Arbuscular mycorrhizal fungi (AMF) provide many benefits in agroecosystems including improved soil tilth, carbon sequestration, and water and nutrient transfer to plants. AMF are known to affect plant nitrogen (N) dynamics and transfer N to plants, but there have been few studies addressing whether the amount of N transferred to plants by AMF is agronomically relevant. We used δ¹⁵N natural abundance methods and δ¹⁵N mass balance equations to estimate the amount of plant N derived from AMF transfer in perennial grasses managed for bioenergy production under different N addition treatments (0, 56 and 196 kg N ha^-1). Differentiation of δ¹⁵N among plant, soil N, and AMF pools was higher than anticipated leading to calculations of 34 to 55% of plant N transferred by AMF in the treatments receiving no N addition to 6 to 22% of plant N transferred to plants in high-N addition treatments. AMF extra-radical hyphae biomass was significantly reduced in the high-N (196 kg N ha^-1) addition treatments, which was negatively correlated to enriched plant δ¹⁵N. Our results suggest that N addition decreases AMF N transfer to plants. When N was limiting to plant growth, AMF supplied agronomically significant amounts of plant N, and a higher proportion of overall plant N. Because differentiation between N pools was greater than expected, stable isotope measurements can be used to estimate N transfer to AMF plant hosts.