million tCO2eq/year. This reduction is partly compensated by an increase of emissions per unit of production, as livestock extensification leads to emission increases of 550 million tCO2eq/year. The importance of the indirect livestock extensification effect is caused by the low efficiency of extensive systems and the exogeneity of forest areas evolutions in the NLU. It emphasises the importance of taking into account indirect effects because of their major role in emission changes.