Effects of Biocontrol Agents Application on Soil Bacterial Community and the Quality of Tobacco.

In this study, to evaluate the effect of different biocontrol agents (BCAs) on the soil bacterial community, we investigated the effects of Bacillus amyloliquefaciens, synthetic bacterial community (Aspergillus niger:Bacillus subtilis:Bacillus licheniformis:Streptomyces microflavus = 3:3:3:1, SynCom), and BCAs combined with lime-nitrogen on soil bacterial community by utilizing 16S rRNA sequencing technology. The sequencing shows that BCAs application can improve the value of Shannon and Sobs index of bacterial community during tobacco rosette and vigorous growing period. With the growth of tobacco, the effect of BCAs on the composition and difference of soil bacterial community structure becomes more and more obvious. In terms of average relative richness, the top six phyla of soil bacterial community are Proteobacteria, Actinobacteria, Chloroflexi, Acidobacteria, Gemmatimonadetes, and Bacteroidetes. Bacillus amyloliquefaciens application can increase the relative richness of Proteobacteria and Bacteroidetes. And the combination between BCAs and lime-nitrogen can increase the relative richness of Gemmatimonadetes and Bacteroidetes. The SynCom also can increase the relative richness of Bacteroidetes, whereas it decreases the relative richness of Acidobacteria. Proteobacteria, Acidobacteria, Gemmatimonadetes, and Bacteroidetes showing an extremely significant correlation with pH and exchangeable magnesium (EMg). BCAs application can improve the tobacco yield, effective leaves, and reducing sugar content that also has extremely significant positive correlation with pH and EMg. In conclusion, the results of our field experiments clearly show that BCAs application can significantly affect the soil pH and EMg by changing most of the dominant soil bacterial species. The richness of Bacteroidetes can serve as an indicator of the changes in soil pH and EMg caused by BCAs application.

Differences in Soil Microbial Community Composition Between Suppressive and Root Rot-Conducive in Tobacco Fields.

Soil microorganism has a profound influence on planting growth and disease suppression. However, the difference in microbial community structure between suppressive and root rot-conducive soil and the mechanism of controlling soil-borne diseases by microorganisms in suppressive soil were not clear. To provide a theoretical foundation for prevention and control of root rot, this paper investigated the change of community structure in rhizosphere soil between suppressive and root rot-conducive tobacco fields. Soil samples were collected during before transplanting, vigorous growing period, and mature period of the tobacco, and bacteria and fungi were analyzed using 16S rRNA and 18S rRNA gene sequencing, respectively. Results showed that bacteria were more sensitive to the change between suppressive and root rot-conducive soil, and fungi were more sensitive to the change of different tobacco growth periods. Compared with conducive soil, tobacco suppressive soil can resist the invasion of pathogens, especially fungi, by regulating soil microbial community structure, and the potential pathogen Boeremia was always lower. Fusarium, the root rot pathogen, decreased rapidly in the mature period in suppressive soil. Moreover, norank_o_Gaiellales and unclassified_f_Trichocomaceae had a critical role in suppressive soil in the process of inhibiting root rot, which was obvious in the mature stage. Overall, the results indicated that the composition and structure of the microbial community significantly altered between suppressive and conducive soil along with the growth of tobacco, and suppressive soil could inhibit the occurrence of soil-borne diseases by boosting beneficial bacteria and inhibiting the potential pathogens.