Determining nitrogen status using a universal critical nitrogen dilution curve for predicting flue-cured tobacco yield and quality

The critical nitrogen (N_c) dilution curve is considered an effective method for crop nitrogen (N) diagnosis, fertilization decision-making, and crop modeling. The N_c dilution curve is typically described using a power function curve (N_c = A₁W^−A2). This study established a universal N_c dilution curve and evaluated its potential application in flue-cured tobacco (Nicotiana tabacum L.) N diagnosis, yield, and quality regulation. Field experiments were conducted involving three tobacco varieties (Honghuadajinyuan, Yunyan87, and K326) and four N application rates (0 to > 90 kg N ha⁻¹). The study was conducted in Southwest China from 2021 to 2022. The universal N_c curve obtained across all experimental data was 3.45 W^−0.39. The parameter A₂ was more susceptible to genotype and environment interactions than parameter A₁. The uncertainty first rapidly decreased and then remained at a lower value as biomass increased. The relationship between leaf biomass and days after transplantation and accumulated growth degree was robust. The N nutrition index (NNI) and accumulated N deficiency effectively determined leaf N status and N demand, respectively. There was an inverse correlation between the NNI, leaf yield, and between the superior and medium leaf ratios. The quantitative relationship can be used to adjust the synergistic production of tobacco yield and leaf quality.