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Arbuscular mycorrhizal (AM) fungi (phylum Glomeromycota) are important components of natural and managed ecosystems. We explored the AM fungal assemblage in a selection of maize fields across a landscape in eastern New York State and characterized their diversity, dominance, and species abundance distribution. In this managed agroecosystem, we could investigate environment-influenced composition and diversity patterns unencumbered by immediate host species effects. We found that AM fungal taxon abundances were distributed lognormally, which suggests that the fungal community structure is shaped in a complex manner by many interacting niche-related factors rather than by only a single factor of disturbance associated with agricultural management. In addition to species abundance distribution, the focal assemblage shared with natural AM fungal communities a pattern of very strong dominance of certain taxa. To quantify this pattern, we developed two new indices “overdominance” and “inequitability”. Contrary to expectations based on observations of natural AM fungal communities, we found that most of the individual field communities were dominated by taxa from within a narrow phylogenetic range. At the landscape scale, we did not find an inverse relationship between the levels of taxonomic richness and phylogenetic relatedness expected in complex communities shaped by competitive exclusion.

The arbuscular mycorrhizal (AM) fungi (phylum Glomeromycota) are important for the functioning of terrestrial ecosystems because of their influence on plant nutrient relations and plant responses to stress. To assess the impact of dispersal limitation and identify niche-related environmental gradients affecting AM fungal community composition and structure, we studied AM fungal communities in an assemblage of maize fields across an eastern New York State landscape. We expressed AM fungal community differences in terms of abundance structure (Bray–Curtis dissimilarities), the presence of unique phylogenetic lineages (UniFrac), and mean phylogenetic relatedness between samples (mean patristic distance, MPD). We did not find strong evidence of dispersal limitation or isolation by distance within or between field sites. To identify environmental factors that may be related to community differentiation, we projected vectors of edaphic variables onto nonmetric multidimensional scaling (NMDS) ordinations of community dissimilarity measures. Of these factors, soil textural components appeared most strongly related to AM fungal community differences. We speculate that this pattern may be explained by the relationship between texture and soil moisture availability. In addition to soil textural components, phylogenetic measures of community differentiation suggested that AM fungal community structure was affected by nutrient concentrations, particularly Mg. Of the two phylogenetic indices of community differentiation, MPD was more consistent and stable with our data, whereas the UniFrac metric failed to be interpretable in several cases. Overall, our data suggest that, rather than phosphorus or pH, soil texture may have an influence on AM fungal community structure over large agroecological scales.