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Microbes play a critical role in soil global biogeochemical circulation and microbe-microbe interactions have also evoked enormous interests in recent years. Utilization of green manures can stimulate microbial activity and affect microbial composition and diversity. However, few studies focus on the microbial interactions or detect the key functional members in communities. With the advances of metagenomic technologies, network analysis has been used as a powerful tool to detect robust interactions between microbial members. Here, random matrix theory-based network analysis was used to investigate the microbial networks in response to four different green manure fertilization regimes (Vicia villosa, common vetch, milk vetch, and radish) over two growth cycles from October 2012 to September 2014. The results showed that the topological properties of microbial networks were dramatically altered by green manure fertilization. Microbial network under milk vetch amendment showed substantially more intense complexity and interactions than other fertilization systems, indicating that milk vetch provided a favorable condition for microbial interactions and niche sharing. The shift of microbial interactions could be attributed to the changes in some major soil traits and the interactions might be correlated to plant growth and production. With the stimuli of green manures, positive interactions predominated the network eventually and the network complexity was in consistency with maize productivity, which suggested that the complex soil microbial networks might benefit to plants rather than simple ones, because complex networks would hold strong the ability to cope with environment changes or suppress soil-borne pathogen infection on plants. In addition, network analyses discerned some putative keystone taxa and seven of them had directly positive interactions with maize yield, which suggested their important roles in maintaining environmental functions and in improving plant growth.

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Bacterial wilt, caused by Ralstonia solanacearum, occurs occasionally during tobacco planting and potentially brings huge economic losses in affected areas. Soil microbes in different management stages play important roles in influencing bacterial wilt incidence. Studies have focused on the impacts of species diversity and composition during cropping periods on disease morbidity; however, the effects of the soil bacterial biomass, species diversity, species succession, and population interactions on morbidity remain unclear during non-cropping periods. In this study, we explored the soil bacterial communities in the non-cropping winter fallow (WF) and cropping late growing (LG) periods under consecutive monoculture systems using 16S ribosomal RNA gene sequencing and qPCR and further analyzed their effects on tobacco bacterial wilt incidence. We found that the bacterial communities in the WF period were significantly different from those in the LG period based on detrended correspondence analysis and dissimilarity tests. Crop morbidity was significantly related to bacterial community structure and to the presence of some genera during WF and LG periods. These genera, such as Arthrobacter, Pseudomonas, Acidobacteria GP6, and Pasteuria, may be potential biological control agents for bacterial wilt. Further analysis indicated that low soil bacterial diversity during the WF period, decrease of bacterial interactions from the WF to LG periods, and low soil biomass during the LG period all have the potential to increase morbidity. In conclusion, an increase of soil bacterial diversity and control of some bacterial abundances in the WF period might be an effective approach in controlling bacterial wilt incidence.