Optimized Nitrogen Application Rate Significantly Increases Total Economic Value and Quality of Flue-Cured Tobacco due to the Improvement of Superior Tobacco Yield

With the improvement of irrigation and transportation infrastructures, single tobacco cultivation has been converted into tobacco–vegetable double cultivation in Yunnan Province. High residual nitrogen (N) levels in soil before tobacco transplanting induced by the excessive N input during the vegetable cultivation season resulted in a reduction of economic income and the quality of flue-cured tobacco. Therefore, the objective of this paper is to describe the optimization of N management and to provide a better understanding of the mechanism of optimal N application rate on the economic benefit and quality of tobacco. A field experiment with six N application rates (0, 45, 60, 75, 90, and 105 kg N ha⁻¹) was carried out with a randomized block design in 2021 and 2022 in Yunnan Province. The economic value and yield, intrinsic chemical and processing quality, leaf growth rate and agronomic characters were determined.

Compared with the currently recommended 105 kg N ha⁻¹ rate, 75 kg N ha⁻¹ significantly increased the total economic value and superior tobacco yields, improved the integrated grade of chemical compounds and resulted in leaf midrib proportions in an appropriate range. The total economic value positively correlated with the superior tobacco leaf yields (R² = 0.91, p < 0.001), while not with medium and inferior leaf yield. The daily leaf growth rate in prosperous growth stage significantly correlated with the yield of superior tobacco and reached its maximum at a N rate of 75 kg N ha⁻¹. The sum of N application rate and soil residual N before transplanting correlated with the total economic value (R² = 0.66, p < 0.05) and superior leaf yield (R² = 0.64, p < 0.05), respectively. Based on the amount of soil residual N before transplanting, the optimal N application rate was 66 kg N ha⁻¹ which was 39 kg N ha⁻¹ lower than the currently recommended N rate (105 kg N ha⁻¹). Our results highlighted that the technical consultants and farmers should adjust the N application rate appropriately according to the residual N amount before transplanting and optimize the water and fertilizer management especially in the prosperous growth stage. An optimized N rate is not only of economic benefit and the improvement of quality of tobacco cultivation, but also environment friendly.