Dynamic Changes Of Rhizosphere Microbial Community Composition And Function In Cut Chrysanthemum Under Continuous Cropping

Cut chrysanthemum,as an ornamental economic crop,has the characteristics of rich flower shape,long storage period,easy propagation and cultivation,etc.Cut chrysanthemum is one of the four famous cut flowers in the world.It is widely grown in China,Holland,Japan etc.However,like soybean,potato,grape and other crops,long-term continuous monocropping will lead to the increase of cut chrysanthemum disease,plant dwarf,inflorescence reduction,seriously affect the quality of cut flowers,causing huge economic losses to growers.In the study of continuous cropping of other crops,the occurrence of continuous cropping obstacle is attributed to the degradation of soil physical and chemical properties,increase of pathogenic microorganisms and allelopathy of plants caused by continuous cropping of single crop for a long time.To deal with the continuous cropping obstacle,the use of crop rotation is an important means to reduce the occurrence of crop continuous cropping obstacle,but it is often with low economic value of crop rotation,which inevitably reduces the economic benefits of farmers.Physical and chemical fumigation are commonly used to eliminate pathogenic microorganisms in soils.Chemical fumigation,in particular,can remove most of the pathogens in soils.However,while eliminating pathogenic bacteria,fumigants also kill beneficial groups in soil,causing great damage to soil ecology.In recent years,biologists are exploring a green and sustainable way to deal with the occurrence of continuous cropping disorder.In this process,the role of plant rhizosphere microorganisms is gradually revealed.Rhizosphere microbial community refers to the collection of bacteria and fungi closely attached to the root surface soil particles,which participate in all stages of plant growth and development.The structure and homeostasis of rhizosphere microbial community are directly related to crop health.The structure and homeostasis of rhizosphere microbial community are directly related to crop health.A simple microbial community composed of beneficial taxa and core microorganisms in the rhizosphere can significantly improve crop quality and yield and reduce the adverse effects of continuous cropping barriers.However,the composition and dynamics of rhizosphere microbial community of cut chrysanthemum,a flower crop prone to continuous cropping disorder,have not been fully revealed.To reveal the causes of continuous cropping disorder and promote the further development of control measures for continuous cropping disorder of chrysanthemum,this study used next-generation amplicon sequencing technology to monitor the dynamic changes of rhizosphere microbial communities in four cut chrysanthemum cultivars during 0-3 years.At the same time,the physicochemical properties such as soil nutrient content,soil salinity and soil enzyme activity were also measured.The results showed that continuous cropping significantly changed the composition and structure of rhizosphere microbial community.Remarkably,the richness of rhizosphere fungal community.Moreover,with the increase of continuous cropping time,the richness of fungal community increased significantly,the number of biocontrol bacteria decreased,and the number of plant pathogenic bacteria increased,resulting in the emergence of "fungal type" community.Prokaryotic microbial functional annotation based on FAPROTAX found that the abundance of functions related to nitrogen and sulfur metabolism and chitin decomposition in rhizosphere of cut chrysanthemum decreased significantly,suggesting that continuous cropping weakened the elemental transformation ability of rhizosphere bacterial community,resulting in a reduction of available nutrients in root system.The fungal function annotation results based on FUNGuild showed that phytopathogenic fungi dominated the rhizosphere soil of the continuous cropping of cut chrysanthemum(even more than 50% in some samples).By comparison,it is found that these groups mainly belong to Acremonium,Plectosphaerellaceae,Fusarium and Cladosporium.Although Fusarium species are the main pathogens responsible for the fusarium wilt disease of cut chrysanthemum,the substantial enrichment of other pathogenic fungi in the rhizosphere may indicate that they collectively lead to the deterioration of cut chrysanthemum traits.Continuous monoculture also reduced the content of ammonium nitrogen and increased the salinity of soil.In the long run,this will lead to insufficient available nutrients in the soil and the occurrence of salinization.It is suggested that the change of microbial community and the deterioration of soil physical and chemical properties may contribute to the occurrence of continuous cropping obstacle of cut chrysanthemum.