Growth, physiology, and nutrient retranslocation in nitrogen-15 fertilized Quercus rubra seedlings

We evaluated gas exchange, chlorophyll index (CI) using SPAD-502 chlorophyll meter, and leaf nutritional responses in one-year-old northern red oak (Quercus rubra L.) container seedlings transplanted into control (unfertilized) or fertilized (0.86 g N plant(-1)) sand culture and grown in a greenhouse for 90 days. We labeled current nitrogen (N) uptake with ((NH4)-N-15)(2)SO4 and directly quantified proportional contributions of N derived from fertilizer (NF) compared with retranslocation or N derived from plant (NP) in leaf growth of red oak seedlings. NF met a greater N demand in leaf growth of fertilized plants while unfertilized seedlings relied entirely on NP for their leaf growth. Fertilization increased leaf dry mass by 67% and new stem dry mass by 253% 90 days after transplanting compared to control seedlings. Specific leaf area (SLA) was not significantly altered but CI increased 90 days after transplanting. Higher leaf N concentration and content in fertilized compared with control seedlings was linked to greater chlorophyll concentrations in the former plants. The higher coefficient of determination (r(2) = 0.72) noted between leaf N concentrations and Cl suggests that the SPAD meter could be a useful tool for assessing leaf N status in northern red oak seedlings. Fertilized seedlings exhibited higher net assimilation, stoinatal conductance, and transpiration rates compared with controls. Increased seedling growth in response to fertilization was related to maintenance of higher gas exchange and greater nutrient uptake, which could improve outplanting success.