Effects of the Cultivation Substrate pH and Ammonium-to-Nitrate Nitrogen Ratio on the C:N:P Stoichiometry in Leaves of Cunninghamia lanceolata and Schima superba

Abstract

We aimed to clarify the nutrient allocation strategies of Cunninghamia lanceolata (CL) and Schima superba (SS) and their differences in N uptake and utilization under acidic stress and heterogeneous allocation of N. One-year-old seedlings of CL and SS were cultured in sands with three pH levels (pH rate 4.5, 5.5, and 6.5) and five different N form ratios (NH4+-N to NO3−-N ratios 10:0, 7:3, 5:5, 3:7, and 0:10) in the greenhouse. The C, N, and P contents and C:N:P stoichiometry in leaves, total biomass, and NH4+-N and NO3−-N contents in the cultivation substrate were analyzed after 160 days. At the pH rate of 4.5, the N contents of CL and SS significantly decreased. However, the N contents of SS increased at the pH rate of 5.5. At the pH rate of 5.5, the N content of CL and SS increased under the cultivation substrate, with ratios of 10:0 and 7:3, and 3:7 and 0:10, respectively. The N content of CL was significantly positively correlated with its P content. The C:P ratios of CL and SS were significantly positively correlated with their C:N ratios and N:P ratios. Under the same treatment, the differences in C:N:P stoichiometry were more significant between CL and SS at the pH rate of 5.5. The NH4+-N content of the cultivation substrate in CL and SS was higher than the NO3−-N content. At the ratio of 7:3, the C:N stoichiometric ratio of CL and C:N:P stoichiometric ratio of SS were negatively correlated with total biomass. At the pH rate of 5.5, cultivation environments with high NH4+-N and NO3−-N amount respectively increased the N content of CL and SS. The CL is susceptible to N limitation, and SS is susceptible to N and P limitation.