Structural Characteristic Of Microbial Communities And Pathogen Succession Patterns In Citrus Orchards Under Intensive Cultivation

Citrus is one of the most important economic crops in the world and the largest category of fruit globally,and has become a pillar industry for farmers to get rich and a healthy product to improve the quality of human life.The intensive agricultural production and land use in citrus cultivation can lead to soil acidification,nutrient imbalance,and destruction of microbial community structure and diversity,which have negative impacts on soil health and plant growth.Recent on intensive citrus cultivation focuses mostly on nutrient regulation and disease detection,with limited understanding of the microbial community and pathogen succession during citrus cultivation,and a lack of exploration of keystone that are resistant to stress and disease in citrus.This study focuses on citrus orchards in Zigui,Hubei Province,which have been planted for 10 years,30 years,and 30 years with grass mulching,and uses adjacent natural forests as controls.High-throughput sequencing technology and quantitative PCR technology are used to study the soil and plant microbial community diversity and structure of the orchards,with the aim of exploring the microbial community structure characteristic and pathogen succession patterns in different planting years and different soil-citrus compartments,and the effects of grass mulching on soil improvement and disease control in citrus orchards,providing a theoretical basis for intensive citrus orchard management and disease control during cultivation.The main conclusions are as follows:(1)The intensive cultivation significantly reduces microbial diversity and increases the network connectivity of microbe in citrus orchards.With increasing planting years,the microbial alpha diversity in bulk soil,rhizosphere soil,and roots all decreased significantly,long-term cultivation only significantly affected the microbial community composition in bulk soil.With increasing planting years,the size and complexity of the microbial network decrease significantly,but the interspecies connectivity and module aggregation increased significantly.In the 10-year citrus orchard,the bulk soil keystones were Nitrolancea,Subgroup＿6,Acetobacteraceae,Sporosarcina and Mortierella elongata.The rhizosphere soil keystones were Fusarium oxysporum and Penicillium pimiteouiense.In the 30-year citrus orchard,the root keystone was Ceratobasidium.(2)The microbial diversity and structure of citrus orchards differ significantly among different compartments.Microbial alpha diversity decreases sequentially from soil to roots,and then to leaves,with no significant difference between bulk soil and rhizosphere soil.The bacterial biomarker change from oligotrophic to copiotrophic as the transition goes from soil to root,and the fungal biomarker change from soil-borne pathogens to leaf pathogens as the transition goes from root system to leaves.The size and complexity of microbial networks decrease sequentially from bulk soil,rhizosphere soil,roots,and then to leaves,but the microbial networks in root and leaves show stronger connectivity and module aggregation.The bulk soil keystone was Subgroup＿6,the rhizosphere soil keystone was Xanthobacteraceae,the leaf keystone was Didymosphaeriaceae.(3)Long-term intensive cultivation causes a significant increase in the abundance of pathogens in citrus orchards.Based on high-throughput sequencing results and FUNGuild functional group,long-term intensive planting can lead to a gradual increase in the abundance of plant pathogen(such as Fusarium,Gibberella,and Penicillium),a gradual decrease in the abundance of pathogenic antagonistic bacteria(such as Bacillus and Methylobacterium),and a gradual increase in microorganism that enhance citrus resistance(such as Burkholderia-Caballeronia-Paraburkholderia and Mycobacterium)in citrus orchards.The results of quantitative PCR show that the copy numbers of Fusarium oxysporum,Fusarium solani,and Colletotrichum gloeosporioides significantly increases with the planting years and shows an accumulation trend in bulk,rhizosphere soil,and roots.(4)Soil properties in citrus orchards significantly affect the abundance of pathogens and microbial community structure.After 30 years of citrus cultivation,soil p H decreased by 2.4 units,total carbon content decreased by 66%,total nitrogen content increased by21%,total phosphorus content increased by 252%,available phosphorus content increased by 1432%,nitrate nitrogen content increased by 135%,ammonium nitrogen content increased by 1784%,and available potassium content increased by 64%.After grass mulching,the soil p H increased by 2.1 units and the total carbon content increased by 64%.The content of soil total carbon,total nitrogen,ammonium nitrogen,nitrate nitrogen,and available potassium is significantly positively correlated with the abundance of some pathogens.Grass mulching treatment can significantly reduce the abundance of Fusarium oxysporum,Fusarium solani,and Colletotrichum gloeosporioides.The results of redundancy analysis indicate that soil available phosphorus,p H,and available potassium content are important environmental factors driving changes in bacterial community structure,while p H,total carbon,and total nitrogen content are important environmental factors that affect fungal community structure.