Effects of nitrogen and phosphorus addition on the stoichiometric characteristics of nitrogen, phosphorus, and potassium in Cinnamomum camphora container seedlings

Method The annual C. camphora container seedlings were used as the test materials, and the exponential fertilization method was used to apply fertilizer treatments including low nitrogen, medium nitrogen, high nitrogen, phosphorus, low nitrogen + phosphorus, medium nitrogen + phosphorus, high nitrogen + phosphorus, and no fertilizer was used as control (CK). After the seedlings stopped growing, the biomass and stoichiometric characteristics of the nutrient elements (nitrogen, phosphorus, potassium) in their roots, stems, and leaves were measured.

Result The biomass of roots reached the highest amount under the single P application. The biomass of stem and leaves as well as the biomass of whole plant under N₃ treatment were significantly higher that other treatments（P<0.05）; The nitrogen mass fraction in roots of C. camphora container seedlings were 23.83 mg/g, 24.712 mg/g and 24.876 mg/g under N₃, N₂P and N₃P treatments, respectively, which were significantly higher than other treatments. The nitrogen mass fraction in stem of C. camphora container seedlings was the highest under N₂P treatment, which was 15.035 mg/g. In leaves, except for single application of phosphorus fertilizer, The nitrogen mass fraction of the other treatments were significantly (P<0.05)higher than those of the control group. The leaf nitrogen mass fractions of N₃ and N₂P treatments were the highest. Phosphorus fertilizer application alone significantly increased the phosphorus accumulation of each organ of C. camphora container seedlings, which was significantly different from that of the control (P<0.05), while single nitrogen application and combined nitrogen and phosphorus application had little effect on phosphorus accumulation of different organs. The combined application of nitrogen and phosphorus significantly increased the N/P of the leaves of C. camphora container seedlings, the N/P values of N₃P treatments were the highest, which was 4.67. The highest N/P values in the roots were N₂P and N₃P treatments, which were 6.18 and 6.16, respectively; N₂P treatment had the highest N/P value in stems, which was 4.27, and on the whole, N₂P and N₃P treatments were more conducive to increasing the N/P value in roots, stems and leaves of C. camphora container seedlings. The accumulation of potassium in the underground part of C. camphora container seedlings was not obviously affected by fertilization, however, the mass fraction of potassium in the aboveground part was significantly affected by the nitrogen and phosphorus addition, especially in the leaves; and the combined application of nitrogen and phosphorus had the greatest effect on the potassium mass fraction of C. camphora container seedlings. Under N₂, N₃ and N₂P treatments, N/K and K/P meet the standard of N/K>2.1 and K/P<3.4 indicating that the roots of C. camphora container seedlings were limited by potassium.

Conclusion Based on the stoichiometric ratio, it is proposed that under the conditions of this study, the factor restricting the growth of C. camphora container seedlings was nitrogen limitation. The single application of nitrogen fertilizer and combined application of nitrogen and phosphorus could significantly increase the biomass of C. camphora container seedlings, change the N/P ratio of various organs, and alleviate nitrogen limitation status.