Soil Profiles Microbial Community Responses To Environmental Disturbance And Growth Of Legumes

Soil microorganisms play an important role in the process of soil formation.However,it still remains poorly understood about the response of underground microbes to the disturbances by agricultural and industrial activities and the change of microbial diversity.In this study,we captured rhizobia in 0-300 cm soil depth profiles by robinia in agricultural land of Yangling to study their distribution and phylogeny.Meanwhile,we analyzed the variation of microbial community after anthropogenic disturbances,including planting robinia and clover,and watering,through 16 S rRNA high-throughput pyrosequencing.We obtained 32 rhizobia isolated from 0-300 cm soil profiles totally by robinia,and strains isolated from arable soil(0-100cm)were dominated(90.6%).16 S rRNA gene sequence analysis showed these strains belong to two genera,including Mesorhizobium and Neorhizobium.Further study based on the phylogenies of multilocus sequence analysis of house-keeping genes recA-atpD-rpoB indicated that the 88% all isolates were Mesorhizobium corresponding to one defined species(Mesorhizobium amorphae,6.25%)and three potential novel species,including Mesorhizobium sp.?(68.5%),Mesorhizobium sp.?(6.25%)and Mesorhizobium sp.?(12.5%).Strain 20-40-1 and 40-60-8 showed closely phylogenetic relationship with type strain Neorhizobium galegae bv.orientalis HAMBI 540 T,and the sequence similarity was 93.7%,the remaining two strains belong to Neorhizobium sp.(6.25%).Based upon the phylogenies of symbiotic genes nodA and nifH,all of the isolates formed a cluster with Mesorhizobium spp.,which were inconformity with the phylogenies of 16 S rRNA gene.Therefore,rhizobia with robinia may have specificly symbiotic genes.Physical and chemical properties of soil appeared significantly vertical variations.The value of pH increased slightly from pH8.21 to pH8.42 with the soil depth increasing,while the soil organic matter,total nitrogen,available potassium and phosphorus level decreased.After planting robinia,the content of total nitrogen increased and the ratio C:N reduced than that in situ soil.The results of 16 S r RNA gene high-throughput sequencing showed that microbial ?-diversity decreased with soil depth,which suggested vertical hierarchy distribution.The soil properties played essential role in shaping microbial community compositon.The relative abundance of Proteobacteria was significantly correlated with pH,and soil arganic matter,total nitrogen,available potassium and phosphorus as Actinobacteria,Chloroflexi and Planctomycetes did.Besides,the distribution of rhizabia was analyzed,we found that the order Rhizobiales were dominated taxa in the alpha-Proteobacteria.Furthermore,the relative abundance of family Bradyrhizobiaceae and Phyllobacteriaceae decreased after disturbance,while the family Rhizobiaceae increased significantly after planting robinia.