Diversity of communities of arbuscular mycorrhizal (AM) fungi present in conventional versus low-input agricultural sites in eastern Pennsylvania, USA

We compared the composition and structure of the communities of arbuscular mycorrhizal (AM) fungi associated with maize (Zea mays L.) and soybeans (Glycine max (L.) Merr.) in a conventional (CON) and two low-input (LI) farming systems to better understand the relationship among AM fungi present in different agricultural systems. One LE system utilized animal manure (LI-AM) and the other green manure (LI-GM) as the nitrogen source. Spores were extracted from rhizosphere soil samples by wet-sieving to perform microscopic identification of the species and to assess frequency of occurrence. These data were used to calculate species richness, Shannon and Wiener index of diversity, and indices of dominance among other ecological measures. The results indicated that 15 consecutive years of farming under the three management practices did not cause many differences among the fungal communities. The majority of the 15 fungal species found throughout the site were present in all treatments. Sporulation of particular fungal species differed among farming systems and/or among hosts, but the general structure of AM fungal communities (according to most ecological measures) was similar for all treatments. Trap cultures were set up for the different treatments and grown for three cycles to try to recover species with low or no sporulation in natural conditions. These results also supported our conclusion about the homogeneity of the communities in the different farming system/plant host combinations, because only one species (Glomus constrictum) that was not found in the field samples sporulated in trap culture pots. Given that differences in sporulation may reflect differential rates of growth, three undescribed species plus Glomus mosseae and Glomus etunicatum were better established, both in the held and in trap cultures, than the other 10 species present in these soils. Also, Gigaspora gigantea accounted for more than 60% of the total volume of spores produced in each treatment, with the exception of conventional plots planted with maize where spore biovolumes were spread much more evenly among several fungal species suggesting that carbon allocation relationships were much more balanced in these plots. The focus of future studies at these sites will be a comparison of the efficacy among the communities in terms of enhancement of plant growth. (C) 2001 Elsevier Science B.V. All rights reserved.