Nitrogen addition method affects growth and nitrogen accumulation in seedlings of four subtropical tree species: Schima superba Gardner & Champ., Pinus massoniana Lamb., Acacia mangium Willd., and Ormosia pinnata Lour

N addition (56, 156, and 206 kg N ha⁻¹ yr^−1as dissolved NH₄NO₃) method (canopy vs soil) did not affect the biomass of N₂-fixers (Acacia mangium Willd. and Ormosia pinnata Lour.), but significantly affected the biomass of non-N₂-fixers (Schima superba Gardner & Champ., Pinus massoniana Lamb.). Coniferous species exposed to N addition on the canopy rather than the soil had higher N accumulation.

Context Previous experiments simulating nitrogen (N) addition in forests were conducted by adding N fertilizer directly to soils, which neglects the fact that N uptake can be done by canopy leaves.
Aims The objective of this study is to examine how different N addition methods (canopy vs soil) affect growth and N accumulation of four subtropical tree seedlings.
Methods An open-air greenhouse experiment was conducted to expose four tree species (Schima superba Gardner & Champ., Pinus massoniana Lamb., Acacia mangium Willd. and Ormosia pinnata Lour.) to different N addition methods (canopy or soil) and N levels (ambient, medium, or high).
Results N addition method affected the biomass of non-N₂-fixers (Schima superba Gardner & Champ. and Pinus massoniana Lamb.), while N₂-fixers (Acacia mangium Willd. and Ormosia pinnata Lour.) were unaffected. N concentrations in the soils and leaves of all trees were significantly increased by the medium and high N additions, and soil N concentrations resulted from N addition via soil rather than the canopy. Although leaf N concentration was significantly affected by N addition method in all trees except for Ormosia pinnata, only N accumulation in Pinus massoniana was significantly affected by N addition method.
Conclusion N addition method affected the biomass of non-N₂-fixers and N accumulation in coniferous species, while it did not affect the biomass of N₂-fixers and N accumulation in broadleaf species.

Keywords
Soil N addition, Canopy N addition, N accumulation, N₂-fixers, Subtropical trees

Publication
Wu, T., Lin, W., Li, Y. et al. Annals of Forest Science (2019) 76: 23. https://doi.org/10.1007/s13595-019-0806-2

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Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.