Antecedent moderate nitrogen fertilization alleviated the effects of drought on growth and leaf photosynthesis of Schima Superba seedlings.

Background: Drought and nitrogen deposition are the major global change factors that alter forest dynamics by affecting tree growth and physiology. However, the impacts of increased nitrogen availability at pre-drought on trees remains poorly understood, and it remains unclear how these responses are coordinated. In this study, we conducted the fertilization-drought microcosm experiment using a widely distributed evergreen broadleaf tree species seedlings (Schima superba) in southern China. The experiment was conducted at 3 stages. First, four levels of N fertilization treatments (without N fertilization-NF, low N fertilization-LF, moderate N fertilization-MF, high N fertilization-HF) were applied for 60 days. Second, all seedlings were allowed to grow under four levels of N fertilization treatments for another 60 days to ensure that the N was absorbed by seedlings. Third, all seedling were subjected to three levels of sustained drought treatments for further 60 days. Traits related to growth and physiology were monitored.

Results: Our findings indicate that drought alone inhibited the growth and leaf photosynthetic rate of S. superba, while N fertilization alone stimulated the growth and leaf photosynthetic rate. Antecedent N fertilization alleviated the drought limitation on growth, due to the increased leaf photosynthetic rate (A_sat) and instantaneous water use efficiency. Moderate N fertilization mitigated the negative effects of drought on A_sat due to improved performance in stomatal conductance, leaf water potential and cell membrane permeability. Additionally, moderate N fertilization increased activities of antioxidant enzymes and osmoprotectants concentration under drought condition.

Conclusions: Overall, our findings suggest that increased N fertilization prior to drought can alleviate the negative effects of drought on growth and physiology, which is dependent on the magnitude of N fertilization and drought stress.