Response of nitrogen use efficiency, N-assimilating enzymes and growth in Cyclocarya paliurus seedlings to different nitrogen nutrition

Cyclocarya paliurus has been used as tea production and medicine in China. In order to identify C. paliurus preference for absorption of nitrogen (N) nutrition and the relationship between C/N ratio and growth, effects of five NO3-/NH4+ ratio treatments on growth, N absorption and metabolism of C. paliurus seedlings were investigated. Compared to NH4+ as a sole N source, the seedlings supplied with a sole NO3- source had higher growth rate, percent of nitrogen derived from nitrogen fertilizer (NDFF), N content and nitrogen use efficiency (NUE). However, mixed nitrate-ammonium nutrition improved growth and NUE in C. paliurus seedling, while an equal mixed ratio nutrition induced the highest NDFF and seedling growth. Nitrate reductase (NR) and nitrite reductase (NiR) activities in leaves and roots showed a decreased trend with decreasing NO3- in growth medium, whereas the highest glutamine synthetase (GS) and glutamate synthase (GOAGT) activities were detected at the equal mixed ratio of NO3-/NH4+. Regression analysis showed that activities of the N-assimilated enzymes were positively correlated with NUE and N content in C. paliurus seedlings, but the seedling growth was negatively and significantly correlated with C/N ratio. Our study demonstrated that C. paliurus preferred NO3- over NH4+ in N nutrition, but an equal mixed ratio of NO3-/NH4+ would best improve seedling growth and nitrogen use efficiency. This study would provide a basis for manipulating nitrogen fertilization in C. paliurus plantations.