Influences Of Simulated Grazing On Soil Microbial Community Composition And Diversity In Typical Grasslands On The Loess Plateau

Soil microorganisms are the most active components of grassland ecosystems,playing an important role in decomposition of organic matter,nutrient cycling and biodiversity maintenance.Grazing is the most direct use of natural grasslands,influencing the structure and function of grassland ecosystem.While many studies have explored the effects of grazing on soil microorganisms,whether and how different pathways of grazing(i.e.defoliation,trampling,dung and urine return)affect soil microorganisms remains largely unexplored.In this study,by performing a simulated grazing experiment in a semiarid grassland on the Loess Plateaut,including control(CK),mowing(M),trampling(T),addition of dung and urine(DU),and M+T+DU(mixed treatment),we investigated the effects of different pathways of livestock grazing on the community composition and diversity of soil fungi and bacteria using highthroughput sequencing technology.By integrating plant community characteristics and soil physicochemical properties,we explored the mechanism of different grazing pathways affecting soil bacterial and fungal communities.We obtained the following results:(1)At phylum and class level,different simulated grazing treatments had no significant effect on the community of dominant groups of soil bacteria and fungi,but changed their relative abundance.In the bacterial community,dung and urine addition and mixed treatment significantly increased the relative abundance of Proteobacteria and Bacteroidetes,but decreased the relative abundance of Actinobacteria,Chloroflexi and Gemmatimonadetes.Dung and urine addition and mixed treatment significantly increased the relative abundance of Alphaproteobacteria and Sphingobacteriia,but decreased the relative abundance of Gemmatimonadetes and Thermoleophilia.In the fungal community,Ascomycota and Basidiomycota were the dominant groups,but different simulated grazing treatments had no significant effect on their relative abundance.At the class level,dung and urine addition significantly decreased the relative abundance of Dothideomycetes,while dung and urine addition and mixed treatment significantly increased the relative abundance of Pezizomycetes.(2)Different simulated grazing treatments had different effects on alpha diversity of soil bacteria and fungi,but had similar effects on beta diversity of soil bacteria and fungi.With regard to bacteria,each simulated grazing treatment significantly increased the ASV richness.Regarding fungi,mowing treatment significantly increased ASV richness,while dung and urine addition and mixed treatment significantly decreased the ASV richness.The results of NMDS showed that,compared to the control,the beta diversity of soil bacteria and fungi was significantly different under dung and urine addition and mixed treatment,but there was no significant difference for mowing and trampling treatment as compared to the control.(3)The network structure characteristics of soil bacteria and fungi were affected by different simulated grazing treatments.Compared with the control,the simulated grazing treatments not only resulted in enhanced cooperative relationships among the bacterial and fungal groups,but also resulted in the changes of the key species in the bacterial and fungal networks.(4)Different simulated grazing treatments had different effects on vegetation characteristics and soil physicochemical properties.Compared with the control,mowing treatment significantly reduced plant aboveground biomass.Mixed treatment significantly reduced plant coverage and aboveground biomass.Trampling and dung and urine addition treatments had no significant effect on the plant community characteristics.Mowing and trampling treatments significantly increased soil organic carbon.Dung and urine addition treatment significantly increased soil organic carbon and nitrate nitrogen.Mixed treatment significantly increased soil organic carbon,total nitrogen and nitrate nitrogen.(5)The community composition and diversity of soil fungi and bacteria were driven by plant community characteristics and soil properties.Correlation analysis showed that soil bulk density was the most important factor affecting the relative abundance of bacteria and fungi.The ASV richness of bacteria was positively correlated with soil nitrate nitrogen,organic carbon,total nitrogen and total phosphorus,while it was negatively correlated with plant aboveground biomass.The ASV richness of fungi was positively correlated with soil p H and bulk density.Random forest model analysis showed that soil total nitrogen and organic carbon were the most important factors influencing bacterial ASV richness,while soil bulk density was the most important factors affecting fungal ASV richness.Soil total phosphorus was consistently the most important factor affecting beta diversity of soil bacteria and fungi.This study investigated the responses of soil bacteria and fungi communities to different pathways of livestock grazing in typical steppe on the Loess Plateau.By integrating plant community characteristics and soil physicochemical properties,we explored the main driving factors of alpha and beta diversity of soil bacteria and fungi.These findings are helpful to further understand the relationships between livestock grazing,plants and soil microbes.This study provides scientific support for the management of typical grassland and the restoration of degraded grassland in the Loess Plateau in the context of increasing human activities and climate change.