Regulating the nitrogen supply of tobacco planting soil through adding exogenous organic carbon to reduce the nicotine content in upper tobacco leaves

Objectives The quality of flue-cured tobacco is still affected by its high nicotine content. Adding available carbon sources to soil can stimulate microbial assimilation and thus regulate the available nitrogen content in soil. Therefore, this study tried to use exogenous carbon to reduce the content of total nitrogen and nicotine in upper leaves of flue-cured tobacco.

Methods Variety NC55 was used as the test material, a pot experiment was carried out, exogenous carbon was added to soil on the 90^th day and 120^th day after the transplantation of flue-cured tobacco. According to the type, rate and time of the exogenous carbon, in total 9 treatments were set up. In G2, C 2000 mg/kg glucose was added on 90^th day after transplanting (carbon content, the same below); in G5, C 5000 mg/kg glucose was added on 90^th day after transplanting; in G2+G10, C 2000 mg/kg and 10000 mg/kg glucose were added on 90^th day and 120^th day after transplanting, respectively; in G5+G10, C 5000 mg/kg and 10000 mg/kg glucose were added on 90^th day and 120^th day after transplanting, respectively; in S2, C 2000 mg/kg sawdust was added on 90^th day after transplanting (carbon content, the same below); in S5, C 5000 mg/kg sawdust was added on 90^th day after transplanting; in S2+G10, C 2000 mg/kg sawdust and 10000 mg/kg glucose were added on 90^th day and 120^th day after transplanting, respectively; in S5+G10, C 5000 mg/kg sawdust and 10000 mg/kg glucose were added on 90^th day and 120^th day after transplanting, respectively; and control (CK) was conventional fertilization treatment without exogenous carbon adding after transplanting. Soil nitrogen contents, total nitrogen and nicotine contents the upper leaves of flue-cured tobacco were measured and compared among different treatments.

Results Applying exogenous carbon significantly reduced nitrogen supply of tobacco-planting soil. The content of mineral nitrogen content in soil decreased with the increase of the rate of exogenous carbon, and the effect was more obvious for applying twice than applying once, the jointly applying of glucose and sawdust was better than singly applying glucose. Applying exogenous carbon twice reduced nicotine and total nitrogen contents in upper leaves of flue-cured tobacco by 0.62–1.40 and 0.71–1.22 percentage points, respectively, which was more significant than applying once.

Conclusions Adding a certain amount of glucose and sawdust respectively in the late-growth stage of flue-cured tobacco has been proved to be efficient in reducing nicotine and total nitrogen contents in the upper leaves of flue-cured tobacco. The reason is the increased C/N ratio limits the nitrogen uptake by tobacco.