UC Irvine

Soils support many vital ecosystem services including water filtration, pollution remediation, carbon sequestration, nutrient cycling, and increased biodiversity. Microbial communities are key regulators of these soil processes and are functionally responsive to shifts in environmental conditions. Global changes including climate change and urbanization are altering soil properties and soil microbial activity. The resulting feedbacks could increase GHG emissions and nitrogen leaching into water systems from both urbanized and natural soils. The aim of this dissertation is to investigate the impact of global changes on the soil microbiome. First, I addressed the impacts of urbanization on soil ecosystems by synthesizing prior literature and developed a framework to assist researchers in answering key questions about the urban soil microbiome. I argue that urban soils offer an excellent opportunity to study fundamental questions about microbial community structure and function under different environmental conditions, with the additional benefit finding methods to improve urban sustainability. Next, I conducted a field experiment applying this framework to soils in a local neighborhood. I constructed a chrono-sequence of yards built across four decades, and characterized the soil and microbial community to provide insight into how urban soils recover from disturbance, and how irrigation, fertilization, and plant type may alter microbial processes compared to an adjacent undeveloped ecosystem. I found that these yard soils, particularly under turfgrass, are wetter and more nutrient-rich compared to undeveloped soils. The chrono-sequence also revealed that urban soils gain more abundant and active microbial communities over time which may result in accumulated soil carbon. Finally, my last chapter explores the impact of drought and nitrogen addition, as may result from fossil fuel burning, on a natural grassland ecosystem. I characterized the microbial community of bulk soils across experimental treatments down to 30cm, and explored the effects of depth, drought, and fertilization on microbial community composition and potential function. I found that depth was the most consistent driver of microbial function, while microbial functions were more resilient to drought and fertilization. An interesting finding from this work was that community composition did not respond to treatments while potential function did. This suggests a need for a trait-based approach to describe microbial communities and predict function from their structure.