Inorganic N addition replaces N supplied to switchgrass (Panicum virgatum) by arbuscular mycorrhizal fungi

Abstract Arbuscular mycorrhizal fungi (AMF) provide many benefits in agroecosystems including improved soil tilth, carbon sequestration, and water and nutrient transfer to plants.AMFare 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 byAMFis agronomically relevant. We used δ15N natural abundance methods and δ15N mass balance equations to estimate the amount of plant N derived fromAMFtransfer in perennial grasses managed for bioenergy production under different N addition treatments (0, 56, and 196 kg N/ha). Differentiation of δ15N among plant, soil N, andAMFpools was higher than anticipated leading to calculations of 34–55% of plant N transferred byAMFin the treatments receiving no N addition to 6–22% of plant N transferred to plants in high‐N addition treatments.AMFextra‐radical hyphae biomass was significantly reduced in the high‐N (196 kg N/ha) addition treatments, which was negatively correlated to enriched plant δ15N. Our results suggest that N addition decreasesAMFN transfer to plants. When N was limiting to plant growth,AMFsupplied 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 toAMFplant hosts.