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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.
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