Effects of canopy N uptake on foliar CO2 assimilation rates and biomass production and allocation in Japanese red pine seedlings

To investigate the direct physiological effects of CNU (canopy nitrogen uptake), three mist solutions (control, N1, and N2 with 0.03, 13.1, and 32.7 kg NH4+-N·ha–1, respectively) were sprayed on Japanese red pine (Pinus densiflora Sieb. et Zucc.) seedlings three times a week for three months. Waterproof sheets protected the surface soil during misting to avoid adding N to the soil. The results show N mist treatments to foliage increased needle N availability in proportion to N dose, which was large enough to cause greater N and chlorophyll content in the needles. This suggests that N is rapidly absorbed, is directly assimilated by the needles, and is used in photosynthesis. These increases resulted in higher maximum net CO2 assimilation rates (Amax) and maximum quantum yield of PSII photochemistry (Fv/Fm) of pine seedlings and subsequently increased bud and root biomass. Increased root biomass reduced the sensitivity of the shoot-to-root ratio to increased N availability in the foliage. In conclusion, our study supported the idea that CNU should be taken into consideration when evaluating the impacts of elevated atmospheric N deposition on forest C sequestration and biomass allocation.