Elucidating the impact of different carbon/nitrogen ratio on soil biochemical properties and rhizosphere bacterial communities of flue-cured tobacco plants

Biochar and nitrogen fertilizer are frequently used together to enhance the soil fertility and crop productivity. This study elucidated, how combined application of biochar and nitrogen fertilizer influence the soil biochemical properties, rhizosphere bacterial communities, and biomass accumulation of tobacco plants through pot and field experiments. The experiments were performed using biochar (0, 600, and 1200 kg/ha) and nitrogen fertilizer (105 and 126 kg/ha) under different carbon/nitrogen (C/N) ratios as follows: B0N105, B600N105, B1200N105, B0N126, B600N126, and B1200N126. The results confirmed that the soil organic carbon pool increased with the increase of biochar dosage under different treatments. The soil C/N ratio was found significantly higher under a high dose of biochar (1200 kg/ha) and low nitrogen rate (105 kg/ha), while decreased under high dose of biochar (1200 kg/ha) and high nitrogen rate (126 kg/ha). Soil nitrate nitrogen and total nitrogen contents, soil enzymatic activities, and biomass accumulation of tobacco plants significantly increased under high dosage of biochar and nitrogen (B1200N126). Analysis of rhizosphere bacteriome through amplicon sequencing of 16S rRNA revealed that the structure, diversity, and composition of rhizosphere bacterial communities dramatically changed under different dosages of C/N ratios. Proteobacteria, Bacteroidetes, Actinobacteria, Firmicutes, and Acidobacteria were highly abundant bacterial phyla in the rhizosphere of tobacco plants under different treatments. Results of co-occurrence network analysis displayed a fewer negative correlation among rhizosphere bacterial communities under high dosage of biochar and nitrogen (B1200N126) than other treatments, which showed a less competition for resources among microbes. In addition, a redundancy analysis further proved a significant positive correlation among soil microbial biomass of carbon and nitrogen, soil biochemical properties, and high dosage of biochar and nitrogen (B1200N126). Thus, we conclude that high dosage of biochar (1200 kg/ha) under high application rate of nitrogen fertilizer (126 kg/ha) enhanced the biomass accumulation of tobacco plants by improving the soil biochemical properties and activities of rhizosphere bacterial communities.