Chronic nitrogen deposition and the composition of active arbuscular mycorrhizal fungi

• Nitrogen (N) deposition decreases AMF root infection and mycelial production.
• We examined effects of increased N on the active soil AMF community using rRNA.
• Active AMF community composition was not affected by increased N deposition.
• Increased soil N decreased abundance of a close relative of Glomus intraradices.
• Significant decrease in rare taxa under increased experimental N deposition.

A growing body of evidence indicates that atmospheric nitrogen (N) deposition can alter the composition and function of arbuscular mycorrhizal fungi (AMF) associated with plant roots. We studied the community of AMF actively transcribing ribosomal genes in the forest floor of northern hardwood forests dominated by sugar maple (Acer saccharum Marsh.) that have been exposed to experimental N deposition since 1994 (30 kg NO3−-N ha−1 year−1). Our objective was to evaluate whether previously observed declines in AM root infection and mycelial production resulted in a compositional shift in the AM fungi actively providing resources to plant symbionts under chronic N deposition. To accomplish this task, we cloned and sequenced the LSU of reverse-transcribed AM fungal rRNA extracted from the forest floor under ambient and experimental N deposition treatments. We found that experimental N deposition did not alter the active community of AMF or AMF diversity, but we did observe a significant decrease in rare taxa under chronic N deposition. Our results indicate that chronic N deposition, at levels expected by the end of this century, can exert a moderate influence on the composition and abundance of AMF associated with plant roots in a wide-spread forest ecosystem in the northeastern North America.