CONTEXT Organic agriculture (OA) is promoted in the European Union (EU) as a sustainable form of agriculture. However, its expansion may be limited by its dependence on external nitrogen (N) resources, such as conventional manure and imported feed, as it prohibits the use of industrial fertilisers. There is currently no consensus on the role of livestock in scenarios of OA expansion, despite their critical contribution to increasing soil fertility through grazing and nutrient cycling via manure and urine. OBJECTIVE Here, we explored the expansion of OA with a view to having 25 % of total agricultural land under OA by 2030, as defined by the EU Green Deal policy, using two development pathways for organic and conventional livestock: business as usual (BAU) and halving the number of livestock (HL). METHODS We modelled N flows for organic, conventional and the entire agricultural system using an existing N budget model applied at the national scale for France. RESULTS AND CONCLUSIONS The results indicated that changing the numbers of livestock species when expanding OA influences N availability. Transitioning to 25 % of agricultural land under OA along with increasing the number of organic livestock in the BAU and HL scenarios decreased total N input, especially industrial fertiliser and feed imports, by 9 % and 28 %, respectively, while decreasing N surpluses by 9 % and 26 %, respectively. In parallel, in the BAU and HL scenarios, biological N fixation increased by 32 % and 61 %, respectively, N output in animal and crop products decreased by 9 % and 30 %, respectively, due to the lower productivity of organic livestock and crops. Notably, this transition implied decreasing human consumption of animal products in the BAU and HL scenarios by 5 % and 30 %, respectively. The scenarios demonstrated the key contribution of livestock and biological N fixation of legumes to the expansion of OA, but further analysis is required, such as considering changes in human diets, farming practices or land use. SIGNIFICANCE This study highlights simultaneous trade-offs of two agricultural systems at the national scale.