Continuous cropping refers to planting a single crop in the same field year after year. This is a vital and common soil management practice in China because of limited arable land resources and lower related agronomic management costs relative to rotation practices . Nevertheless, crop continuous cropping has tremendous pressure on the soil's capacity and negatively affects soil function including serious soil sickness, ecosystem degradation, loss of productivity [2, 3]. Thus, it is crucial to find indicators for early evaluation of soil health decline for the sustainable management under continuous cropping system.
Cut chrysanthemum (Chrysanthemum morifolium), which is native to Northeastern Europe and Asia, is the oldest ornamental plant and an important herb and known for a wide range of biodiversity and attractive colors . Cut chrysanthemums are in great demand worldwide and have great export value because of their broad use, such as in tea, medicine, ornaments, and food . China is one of the important exporters of cut chrysanthemums in the world, and the cultivated area is gradually expanding. Continuous cropping of cut chrysanthemum has become the mainstream cultivation practice to obtain higher economic benefits. However, long-term monoculture has negative impacts on soil physical and chemical properties (abiotic factors), thereby threatening chrysanthemum quantity and quality. The soil abiotic characteristics including soil moisture, pH, nutrient, and organic matter (SOM) mediate the alteration of soil microorganisms [3, 6, 7], which are likely to be sensitive to continuous monoculture. However, the abiotic and biotic mechanism and their interactions behind the adverse effects of continuous monoculture remains unclear.
Soil microbes play an extremely vital role in soil elements cycle and ecosystem functions, regulating the response of soil ecosystems to human disturbance [8, 9]. Healthy soil is the fundamental guarantee for plant growth and food security in agriculture . Given the significance of soil microorganisms for specific soil functions , they must be considered when exploring the mechanisms behind the response of agricultural soil systems to continuous cropping cultivation. Increasing numbers of studies have shown that soil enzyme activities and soil microbial composition maintain a certain relationship in response to continuous cropping [12, 13]. The effects of continuous cropping on soil microbial community have been accessed in crops and vegetables [14, 15, 16]. For example, Ali et al.  reported that rhizospheric soil microbial community diversity significantly reduced during the cucumber continuous cropping. Nevertheless, the dynamic successions of microbial communities of facility horticultural plant rhizosphere soil are less understood . To date, it remains unknown how the response of rhizospheric soil microbial community to long-term cut chrysanthemum continuous cropping.
There are two relationships between species in the microbial community, either competition for resources and space  or mutualism . Microbial co-occurring network can reveal the relationships between microbial species and explain the assembly of complex microbial communities in various environments such as oceans and sandy land . Network analysis can also reveal why certain groups of microbes appear together consistently, or whether some groups of microbes are more important to keep the network stability in response to environmental disturbances . In addition, the complexity of the network is an important indicator of the stability and the function of the ecosystem . Therefore, an unknown question is how the complexity of soil microorganism, as expressed by network connectivity, changes with continuous cropping.
Here, this study focused on how the abundance, diversity, composition, and network complexity of rhizospheric soil bacteria change in response to cut chrysanthemum continuous cropping. Moreover, we determined whether alterations in bacterial community caused by continuous cropping are linked with soil properties. We hypothesized that continuous cropping would adversely affect soil bacterial abundance, diversity, and network complexity. The diversity and composition of soil bacteria were quantified by using 16S rRNA gene amplicon sequencing. The total amounts of bacteria were applied using the dilution-plate method. The objectives of this study were to (1) evaluate the changes of enzymatic activities to continuous cropping; (2) compare the bacterial population, composition, and diversity in different continuous cropping years; and (3) assess the correlations between the bacterial community and soil properties. These results provided a comprehensive understanding of the response of soil microbial community to cut chrysanthemum continuous cropping, and may conducive to improving agricultural strategies by regulating the community function of soil microbiome to reduce the adverse impacts of long-term monoculture. These findings provided a scientific basis for a comprehensive understanding of the effects of chrysanthemum continuous cropping on soil bacterial communities, and could conducive to lower continuous cropping obstacles by regulating the soil microflora.