Optimized Nitrogen Application Rate Significantly Increases Total Economic Value and Quality of Flue-Cured Tobacco due to the Improvement of Superior Tobacco Yield
Published Online: Jun 09, 2024
Page range: 136 - 147
Received: Jul 10, 2023
Accepted: Jan 17, 2024
DOI: https://doi.org/10.2478/cttr-2024-0003
Keywords
© 2024 Yi Pu et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
The tobacco industry is the largest tax contributor in China, accounting for 8.7% of the Chinese total tax revenue in 2022 (1). The annual cultivation area of flue-cured tobacco in China is 1.01 million hectares with an annual tobacco yield of 2.13 million tons. Yunnan Province is the most important tobacco-producing area, well known for its high-quality tobacco. The cultivation area accounts for about 40% of the total production in China (2). With the increasing demand for land use for vegetable production in winter and spring, and the improvement of irrigation and transportation infrastructure, annual single tobacco mono-cropping has gradually been converted into vegetable–tobacco double cultivation in Yunnan Province. The growth period of vegetables is from October to February, while tobacco cultivation is from April to August. Excessive use of fertilizer and water for vegetable production has become a common phenomenon, aiming to achieve high yields of vegetables. As a result, the soil residual nitrogen (N) amount increases before the tobacco transplanting in the late spring. Thus the recommended optimal N rate might not be suitable for the production of tobacco of high quality (3). Based on the amount of soil residual N before tobacco transplanting, optimizing N management and understanding the mechanism of the effect of the optimal N application rate on plant growth are an important issue, not only for the economic benefit and quality of tobacco, but also for a better protection of the environment.
Yunnan Province has unique climatic conditions, and it is of high economic benefits to cultivate open field vegetables in winter and spring after tobacco-harvesting. However, in order to achieve high yields as well as an economic income and due to N leaching induced by flooding irrigation, the average N application rate is as high as 450 kg ha−1 season−1 (4), which exeeds plant uptake by 3–4 times (5). For this reason the amount of residual N in soil increases year by year (6). This practice not only wastes fertilizer resources, but also increases the risk of water and air pollution (7–8). In addition, the soil quality, such as the deterioration of soil structure and cultivability (9), decreases the soil pH and beneficial microorganisms (10, 11, 12). Furthermore, it leads to a decrease in plant resistance and imbalance of soil nutrients (8, 13) and especially to an increase of soil residual N, which affects the production of high-quality tobacco (14).
Flue-cured tobacco is very sensitive to nitrogen (15–16). Insufficient N supply leads to slow growth rates, leaf yellowing and premature aging, poor tobacco quality and low yield while oversupply of N results in late ripening, abnormal yellowing of tobacco leaves, and substantial reduction of tobacco quality and economic value (17–18). Soil N is the main source of N absorbed by flue-cured tobacco, accounting for over 50% of total N uptake (19). Excessive soil residual N will inevitably result in the optimal N rate recommended by the demonstration experiments of tobacco industry over the years, which might not meet the requirements for the producing high-quality tobacco. The results of a large soil sample survey showed that the residual inorganic N amount has increased by 93% in the tobacco–vegetable double cultivation system in the past 10 years (3), which is the highest classification standard of soil nutrients in tobacco field (20). In order to meet the tobacco N requirement at different growth stages, it is necessary to re-optimize N application rate and to establish the nitrogen-effect-equation based on the current soil residual N amount and plant demand at different growth stages.
The tobacco growth stages are roughly divided into four periods, namely resetting, prosperous, mature and harvesting stage (19). Among them, in the prosperous stage, the tobacco middle and upper leaves grow rapidly, which is the key period to determine the economic value and quality of flue-cured tobacco (19). Compared with lower tobacco leaves, middle and upper leaves have a higher economic value with better chemical and processing qualities (18), such as higher contents of nicotine and total nitrogen as well as a lower leaf midrib proportion. Ensuring timely and appropriate N supply in the prosperous growth stage plays a decisive role in obtaining high quality and good yields of flue-cured tobacco. The evaluation indexes of high-quality flue-cured tobacco include commodity quality, intrinsic chemical and processing quality (21). Commodity quality determines the sales price of flue-cured tobacco, ranging from 5 to 50 RMB kg−1 (0.8 to 6.6 U.S. Dollars kg−1), which is the most important concern for farmers. The intrinsic chemical quality determines the flavor of flue-cured tobacco, which mainly includes six indexes and is important to flue-cured tobacco purchasing enterprises. Leaf midrib proportion as an important parameter of processing quality, is a major concern for flue-cured tobacco processing enterprises (22). Excepting the varieties and ecological conditions of cultivation areas, the above three categories of indicators are mainly affected by fertilization management (23), especially by the amount and time of N application.
