Date of Award

Fall 2025

Project Type

Dissertation

Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

Jessica G Ernakovich

Second Advisor

Staurt Grandy

Third Advisor

Caitlin Hicks Pries

Abstract

Permafrost, or perennially frozen soil, covers approximately 25% of the northern hemisphere and contains over half of the world’s soil organic carbon stocks. Permafrost-affected regions are rapidly warming, resulting in ecosystem changes including permafrost thaw and a shift from sedge-dominated to shrub-dominated plant communities. As permafrost thaws, it releases previously unavailable compounds to plants and microorganisms. Plant root inputs can then stimulate the decomposition of thawed permafrost, increasing greenhouse gas emissions in a process called priming. Permafrost thaw increases the decomposition of previously frozen soil organic matter (SOM); however, the extent of decomposition is highly dependent on plants and their associated microbial communities. My research addresses this knowledge gap by examining critical interactions between microbial communities, plant roots, and permafrost mineralogy, to determine how they synergistically influence soil priming. First, I ran an exploratory study analyzing the rhizosphere, or root microbiomes, of common tundra plants in Northern Alaska. I found that soil mineralogy, host plant species, and mycorrhizal fungal composition, were strong drivers of rhizosphere bacterial and fungal compositions. Second, I incubated native permafrost with shrub or sedge rhizosphere inoculants while adding simulated root exudates or water daily. Using quantitative stable isotope probing, I found that growth rates and substrate preferences varied greatly where shrubs preferred exudate carbon and sedges preferred SOM. Third, I used an isotope labelling cabinet where I examined the impacts of plant type, permafrost mineralogy, and elevated atmospheric CO2 on priming. I found that plants show strong temporal controls on priming and that shrubs cycle a disproportionate amount of plant-derived carbon while sedges a disproportionate amount of SOM-derived carbon. Finally, I synthesized my findings into a new conceptual framework highlighting important mechanisms in shrub and sedge microbial communities and how they impact carbon cycling.

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