[Objective] Nitrogen, a critical limiting nutrient in aquatic ecosystems, exhibits spatiotemporal heterogeneity that profoundly influences water quality dynamics and eutrophication evolution in the Danjiangkou Reservoir. This study aims to clarify the spatiotemporal differentiation patterns and driving mechanisms of key nitrogen forms in Danjiangkou Reservoir, focusing on seasonal (non-flood vs. flood) and vertical (surface, middle, bottom) variations. It focuses on critical yet underexplored issues, including seasonal dynamics of nitrogen forms, vertical data gaps, and tributary-reservoir coupling, to directly support the water quality security goals of the South-to-North Water Diversion Project. [Methods] This study systematically collected surface water samples from tributaries discharging into the reservoir and stratified samples (surface, middle, and bottom layers) during the non-flood (March 2024) and the flood (September 2024) seasons. By quantifying total nitrogen, nitrate, ammonium, and dissolved organic nitrogen, we investigated the spatiotemporal differentiation mechanisms of nitrogen forms. [Results] In the non-flood season, nitrate nitrogen ($\mathrm{NO}_{3}^{-}-\mathrm{N}$) was the dominant form of nitrogen in the water of the Danjiangkou Reservoir area and its inflow tributaries, accounting for 63.3% to 88.6% of the total nitrogen and holding an absolute advantage. In the longitudinal direction of the reservoir area, after the inflow from the Han Reservoir entered the Danjiangkou Reservoir, the concentration of $\mathrm{NO}_{3}^{-}-\mathrm{N}$ generally showed a decreasing trend along the direction of water flow. In the vertical direction, the average concentration of $\mathrm{NO}_{3}^{-}-\mathrm{N}$ followed the order of bottom layer > middle layer > surface layer, which may be attributed to photochemical denitrification in the surface water. Dissolved organic nitrogen (DON) was the second most abundant form of nitrogen; its concentration in the Han Reservoir was significantly higher than that in the Dan Reservoir. Vertically, DON concentration showed a decreasing trend toward the bottom in the Han Reservoir, while it showed an increasing trend toward the bottom in the Dan Reservoir. This phenomenon may be caused by the adsorption of DON by fine suspended sediments and their transport to the reservoir bottom under the slow flow conditions of the Dan Reservoir. After entering the flood season, DON concentrations in the inflow tributaries increased significantly. In both the Danjiangkou Reservoir area and its tributaries, the dominant forms of nitrogen became $\mathrm{NO}_{3}^{-}-\mathrm{N}$ and DON, with $\mathrm{NO}_{3}^{-}-\mathrm{N}$ accounting for 13.90% to 90.44% and DON accounting for 1.04% to 74.18%. Compared with the non-flood season, the content of $\mathrm{NO}_{3}^{-}-\mathrm{N}$ decreased significantly, which may be due to the enhanced photochemical denitrification in the surface water and microbial denitrification in the bottom water of the reservoir during the flood season. In the longitudinal direction, the distribution of $\mathrm{NO}_{3}^{-}-\mathrm{N}$ showed no obvious regularity; however, in the vertical direction, the average concentration of $\mathrm{NO}_{3}^{-}-\mathrm{N}$ still followed the order of bottom layer > middle layer > surface layer. [Conclusion] The results show that nitrate nitrogen is the main form of nitrogen in the water body during the non-flood season of the reservoir. After entering the flood season, the content of dissolved organic nitrogen increases significantly and jointly dominates the occurrence of nitrogen in the water body with nitrate nitrogen. Longitudinally, after the water from the Han Reservoir enters the Danjiangkou Reservoir, the concentration of nitrate nitrogen generally shows a decreasing trend along the direction of water flow, reflecting the regulatory effect of the hydrodynamic process on the transport of nitrogen. Vertically, the average concentration of nitrate nitrogen in both the non-flood season and the flood season shows the typical pattern of bottom layer > middle layer > surface layer, and the vertical difference in the flood season is more significant. This pattern may be closely related to the reduction of nitrate nitrogen by photochemical denitrification in the surface water body and the inhibition of vertical mixing due to thermal stratification in the flood season.