Objectives The study aimed to investigate the effects of pH levels and ammonium to nitrate ratios on seedling growth and the photosynthetic characteristics of subtropical coniferous and broad-leaved tree species, Cunninghamia lanceolata and Schima superba. This research could provides theoretical support for site selection and nitrogen fertilizer management in mixed forests of C. lanceolata and S. superba.

Methods One-year-old C. lanceolata and S. superba seedlings were used as experimental materials in a sand culture nutrient solution experiment. Three pH levels (4.0、5.0、6.5)for the cultivation substrates were setup. Under each pH level, five ammonium to nitrate ratios (NH₄⁺-N : NO₃⁻-N) were established: 10∶0 (R₁), 7∶3 (R₂), 5∶5 (R₃), 3∶7 (R₄), and 0∶10 (R₅), all the treatments were supplied with a constant total nitrogen concentration of 2 mmol/L during the seedling growth period. Growth indicators and photosynthetic characteristics including net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), and water use efficiency (WUE) of seedlings were measured of seedlings were measured after culture for 120 days. Experimental treatments were terminated at 160 days, followed by assessment of C. lanceolata and S. superba seedling growth parameters.

Results The growth in height and ground diameter of both C. lanceolata and S. superba seedlings generally increased with the rising pH. The growth of both seedlings at the pH 4.0 levels was significantly lower by 50.5%−66.5% and 50.3%−73.7% compared to the pH 5.0 and 6.5 levels, respectively. pH 4.0 significantly inhibiting C. lanceolata's biomass accumulation compared to pH5.0 and 6.5, while the biomass of S. superba peak at the 5.0 level. The leaf P_n, T_r, G_s of C. lanceolata were highest at the R₃ ratio across all the three pH levels, with P_n surpassing other ratios by 62.9%−110.0%, 29.2%−55.9%, and 16.5%−40.5%, respectively. For S. superba, the P_n, T_r, G_s showed significantly higher values in the R₄ ratio than other four ratios under the pH=5.0, WUE for S. superba was highest at the pH 5.0 level, exceeding levels at pH 4.0 and 6.5. Under the same pH levels, the overall height and ground diameter of C. lanceolata were higher at the R₁ and R₂ ratios compared to the R₄ and R₅ ratios, whereas S. superba was greater at the R₄ and R₅ ratios than at the R₁ and R₂ ratios. Treatments with higher ammonium nitrogen concentrations increased the total biomass of C. lanceolata but reduced the root-to-shoot ratio, while treatments with higher nitrate nitrogen concentrations enhanced the total biomass of S. superba. For both tree spices, P_n, T_r, and G_s were generally higher under mixed ammonium to nitrate ratios than under single-form nitrogen ratios. Principal component analysis (PCA) revealed that C. lanceolata exhibited optimal performance under the (5∶5 ratio) at pH 5.0 treatment, while S. superba demonstrated superior growth under the pure nitrate treatment (3∶7 ratio) at pH 5.0

Conclusions Mixed plantations of C. lanceolata and S. superba should avoid strongly acidic soils (pH 4.0), pH 5.0 is benefit for the growth of both C. lanceolata and S. superba. While in this level, the suitable ammonium to nitrate ratios for C. lanceolata and S. superba are 5∶5 and 3∶7 respectively. In summary, both C. lanceolata and S. superba prefers mixed supply of ammonium and nitrate nitrogen over single-form nitrogen for seedling growth.