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Decline of arbuscular mycorrhizal fungi in northern hardwood forests exposed to chronic nitrogen additions

van Diepen, Linda T.A., Lilleskov, Erik A., Pregitzer, Kurt S., Miller, R. Michael
The new phytologist 2007 v.176 no.1 pp. 175-183
Acer, aboveground biomass, carbon, correlation, energy, fatty acids, hardwood forests, mycorrhizal fungi, nitrogen, phospholipids, roots, soil, vesicular arbuscular mycorrhizae
Arbuscular mycorrhizal (AM) fungi are important below-ground carbon (C) sinks that can be sensitive to increased nitrogen (N) availability. The abundance of AM fungi (AMF) was estimated in maple (Acer spp.) fine roots following more than a decade of experimental additions of N designed to simulate chronic atmospheric N deposition. Abundance of AMF was measured by staining and ocular estimation, as well as by analyzing for the AMF indicator fatty acid 16:1ω5c in phospholipid (biomass indicator) and neutral lipid (lipid storage indicator) fractions. Arbuscular mycorrhizal fungal biomass, storage structures and lipid storage declined in response to N addition measured by both methods. This pattern was found when AM response was characterized as colonization intensity, on an areal basis and in proportion to maple above-ground biomass. The phospholipid fraction of the fatty acid 16:1ω5c was positively correlated with total AMF colonization and the neutral lipid fraction with vesicle colonization. Decreased AMF abundance with simulated N deposition suggests reduced C allocation to these fungi or a direct soil N-mediated decline. The fatty acid (phospholipid and neutral lipid fractions) 16:1ω5c was found to be a good indicator for AMF active biomass and stored energy, respectively.