Nitrogen Deposition and Light Interactively Influence Growth and Physiological Performance of the Endangered Tree Horsfieldia hainanensis

Increasing atmospheric nitrogen (N) deposition is a major global environmental change driver affecting plant growth and development, yet its impacts on the physiological processes of tree species remain poorly understood. This study investigated the interactive effects of N deposition and light availability on growth, biomass allocation, and physiological-biochemical traits of Horsfieldia hainanensis seedlings. Six N treatments (no N: CK; four exponential levels, E1-E4: 2500-5500 mg·seedling-¹; and a conventional level, CF: 3800 mg·seedling-¹) were applied in a two-factor randomized design with three shading levels (60%, 40%, and 20% of natural light; no full-light or complete shading controls due to seedling mortality within 7 days), achieved using adjustable shade structures and verified with a lux meter to simulate realistic environmental change scenarios. Results showed that both light and N significantly influenced seedling growth, biomass, photosynthetic capacity, pigment content, and stress-related biochemical traits, with strong interactive effects. Moderate shading (40%) combined with moderate N supply (E3: 4500 mg·seedling-¹) optimized stem diameter and height growth, total biomass accumulation, and photosynthetic efficiency. Severe shading (20%) limited growth regardless of N, while high N under high light (60%) caused imbalanced biomass allocation. Antioxidant enzyme activities and osmotic adjustment substances were differentially regulated by light and N, enhancing stress tolerance under specific combinations. Multivariate and correlation analyses highlighted the integrative role of light-N interactions in coordinating growth, physiology, and stress responses. These findings provide novel, evidence-based guidance for optimizing nursery and field management practices to enhance the establishment, survival, and conservation of H. hainanensis under rising nitrogen deposition in subtropical forests.