Symbiotic dinitrogen fixation by legume trees represents a substantial N input in agroforestry systems, which may benefit the associated crops. Applying 15N labelling, we studied N transfer via common mycelial networks (CMN) and root exudation from the legume tree Gliricidia sepium to the associated fodder grass Dichantium aristatum. The plants were grown in greenhouse in shared pots in full interaction (treatment FI) or with their root systems separated with a fine mesh that allowed N transfer via CMN only (treatment MY). Tree root exudation was measured separately with hydroponics. Nitrogen transfer estimates were based on the isotopic signature of N (δ15N) transferred from the donor. We obtained a range for estimates by calculating transfer with δ15N of tree roots and exudates. Nitrogen transfer was 3.7-14.0 and 0.7-2.5% of grass total N in treatments FI and MY, respectively. Root δ15N gave the lower and exudate δ15N the higher estimates. Transfer in FI probably occurred mainly via root exudation. Transfer in MY correlated negatively with grass root N concentration, implying that it was driven by source-sink relationships between the plants. The range of transfer estimates, depending on source δ15N applied, indicates the need of understanding the transfer mechanisms as a basis for reliable estimates.