A field experiment was conducted from 2021 to 2022 in Yuxi City, which is a major area producing high-quality flue-cured tobacco in Yunnan Province. Parameters as yield and economic value, intrinsic chemical and processing quality, daily leaf growth rate, agronomic characters and soil N amount were determined. Objective of this study was to optimize N management and understand the mechanism of the effect of the optimal N application rate on plant growth for the better economic benefit and high quality of flue-cured tobacco.
Our hypotheses were:
Yield decrease of superior leaf tobacco results in the reduction of total economic value and quality, caused by excessive or deficient N application rate. The optimal N application rate including soil residual N might be significantly lower than the currently recommended rate. The nitrogen effect equation including soil residual N can be used to guide the cultivation of high-quality flue-cured tobacco.
The experimental site is located in Guojia Village (N24°38′25″, E102°52′26″), Yuxi City, Yunnan Province, with an altitude of 1730 m. The average precipitation, temperature and sunshine duration during the growth period is 126 mm, 20.5 °C, and 165 h (see supplementary tables at the end of publication, Table S1).
Prior to the experiment beginning in 2021, soil tests of the 0–20 cm topsoil showed:
bulk density: 1.35 g cm−3, pH: 6.28, organic matter: 35.0 g kg−1, alkali-hydrolyzed N: 188 mg N kg−1, available phosphorus: 67.4 mg P kg−1, and available potassium: 375 mg K kg−1.
Before our experiment began, the previous vegetable crop was fresh faba beans with an N application of 180 kg N ha−1. The field experiments were conducted from April 22nd to August 30th, 2021, and from April 25th to August 30th, 2022. Six N application rates (0, 45, 60, 75, 90 and 105 kg N ha−1) were set in a completely randomized block with three replications and a total of 18 plots. Each plot was 9.6 m long and 7.8 m wide.
On April 5th, 2021 and 2022, the raised beds were set up after deep ploughing of 20 cm and rotating ploughing twice, with a height of 30 cm and a width of 60 cm. On April 23rd, base fertilizer was applied in small holes with a diameter of 10 cm and a depth of 5 cm, where the seedling would be transplanted. On the next day, 45 days old seedlings of local popular tobacco variety “Cinnabar Yan No. 2” were transplanted with plant space of 0.6 m and row space of 1.2 m. Immediately after transplanting the seedling, 2 L water were irrigated for each planting hole and insecticide beta-cyfluthrin (Bayer, Leverkusen, Germany) for controlling tobacco aphids was sprayed. Hereafter black plastic film (15 µm in thickness) was covered immediately to avoid evaporation and weeds as well as to increase soil temperature. On May 21st, the plastic film was removed and soil was added around the seedlings. On June 1st, the topdressing fertilizer was applied. On June 25th, the tobacco was topped. 20 to 30 mm water was given every 5–7 days by drip irrigation system according to the precipitation and soil moisture. On the 10th, 25th and 40th day after transplanting, dimethomorph (BASF, Ludwigshafen, Germany) and fluopicolide and propamocab hydrochloride (Bayer, Munich, Germany) were sprayed to control black root rot and black shank, respectively.
According to N application rates, special compound fertilizer “Hongye Brand” for flue-cured tobacco was used with a ratio 1:0.5:2 of N:P2O5:K2O (Honghe Henglin Chemical Co., LTD, Yunnan, China). For control treatment (0 kg N ha−1), only phosphorus and potassium fertilizer were applied. The total application amount of phosphorus (45 kg P2O5 ha−1) and potassium fertilizer (270 kg K2O ha−1) were the same for all treatments. The insufficient phosphate and potassium fertilizer in each treatment were supplemented with calcium superphosphate (16% P2O5) and potassium sulfate (51% K2O). The ratio of base to topdressing fertilizer was 1:1. The base fertilizer was applied in the planting hole (25 cm in diameter, 15 cm in depth) and mixed with approximately 0.5 kg soil. Topdressing fertilizer was dissolved in water and then poured into the planting hole.
