Tests for negative plant-soil feedback effects within a community context
Creators: Oppon, Kenneth
Publisher: name: University of Alberta Library
Year: 2023

<p>The coexistence of numerous species despite competing for similar resources remains a central paradox within the field of ecology. Differentiation of species interactions with resources and their environment has been proposed as a mechanism by which species reduce competitive interactions, thus enabling coexistence between potential competitors. Recent investigations within plant communities suggest that negative feedbacks between plants and their soils could be an important contributor to this differentiation. These plant-soil feedbacks occur when plants condition their soils in ways that disproportionately disadvantage their own species members compared to others, creating conditions that could promote coexistence between species. Tests for these negative plant-soil feedbacks have often focused on the effects of plant associated soil microbes, as the accumulation of host-specific soil enemies have often been implicated as the cause of these feedbacks. However, changes to abiotic soil properties could impact biotically driven feedbacks or even drive feedbacks themselves. Moreover, previous feedback studies have primarily focused on population-level changes to soil properties and their subsequent effects, leading to limited understanding of how abiotic soil components and interspecific interactions within communities interact with biotically driven soil feedbacks.<br>
In this thesis, I investigated the presence of negative plant-soil feedbacks within communities by examining the impact of soil origin on community structure. Using a mesocosm experiment with field-collected turfs from 84 communities, I assessed whether community structure would vary when provided with their own soils, soils conditioned by another community, and soils of mixed origin. Additionally, I manipulated the soil composition by growing communities with field soils or with field soil as live inoculants, enabling the isolation of biotic soil effects to discern the role of soil microbes and abiotic soil properties in plant-soil feedback effects. <br>
I found evidence of negative plant-soil feedback mediated through soil microbiota as communities grown with their own live inocula soils were less even and species rich than communities grown with live inocula from a different community. These findings suggest that soil specialist enemies impact plant growth and could contribute to species coexistence within these communities. Additionally, I observed a positive plant-soil feedback for Bromus inermis, an invasive species in North American grasslands, which exhibited a significantly stronger growth response to its own soil microbes compared to those from other communities. This positive feedback may be a key factor in preventing the coexistence of B.inermis with other species by promoting its competitive advantage through the soil microbiota. However, soil origin did not strongly impact community species richness, evenness or the growth response of B.inermis when communities grew with field soils. Thus, while evidence of soil biota mediated plant soil feedbacks was found, other soil components that vary, such as resources and chemistry, appear to have a larger effect size, masking subtle biotic differences. Together, these findings suggest that plant-soil feedback driven by soil microbes may not be a principal determinant of species coexistence. </p>