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

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Bibliographic Details
Title:	Investigating Spatial and Temporal Nitrogen Dynamics in a Forested Headwater Stream Over the Course of an Annual Drying Event.
Authors:	Zarek, Kaci^1,2 (AUTHOR), Jones, C. Nathan¹ (AUTHOR) natejones@ua.edu, Peterson, Delaney M.¹ (AUTHOR), Plont, Stephen¹ (AUTHOR), Shogren, Arial J.¹ (AUTHOR), Tatariw, Corianne³ (AUTHOR), Speir, Shannon L.⁴ (AUTHOR), Mortazavi, Behzad⁵ (AUTHOR), Burgin, Amy J.⁶ (AUTHOR)
Source:	Journal of Geophysical Research. Biogeosciences. Apr2025, Vol. 130 Issue 4, p1-20. 20p.
Subject Terms:	Water temperature, Nitrogen cycle, Water quality, Water sampling, *Denitrification, Autumn
Abstract:	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. Plain Language Summary: Headwater streams are small but constitute a significant proportion of global river networks. Even in the wettest and more humid regions, headwater streams are experiencing increased drying, resulting in streams drying out or ceasing to flow. While stream drying has been observed, it is unclear how it will impact ecosystems and water quality. In this study, we used a combination of high‐frequency sensor data, water chemistry sampling across the watershed, and lab experiments to determine the impact of stream drying on nitrogen cycling. We found that drying conditions during the summer and early fall lead to increases in nitrogen removal rates, and these conditions were linked specifically to increased temperatures, reduced streamflow, decreased dissolved oxygen, and increased dissolved inorganic nitrogen availability. Taken together, our findings are important for understanding how headwaters and their associated ecosystem functions in wet and humid regions will respond to increased stream drying. Key Points: Seasonal NO3− concentration patterns were most strongly correlated with watershed‐scale connectivity during the rewet period and streamflow during the dry‐down periodThroughout the watershed, net denitrification conditions were more variable across seasons than spaceAcross the annual drying event, the seasonal dry‐down period experienced highest denitrification potential rates [ABSTRACT FROM AUTHOR]
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Database:	GreenFILE

Description
Abstract:	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. Plain Language Summary: Headwater streams are small but constitute a significant proportion of global river networks. Even in the wettest and more humid regions, headwater streams are experiencing increased drying, resulting in streams drying out or ceasing to flow. While stream drying has been observed, it is unclear how it will impact ecosystems and water quality. In this study, we used a combination of high‐frequency sensor data, water chemistry sampling across the watershed, and lab experiments to determine the impact of stream drying on nitrogen cycling. We found that drying conditions during the summer and early fall lead to increases in nitrogen removal rates, and these conditions were linked specifically to increased temperatures, reduced streamflow, decreased dissolved oxygen, and increased dissolved inorganic nitrogen availability. Taken together, our findings are important for understanding how headwaters and their associated ecosystem functions in wet and humid regions will respond to increased stream drying. Key Points: Seasonal NO3− concentration patterns were most strongly correlated with watershed‐scale connectivity during the rewet period and streamflow during the dry‐down periodThroughout the watershed, net denitrification conditions were more variable across seasons than spaceAcross the annual drying event, the seasonal dry‐down period experienced highest denitrification potential rates [ABSTRACT FROM AUTHOR]
ISSN:	21698953
DOI:	10.1029/2024JG008522