Characteristics Of Soil Microbial Community Along The Soil Profile Of Typical Vegetation In Mount Segrila

Microbial communities in subsoil are important for the regulating terrestrial biogeochemical cycle and the cycling and transformation of nutrient and energy in deep soil.However,knowledge of microbial communities in the subsoil is inadequate.As one of the main alpine ecosystem regions,the Tibetan Plateau is extremely sensitive to environmental changes due to the harsh topography and unique climatic conditions.As a main activity index of soil,the community composition and diversity of soil microbes can respond rapidly to environmental change.Therefore,it is of great significance to study the characteristics of microbial community in the subsoil of the Tibetan Plateau,which will promote further understanding of soil microbial communities and their ecosystem functioning in alpine ecosystems.In this study,topsoil(0-20 cm)and subsoil(40-60 cm)samples were collected from typical forest and alpine meadow in Mount Segrila,Tibet.Methods of Illumina Miseq high-throughput sequencing and microbial molecular ecological network analysis were used to investigate the variations in diversity,community composition and co-occurance network of fungi and bacteria between the topsoil and subsoil in alpine ecosystems.The results are as follows:(1)Compared to the topsoil,p H increased significantly(in forest)whereas contents of soil organic carbon(SOC)and total nitrogen(TN)decreased significantly in the subsoil.Contents of SOC,TN and soil water content(SWC)were significantly higher in alpine meadow than in forest;wherease p H was significantly lower in forest than in alpine meadow.The diversity indices of microbial communities were significantly affected by soil depth.Compared to the topsoil,fungal richness(Chao index)and bacteria Shannon index decreased significantly in the subsoil of forest and alpine meadow,respectively.Results from Pearson correlation analysis showed that the diversity indices of bacteria and fungi in the topsoil and subsoil were positively correlated with average annual temperature(MAT),growing season precipitation(GSP),p H and the ratio of SOC to TN(C:N),and negatively correlated with TN,SOC and SWC.(2)The composition of soil microbial community was significantly different between topsoil and subsoil in Mount Segrila,and such differences varied with vegetation types.Compared to the topsoil,the relative abundances of Actinobacteria and Bacteroidetes decreased significantly in the subsoil.The relative abundances of Chloroflex and Nitrospirae significantly increased in the subsoil in both forest and alpine meadow,wherease Acidobacteria and Gemmatimonadetes were comparatively abundant in the subsoil of alpine meadow.There was no significant difference in the relative abundance of four fungal phyla between topsoil and subsoil in both forest and alpine meadow.Nonetheless,in the subsoil of forest the relative abundances of Dothideomycetes and Tremellomycetes significantly decreased while Sordariomycetes and Agaricomycetes increased.Non-metric multidimensional scaling(NMDS)analysis and Mantel tests showed that climate(MAT,GSP)and soil properties(e.g.p H,TN,SWC)were the main factors affecting the composition of bacterial and fungal communities in the topsoil and subsoil.Particularly,MAT,p H and GSP were identified as the key factors that drove the variations in bacterial and fungal communities in Mount Segrila.(3)Compared to the topsoil,the average clustering coefficient(avg CC)of fungal and bacterial co-occurrence networks increased in the subsoil,indicating that microbial networks become more organized in the subsoil.In the subsoil the average degree(avg K)of bacterial network was higher than in the topsoil in forest,but was lower in alpine meadow,indicating that the bacterial network was more complex in the subsoil of forest and in the topsoil of alpine meadow.In contrast to bacterial network,the avg K of fungal network in the subsoil decreased in the forest but increased in the meadow,indicating that the fungal network was more complex in forest topsoil and meadow subsoil.Positive links(62.42-88.17% of total links)dominated in all bacteria networks,and the proportion of positive links increased with soil depth.In contrary,negative links(64.5-75.35% of total links)dominated in all fungal networks,and the proportion of negative links increased with soil depth in forest but decreased in the subsoil of alpine meadow.(4)Keystone taxa in bacterial and fungal networks varied with soil depth and vegetation types.In forest,Proteobacteria and Dothideomycetes(in the topsoil),Chloroflexi,Actinobacteria and Leotiomycetes,Agaricomycetes(in the subsoil)were the keystone taxa in bacterial and fungal networks,respectively.In alpine meadow,Acidobacteria and Leotiomycetes(in the topsoil),Proteobacteria and Agaricomycetes,Sordariomycetes(in the subsoil)were the keystone taxa in bacteria and fungi networks,respectively.For all bacteria and fungal networks,more than 97.5% of the nodes were identified as the peripherals.Nodes that identified as module hubs and connectors accounted for less than 2% in all the bacterial and fungal networks,yet they play an important role in maintaining the stability of microbial network.These functional groups mainly include Acidobacteria,Proteobacteria,Actinobacteria,Leotiomycetes,Agaricomycetes and Dothideomycetes.