Controls on Retention, Transformation, and Export of Dissolved Nitrogen and Phosphorus in Agricultural Streams across Spatial and Temporal Scales

Agricultural land use has increased the availability of nitrogen (N) and phosphorus (P) in the biosphere via the use of synthetic fertilizers. Moreover, landscape alterations that enhance drainage, including subsurface tiles and stream channelization, and sustain agriculture, also accelerate the transfer of dissolved nutrients from fields to streams. Farming practices across the midwestern U.S. “corn belt” contribute to water quality degradation in both the Gulf of Mexico and the Great Lakes, and changes in the timing and intensity of storms under climate change will likely increase nutrient export from highly managed agroecosystems. Thus, a critical need exists for research exploring the interactions between local environmental context and spatiotemporal variation, to improve understanding of how conservation and restoration activities can buffer the impacts of agriculture on freshwater. The overarching goal of my dissertation is to identify controls on retention, transformation, and export of dissolved N and P in agricultural streams using both monitoring and experimental approaches. My research also adds insight into how these controls are impacted by management interventions, such as conservation or restoration, to support stream ecosystem function.

I found that substrate, light, and streamflow interact to control N and P removal from the water column. Additionally, I documented that the accumulation of organic matter following floodplain restoration controls nitrate-N removal via denitrification, and that floodplains can significantly increase nitrate-N removal at the watershed-scale; however, the relative increase in removal depends on floodplain surface area and inundation frequency. I also found that increasing temperature and carbon availability enhanced the production of nitrous oxide, a greenhouse gas, from stream sediments, due to incomplete denitrification. As such, changes in global temperature and carbon availability may increase nitrous oxide emissions from streams. At the watershed-scale, extensive tile drainage and widespread fertilizer use enhanced the transport of nitrate-N during storms, but higher cover crop coverage reduced nitrate-N export in spring. My dissertation highlights how both regional variation and watershed characteristics influence the role of local environmental context in controlling nutrient cycling in agricultural landscapes. Furthermore, management that enhances ecosystem resilience can mitigate the impacts of a changing climate on downstream nutrient loss.