Effects Of Continuous Cropping Of Glycyrrhiza Glabra L. On Soil Microbial Community Structure And Root Rot Pathogens

Glycyrrhiza glabra L.is one of the original medicinal plants of Glycyrrhiza in the Pharmacopoeia of the people’s Republic of China(2020 Edition),which has high economic and ecological value.It was found that G.glabra suffered from serious continuous cropping obstacles,that is,the continuous cropping of G.glabra often leads to stunting plant growth,yield decline and root rot frequently development.However,the mechanism remains unclear.Thus,high-throughput sequencing technology was used to sequence the 16 S r DNA and ITS sequences of uncultivated soil without planting G.glabra(Control),the rhizosphere soil of 1-year-old G.glabra(Gg1)and the rhizosphere soil of 5-year-old G.glabra(Gg5)in the same plot.Then,we comparatively analysed the differences of microbial community structure between rhizosphere soil of G.glabra and control group,as well as between different planting years of rhizosphere soil of G.glabra.The effects of aqueous extracts from roots,stems and leaves and root exudates of G.glabra on the growth and reproduction of root rot pathogens(Fusarium oxysporum and F.solani)were investigated by exogenous addition method.On this basis,the contents of 9 phenolic acids in the leaves of G.glabra with potential promoting effect on the pathogen of root rot were determined,and the effects of these compounds on the mycelial growth,sporulation and spore germination of F.oxysporum and F.solani were studied,in order to explain the causes of continuous cropping obstacles of G.glabra and provide theoretical basis for exploring reasonable mitigation measures.The main results are as follows:G.glabra continuous cropping increased the richness of bacterial community and decreased the richness of fungal community in rhizosphere soil(P > 0.05).Principal co-ordinates analysis indicated that there were significant differences between the rhizosphere soil microbial composition of G.glabra and the control group,and the planting years of G.glabra significantly affected the microbial community composition in rhizosphere soil.At the phylum level,compared with Gg1,the relative abundance of Blastocladiomycota decreased by 83.16%,and that of Gemmatimonadetes and Acidobacteria increased by 70.99% and 61.08%,respectively,in the rhizosphere soil of G.glabra after 5 years of continuous cropping(Gg5).At the genus level,compared with Gg1,the relative abundance of beneficial microbes Arthrobacter,Pseudomona and Naganishia decreased by 64.08%,31.30% and 63.58%,and the relative abundance of pathogenic microbes Fusarium and Thanatephorus increased by 78.08% and 2077.70%,respectively,in the rhizosphere soil of G.glabra under continuous cropping for 5 years(Gg5).In conclusion,the change of microbial community structure in rhizosphere soil of G.glabra,the decrease of beneficial microbes and the increase of pathogenic microbes may be one of the important reasons for the continuous cropping obstacle of G.glabra.The pathogenicity test showed that F.oxysporum and F.solani had strong pathogenicity to G.glabra,and could cause typical symptoms of root rot disease.The aqueous extracts of the roots and stems and root exudates of G.glabra significantly inhibited the growth and reproduction of F.oxysporum and F.solani,and the higher the concentration,the stronger the inhibitory rate.However,the aqueous extracts of leaves significantly promoted the mycelial growth and sporulation of the two pathogens.In the concentration range of 0.005-0.02 g/m L,the promotion effects of the aqueous extracts from the leaves of G.glabra on the colony diameter and spore yield of F.oxysporum were up to 14.50% and 24.10%,and the promotion effects on the colony diameter and spore yield of F.solani were up to 7.92% and 13.46%,respectively.Therefore,the large accumulation of G.glabra leaves as litters in farmland may be another reason for the frequent occurrence of root rot in continuous cropping of G.glabra.It is recommended to harvest the leaves each year before they fall and remove them from the field in time.Based on previous reports on phenolic acids that play a key allelopathic role in continuous cropping obstacles of crops and phenolic acids in Glycyrrhiza,the contents of chlorogenic acid,gallic acid,salicylic acid,phydroxybenzoic acid,sinapic acid,ferulic acid,p-coumaric acid,caffeic acid and cinnamic acid in the leaves of G.glabra were determined by ultra performance liquid chromatography-mass spectrometry.The results showed that there were only salicylic acid,p-hydroxybenzoic acid,p-coumaric acid,ferulic acid,sinapic acid,chlorogenic acid and gallic acid in the leaves of G.glabra,and the content of ferulic acid was the highest.Among the seven phenolic acids,only p-coumaric acid promoted the mycelial growth of F.solani,while other phenolic acids inhibited the growth and reproduction of F.oxysporum and F.solani in varying degrees.Therefore,we speculate that in addition to the nine phenolic acids detected in this experiment,other allelochemicals may promote the growth and reproduction of F.oxysporum and F.solani,leading to frequent root rot of G.glabra.