Date of Award

2017

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Biological Sciences (MS)

Administrative Home Department

Department of Biological Sciences

Advisor 1

Amy Marcarelli

Committee Member 1

Stephen Techtmann

Committee Member 2

Joseph Bump

Abstract

It is frequently assumed that N2 fixation and denitrification do not co-occur in streams because each process should be favored under different concentrations of reactive nitrogen. Yet, both N2 fixation and denitrification have been found to co-occur in marine and coastal ecosystems despite their differences in nitrogen requirements, and we cannot evaluate this assumption for streams because both processes are rarely quantified together. We asked if these processes could co-exist by measuring rates of N2 fixation using acetylene reduction, denitrification using acetylene block, and N2 flux using membrane inlet mass spectrometry on rocks and sediment in 8 southeastern Idaho streams encompassing a dissolved inorganic nitrogen (DIN) gradient of 6-615 µg/L. N2 flux rates on rocks had a mean of -12,000 ± 4,900 µg/m2/h and on sediment of -2,400 ± 12,000 µg/m2/h, which were significantly different. N2 fixation rates were not significantly different among rock and sediment substrate with means of 22.9 ± 54.4 and 2.2 ± 2.0 µg/m2/h, respectively. Unamended denitrification rates were significantly different among rock and sediment substrates with means of 3 ± 7 and 2248 ±1565 μg/m2/h, respectively. Amended denitrification rates were also significantly different among substrates with a mean of 352 ± 690 µg/m2/h on rocks and 18,100 ± 6287 µg/m2/h on sediment. DIN concentration was not a significant predictor of unamended denitrification rates, but was a significant predictor of N2 flux and N2fixation rates on rocks in 2016, and amended denitrification rates on sediments in 2015 and 2016, indicating that DIN concentration alone cannot predict occurrence of processes on all substrates at all times. Multiple linear regression models relating environmental variables to measured rates showed that carbon and phosphorus availability were important predictors of denitrification rates and phosphorus, carbon, and light availability were important predictors of N2 flux rates across all sites. No significant model was produced for N2 fixation rates. Environmental characteristics measured at the scale of entire stream-reaches may not be at a fine enough spatial scale to characterize and predict the co-occurrence of these processes within stream reaches. N2 flux is balanced by the rates of N2 fixation and denitrification, and in order to better understand the fluxes and cycling of N through stream ecosystems we need to examine the co-occurrence of these processes.

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