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

During an N-deposition simulation experiment, we showed that low to medium addition of N had beneficial effects on growth and photosynthetic rates of Fraxinus mandshurica Rupr. 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.

Fraxinus mandshurica, Photosynthetic response, Nitrogen deposition, Rubisco, Biomass production

Wang, M., Zhang, WW., Li, N. et al. Annals of Forest Science (2018) 75: 1.

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