Construction and verification of a critical nitrogen dilution curve for flue-cured tobacco under tobacco-rice rotation systems in southeast China

Objectives This study aims to construct a critical N dilution curve for flue-cured tobacco and use the same to rapidly diagnose and evaluate the N nutrition status of flue-cured tobacco.

Methods Three field experiments were conducted in two locations for two years. Six N application rates (0, 45, 90, 135, 180, and 300 kg/hm²) were setup in each experiment. The shoot and leaf dry matter accumulation recorded on different days after transplanting were analyzed. The critical N dilution curve equation and dry matter accumulation equation of shoot and leaves were established using N concentration and dry matter accumulation in plants at different growth stages. Combined with the non-destructive determination of N concentration based on the visible spectrum platform of UAV, the N nutrition index (NNI) was calculated and used to determine the N nutrition status of flue-cured tobacco plants.

Results N application (P < 0.05) increased shoot and leaf dry matter accumulation of flue-cured tobacco, with variation among the N treatments. The N concentration in shoot and leaves decreased across the growth stages of flue-cured tobacco. The power equation well described the relationship between the critical N concentration and dry matter accumulation in the shoot and leaves. Independent validation of the model showed that observations for the N deficient group were below the critical N dilution curve, while those of the sufficient N group were near the curve. The RMSEs of the simulated and actual critical N concentrations for the shoots and leaves were 0.55 and 0.44, and the corresponding n-RMSEs were 25% and 17%, showing the high stability of the model. The leaf NNI of flue-cured tobacco gradually increased with an increasing N application rate. The leaf NNI was higher than 1 when N application rate reached surplus level (135 kg/hm²).

Conclusions The constructed leaf critical N dilution curve for flue-cured tobacco under tobacco-rice rotation system is Nc = 3.2339 × DM_leaves^–0.475, which is accurate with low n-RMSEs. This is verified by leaf NNIs that coincide with the N application levels, suggesting the curve could be used to rapidly diagnose the N status of flue-cured tobacco in the study area.