Plant samples were collected at 35th, 70th and 105th days after transplanting. Three tobacco plants with even growth were selected from the same position in each plot each time and numbered with the label. The tobacco plants were cut close to the soil surface, put into nylon mesh bags and brought back to the laboratory, put in the oven for 30 min at 105 °C and dried to constant weight at 75 °C. The dry weight of the leaves was measured.
Soil samples of 0–20 cm was collected by a soil auger with a diameter of 38 mm in the middle of the two plants. Airdried soil was sieved through a 2-mm diameter sieve. 10 g soil was extracted with 100 mL 0.1 mol L−1 CaCl2 solution. The NO3−-N and NH4+-N concentrations were determined by an auto-analyzer (AA3, Hamburg, Germany).
During the harvest period July 25th to August 25th, the matured tobacco leaves were collected on four separate dates within a 10 days interval. All the leaves of each plot were tied and flue-cured according to the national standard (GB) for flue-cured tobacco (GB 2630-1992) and for curing tobacco (GB/T 23219-2008) independently. After flue-curing, the tobacco leaves were graded according to GB 2635-92 (Table S2). The dry weight of flue-cured tobacco of each grade was recorded respectively for the subsequent calculation of the tobacco economic value and yield. The total economic value of flue-cured tobacco was calculated as Σ yield of different grades per hectare × responding price (Table S4). The nitrogen economic benefit (NEB) of flue-cured tobacco = (the economic value of each N rate – the economic value of 0 kg N ha−1) / the N rate of each treatment.
Twenty-five tobacco leaves of X2F, C3F and B2F grades (Table S2) were selected from the harvested and flue-cured tobacco on July 25th, August 15th and August 25th, respectively, for the subsequent determination of leaf midrib proportion and chemical quality. From each grade of the tobacco samples in each plot, 10 flue-cured leaves were randomly selected, using the near infrared spectrum analyzer (IAS-8120, Xunjie Guangyuan Technology Co., LTD., Wuxi, China). It measured the water content of the 10 pieces of tobacco leaves paralleled to the middle line of the main vein 1/4, 2/3, 3/4, respectively, taking the average value recorded as water content. Then, the 10 pieces of tobacco leaves were manually separated into laminas and midribs (side veins and main veins with a diameter < 1.5 mm at leaf tip are regarded as laminas), and the laminas and midribs were separately weighed for calculating the leaf midrib proportion. In addition, 5 flue-cured tobacco leaves were randomly selected from each plot and grade. After removing the leaf midrib, they were cut into 0.8 ± 0.1 mm slices, which were baked to constant weight at 60 °C. Then a pulverizer (Cyclone Mill-Twister type, Retsch, Germany) was used and mixed through a 0.25 mm mesh screen. The quality parameters (total sugar, reducing sugar, total nitrogen, nicotine, potassium oxide, chlorine) were determined by Fourier Transform near infrared spectrometer (Antaris II, Thermo Fisher Scientific, Waltham, MA, USA). Integrated grade of tobacco intrinsic chemical quality = scores of various quality parameters × weights of various quality parameters, specific scoring standards and calculation formulas are shown in Table S4.
With the increase of N rate, the economic value of flue-cured tobacco and N economic benefit first increased and then decreased (Figure 1). At the N rate of 75 kg N ha−1, the economic value was 122 × 103 RMB ha−1 with N economic benefit (NEB) of 1836 RMB kg−1 N which was significantly higher than those of other N rates. Compared with the currently recommended N rate of 105 kg N ha−1, economic value and NEB were significantly increased by 37.0% and 32.8% at the N rate of 75 kg N ha−1, respectively.
Figure 1.
With the increasing N rate, the integrated grade of chemical quality showed a parabolic trend, increasing up to its highest level at the N rate of 75 kg N ha−1 and then decreasing (Figure 2). With the increase of N rate, the contents of nicotine, total nitrogen and potassium K increased first and then decreased (Table S3), but the total sugar and reducing sugar showed a trend of decreasing first and then increasing, while the chlorine content had no significant change. Except for K, the other five parameters of chemical quality mentioned above were in the suitable range at the N rate of 75 kg N ha−1.
