Understanding and maximising the environmental benefits associated with nitrogen in legumes, at the level of the crop, the crop rotation and the farm

Legumes constitute a very important family of flowering plants, both herbaceous and tree-like, the Fabaceae, one of whose key characteristics is their ability to form a partnership (symbiotic association) with a class of bacteria (Rhizobium sp.) capable of fixing inert atmospheric nitrogen (N₂) and converting it into biologically usable nitrogen (reactive). This nitrogen is then made available to the rest of the ecosystem through a variety of recycling pathways, via the soil or animal manure. Legumes are grown as pure stands or in mixtures, for their herbaceous biomass or their seeds, in crop rotations or grassland systems of varying durations. The aims of this article are to review the environmental benefits associated with nitrogen, to clarify the associated risks of nitrate leaching at the plot level and of eutrophication at the farm level, and to draw some conclusions regarding their potential role in sustainable production systems.