Objectives Effects of planting and utilizing green manure on soil nutrients, yield, and quality of tobacco in the erosion area of Fuxian Lake in Yunnan, were studied to clarify the resources of suitable local green manures, in order to provide reference for optimizing the local tobacco planting system.
Methods The field experiment was conducted from 2019 to 2021, with four treatments: winter fallow, rape (Brassica campestris), common vetch (Vicia sativa L.), and smooth vetch (Vicia villosa Roth var. glabrescens). We measured the following indicators: soil properties after returning green manure to the field and at harvesting of flue-cured tobacco; the agronomic traits and yield of tobacco, and the proportion of top grade tobacco; the chemical composition, appearance and sensory quality, and the output value of tobacco leaves.
Results Compared to winter fallow in 2020, the soil organic matter was not significantly changed, the alkali hydrolyzed nitrogen (AN), available phosphorus (AP) and available potassium (AK) slightly decreased before tobacco planting. After tobacco harvest, the average soil organic matter AN, AP and AK increased by 3.2 g/kg, 6.8 mg/kg, 10.1 mg/kg, and 84.8 mg/kg, respectively, with the highest increment in treatment of smooth vetch. Before planting and after harvest of tobacco in 2021, all the soil organic matter and available nutrient contents under green manure treatments were significantly higher than those under winter fallow. Among the green manure treatments, soil organic matter was the highest in rape treatment, and the highest available nutrients were in smooth vetch treatment. The soil Cl content under winter fallow was significantly higher than under green manure treatment. In 2020 and 2021, green manures increased the average tobacco yield by 13.8% and 22.6%, respectively, and the proportion of top grade tobacco by 11.2% and 24.7%, andsmooth vetch was recorded the highest yield increase. Comparing to winter fallow in 2021, the plant height, stem girth, and maximum leaf width under green manure treatments were increased by 26.6 cm, 2.2 cm, and 6.9 cm, respectively; The appearance quality of the middle and upper leaves of tobacco was improved by 5.0−5.7 points and 4.5−5.8 points, and the sensory quality was increased by 2.0−3.2 points and 3.6−4.2 points, respectively; The average price of tobacco were significantly increased by 7.4%−11.9% in the second year, and the output value significantly increased by 30.5%−38.0%. Among the manure treatments, smooth vetch performed better than the others in all above indices. Retuning green manure reduced the difference in total sugar and reducing sugar content in tobacco leaves, and increased their K content. Random forest regression analysis showed that soil organic matter, Cl ion and AK content, as well as plant height, maximum leaf width and stem circumference of flue-cured tobacco were the key factors affecting the yield of flue-cured tobacco.
Conclusions Planting and returning green manure to tobacco field could improve soil nutrient availabilty, thereby improve the proportion of high-quality tobacco and the quality of tobacco, significantly improve economic benefits of tobacco production, and the promotion effect will be more obvious on the second year of green manure returning. Among the test green manures, smooth vetch (Vicia villosa Roth var. glabrescens) shows the most significant effect, so is recommended as green manure in the erosion area of Fuxian Lake in Yunnan Province.