This study demonstrated that tobacco bacterial wilt can be reduced by rotation of tobacco and radish. The disease index was the lowest after two years of tobacco–radish rotation, followed by radish rotation for three years. Furthermore, 16S amplicon sequencing revealed that the relative abundance of Ralstonia was higher in the CK than in the SY and TY.
It is well known that deteriorating soil physical properties lead to the decline in tobacco yields (Xiao et al. 2024).But rotation can improve the supply of soil nutrients, which may be related to changes in soil enzyme activity, thus alleviating the continuous cropping barriers(Liu et al. 2024). Our results revealed the changing trend of available nutrients in continuous cropping tobacco field with the different years of tobacco-radish rotation. The tobacco-radish rotation increased the content of pH, OP, AK in soil, the results of post-test showed that the difference of pH was significant (P < 0.05) .However, the content of AN and OM was significantly decreased, The tobacco-radish rotation system yields more crops per unit of land area, which to some extent increases the pressure on the soil nutrient supply, to some extent, this nutrient uptake pressure was magnified with the increase of rotation years. In the process of tobacco-radish rotation, it is necessary to pay attention to the supplement of nitrogen fertilizer and organic matter, this is conducive to the formation of a more stable soil nutrient status to ensure the growth of subsequent plant needs.
Soil microbial diversity plays a crucial role in supporting multiple ecosystem functions (Wu et al. 2019). Our results showed that crop rotation significantly increased bacterial community richness in different years. The PCoA analysis showed that the treated sample groups were significantly separated, with FY being closer to CK, followed by SY, and TY being further from the control sample point, showing a trend towards greater differences between the bacterial communities and the control with increasing crop rotation years. The composition of the bacterial community was the same as that of the CK, but the abundance of bacteria differed. The relative abundance of Firmicutes in the FY, SY and TY treatments was greater than that in the tobacco monocropping treatment. Studies have shown that the relative abundances of Firmicutes, Chlorobi and Zygomycota decrease in soils that are susceptible to bacterial wilt, while in soils with disease inhibition properties, the relative abundances of those bacterial phyla rised (Gong et al. 2024). The relative abundance of Ralstonia decreased after different years of tobacco–radish rotation, while the relative abundance of Sphingomonas increased in the FY, SY, and TY treatments increased the most, which was significantly greater than that in the CK. In this study, Sphingomonas tended to increase in soil with reduced disease (Zhang et al. 2017a). Moreover, the differences between the bacterial genera norank_f__Gemmatimonadaceae and raul's species were negatively correlated, which could be due to the inhibitory action of potential bacterial wilt. At present, large amounts of sphingosine have been proven to increase plant growth and improve plant resistance. The results showed that the tobacco- radish rotation was beneficial to the formation of microbial community, which decreased the relative abundance of pathogenic bacteria, and increased the relative abundance of beneficial bacteria, etc. At the same time, with the increase of tobacco-radish rotation years, this“Effect” was amplified.
The coexistence network can be used to study interactions between microorganisms, thus helping us to understand the response of microbial communities to external disturbances (de Vries et al. 2018; Wang et al. 2022; Wei et al. 2019b; Yi et al. 2023). The analysis of the coexistence network revealed that Ralstonia appeared in the CK network (purple module) and TY network (grey module) but not in the SY and TY networks. Moreover, compared with those in the control group, Streptomyces appeared in the first module of the FY network, Bacillus and Tumebacillus appeared in the first module of the SY network, while no bacterial genera were found in the first module of the CK network. The results showed that tobacco antagonism was more important in the SY network. The number of nodes, variables, and connectivity of the SY network were greater than those of the CK network, and tobacco–radish rotation was conducive to the formation of a more stable network structure in soil, which enhanced the resistance of rhizosphere microorganisms to disturbances. The results showed that tobacco-radish rotation was beneficial to the formation of a more complex bacterial community network, and enhanced the “Communication” and “Cooperation” of soil microorganisms, rotation also reduces the “Importance” of pathogens in the network and increases the role of beneficial bacteria in the network, which may also be responsible for the reduction of disease.
Changes in soil chemical conditions affect the structural composition of soil microorganisms to a certain extent (Zheng et al. 2023). Previously reported that soil acidification can reduce the abundance of carbon- and nitrogen-metabolizing bacteria such as Nitrospirae, Firmicutes, Chlorobi and Zygomycota, which are prone to bacterial wilt (Liu et al. 2022; Niu et al. 2017b; Shen et al. 2018).These effects are believed to be due to its nitrogen fixation, phosphate dissolution and plant growth hormone production functions (Asaf et al. 2020; Ding et al. 2021; Kim et al. 2020), and Sphingomonas have also been developed into antagonistic agents for the prevention and control of tobacco bacterial wilt (Li et al. 2021), which is consistent with the conclusions reached in this study. Changes in bacterial community composition and structure are mainly driven by changes in soil properties (Zhang et al. 2017a). The results of the RDA showed that pH and alkali-hydrolysed nitrogen were the main reasons for the difference in the bacterial community composition between the tobacco and radish rotation treatments and the control. The results of correlation analysis showed that the change of soil physical and chemical properties may be the important reason for the increase of beneficial bacteria genius and the decrease of pathogen pressure.
In summary, the tobacco-radish rotation can improve the physical and chemical properties of soil, reshape the soil bacterial community, enrich the beneficial microorganisms in the soil rhizosphere, reduce the number of pathogenic bacteria in soil, and alleviate the occurrence of tobacco bacterial wilt. (Fig. 6) .