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Agricultural management is known to influence the abundance of arbuscular mycorrhizal (AM) fungi in the soil but relatively little is known about how this impacts the functioning of AM symbioses in crop plants, particularly in arid environments. We compared the abundance of spores and the mycorrhizal infection potential (a bioassay to measure total density of AM fungal spores, hyphae, and infected roots), in low-input and high-input agricultural systems managed by Native Americans in southwestern USA. These field studies were complemented with two greenhouse experiments to assess: (1) the mutualistic functioning of AM fungi and other soil organisms from high-input, low-input, and unmanaged ecosystems and (2) the mycorrhizal responsiveness of landrace and hybrid maize. We found significantly more AM fungal spores in high-input maize fields than in low-input maize fields, alfalfa fields, or unmanaged soils. However, the MIP bioassay indicated that the density of AM fungal propagules was not significantly different in the low- and high-input systems or the unmanaged-native soils. When maize was grown in field soil brought into the greenhouse. AM fungi and communities of other soil organisms did not benefit plant growth in high fertility soil (Experiment 1), but they did improve maize growth in low fertility soil (Experiment 2). Landrace maize was more responsive to mycorrhizas than hybrid maize, and novel soil inoculum was more beneficial than inoculum from sites where the crop and organisms have long coexisted. These findings suggest that agricultural management practices may influence the propagule density and symbiotic functioning of AM symbioses in arid environments; and that AM fungi and other soil organisms may play a more important role in crop performance in low fertility soil than in high fertility soil. (C) 2010 Elsevier B.V. All rights reserved.