Date of Award

Summer 8-17-2018

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Forest Resources

Advisor

Ivan J. Fernandez

Second Committee Member

Jean D. MacRae

Third Committee Member

Sarah J. Nelson

Additional Committee Members

Tsutomu Ohno

Aaron R. Weiskittel

Abstract

Natural and anthropogenic disturbances have altered the N cycle on the regional to global scale. Although numerous studies have examined the effects of chronic atmospheric N deposition on ecosystem processes, less is known about intra-annual variability of these processes, or how these responses might be altered under conditions of changing climate. The work described in this dissertation focuses on the effects of a changing chemical and physical climate, particularly terrestrial N processes (a) during Maine’s changing winters, and (b) in response to chronic elevated N additions.

Subnivean winter soil processes were examined at the Dwight B. DeMerritt Forest (DBDF) and the Bear Brook Watershed in Maine (BBWM). In addition, we conducted an experimental snow removal study for two consecutive winters at DBDF to simulate reduced snowfall and induce soil freezing. Soil microbial processes of N mineralization and immobilization continued under the snowpack (reference plots), and soil freezing and thawing events increased C and N concentrations (treated plots), which we took as evidence of microbial cell lysis.

BBWM is a long-term paired-watershed experiment designed to study the effect of elevated N deposition and acidification. Research at the site has studied the evolution of ecosystem response to treatments and ambient changes in the chemical and physical climate over time, in particular, declining S and N deposition and a warmer, wetter climate in the Northeast. Here, we synthesize results from three decades of research on streams, soils, and vegetation to describe the long-term effect of N deposition at BBWM. While N leaching and export increased almost immediately in response to the treatments, labile soil N did not increase until the fifth year of treatment. Labile N became increasingly available in West Bear over time; after 25 years of treatment, treated soils had 10X more extractable ammonium and 200X more extractable nitrate than the reference watershed soils.

The research presented here fills a critical gap in our understanding of New England temperate forests. Our findings highlight the importance of long-term ecological research to understand ecosystem response, which is especially important in light of the current period of rapid environmental change.

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