Date of Award

Spring 1-1-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Noah Fierer

Second Advisor

William D. Bowman

Third Advisor

Nolan C. Kane

Fourth Advisor

Catherine A. Lozupone

Fifth Advisor

Steven K. Schmidt

Abstract

Plants and soil are fundamental components of terrestrial ecosystems, and microorganisms play key roles in the health of plants and in the ecosystem processes that take place in soils. Thus, in order to understand the functioning of terrestrial ecosystems, it is critical that plant and soil associated microbial communities are studied. Yet, remarkably little is known about the general distributions of plant and soil communities and the factors determining their structure. Here, I present an exploration of the biogeography of plant and soil microbial communities through four independent studies. In the first, I show how bacterial community structure varies predictably throughout the leaves and bark in individual Ginkgo biloba trees. Next, I investigate whether domestication of the sunflower plant (Helianthus annuus) has led to differences in root and rhizosphere fungal and bacterial communities. The results demonstrate that domestication has affected fungal but not bacterial communities. Third, using two complementary experiments set in a grassland, I show that plant species identity causally affects soil fungal, bacterial, protistan, and metazoan community composition and that the composition of soil communities in field samples were predictable based on plant community composition. Fourth, I show that increased inputs of nitrogen and phosphorus had consistent effects on soil fungal and bacterial communities in grasslands located around the globe. I describe how the observed effects relate to the function and ecology of the belowground community members and concomitant shifts in the composition of plant communities. Together, these studies reveal general patterns in plant and soil associated microbial communities and demonstrate important factors determining the structure of these communities. These results will help enable a predictive understanding of the biogeography of plant and soil microbial communities and will lead to a better understanding of terrestrial ecosystems.

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