Effects of canopy N uptake on foliar CO ₂ 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 N ₂ with 0.03, 13.1, and 32.7 kg NH ₄ ⁺-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 CO ₂ assimilation rates (A _max) and maximum quantum yield of PSII photochemistry (F _v/F _m) 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.