Linking Plant and Soil Microbial Diversity via Plant Functional Trait Ecology

Abstract

Plant–microbe interactions are central to ecosystem functioning, yet our understanding of how plant diversity relates to soil microbial diversity remains rudimentary. Cover crops are increasingly promoted for their potential to enhance soil health and microbial diversity in agroecosystems, but variability in their effects highlights the need for a more mechanistic, trait-based understanding. This dissertation examines how various attributes of plant diversity influence microbial diversity, and if the patterns observed at species scale can be extended to mixed communities. Using a combination of monocultures and intentionally designed plant mixtures, I evaluated the relationships between microbial diversity and both taxonomic and functional plant diversity using greenhouse mesocosms and field-based studies. In Chapter 2, I quantified plant functional traits (PFTs) for 29 agriculturally relevant species and assessed their relationships with soil microbial community structure. While plant functional traits such as biomass, root diameter, and root tissue nutrients explained variation in bacterial communities, PFTs did not relate to differences in fungal communities. However, specific bacterial and fungal taxa showed strong associations with particular PFTs. Chapter 3 extended these analyses to plant mixtures designed to test either PFT diversity or expected soil microbial diversity. My results showed that the mixtures designed to exhibit distinct PFT composition based on their species-level traits, did indeed vary in their community-level PFT composition. Furthermore, the same mixtures also harbored distinct soil bacterial and fungal communities. However, I could not identify specific traits driving these relationships, suggesting the overall trait diversity is more important than any individual trait for relating PFTs to soil microbial diversity. In Chapter 4, I evaluated whether plant species or trait diversity relates to corresponding soil microbial diversity in semi-natural herbaceous mixed plant communities in a conventionally managed agricultural field over a typical winter cover crop growing period. Variation in PFT composition was positively related to the differences in plant species composition. Although bacterial alpha diversity showed some associations with root traits like %N and C:N ratio, microbial beta-diversity was not well explained by PFT or species diversity. Together, these findings underscore the importance of specific plant traits in structuring microbial communities, while highlighting the complexity of predicting microbial outcomes from plant community composition and diversity, especially across different spatial and temporal scales. This work supports the integration of trait-based frameworks into designing plant mixtures such as cover crop polycultures and points toward future research linking PFTs to microbial functional potential and agroecosystem services.