Soils provide many essential functions that underpin the delivery of key ecosystem services such as nutrient cycling. While many factors regulate the functioning of soil, biotic interactions between plants and soil microorganisms are of central importance. Plant traits are increasingly being used to interrogate these interactions and provide a framework for understanding their importance for soil functioning, with root traits being of key interest given their intimate association with soil. However, important knowledge gaps remain that limit our understanding of the interactions between root traits and the soil microbiome that drive soil processes. This thesis presents a combination of glasshouse and field-based studies examining how root traits determine soil ecosystem processes in grassland soils, with a focus on rhizosphere fungal community composition, plant-soil feedbacks (PSFs) and soil organic matter (SOM) dynamics. First, I provide a review of the current status of root trait research before discussing the role of root traits in structuring the rhizosphere mycobiome and determining PSFs and SOM dynamics. In chapter 2 I report results from a glasshouse study showing root traits are key determinants rhizosphere fungal community composition, with strong links between root traits and arbuscular mycorrhizal fungi. Chapter 3 describes a related study in which I found root traits to be poor predictors of the direction and magnitude of PSFs. Finally, in chapter 4 I used a long-term grassland biodiversity restoration experiment to show that root traits are largely unrelated to SOM dynamics. Rather, the quantity and quality of SOM and microbial community composition were primarily determined by the land management practices implemented. My findings suggest that that in a glasshouse environment, root traits can provide important mechanistic insights into plant-soil interactions. However, in the field, under realistic land management scenarios, soil abiotic factors likely dilute the role of root traits in determining soil processes. The data presented in this thesis will aide with the development of predictive frameworks of how plant functional diversity drives soil functions via effects on the soil microbiome. Furthermore, these results will be of key interest to land managers and ecologists with the aim of managing grassland soils to improve SOM content and soil microbial diversity.