Excess nitrogen (N) from terrestrial landscapes poses environmental challenges as it moves via surface runoff and groundwater flows into aquatic ecosystems. Managing and anticipating the environmental challenges associated with these altered N inputs from terrestrial to aquatic ecosystems requires a deep understanding of how N is biogeochemically transformed, retained, and/or transported in streams and rivers. Here, I used long-term data on surface stream and groundwater chemistry as well as discharge to determine the main sources of N and estimate the N mass balance of a 1.4 km boreal stream reach. The goal was to evaluate daily net uptake or production rates of different N forms (ammonium - NH4-N, nitrate NO3-N and dissolved organic N - DON) throughout the seasons, and assess physical and chemical factors that may drive changes in net processing. The mass balances analysis revealed distinct patterns in net uptake among N forms. Notably, there was clear evidence of NH4-N and DON uptake (removal) in the stream, while NO3-N processing patterns showed neither clear uptake nor production. Further, variation in net uptake for NH4-N and DON was positively related to stream DOC, DOC:DIN, and C:N ratios, indicating that carbon rich conditions promoted greater N demand in this ecosystem. By comparison, variations in net NO3-N uptake or production at the reach scale were only weakly correlated with these carbon rich conditions. Finally, I assessed these patterns within the nutrient processing domains (NPDs) framework to characterize the behavior/character of the study reach. Accordingly, during the open water season, the stream reach acted mostly as a consumer for both NH4-N and DON, while on many dates it acted as a weak enhancer for NO3-N. These findings contribute to the broader understanding of N dynamics in boreal stream ecosystems and emphasize the complex interplay among organic and inorganic N forms, carbon dynamics, and nutrient processing in these environments. This knowledge is crucial for effective environmental management and conservation efforts in the region.

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