Objectives Light intensity and nitrogen supply heavily affect the growth of Cunninghamia lanceolata seedlings. We measured the growth and physiological indices of Cunninghamia lanceolata seedlings under different light intensity and nitrogen application rates.

Methods A two-factor randomized block design pot experiment was conducted in the state owned Cunninghamia lanceolata seedling forest, located in Shunchang City, Fujian Province, one-year-old seedlings was used as test materials. Four shading levels were applied to simulate natural light reduction: 0% shading (L0, full sunlight), 25% shading (L1), 55% shading (L2), and 80% shading (L3). Three N levels were administered per seedling: N 0 g (N0, control), N 4 g (N1, moderate), and N 6 g (N2, high). The seedlings were exposed to the experimental conditions for one year, with regular maintenance to ensure consistent growth conditions. Then seedling height and ground diameter were measured, the antioxidant indices in leaves were analyzed to calculate the resistance to stress, and the endogenous hormone contents in apical buds were measured as well. The promotion effect of treatments was screened by subordination function method.

Results Compared with L₀N₀, suitable shading promoted the height and ground diameter growth of C. lanceolata seedlings, nitrogen application further enhanced the promoting effect of shading. L₂N₁ was recorded the most suitable light-nitrogen combination, with seedling height and ground diameter increased by 139.54% and 89.84%, compared to L₀N₀. Both shading and nitrogen application increased the activities of peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD), reduced the accumulation of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) in the leaves C. lanceolata seedlings, thereby enhancing the seedlings’ total antioxidant capacity. L₂N₁ and L₂N₂ were tested higher CAT, SOD, POD activities and total antioxidant capacities, with the increment in L₂N₁ by 131.67%, 24.26%, 625.70%, and 141.84%, and decline in H₂O₂ and MDA contents by 72.71% and 66.42% as compared with L₀N₀, respectively. Shading led to lower cytokinin (CTK) and abscisic acid (ABA) content in the buds and gibberellic acid (GA) and ABA content in the stems of C. lanceolata, but higher ratios of CTK/ABA, GA/ABA, and IAA (indoleacetic acid) /ABA. N application offset the inhibition of hormones by shading, and significantly improved the contents of IAA, GA and ABA, as well as the ratios of CTK/GA, GA/ABA and IAA/ABA in buds and stems of C. lanceolata seedlings (P<0.01), and the effect was not significantly different between N₁ and N₂. The subordination function analysis results showed that the most suitable light-nitrogen combination treatment for the growth of C. lanceolata seedlings was L₂N₁ treatment.

Conclusions An appropriate light-nitrogen combination can effectively increase the IAA and GA contents and the IAA/ABA and GA/ABA ratios, decrease the CTK/GA ratio in buds, and enhance the antioxidant enzyme activity, which results in high scavenging capacity, as a consequencece promoting the growth of C. lanceolata seedlings. Under the test conditions, the suitable light intensity and nitrogen application rate for high quality C. lanceolata seedling production are 542.68 μmol/(m²·s) and 4 g/seedling, respectively.