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

Abstract

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.