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 to 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.