Headwater streams can process a large proportion of nitrogen (N) inputs from their watersheds, and retention or transformation by instream biota can significantly influence the magnitude and timing of N export to downstream systems. The challenge of identifying pattern in processes at various scales is foundational to ecology, but the majority of stream studies have measured biogeochemical processes at the reach scale, mostly in headwaters. It is challenging to empirically measure N transformations at an adequate spatial density for incorporation into watershed scale models, and field studies that investigate N biogeochemistry along a size gradient from streams to rivers are a current gap in the literature. Finally, improved understanding of controls on N processing will inform restoration and management strategies that mitigate the export of N pollution from impacted watersheds; floodplain restoration is one increasingly common strategy. The primary objective of my dissertation is to improve understanding of the fate of N in stream networks and their floodplains by identifying biogeochemical controls that vary as a function of spatial and temporal scale in the context of land use change.
To begin, I simultaneously measured both gaseous end products of denitrification while addressing the environmental factors that have been shown to drive the ratio of the production of these gases in constructed floodplains. Then I scaled this biogeochemical process to the reach scale to assess the impact of surrounding land use, season, and stream size on denitrification and assimilatory nitrate removal in two watersheds of contrasting land use. To better understand reach scale processes, I tested the effect of streambed substrate, biological structure, and stream flow on gas exchange in low-gradient experimental streams. In addition, I collected seasonal, high-density, synoptic water chemistry data from two watersheds of contrasting land use to assess the impact of land use and seasonality on nitrate concentrations and nitrous oxide flux throughout the stream network. Finally, I explore the body of literature around N and sediment retention in floodplains to assess the impact of floodplain restoration efforts in the Wabash River basin since the early 1970s.