Invasive plants are globally recognized as a problem due to their negative impacts on biodiversity, ecosystem function, and agricultural production. The myriad of interactions between plants, soil microorganisms, and nutrients are well recognized, but little is known about how these plant-soil feedbacks affect vegetation dynamics and invasive species. The following PhD research studied the interactions between invasive plants and the soil to improve our understanding of complex systems and assist in the development of innovative management and control strategies. To augment our understanding of plant-soil feedbacks I studied decomposition, soil microbial diversity, and vegetation while manipulating soil microorganisms and plant competition in field and greenhouse experiments.

Chapter 1 provides a summary of the impacts of invasive plants on soil properties, nutrient cycling, and microbial communities. The chapter is currently in press for a CABI publication edited by Thomas Monaco and Roger Sheley (Invasive Plant Ecology and Management: Linking Processes to Practice). Chapter 2 uses soil conditioning by specific plant species and inoculation with the soil microbial communities of several plant communities to study the growth and competition of a native and invasive plant species. The results of several greenhouse studies document how a native plant is only impacted by interspecific competition and avoids effects from soil microbes, while an invasive species avoids any negative impacts of exploitative competition, but is affected by soil microorganisms (interference competition). These results highlight the variability of competitive effects and how different types of competition may influence plant invasion. An improved understanding of the role of pathogens and interference competition in plant community dynamics could assist in invasive plant management practices based on manipulation of species’ specific pathogen accumulation.

Field-based studies in three US states used inoculation with the soil microorganisms of adjacent native plant communities to study the influence of inoculation on the restoration of native plants in roto-tilled invasive knapweed infestations (Chapter 3). I compared alternative hypotheses that native plants would benefit from potential mutualisms with the native soil inoculation or that invasive plants would have reduced pathogen loading when inoculated with soil from a native plant community. Results highlight species and site specific responses of inoculation with whole soils from native plant communities. Spotted knapweed (Centaurea stoebe) was negatively impacted by native plant soil inoculation, while Russian knapweed ( Acroptilon repens) responded positively. Inoculation was shown to impact plant growth, soil nutrients and microorganisms, although the variability of responses requires additional research before any applications in invasive plant management.

Litter decomposition experiments at three sites in Colorado utilized gradients between native and invasive knapweed vegetation to study the impacts of knapweed populations on decomposition of different litter types and the microbial communities of litter and soil (Chapter 4). I hypothesized that invaded vegetation would decompose litter more rapidly and that a positive feedback would exist in the decomposition of knapweed litter in knapweed vegetation. The results showed increased amounts of litter decomposition inside of knapweed infestations and distinctly different soil and litter microbial communities between the sites, but not between vegetation types within a site. Understanding the interactions of invasive knapweed with soil, litter, and microbes could improve our ability to manage ecosystems through the manipulation of primary ecosystem processes, such as decomposition, litter inputs and quality, nutrient cycling, and microbial activity.

The research presented in this dissertation provides baseline information on the interactions and feedbacks between vegetation, microorganisms, and nutrients with a unique focus on invasive plant management. Results highlight the potential of inoculation as a tool to impact specific vegetation, although main challenges exist in the application as a management tool due to species’ specific results and variability of impacts. The combination of competition and inoculation studies illustrates the responses of different species to exploitative (inter- or intraspecific competition) and interference (microbial interactions) competition and may help to elucidate why certain species are invasive. Through understanding when and why competition is most powerful, it may be possible to promote invasive plant management strategies that are based on maximizing competition. An overarching goal of this research is to promote management of invasive and native plant communities that develops sustainable and resilient systems through the manipulation of ecosystem processes, instead of top-down control strategies that are disruptive, expensive, and only applicable on small scales. Management of ecosystem processes requires in-depth knowledge of specific systems and must take into account the variability of vegetation, soils, and microorganisms before use as a potential management tool. The utility of the research presented in this dissertation requires a paradigm shift away from our innate desire to manage for what humanity has known and instead offers creative approaches to managing the novel ecosystems that humans have created throughout the earth.