Archived soil samples (1937-1999) and historic air quality data from the Los Angeles Basin were used for reconstructing the record of change between atmospheric NOx loads, soil δ15N values and the diversity of arbuscular mycorrhizae (AM), which are ubiquitous plant - fungus mutualists that control plant community productivity. A tripling of atmospheric NOx loads between 1937 and the 1970s was paralleled by soil nitrogen enrichment (δ15N = 3.18). From 1975 onwards, atmospheric NOx declined, but soils became nitrogen saturated (δ15 N = -4 and NO3-nitrogen = 171mgkg-1). The shifts in the AM community followed 28 years of atmospheric nitrogen enrichment and coincided with the onset of soil nitrogen saturation. Such changes were manifest in the loss of AM productivity, species richness (one species per year), three genera (Acaulospora , Scutellospora and Gigaspora) in the spore community and Gigaspora within the roots. Nitrogen enrichment also enhanced the proliferation of potentially less mutualistic species of Glomus. Autoregressive models implied that such patterns will persist and be driven by soil nitrogen cycling patterns. Chronic nitrogen enrichment from air pollution thus alters the diversity and mutualistic functioning of AM communities, which, in turn, may influence the plant community.
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