Distribution Of Bacterial Community Composition And Function Along Soil Profiles In Fluvo-aquic Soil Of North China Plain

Soil microorganisms play a crucial role in driving the nutrient cycling and energy flow in soils as well as maintaining soil structure.It is suggested that a large number of microorganism exist in the deeper layers of soil.However,little is known about the microbial community composition,functionality and the mechanisms of community assembly in deep soil layers.In this study,soil samples were collected from 14 layers(0-0.1,0.1-0.2,0.2-0.4,0.4-0.6,0.6-1,1-1.4,1.4-2,2-3,3-4,4-5,5-6,6-7,7-8,8-10 m)of soil profiles(with depth of 10 m)that were set up for long-term observation at the Yucheng Comprehensive Experimental Station,Chinese Academy of Sciences.16 S r DNA high-throughput sequencing technology was used to analyze variations in bacterial community composition and assembly mechanism,potential functions and molecular ecological network structure with soil profile depth.Results of this study will provide novel insight into the depth-related characteristics of community composition and functionality of soil bacteria,which will help to promote the exploitation of microbial resources in deep soils.The results are as follows:(1)Soil depth significantly affected the bacterial diversity and community composition.Bacterial richness first decreased significantly with increasing soil depth from 0-6 m,then slightly increased from 6-10 m.Similarly,bacterial Shannon diversity first decreased significantly with soil depth from 0-7 m,then slightly increased from 7-10 m.The relative abundances of Proteobacteria,Actinobacteria,Acidobacteria,Bacteroidetes and Planctomycetes decreased significantly with increasing soil depth,whereas Chloroflexi,Firmicutes,Gemmatimonadetes and GAL15 were more abundant in deeper soil layers.The relative abundances of Actinobacteria,?-Proteobacteria and?-Proteobacteria decreased significantly with increasing soil depth,whereas Clostridia,NC10 and Bacilli were more abundant in the subsoil.The structure of bacterial community varied significantly with profile depth.PCo A plot showed that there were significant differences in bacterial community composition between depth of 0-0.4 m,0.4-3 m and 3-10 m.The phylogenetic diversity of soil bacteria significantly decreased with increasing soil depth.Results from the calculation of ?NTI showed that deterministic processes and stochastic processes comprised 83.3% and 16.7%,respectively,for the community assembly of bacteria in the top soil layer(0-0.4 m),indicating that environmental factors may have exerted predominant effects on the changes in bacterial community at this profile layer.As profile depth increases,the proportion of deterministic processes decreased to58.1%—66.9% while that of stochastic processes increased to 33.1%—41.9%.This indicates a greater probability of random distribution and dispersal of bacterial communities in deeper soil layers.(2)Results from PICRUSt analysis showed that the relative abundances of membrane transport,carbohydrate metabolism and amino acid metabolism significantly decreased with increasing soil depth.Nucleotide metabolism and Replication and repair were more abundant in the intermediate layer of soil profile(0.6-3 m).The relative abundances of cell motility and signal transduction significantly increased with increasing soil depth.In addition,soil depth exerted significant influence on the abundance of nitrogen cycling related genes(hao,nirk,nor B,nap A,nas A,nas B,nir B,nrf A)and phosphorus cycling related genes(ybe Y,pho R,pho H,ppa,Int,pst B,pst A,pst C),but the depth-related changes of individual gene were not identical.The relative abundances of functional genes were significantly correlated with the relative abundances of major bacterial taxa such as Actinobacteria,Bacteroidetes and Chloroflexi.There was a positive correlation(P < 0.001)between the dissimilarity of the composition of KEGG pathway genes with soil depth(?-diversity)and with the dissimilarity of bacterial community structure along the soil profile.This indicates a strong linkage between microbial community composition and potential functionality.(3)Characteristics of microbial network structure varied with soil profile depth.The average clustering coefficient(avg CC)and average degree(avg K)decreased with increasing soil depth,indicating that the bacterial network was comparatively more complex in the top layer of soil profile.The links between OTUs that constructed bacterial network were largely positive links(71.6%-87.99%),and this proportion increased with soil depth,indicating that the positive interactions between species become stronger in the subsoil.The average path distance(GD)and modularity in the top soil layer were greater than those in the subsoil,indicating that bacterial network in the top layers is more stable in response to environmental changes.The keystone taxa in bacterial network varied with soil profile depth.Proteobacteria,Acidobacteria and Chloroflexi were identified as the keystone taxa in networks of depth 0-0.4 m,0.4-3 m and 3-10 m,respectively.About 97% of the OTUs that presented in bacterial network were identified as peripherals nodes.The number of OTUs that identified as module hubs and connectors decreased with increasing soil depth.