Symbioses with mycorrhizal fungi and nitrogen-fixing bacteria (NFB) enhance nitrogen (N) acquisition in host plants and may promote N transfer to neighbouring plants through mycorrhizal networks (MN). Nevertheless, the extent and mechanisms of this transfer remain unclear. On the basis of a synthesis of 15N labeling studies, we show that MN and NFB synergistically enhanced interplant N sharing. In the presence of MN, N transfer from N-fixing donors to non-N-fixing receivers increased by an average of 9.7-fold, accounting for 5.61% of the total N in receiver plants. Moreover, greater amounts of N were transferred from N-fixing plants towards their phylogenetically distant plants. Source-sink gradients driven by differences in N content between neighbouring plants further promoted N transfer. Together, our findings highlight the ecological significance of an expanded MN framework in explaining interplant N sharing and provide new insights into how symbiotic guild interactions promote species coexistence and biodiversity maintenance.