Shifts in arbuscular mycorrhizal communities along an anthropogenic nitrogen deposition gradient

We evaluated arbuscular mycorrhizal (AM) species diversity and abundance in nine locations along an anthropogenic nitrogen deposition gradient in coas tal sage scrub (CSS) vegetation in southern California. The primary polluta nts were nitrogen oxides derived from vehicular emissions. Extractable soil N on the gradient ranged from 5 to 87 mu g/g during the summer months. For comparative purpose, we also assessed AM communities in nitrogen-fertilize d (60 kg N.ha(-1). yr(-1)) and unfertilized plots. Nitrogen enrichment indu ced a shift in AM community composition. In particular, an increasing input of nitrogen was associated with the displacement of the larger-spored spec ies of Scutellospora and Gigaspora (due to a failure to sporulate) with a c oncomitant proliferation of small-spored Glomus species (e.g., Glomus aggre gatum, Glomus leptotichum). A subsequent reduction in species richness and diversity las measured by Shannon-Wiener index) accompanied eutrophication. Nitrogen enrichment also significantly reduced spore abundance, modified t he timing of AM spore production in the most eutrophied site, and reduced h yphal and vesicular root infection. The fertilization experiment yielded si milar patterns to those found along the gradient, and hence nitrogen input most likely explains the relationship between anthropogenic pollution and s hifts in the AM communities. Such changes also indicated that AM species we re sensitive indicators of nitrogen enrichment. The CSS is currently underg oing a conversion to Mediterranean annual grasslands, especially in the mor e urban polluted areas, and the shifts in the mycorrhizal fungal community may facilitate grass dominance in this system.