The leaf midrib proportion significantly decreased with the increase of N rates (Figure 3). Compared with the currently recommend rate of 105 kg N ha−1, the leaf midrib proportion increased by 3.6–4.4% at 75 kg N ha−1 for different grades, but it was lower than that at low N rates. Regardless of N rates, the leaf midrib proportion of B2F tobacco leaves was significantly lower than that of C3F and X2F tobacco leaves (Figure 3).
Figure 2.
Similar to the economic value, the total yield and yield of superior flue-cured tobacco first showed a trend of increase and then decreased with a higher N rate (Figure 4). The total yield and yield of superior flue-cured tobacco peaked to 3690 kg ha−1 at the N rate of 75 kg N ha−1 (Figure 4), which significantly increased by 27.3% and 37.0% compared with the currently recommended N rate of 105 kg N ha−1, respectively.
Figure 3.
Figure 4.
However, the yield of medium and inferior flue-cured tobacco was not significantly affected by the N rate. There was a significant positive correlation between the total economic value and the yield of superior flue-cured tobacco (Figure 5; R2 = 0.91, p < 0.05), while there was no significant correlation with the yield of medium and inferior flue-cured tobacco.
The leaf growth rate at the prosperous growth stage (resetting - topping) was significantly higher than that at the resetting (transplanting - resetting) and maturing stage (topping - harvesting) (Figure 6). The maximum leaf growth rate appeared at the N rate of 75 kg N ha−1 regardless of growth stages. Compared with the current recommendation of 105 kg N ha−1, the leaf growth rate at 75 kg N ha−1 was significantly increased by 17.8%. There was a significant positive correlation between the yield of superior tobacco and the leaf growth rate at the prosperous growth stage (Figure 7; R2 = 0.66, p < 0.05).
Excluding two data points outside the 95% confidence zone, the sum of applied N and the residual soil inorganic N before transplanting showed a significant positive correlation with the total economic value of flue-cured tobacco and the yield of superior tobacco, respectively (Figure 8). Based on the total economic value and the yield of superior tobacco, the sum of N rate and residual soil N determined by the nitrogen effect equation was 130 kg N ha−1, in which the optimal N rate was 66 kg N ha−1 and soil inorganic nitrogen content before transplantation was 64 kg N ha−1 (Figure 8).
Compared with the N rate of 105 kg N ha−1 recommended by the tobacco industry after years of field experiments, the optimal N rate of 75 kg N ha−1 determined in this paper significantly improved the leaf growth rate during the prosperous growth stage, and increased the total economic value of flue-cured tobacco, the yield of superior tobacco and the integrate grade of intrinsic chemical quality as well as the processing quality. The equation of N effect for the recommendation of optimal N application rate should include the amount of residual soil N before transplanting.
The economic value and quality of flue-cured tobacco are very sensitive to nitrogen supply. Suitable N application rates significantly increase the yields of superior flue-cured tobacco (Figure 4) (14). At the N rate of 75 kg N ha−1, the economic value, the N economic benefit and the yield of superior flue-cured tobacco are significantly higher than those of all other N rates including the currently recommended 105 kg N ha−1 (Figure 1 and Figure 4). This is closely related to the effect of N supply at different growth stages on the commodity grade of flue-cured tobacco (24) which determines the sale price. The average sale price of superior tobacco is about 8 times higher than that of inferior tobacco (Table S2). This verified well by the significant positive correlation between the yield of superior tobacco and the total economic value of flue-cured tobacco (Figure 5). Besides the commodity quality of flue-cured tobacco, the chemical and processing qualities are the most affeceted indexes of tobacco purchasing and processing enterprises. The integrate grade of chemical quality is an important index to evaluate the coordination of chemical components of flue-cured tobacco (23). The integrate grade of chemical quality of C3F tobacco leaf, the most representative of superior flue-cured tobacco, was significantly higher at the N rate of 75 kg N ha−1 than that of all other N rates including the currently recommended N rate of 105 kg N ha−1 (Figure 2; Table S3). Leaf midrib proportion is an important index to evaluate the processing quality of flue-cured tobacco. If the leaf midrib proportion is too high, it will reduce the percentage of leaf strips yield. If it is too low, it will reduce the tensile strength and machinability (25). The leaf midrib proportion of flue-cured tobacco decreased significantly with increase of the N rate (Figure 3). The leaf midrib proportion of 75 kg N ha−1 was between high and low N rates. Therefore, optimizing the N management is the key to improve the yield and quality of superior flue-cured tobacco (Figure 4 and 5).
