Nitrate Loads From Land to Stream Are Balanced by In‐Stream Nitrate Uptake Across Seasons in a Dryland Stream Network.

Exploring nitrogen dynamics in stream networks is critical for understanding how these systems attenuate nutrient pollution while maintaining ecological productivity. We investigated Oak Creek, a dryland watershed in central Arizona, USA, to elucidate the relationship between terrestrial nitrate (NO3−) loading and stream NO3− uptake, highlighting the influence of land cover and hydrologic connectivity. We conducted four seasonal synoptic sampling campaigns along the 167‐km network combined with stream NO3− uptake experiments (in 370–710‐m reaches) and integrated the data in a mass‐balance model to scale in‐stream uptake and estimate NO3− loading from landscape to the stream network. Stream NO3− concentrations were low throughout the watershed (<5–236 μg N/L) and stream NO3− vertical uptake velocity was high (5.5–18.0 mm/min). During the summer dry (June), summer wet (September), and winter dry (November) seasons, the lower mainstem exhibited higher lateral NO3− loading (10–51 kg N km−2 d−1) than the headwaters and tributaries (<0.001–0.086 kg N km−2 d−1), likely owing to differences in irrigation infrastructure and near‐stream land cover. In contrast, during the winter wet season (February) lateral NO3− loads were higher in the intermittent headwaters and tributaries (0.008–0.479 kg N km−2 d−1), which had flowing surface water only in this season. Despite high lateral NO3− loading in some locations, in‐stream uptake removed >81% of NO3− before reaching the watershed outlet. Our findings highlight that high rates of in‐stream uptake maintain low nitrogen export at the network scale, even with high fluxes from the landscape and seasonal variation in hydrologic connectivity. Plain Language Summary: Exploring nitrogen dynamics in desert streams is critical for understanding how these systems can reduce pollution while maintaining healthy ecosystems. We examined how seasons and human activities, like farming and development, affect nitrogen pollution in a desert stream in Arizona, USA. We found that in seasons with little rain, nitrogen delivery to the stream was high in areas where irrigation is common. However, in‐stream nitrogen remained low because of the high capacity of the stream algal and microbial community to remove nitrogen from the water. This reveals that plants and microbes play a vital role in regulating nitrogen in deserts. This study has broad implications beyond this particular desert stream, emphasizing the importance of understanding complex interactions between human activities, water, and stream microbes. By studying these interactions, we can better manage and preserve desert streams with changing climate and human pressures. Key Points: Nitrate loading to a dryland stream network varies spatially and seasonally due to hydrologic connection to landscape nutrient sourcesHigh in‐stream nitrate uptake maintained low stream nitrate concentration and watershed export even when landscape inputs were highIn drylands, hydrologic disconnections in space and time between nitrate sources and the stream requires a flexible modeling approach [ABSTRACT FROM AUTHOR]