Mycorrhizal symbioses influence grassland productivity, community structure, nutrient dynamics, and belowground carbon sequestration.  Species and genera of arbuscular mycorrhizal (AM) fungi are known to vary in their habitat requirements and in their effects on plant communities and ecosystem processes; yet little is known about the autecology of these organisms because their distributions are difficult to study in natural ecosystems.  We studied the species composition of AM fungal spores in soils collected from long-term grazed and un-grazed plots at eight sites which form natural soil fertility and precipitation gradients in the ancient grassland ecosystem of the Serengeti National Park in Tanzania. Our goal was to gain a holistic understanding of the factors that control the structure of AM fungal communities in natural grasslands. We tested the main treatment effects (site and grazing) with an ANOVA model and tested the relative influence of interactive biotic and abiotic factors on AM fungal communities using structural equation models.  
Results/Conclusions

Site influenced the structure of AM spore communities and precipitation was a particularly strong predictor of many AM fungal response variables. Specifically, spore abundance was highest in the most water-limited sites and AM spore diversity, richness and evenness were greatest in sites where water was less limiting. Grazing did not significantly influence spore communities and no treatment interactions were observed. Of the 53 taxa of AM fungal spores observed, 22 were present in more than 80% of the plots. We found multiple co-dominant species, and frequency and abundance of taxa varied among sites. For example, within the Gigasporaceae family; Scutellospora spores were most abundant in dry sites, while Gigaspora spores and auxiliary cells were most abundant in mesic sites, which may indicate niche partitioning and a shift in resource allocation in response to water limitation. We used structural equation modeling to determine the relative influences of soil properties, rainfall, and plant productivity on several metrics of AM fungal biomass and structure. Many factors, including rainfall and soil organic matter, were of importance. Our findings demonstrate that multiple factors influence the autecology and synecology of AM fungi, which has implications for their management in natural as well as agricultural ecosystems.