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Summary • We enumerated arbuscular mycorrhizal (AM) fungal spore communities for 3 yr as part of a long-term CO2 enrichment experiment at Cedar Creek, Minnesota, USA. Complete factorial combinations of two levels of CO2 and N, and 16 perennial plant species grown in monoculture and 16-species polyculture were arranged in a split-plot design. • In 1998–2000, spore communities were quantified under monocultures of eight plant species. In 2000, measurements were expanded to include monocultures and polycultures of all of the plant species. • Under plant monocultures, only Glomus clarum responded significantly to CO2 elevation out of 11 species present. This response was not detectable under plant polycultures. Glomus clarum was also significantly more abundant under plant polycultures. Nitrogen addition had small negative effects on AM fungal spore abundance and species richness in 2000. The interaction of CO2 and N did not affect arbuscular mycorrhizal fungal spore communities. • We show that CO2 enrichment and plant species richness impact arbuscular mycorrhizal fungal community structure. These findings are important because altered symbiotic functioning may result. [ABSTRACT FROM AUTHOR]