<p>The introduction of herbaceous or woody nitrogen-fixing species into mixed systems generally enhances the aboveground productivity of non-fixing species through facilitation processes, while reducing the need for external inputs. Tree aboveground growth is closely linked to root development, with coarse roots playing a key role in anchoring the tree to the soil and fine roots being essential for water and nutrient absorption. This study examined total root biomass, its allocation among root diameter classes, their respective nitrogen concentrations and the partitioning between below- and aboveground biomass, in agroforestry and forest mixture systems incorporating either woody (alder) or herbaceous (alfalfa) nitrogen-fixing species in association with grass or poplar. The study was carried out at a 10-year trial site in northeastern France including two agroforestry systems (poplar/alfalfa; alder/grasses), the poplar-alder association and their respective monocultures. Trees were excavated and roots were classified into three diameter categories: fine, medium and coarse. No significant differences in root biomass were observed between the mixed systems and the monocultures. However, a nitrogen enrichment of the poplar roots was detected in the agroforestry system, without any increase in root biomass. This was likely due to a reallocation of resources toward aboveground growth. In the alder-based agroforestry system, a trend toward a higher root-to-shoot ratio suggested increased competition with grasses. Finally, no root-level facilitation was detected in the alder–poplar mixture, likely due to the non-limiting soil conditions which could have promoted positive interspecific interactions.</p>