Abstract: In recent years, herbaceous plants such as Deyeuxia angustifolia have invaded the shrub tundra of the Changbai Mountains, forming three representative vegetation types: Rhododendron aureum, Rhododendron aureum - Deyeuxia angustifolia coexistence, and Deyeuxia angustifolia. Non-structural carbohydrates (NSC) are important indicators of plant carbon uptake and consumption, reflecting the overall carbon supply status and stress resistance of plants. To elucidate the environmental adaptation mechanism of plants and empirically demonstrate the role of nitrogen deposition in vegetation change, a nitrogen deposition simulation experiment with three different nitrogen application levels was conducted to compare the differences in aboveground non-structural carbohydrate synthesis of Rhododendron aureum and Deyeuxia angustifolia in the three vegetation types on the tundra of Changbai Mountains and the differences in the decomposition of NSC in their litter. The results showed that: (1) Nitrogen deposition drove the organ-specific allocation of NSC in plants. In Rhododendron aureum, stem starch, soluble sugars, and total NSC content significantly increased with increasing nitrogen application.In contrast, the corresponding components in leaves and stems of Deyeuxia angustifolia showed a downward trend, indicating that their stems served a key role in carbon storage under nitrogen enrichment conditions. (2) Nitrogen deposition exerted "type-dependent" effects on litter decomposition: the decomposition rates of the three litters were ranked as Deyeuxia angustifolia > mixed > Rhododendron aureum; nitrogen deposition accelerated the decomposition of single litter by alleviating its nitrogen limitation (lowering C/N ratio), but inhibited the decomposition of mixed litter. In addition, nitrogen deposition promoted the accumulation of starch in litter, while inhibited the release of soluble sugar and NSC, suggesting that external nitrogen input may affect the decomposition process by changing the metabolic priority of carbon components. This study revealed that nitrogen deposition strongly mediated the carbon–nitrogen coupling cycle in tundra ecosystems by regulating NSC allocation and litter decomposition processes. These regulatory effects were driven by coordinated plant physiological responses and decomposition dynamics. The findings provided critical theoretical support for predicting changes in the structure and function of high-altitude cold ecosystems under continued global nitrogen enrichment.