Undergraduate Honors Theses

Thesis Defended

Spring 2016

Document Type


Type of Thesis

Departmental Honors


Ecology and Evolutionary Biology

First Advisor

Dr. William Bowman

Second Advisor

Dr. Noah Fierer

Third Advisor

Dr. Norman Pace

Fourth Advisor

Dr. Pieter Johnson

Fifth Advisor

Dr. Nick Schneider


Increasing levels of reactive nitrogen (N) deposition have been shown to affect bacterial and fungal abundances in soils across a wide variety of terrestrial ecosystems, potentially influencing stabilization of excess inorganic N and associated detrimental ecological effects. However, few studies have investigated these changes in sensitive alpine systems or considered potential seasonal variation in microbial abundance. In the present study, soil samples were collected periodically throughout the course of a single growing season from plots subjected to long-term experimental N additions located in an alpine tundra dry meadow community on Niwot Ridge, Colorado. To examine recovery potential of plots impacted by long-term elevated N deposition, half of each experimental plot stopped receiving N treatment in 2007. Fungal to bacterial ratio (fungi:bacteria) was measured to estimate potential N leaching and destabilization of soil microbial communities due to increased N deposition. Quantitative polymerase chain reaction (qPCR) analysis was used on bulked soil samples to quantify both the effects of chronic N amendment on fungi:bacteria and recovery potential of fungi:bacteria in those same soils following cessation of N additions. Fungi:bacteria was found to change differently in response to N addition at different points during the growing season, driven primarily by changes in fungal abundance, with a decreased fungi:bacteria during the early growing season, no change to fungi:bacteria during the late growing season, and an increased fungi:bacteria during senescence. An overall decrease in microbial biomass with N addition was also observed, independent of growing season. The decrease in fungi:bacteria during the early season had a greater magnitude than the increase in fungi:bacteria during the late season, possibly inhibiting this community’s ability to act as an N sink overall. Additionally, there was no indication of recovery of microbial biomass from cessation of the N treatment after 7 years, indicating that the lowered capacity of the soil to stabilize inorganic N due to both the decrease in overall microbial biomass and fungi:bacteria may persist even if N deposition rates decrease to previous levels. These results suggest that increasing N deposition will be a significant factor driving changes in microbial abundance and fungi:bacteria in alpine tundra soils, possibly leading to long-term destabilization of alpine soil microbial communities and inorganic N storage.