In-stream nutrient uptake is essential in regulating nutrient transport in fluvial ecosystems, yet knowledge of interactions between multiple nutrient uptake still needs to be improved. To reveal the characteristics of interactions between N and P uptake and their seasonal variability, we conducted a series of short-term (i.e., minutes to hours) nutrient addition experiments, including individual nitrogen (N) and phosphorus (P) and combined N plus P additions, in a vegetated forested stream during two contrasting seasons (i.e., the late spring and autumn). Moreover, we explored the responsiveness of N or P uptake rates to their concentrations under the conditions of varying ratios of nitrate-N (NO 3 ¯-N) to ammonium-N (NH 4 +-N) in all N additions. We found a significant interaction between N and P retention and uptake in the stream where it was P-limited. Across all experiments, the responsiveness of NH 4 +-N uptake to increased P concentrations increased with decreasing ratios of NO 3 ¯-N/NH 4 +-N additions in late spring or autumn. In contrast, NO 3 ¯-N uptake showed an opposite pattern. Partial least squares regression results suggested that the hydromorphology (e.g., discharge, channel heterogeneity, aquatic macrophytes) was the main driver and powerfully impacted the whole-reach N and P uptake and their interactions. • Obvious interactions between N and P uptake in a vegetated stream. • Responsiveness of N uptake to increased P varied as added N forms shifted. • Hydromorphology strongly impacted N and P uptake and their interactions. [ABSTRACT FROM AUTHOR]