Photosynthesis and growth responses of Fraxinus mandshurica Rupr. seedlings to a gradient of simulated nitrogen deposition

Abstract

Key message: During an N-deposition simulation experiment, we showed that low to medium addition of N had beneficial effects on growth and photosynthetic rates ofFraxinus mandshuricaRupr. seedlings, while beyond a threshold of 80 kg N ha−1 year−1, performance plateaued and even declined at higher immissions. Context: Temperate forests are shifting from naturally N-limited toward N-saturated status with increasing N deposition. Yet, our knowledge regarding how seedling growth and physiology respond to excessive N input in temperate tree species remains very limited. Aims: The objective of this study was to examine growth and photosynthetic responses of F. mandshurica seedlings to a gradient of simulated N deposition. Methods: We conducted a 4-year study to investigate growth and photosynthetic responses of F. mandshurica seedlings to a large gradient of simulated N deposition (0, 20, 40, 60, 80, 100, and 120 kg N ha−1 year−1). Biomass accumulation and allocation, photosynthetic gas exchange, expression, and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in leaves were determined during the fourth growing season. Soil biochemical properties were measured to link them to the alterations in growth and photosynthetic traits across the N addition gradient. Results: Seedling growth and photosynthesis were dependent upon the rates of N deposition. The maximum rate of carboxylation (Vc,max) and the net photosynthetic rate under saturating light (Asat) reached a maximum under 60 kg N ha−1 year−1. By contrast, high-level N inputs (100 and 120 kg N ha−1 year−1) resulted in suboptimal values in biomass and photosynthetic activity. Nitrogen deposition also modulated the activity and expression of Rubisco in leaves with a maximum around 80–100 kg N ha−1 year−1. Redundancy analysis (RDA) showed that the changes of seedling growth and photosynthesis along the gradient of N deposition were mostly attributed to the variations of soil pH and total N content. Conclusion: Our data suggest that the threshold of N deposition is about 80 kg N ha−1 year−1 for F. mandshurica seedlings in this region. Excessive N input decreased performance on the seedling growth and photosynthesis.