Soil Microbial Diversity In The Rhizosphere Of Typical Desert Halophytes

Soil water and salt is often inseparable in the arid regions of northwest China,and the formation of soil environmental gradients that interact with soil water and salt were driven by the spatial heterogeneity of vegetation and resources.The changes in soil water and salt regimes can exert great impacts on the structure and function of ecosystem in arid desert areas.It was reported that soil microorganisms have relatively sensitive responses to the alterations in soil water,salt and their interactions and possess the ability to predict the variations in proximal environments.The adaptative strategies adopted by soil microorganisms may vary with the degrees of drought and salinity.The separate effects of soil water and salt on rhizosphere microbes have been extensively studied,but few studies have explored the response process and mechanism of rhizosphere soil microbes to the coaction of soil water and salt.The knowledges of varying mechanisms of structural and functional diversity of microbial communities in rhizosphere soil in the context of changing water and salt regimes contribute to the understanding of the stability of ecosystem function in arid and semiarid regions.In the present study,three sites with contrasting soil water and salt contents and vertical distance of every 1.2 km to the bank of Aqikesu river were set in Ebinur Lake Wetland National Nature Reserve in Xinjiang Uygur Autonomous Region of China.Three halophytic shrub species(Reaumuria songarica,Nitraria tangutorum and Alhagi sparsifolia)in each site were selected as the research object.In the present study,by using 16 S amplicon high-throughput sequencing technology and Biolog-ECO plate technique,we analyzed the variations in environmental factors in the rhizosphere soil of three speciess along the soil watersalt gradient,explored the structural and functional diversity of rhizosphere microbes,indentified the effects of each environment factor on the structural and functional diversity.(1)With the decreases in water and salt contents,the contents of soil organic matter,total nitrogen,alkaline nitrogen,total phosphorus,available phosphorus and urease decreased in the rhizosphere soil of three species,and there were no significant differences in the contentd of environmental factors among three plant species in each water-salt site.(2)With the decreases in water and salt contents,the abundances of dominant strains were more prominent in the rhizosphere bacterial communities of three plant species,especially Actinomycetes at the and the phylum level and Gammaproteobacteria at the class level.The community structure of archaea tends to be simple with the decreases in water and salt content,Euryarchaeota is the dominant phylum and Halobacteria is the dominant class.Compared with the other two species,the abundances of dominant strains of the Nitraria tangutorum rhizosphere at the phylum and the class level are the highest under the medium water-salt condition.There are many strains in the fungal community structure that have not been identified,Ascomycota and Sordariomycetes is the dominant phylum and class,respectively.The alpha diversity of the fungal community in Reaumuria songarica rhizosphere was the highest under the condition of medium water-salt,and the alpha diversity of bacteria,archaea and fungi in the rhizosphere of Nitraria tangutorum was the highest under the condition of low water-salt.The alpha diversity of Alhagi sparsifolia rhizosphere bacteria,archaea and fungal communities was highest under high water-salt condition.The richness of rhizosphere microorganisms in Reaumuria songarica and Alhagi sparsifolia was similar,and with the decreases in water and salt contents the richness of bacteria shows a tendency to decrease first and then increase,the richness of archaea in Reaumuria songarica rhizosphere shows a gradual decrease.The richness of fungi in Reaumuria songarica increased gradually with the decreases in water and salt contents,and the Alhagi sparsifolia exhibited the opposite trend.The richness of Nitraria tangutorum rhizosphere microorganisms showed different pattern: with the decreases in water and salt contents,bacteria gradually decreased but archaea gradually increased,fungi decreased first and then increased.The analysis of UPGMA showed that the microbial differences among plant species under the same water and salt condition was relatively small,while the microorganisms of the same plant were quite different along the water-salt gradient.(3)With the decreases in water and salt contents,the community evenness index(McIntosh)of microorganisms in the rhizosphere of three plant speciess is invariably consistent with the change trend of average color change rate(AWCD).In tetms of the utilization of the carbon sources in the rhizosphere,Nitraria tangutorum,Reaumuria songarica and Alhagi sparsifolia reached the maximum utilization under the low,medium and high water-salt condition,respectively,which is consistent with the variations in alpha diversity of microbial communities and indicated a potential link between microorganisms in structure and function.Amino acids,carbohydrate,polymer are the main types of carbon sources utilized by soil microbe in the rhizosphere of three plant speciess.(4)Compared with vegetation type,soil water and salt gradient can change soil environmental factors,soil structural and functional diversity of microbial communities.With the changes in water and salt,environmental factors will also exert impacts on microorganisms.Total salt,ammonium nitrogen,nitrate nitrogen,alkaline phosphatase and beta-glucosamine content were the environmental factors that had the greatest impacts on bacterial communities;archaea is mainly affected by total salt and total phosphorus content,while fungi are mainly regulated by nitrate nitrogen and total phosphorus content.Beta-glucosamine is the decisive factor that exerts the profound influences on soil microbial absorption and utilization of carbon sources.In conclusion,in the study area where soil is subjected to twofold stresses from drought and salinity,Reaumuria songarica,Nitraria tangutorum and Alhagi sparsifolia rhizosphere soil microorganisms have strong adaptative ability to adapt to adversity.The contents of water and salt regulated the microenvironment in rhizosphere and altered the composition of microbial community,which may change the metabolic activity and functional diversity of soil microbial communities.