Aims Belowground carbon transfer from plant to plant has been extensively described, but such transfer for nitrogen has been less thoroughly investigated when the donor is a non-N 2-fixing species. This study, applied to forest regeneration, aimed to determine whether tree seedlings facilitated neighbouring grass growth through nitrogen transfer at an early stage of development, thus facilitating nitrogen acquisition by understory species. Methods Quercus petraea seedlings were planted in pots either sole-grown or mixed-grown with Molinia caerulea tufts or another oak seedling. 15 N-urea pulse-chase labelling (cotton wick method) was performed in oak shoots and the fate of 15 N in each soil and plant compartment was tracked for one year. N transfer pathways were investigated using two degrees of physical separation between root systems. Results Molinia dry weight was higher when mixed-grown with oak seedlings than when sole-grown. Increase in grass dry weight correlated with N transfer from donor oak to receiver Molinia. Interestingly, the presence of Molinia increased N rhizodeposition of oak. N allocation in donor oak towards root in winter and shoot in spring was enhanced. Conclusions Oak seedlings facilitated Molinia growth through rapid N transfer, underlining the ability of non-N 2-fixing species to supply N to neighbours. 15 N allocation within donor oak and its rhizodeposition depended on neighbour identity.