Objectives Reducing soil nitrogen supply at the later growth stage of flue-cured tobacco is very important to improve the quality of its leaves. Microbial immobilization plays a vital role in regulating the available N content in the soil and is affected by the quality and quantity of organic carbon supply. Therefore, this study explores the effects of different organic carbon sources in reducing soil mineral N.

Methods We used the laboratory incubation method to test the yellow soil collected from the maize-tobacco rotation field in Guizhou Province. The five organic carbon sources selected were maize straw, wheat straw, reed straw, sawdust, and glucose. Each organic carbon source was added at four levels of pure carbon (C) 0, 2.0, 5.0, and 10.0 g/kg. Before incubation, NH₄NO₃ was added to each treatment at N 0.2 g/kg to simulate the mineral nitrogen in the soil at the late growth stage of flue-cured tobacco. Gas samples were collected to monitor CO₂ and N₂O emissions on the 1, 3, 5, 7, 9, 14, 21, and 30 days of incubation. Soil samples were collected five times to determine mineral N, organic carbon, and microbial biomass carbon contents.

Results Compared with the no carbon source treatment, the addition of carbon significantly improved the soil respiration rate. It increased the cumulative CO₂ emission by 64.8% to 729.3%, and the microbial biomass carbon content significantly was increased. There was a significant positive correlation between the microbial biomass carbon content and the amount of carbon added to the soil. Mineral N content was (P < 0.05) reduced by adding carbon sources. The reduction was 20.7%–55.9% in maize straw, 24.4%–99.8% in wheat straw, 21.4%–99.7% in reed straw, 31.4%–99.9% in sawdust, and 44.3%–84.8% in glucose at the end of the incubation period. This corresponded to reducing mineral N per gram of organic carbon by 11.4 mg in maize straw, 20.8 mg in wheat straw, 20.8 mg in reed straw, 20.5 mg in sawdust, and 16.1 mg in glucose. When carbon addition was less than or equal to 5.0 g/kg, the total reduction of mineral N was positively correlated with the C/N ratio of the organic carbon sources (P < 0.01). In addition, straw addition significantly reduced the N₂O emission by 78.7%–96.5%, while glucose addition increased N₂O emission by 153.6%–298.6%, compared to the control.

Conclusions The C/N ratio of organic carbon sources determines the potential reduction of soil mineral N content. Adding organic carbon to tobacco-planting soil could reduce the content of soil mineral N, and the decline increases with an increase in the rate of organic carbon addition. In addition, adding straw-based carbon sources could reduce N₂O emissions. However, glucose does not have the potential to reduce N₂O emissions.