Date of Award

Winter 12-20-2016

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Environmental Sciences

Advisor

Jean D. MacRae

Second Committee Member

Kevin S. Simon

Third Committee Member

Ivan J. Fernandez

Additional Committee Members

Brian McGill

Amy Burgin

Abstract

Microbes drive biogeochemical cycles through their metabolic activity. However, human-driven disturbances change microbial community composition and activity, altering ecosystem function. I investigated the impact of three types of disturbance, atmospheric deposition, dairy pasturing, and snowpack loss, on microbial community composition and activity to evaluate the relationship between microbial disturbance response and ecosystem function. Atmospheric nitrogen (N) and acid deposition on forest soils increases soil N availability and reduces soil pH, changing both soil microbe nutrient requirements and community composition. I evaluated microbial response to long-term deposition at the Bear Brook Watershed in Maine (BBWM) a paired watershed experiment in which one watershed has been subjected to bimonthly additions of ammonium sulfate to simulate the effects of acid and N deposition. I found that long-term deposition has increased microbial demand for carbon (C) and reduced fungal biomass, potentially altering organic matter decomposition. Agriculture increases ecosystem N loads with the addition of the physical disturbance of land use change. I conducted a study in a New Zealand dairy pasture to identify the factors that regulate denitrification—a microbial process that converts N to a biologically unavailable form. I found that pasture soils had a greater capacity for denitrification than native forest soils. The microbial community in pasture soils had more taxa capable of denitrification, indicating that disturbance altered the functional capacity of the community. In Maine, climate change is predicted to decrease snowpack, altering seasonal soil temperature and moisture regimens. In December 2014, I established experimental snow removal plots in Old Town, Maine. I found that that snow removal temporarily increased soil carbon dioxide flux during the spring thaw, but the microbial community composition was not affected. My results indicate that the duration and nature of disturbance influences the relationship between microbial community composition and ecosystem function. Both long-term N deposition and dairy pasturing, which occurred on the order of decades, altered the microbial communities and ecosystem biogeochemical processing. However, one winter of snow removal induced soil freezing did not have a lasting effect on community composition or activity. These findings demonstrate the importance of microbial communities in moderating ecosystem disturbance response.

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