Objectives Investigating 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, to 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 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 pH 6.5 levels, respectively. At pH 4.0, the biomass accumulation of C. lanceolata was significantly inhibited compared to pH 5.0 and pH 6.5, while the biomass of S. superba peaked at the pH 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. Under the R3 ratio, P_n surpassed that of other ratios by 62.9%−110.0% (pH 4.0), 29.2%−55.9% (pH 5.0), and 16.5%−40.5% (pH 6.5), 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 those at pH 4.0 and pH 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 the overall growth of 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. Principal component analysis revealed that C. lanceolata exhibited the best growth under the treatment with a pH of 5.0 and an ammonium-to-nitrate ratio of 5∶5, while S. superba showed optimal growth under the treatment with a pH of 5.0 and an ammonium-to-nitrate ratio of 3∶7.

Conclusions Mixed plantations of C. lanceolata and S. superba should avoid strongly acidic soils (pH 4.0), pH 5.0 is beneficial 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 prefer a mixed supply of ammonium and nitrate nitrogen over single-form nitrogen for seedling growth.