Date of Award

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Natural Resources

First Advisor

Carol Adair

Abstract

Soil denitrification is a critical component of nitrogen (N) cycling on Earth. It is a microbially-mediated process that removes N from soils by reducing nitrate (NO3-), a highly bioavailable molecule and significant contributor to eutrophication, to gaseous forms of N (N2 or N2O). Riparian areas, which are located at the interface between terrestrial and aquatic ecosystems, are areas of elevated denitrification rates, as they frequently exhibit favorable conditions for this process. Denitrification provides a critical ecosystem service by reducing N inputs to streams and rivers. However, this process is highly variable in time and space, making it difficult to predict when and where riparian soil denitrification occurs and contributes significantly to N removal. It is therefore critical to understand when, where, and how the controls on this process fluctuate. Changes to physical and chemical soil conditions (e.g., oxygen (O2) and substrate availability, soil moisture and temperature) modulates the biological response through expression of functional denitrification genes, which strongly controls instantaneous denitrification rates. This dissertation therefore targets both physical and chemical (i.e., environmental) controls on denitrification, as well as the biological indicators of this process, including the abundance of functional denitrification genes and denitrifier community composition.

I first address the heterogeneity of riparian soil O2, a key control on denitrification, as this process can only occur in the absence of O2. I use a machine-learning approach to pinpoint combinations of soil conditions that lead to contrasting O2 regimes. While it is often assumed that high soil moisture leads to low soil O2, my results indicate that this is not always the case. Riparian soil O2 varies seasonally and with soil moisture, but variable combinations of soil and site-specific hydrologic conditions complicate the relationship between soil water content and O2. Next, I assess seasonal and spatial (i.e., landscape position and adjacent land use) variability in denitrifier community composition using a cutting-edge DNA sequencing technology. The results indicate that denitrifier community composition is stable across seasons but is sensitive to contrasting soil moisture and O2 regimes within a given site. Finally, I assess links between environmental and biological (i.e., functional nitrification and denitrification gene abundance) controls on denitrification rates during the spring snowmelt period. The results suggest that the utility of gene abundance in predicting the denitrification capacity of soils is site-specific.

The findings from this dissertation indicate that soil O2 generally fluctuates seasonally, with soil moisture, and that low O2 events most frequently occur under cool and wet soil conditions. However, there are deviations from this pattern, which highlight problems associated with predicting soil O2 solely based on soil moisture. This dissertation also provides evidence that denitrifier community composition is strongly regulated by soil conditions that control the availability of electron acceptors. The dissertation results also suggest that the utility in incorporating the biotic community to assess denitrification variability is site-specific, as we observed strong links between gene abundance and denitrification rates at only one study site with unique hydrologic features.

Language

en

Number of Pages

148 p.

Available for download on Friday, October 07, 2022

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