Effects of different nitrogen forms and concentrations on seedling growth traits and physiological characteristics of Populus simonii × P. nigra

Numerous growth and physiological variables of 3-week-old Populus simonii × P. nigra seedlings were assessed after treatment with either nitrate nitrogen (NO 3 − –N) (0.1, 0.5, 1, 5, or 10 mmol·L −1 ) or ammonium nitrogen (NH 4 + –N) (0.1, 0.5, 1, 5, or 10 mmol·L −1 ) to determine the best nitrogen form and concentration to optimize growth, biomass allocation, pigment content, and photosynthetic capacity. The results of combining membership function and an evaluation index suggested that, 5 mmol·L −1 nitrogen, regardless of the form, yielded the highest comprehensive evaluation index and good growth. In addition, a Pearson correlation analysis and network visualization revealed that the total mass, shoot mass, root mass, leaf dry mass, plant height, leaf area, chlorophyll a and total chlorophyll had a physiological index connectivity degree ≥ 15 for both nitrogen forms. Net photosynthetic rate, stomatal conductance, transpiration rate, maximum photochemical efficiency of PSII, total nitrogen content, ground diameter, chlorophyll b , and carotenoid were unique indices for evaluating NH 4 + –N-based nutrition, which could provide a theoretical basis for evaluating the effects of nitrogen fertilizer on seedlings, cultivation periods, and stress tolerance in P. simonii × P. nigra .