Investigating Spatial and Temporal Nitrogen Dynamics in a Forested Headwater Stream Over the Course of an Annual Drying Event

Headwater streams make up nearly 80% of the stream network by length and play an important role in shaping the physical, chemical, and biological functions of downstream waters. In the Southeastern US, these systems are beginning to experience increases in the frequency and duration of drying, but it is unclear how increased drying will impact downstream water quality. To begin to address this issue, we examined nitrogen dynamics in a forested headwater stream across an annual drying event. The annual drying event was divided into seasonal wet (March–June), dry-down (June–October), and rewet (November–March) periods. We used a combination of water quality sensors at the watershed outlet, spatially distributed synoptic sampling of net denitrification conditions and physicochemical variables across the watershed, and a series of denitrification potential experiments to examine variation in biogeochemical state across the annual drying event. At the watershed outlet, nitrate concentrations were positively correlated with increased watershed-scale connectivity during the rewet period, while nitrate concentrations were positively correlated with increased streamflow during the dry-down period. Throughout the watershed, net denitrification conditions varied more across seasons than space, with greater net denitrification conditions during the dry-down period. Further, net denitrification conditions were positively correlated to stream temperature, nitrate, and ammonium concentrations but inversely related to streamflow. Finally, denitrification measurements confirmed the dry-down period experienced the highest denitrification rates. Our results highlight the connection between stream drying and nitrogen dynamics in humid systems, providing key information for developing predictive understanding of headwater streams.