Responses of nitrogen metabolism, photosynthetic parameter and growth to nitrogen fertilization inCyclocarya paliurus

Cyclocarya paliurus is a multi-function tree species and its leaves have been used as tea production and medicine in China. To meet the huge requirement of C. paliurus leaf production, many C. paliurus plantations are being developed in the sub-tropical region of China where nitrogen (N) availability is limited. Therefore, N fertilization is crucial to improve the plantation productivity and its leaf biomass production. However, only few studies were conducted to examine effects of N fertilization on C. paliurus seedling growth, while responses of nitrogen metabolism and growth to nitrogen fertilization in C. paliurus remain unknown, especially no field study has been carried out to verify the N fertilization effects. This study combined hydroponic culture with field trial together to evaluate the effects of four N supply levels on C. paliurus growth, photosynthesis, C:N:P stoichiometry and N metabolism, while the relationship between stoichiometric characteristics and growth was also discussed. N supply levels significantly affected N metabolism, photosynthesis parameters, C:N:P stoichiometry and growth of C. paliurus. As N supply increased, leaf N content and NDFF enhanced via modulating N metabolism, but the nitrogen use efficiency (NUE) decreased. Leaf N content was positively correlated with the NDFF and relative gene expression of N-assimilated enzymes, while negatively associated with NUE. Compared to the soil stoichiometric characteristics, leaf C:N:P stoichiometry was more strongly related to the growth of C. paliurus plantations at the research sites, and could be used as diagnostic indices of key nutrient availability in the soil. Based on the results from both hydroponic culture and field trial, the threshold values of C/N, C/P and N/P in the leaves are recommended for adding N and P fertilizer in C. paliurus plantations. Findings from the present study provide a theoretical basis for optimizing the application of N fertilizer in C. paliurus plantations to achieve high growth under N-limiting region, but the optimal quantity of N addition would vary with the plantation ages and site conditions.