Date of Award:

5-2020

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Committee

Bonnie G. Waring

Committee

Karen H. Beard

Committee

Trisha B. Atwood

Abstract

Herbivory by migratory animals in high-latitude ecosystems is known to impact greenhouse gas emissions from soils. However, few studies quantify the relationships between changes herbivores make to plant communities and soil conditions, and the biological interactions soil organisms have with their environment that result in changes to greenhouse gas emissions. These relationships are important to understand because they capture important carbon-climate feedbacks that may have implications for climate change policy and land management decisions, especially since high-latitude systems are experiencing unprecedented changes in climate.

In the Yukon-Kuskokwim (Y-K) Delta in western Alaska, herbivory by migratory geese affects the magnitude of greenhouse gas emissions coming from soils, but the mechanisms driving these relationships are poorly understood. To determine these mechanisms, variation in soil environments between adjacent grazed and un-grazed sites were compared to variation in soil environments across a landscape-scale gradient of plant communities to better understand the magnitude of differences in soil environments created by grazing. Soil environment characteristics measured included soil pH, moisture, total organic carbon and nutrients, and microbial community structure and dynamics. We also performed an incubation experiment on soils from grazed and un-grazed sites to assess the mechanistic drivers of changes in greenhouse gas emissions by manipulating soil environment characteristics that change with herbivory in the field: soil moisture, temperature, and nutrient content.

We found that soil environments between adjacent grazed and un-grazed sites had nearly as much variation as soil environments across the landscape, including in microbial communities. From the incubation experiment, greenhouse gas emissions increased with temperature and nutrient content, but there was no synergistic effect of moisture. Moreover, the effects of temperature and nutrients on greenhouse gases was increased in soils from grazed sites. The differences in the greenhouse gas emissions were not due to differences in absolute abundances of soil microbes. Instead, the results suggest that differences in relative abundances of soil microbial taxonomic groups with known differences in physiological traits or life-history strategies may account for the observed differences in greenhouse gas emissions.

These results have major implications for high-latitude ecosystems because these ecosystems are warming twice as fast as lower-latitude ecosystems, suggesting that greenhouse gas emissions will increase in grazed sites and contribute to positive feedbacks in climate. These results also suggest that relationships among herbivores, soil microbial communities, and belowground carbon cycling are important to capture ecological relationships that impact global climate.

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