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

Abstract

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. 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. 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. 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). 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. 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.