The arbuscular mycorrhizal symbiosis links N mineralization to plant demand

Arbuscular mycorrhizal (AM) fungi facilitate inorganic N (NH4 (+) or NO3 (-)) uptake by plants, but their role in N mobilization from organic sources is unclear. We hypothesized that arbuscular mycorrhizae enhance the ability of a plant to use organic residues (ORs) as a source of N. This was tested under controlled glasshouse conditions by burying a patch of OR in soil separated by 20-mu m nylon mesh so that only fungal hyphae can pass through it. The fate of the N contained in the OR patch, as influenced by Glomus claroideum, Glomus clarum, or Glomus intraradices over 24 weeks, was determined using N-15 as a tracer. AM fungal species enhanced N mineralization from OR to different levels. N recovery and translocation to Russian wild rye by hyphae reached 25% of mineralized N in G. clarum, which was most effective despite its smaller extraradical development in soil. Mobilization of N by G. clarum relieved plant N deficiency and enhanced plant growth. We show that AM hyphae modify soil functioning by linking plant growth to N mineralization from OR. AM species enhance N mineralization differentially leading to species-specific changes in the quality of the soil environment (soil C-to-N ratio) and structure of the soil microbial community.