Date of Award

Winter 2022

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

Ruth K Varner

Second Advisor

Jessica G Ernakovich

Third Advisor

Stephen Frolking

Abstract

Northern peatlands are both globally important carbon (C) stores and sources of methane (CH4). The impacts of climate change including warming, changing precipitation and hydrology, shifts in vegetation, and thawing permafrost may increase the vulnerability of the northern peatland C stock, including the amount of C lost to the atmosphere as CH4. Variation in peatland water table depth strongly influences CH4 cycling, as water table levels largely control redox conditions and therefore rates of anaerobic CH4 production (methanogenesis) and aerobic CH4 oxidation (consumption, methanotrophy). As CH4 emissions reflect the balance of methanogenesis and methanotrophy, changes in water table depth due to landcover change, microtopography, or changes in precipitation strongly influence CH4 emissions across northern peatlands. Vegetation also impacts peatland CH4 cycling by affecting the quality and quantity of labile C inputs and the exchange of CH4 and oxygen between the atmosphere and the subsurface. As northern peatlands have high spatial heterogeneity in water table levels and vegetation, CH4 emissions often vary strongly across the landscape.The first portion of this dissertation examines how site-scale landscape heterogeneity impacts belowground CH4 cycling. This work was conducted in a thawing permafrost peatland (Stordalen Mire, 68°21’N, 19°02’E) and a temperate fen (Sallie’s Fen, 43°12.5′N, 71°3.5′W). At Stordalen Mire, I examined how lineage abundance and activity of methane oxidizing bacteria (MOB) vary with permafrost thaw. I found that MOB clustered across the thaw gradient according to their redox niches, and that MOB community composition was a strong predictor of CH4 oxidation rates. At Sallie’s Fen, I investigated belowground CH4 cycling across hummocks and lawns, and found that CH4 cycling in hummocks was more strongly regulated by CH4 oxidation whereas in lawns CH4 production was the more dominant process. This portion of my dissertation emphasizes the importance of considering site-scale landscape heterogeneity in efforts to upscale observations of CH4 cycling. The work conducted at Stordalen Mire also highlights how shifts in MOB communities impacts rates of CH4 oxidation, demonstrating the need for more explicit model representation of major MOB clades with differing traits. The second half of this dissertation investigates how variation in precipitation affects peatland CH4 cycling and emissions. This work was conducted at Sallie’s Fen, where I measured CH4 emissions and their isotopic composition (δ13C-CH4) as well as the stable isotope composition of CH4 and carbon dioxide (CO2) dissolved in peat porewater across two summers marked by severe drought conditions (2020) and record rainfall (2021). Drought conditions significantly reduced CH4 emissions and increased emitted δ13C-CH4 in late summer, but heavy precipitation only had a transient impact on CH4 emissions. There was significant seasonal variation in δ13C-CH4, underscoring the importance of continued monitoring of wetland δ13C- CH4 to inform CH4 source partitioning efforts. Belowground, variation in porewater CH4 and CO2 across microtopography and depth was more pronounced in the drought year, as flooding after rainstorms muted variation in water table depth across the site. The seasonality of CH4 cycling also differed between the dry and wet years. From both an above and belowground perspective, drought had a stronger impact on CH4 cycling, suggesting projected drying across permafrost-free peatlands due to rising temperatures will significantly impact CH4 emissions.

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