As Neotropical forests are increasingly converted to agriculture (especially pastures), little is known about the impacts on microbial biodiversity. To assess such impacts, I compared spore abundance and diversity of arbuscular mycorrhizal fungi (AMF) in soils from lowland evergreen forests and pastures in Nicaragua and Costa Rica. Species composition, dominance–diversity curves, and Simpson's diversity indices were similar for both forest and pasture soils. Of 28 distinct fungus morphospecies, 11 produced more spores in pasture, while only 1 produced more spores in forest. According to species-accumulation curves, local AMF species richness did not significantly decline following conversion of forest to pasture. Because pastures contained a surprising abundance and diversity of AMF spores compared to native forest, a lack of mycorrhizal fungi is unlikely to limit plant succession, restoration, or reforestation in the pastures studied. At the regional scale, however, species-accumulation curves showed significantly greater gamma (G) diversity of spores in forest. In addition to these trends in diversity, species that sporulated more in pasture tended to have small spores, while the one species that sporulated more in forest had the largest spores. Similarly, only large-spored fungi (>300 µm) showed any seasonal variation in spore abundance, being more common in the wet season.