Abstract: Nitrogen (N) deficiency is a critical factor limiting natural regeneration in coastal shelterbelt forests, but the influence of different N forms on seedling establishment under varying light conditions remains poorly understood. This study investigated the effects of N forms and N concentrations on Ligustrum compactum seedlings under simulated canopy gap conditions using a three-factor design: N form (NO_₃^⁻-N, NH_₄^⁺-N, mixed N), N concentration (30 and 60 kg ha^⁻1 a^⁻1), and light intensity (30%, 60%, and 90% full sunlight). Results showed that N addition significantly promoted seedling growth, net photosynthesis rate, and water use efficiency; however, the effects varied among N forms and concentrations. Overall, NO₃⁻-N or mixed N were more favored by L. compactum seedlings; however, the N preference was altered by light intensity and N concentration. For instance, L. compactum showed greater NO₃⁻-N or mixed N preference under low and medium light intensities, while displaying more NH4+-N preference under high light intensity. N concentration also affected the growth and N preference of L. compactum seedlings, but the variance explained by N concentration was lower than that of light intensity. Leaf C, N, P stoichiometry exhibited stronger correlations with seedling’s morphological trait plasticity than those of leaf gas exchange, and further analysis demonstrated that leaf C:P and N:P were the top two critical factors affecting seedling growth, indicating that the coordination and balance among C, N, P elements were more important in explaining the seedling growth under N addition. Therefore, our results clarified that the N preference of L. compactum seedlings could be altered by light intensity and revealed that leaf C, N, P ratios were stronger predictors than leaf gas exchange parameters for explaining the N effects on seedling performance. These findings demonstrated the mechanisms of light-N interactions affecting seedling performance, providing practical guidance for optimizing N fertilization and improving natural regeneration in canopy gaps of degraded coastal shelterbelt forests.

Key words: Nitrogen form, Light intensity, Seedling performance, Leaf C, N, P, Leaf gas exchange, Coastal forest