The Effect Of Nitrogen Nutrition Forms On Fusarium Wilt And Soil Fungal Community Of Cucumber

Fusarium wilt is a soil-borne disease caused by different specialized forms of Fusarium oxysporum,which leads to a seriously decrease in crops yield as well as quality and induces huge economic losses.Ammonium(NH4+)and nitrate(NO3-)are two of the major inorganic nitrogen forms for plant absorption from soil.The occurrence of plant diseases is affected by nitrogen forms through regulating spore germination and hypha growth of pathogen,plant and pathogen metabolism and host plant resistance.Effects of nitrogen forms on the growth of Fusarium oxysporum and the occurrence of Fusarium wilt have been widely studied,while the role of soil rhizosphere microorganisms affected by nitrogen forms in regulating Fusarium wilt occurrence and the related mechanisms remain unclear.In order to explain the pathogenic mechanism of Fusarium wilt from the perspective of rhizosphere microbes,our present study explored the characteristics of rhizosphere microbial community related to the occurrence of Fusarium wilt,and the results of which would provide theoretical basis for the prevention and control of Fusarium wilt.In the present study,rhizosphere soils from healthy and diseased plants were collected to analyze the response of rhizosphere soil fungal communities and its relationship with Fusarium wilt occurrence.By supplying different nitrogen forms and inoculating pathogen in cucumber plants,the effects of nitrogen forms on the structure,diversity and function of soil fungal communities were analyzed,which were further coupled with soil environmental factors(physical and chemical properties,soluble organic matter)to analyze the mechanisms underlying the inhibition effect of NO3-on Fusarium wilt.Moreover,by using split-root and metabolome analysis,soil samples and root exudates at different distances from the cucumber root of the rhizosphere space were collected,the effects of nitrogen forms and pathogen inoculation on soil metabolites and root exudates were analyzed,and key metabolisms and potential allelochemical were screened to explore their regulatory mechanisms on the occurrence of cucumber Fusarium wilt.The major results are listed as follows.1.The occurrence of Fusarium wilt significantly increased the abundance of Fusarium oxysporum,while decreased the fungi species richness and diversity in the rhizosphere soil of cucumber and tomato.Compared with the soil of healthy plant,the relative abundance of Ascomycota increased while Basidiomycetes decreased in the rhizosphere of cucumber and tomato,moreover,the relative abundance of Basidiomycota in the rhizosphere of bitter melon was significantly higher than that in cucumber and tomato.The proportion of Gymnopilus significantly negatively correlated with the number of Fusarium oxysporum while positively correlated with the species richness of fungi.Saprophytic ascomycetes(Trichoderma,Penicillium)and basidiomycetes(Gymnopilu)in the rhizosphere of healthy plants played antagonistic roles against pathogen.2.Compared with non-inoculation treatment,the increasing in leaf temperature and disease index of nitrate-supply plants was lower than that in leaf with ammonium-supply(.The Alpha diversity of rhizosphere fungi was reduced after FOC inoculation in ammonium treatment rather than in nitrate treatment.The more active species and interactions between them in rhizosphere soil with nitrate supply werre important for the inhibition of pathogen invasion.3.Different nitrogen forms supply caused the increase of corresponding forms of nitrogen in rhizosphere soil.The inoculation of pathogen induced the increase in pH and microbial C/N ratio in rhizosphere soil.The response of carbon deposits in rhizosphere soil to nitrogen forms was more significant than that in bulk soil and were driven by soil physical and chemical properties.Different nitrogen forms drived the response of fungal communities to the pathogen invasion by affecting the environmental factors of rhizosphere soil and therefore affecting the occurrence of cucumber Fusarium wilt.4.FOC ioculation resulted in an increase in the content of carbohydrates and esters in the ammonium treated soil,however,all the carbon deposits were significantly reduced in nitrate treated soil after FOC ioculation.With the increase in the sampling distance from the roots,carbon deposits contents tended to be similar.Moreover,FOC ioculation decreased root exudates levels in nitrate treated soil,and significant differences in root exudates components were observed between ammonium treated soil and other treatments.Therefore,when nitrate was supplied,more stable root exudates and higher levels of ketones were produced from shoots,which further affected the allelopathy related to pathological fungi and contributed to the inhibition of Fusasium wilt.5.Compared with amino acids and organic acids,carbohydrates promoted the spore germination of Fusarium oxysporum efficiently,while amino acids was advantage for the production of toxins comparing with sugars and organic acids.6.FOC inoculation only resulted in a significant increase in the total number of fungi in soil cultured with pure ammonium.With the prolongation of inoculation time,the total fungi number continued to decrease in the soil.The ability of soil microbial in treatment A and AN to metabolize acids and the niche width of soil microbial nutrition were significantly increased at the initial stage after FOC inoculation,while at later stages,the ability of soil microbial in treatment A to metabolize majority of carbon sources sharply decreased,and the niche width of soil microbial nutrition for treatment AN and N were gradually rised and then kept stability.Therefore,the pathogen utilized the carbon source effectively in the ammonium supplied soil for occuping the niche and further promoted the spore germination and toxin production by changing the metabolism of sugar,amino acid and organic acid,which eventually resulted in the successful colonization of the pathogen.In conclusion,the environmental factors in rhizosphere soil,the allelopathy of rhizosphere carbon deposits,and the ability of pathogen to colonize,grow and produce toxins together affected the responses of rhizosphere fungi diversity under different nitrogen forms.The higher rhizosphere soil fungi diversity and stronger interaction within the communities with nitrate supply inhibited the invasion of pathogen and the occurrence of Fusarium wilt efficiently.