Identifying drivers of soil fungal community composition and consequences for litter decomposition

Microorganisms are among the most diverse taxa on earth, but the assembly rules that structure microbial communities remain unclear. It is particularly critical to determine the factors influencing soil fungal composition and diversity. Diversity of saprotrophic fungi can affect soil decomposition rates while arbuscular mycorrhizal fungal diversity and composition can impact aboveground primary productivity. My dissertation focused on determining how spatial, environmental and biotic filters act to structure soil fungal communities at global, regional and local scales and how fungal diversity can affect decomposition rates in situ.;In Chapter 2, I discovered arbuscular mycorrhizal fungal composition is structured equally by geographic distance, environmental factors and plant communities at the global scale (up to 5% of variance explained by each). In Chapter 3, I expanded my work on dispersal to determine the relative importance of airborne dispersal on soil fungal community composition. I found that soil fungal communities differed mostly in response to environmental factors such as soil moisture, and soil nitrate. Airborne fungal assemblages, in turn, differed mostly over time. Neither group differed spatially throughout southern California. Furthermore, the spore morphology of air-dispersed fungal taxa was correlated to spatial range. Small spores from fungi with aboveground fruiting bodies were dispersed farther than large spores from fungal taxa with belowground fruiting bodies. I then expanded this work to examine trends in spatial and abiotic filtering at a variety of scales. I found that environmental filtering explained more of the variance in soil fungal community composition at smaller scales, while geographic space explained more of the variance in soil fungal communities at larger scales. These results suggest that dispersal limitation may become increasing important at larger scales.;In Chapter 4, I examined how biotic interactions between experimentally assembled fungal communities contributed to litter decomposition rates over time. Fungal assemblages that were more distantly phylogenetically related decomposed faster than closely related assemblages. This trend seemed to be driven by two processes: (1) competition between closely-related fungal isolates and (2) phylogenetic clustering of cellulose decomposition. I then related this trend to large-scale patterns of soil fungal community structure. At 52 sites across the western hemisphere, most fungal communities were significantly clustered (i.e., more closely related than expected by chance). Therefore, in these systems carbon cycling may be slower than in fungal communities that are more distantly related.;Overall, I have shown that the drivers of soil fungal community assembly differ at the global, region and local scales with dispersal being most important at larger scales and abiotic factors being more important at regional and local scales. Moreover, I've shown that the phylogenetic structure of fungal communities can influence carbon cycling rates and that this may have an impact at the global scale as most fungal communities are phylogenetically clustered.