Figure 5.
Figure 6.
Figure 7.
Superior tobacco mainly includes five grades, including upper (B1-2F) and middle (C1-3F) leaves mainly formed during the prosperous stage (resetting - topping) (Table S2).
Figure 8.
The prosperous stage is the key period for the formation of flue-cured tobacco yield and quality (19). The leaf growth rate of flue-cured tobacco in the prosperous stage is significantly higher than that in both stages of resetting and maturing (Figure 6), and the peak of N uptake also occurs in this stage (26). The leaf growth rate of prosperous growth stage was significantly higher at 75 kg N ha−1 than that of other N rates (Figure 6), and showed a significant positive correlation with the yield of superior tobacco (Figure 7). Therefore, we can conclude that the leaf growth rate of the prosperous growth stage is the key to determine the yield of superior tobacco and the total economic value (Figure 5). Optimizing the N supply rate with attention of soil residual N can not only promote leaf growth and improve the yield and quality of superior tobacco (Figure 4), but also help to avoid late ripening and the abnormal yellowing of tobacco leaves, which leads to a reduction of tobacco quality and sale prices (27) (Figure 2; Table S2).
The high quality and yield of flue-cured tobacco are the results of the combined action of N from soil and fertilizer, and at least 50% of the N absorbed by tobacco plants comes from the soil (28). Therefore, to recommend reasonable N application rates based on the residual soil N before transplanting, it should be the basic principle to optimize N management of flue-cured tobacco. This was well verified by the significantly positive correlation between the sum of soil N and N rate and the economic value and yield of superior flue-cured tobacco respectively (Figure 8). When the sum of soil N and N rate was 130 kg N ha−1, the economic value and the yield of superior flue-cured tobacco reached the maximum (Figure 8). However, the currently reported nitrogen effect equation often ignored the effect of soil N (29). In particular, with the improvement of irrigation and transportation infrastructure, single tobacco cultivation has been converted into tobacco and vegetable double cropping.
The excessive N input in vegetable production results in a large amount of soil residual N before tobacco transplanting, which will not only affect the quality and economic value of flue-cured tobacco, but also leads to a high nicotine content and poor smoking qualities (30). Too high or too low N application rates will affect the economic value and quality of flue-cured tobacco. Except the N rate, the percentages of base- and topdressing fertilizer also have a crucial impact (23). The prosperous stage has the highest leaf growth rate (Figure 6) and is the key growth stage for the formation of superior C3F and B2F tobacco leaves, which have reasonable chemical (Figure 2, Table S3) and processing qualities (Figure 3). Therefore, ensuring timely and sufficient N supply in the prosperous stage is the key to improving the chemical quality and sensory quality of flue-cured tobacco (31–32). If residual soil N was high before transplanting (Table S5), one should not only reduce the total N application rates, but also appropriately reduce base fertilizer rates, while increasing topdressing fertilizer rates to improve leaf growth rate in the prosperous stage (Table S6). On the contrary, the total N rate and proportion of base fertilizer should be appropriately increased when the residual soil N is low. In conclusion, optimizing the total N rate and the proportion in different growth stages according to soil residual N content before transplanting and N effect equation is the key to improve the total economic value and quality of flue-cured tobacco.
Conversion of single-tobacco-cultivation into tobacco– vegetable double production leads to high residual soil N before tobacco transplanting due to the excessive N fertilizer application rate in vegetable production. The currently recommended N rates of 105 kg N ha−1 without consideration of soil residual N is no more suitable for the production of flue-cured tobacco with high quality and economic value. According to the current soil residual N, our results demonstrate that the optimal N rate should be reduced to 66 kg N ha−1, which significantly increases the economic value, yield and quality of flue-cured tobacco due to the improvement of superior tobacco yield and the daily leaf growth rate especially in the prosperous stage. Therefore, it is suggested that consultants and tobacco farmers should measure their soil residual N content before transplanting in order to determine the total N application amount and to adjust the N application ratio at different growth stages according to the nitrogen effect equation obtained in this paper.