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International audience; Organic farming practices are a possible way to reconcile agricultural production and water quality, but the comparative merits of conventional and organic practices regarding the nitrogen (N) issue are complex and still being debated. This study was designed to investigate the choice governing conversion to organic farming of en experimental mixed dairy farm in Lorraine (eastern France) and to highlight the assoicated changes in N fluxes. The study was conducted over a suitably long period of time at a farm scale integrating the diversity of practices in one conventional and then two autonomous organic systems (a grazing livestock system and a mixed dairy system). Two indicators were used to draw up the main features. First, the farm-gate N budgets show the the organic farm has a balance twice as low as the conventional system. Second, the soil surface balance exhibited substantial differences between functional zones. Finally, this study shows that an accurate estimation of soil surface balance requires taking into account the spatial distribution within the farm area based on practices and their rationale.