The Microbial Community Structure And Function Of Biological Soil Crusts In Typical Sandland Areas

Biological soil crusts(BSCs)are the important part of the surface landscape in arid sandland areas,which are of critical importance maintaining the stability of the desert ecosystem.As the important components of BSCs,microorganisms play critical roles in sustaining the structure and function of BSCs and promoting the material cycling of ecosystem.BSCs are widely distributed in the large-area sandland areas with complex and diverse natural conditions in the north of China,forming a distinctive ecological gradient.In this study,both the amplicon and metagenomics sequencing approaches were used to analyze the microbial community structures and functional genes of different developmental stages of BSCs in three typical sandland areas(Mu Us Sandy Land,Gonghe Basin sandy land and Gurbantunggut Desert).We investigated the changing regularity of microbial community structure and function genes with BSC development,compared the differences of BSC microbial communities in different shrub communities and clarified the distribution and construction mechanism of microbial community structures and functions of BSCs at the regional scale.The main conclusions are as follows:(1)With BSC development,bacterial diversity significantly increased,but fungal diversity had no significant changes.The bacterial community of BSCs was dominated by Proteobacteria,Actinobacteria,Cyanobacteria and Acidobacteria,and the fungal community of BSCs was dominated by Ascomycota,Basidiomycota and Chytridiomycota.As water and nutrient conditions improved with BSC development,the relative abundance of the oligotrophic bacteria significantly decreased,whereas the relative abundance of the eutrophic bacteria significantly increased.For fungi,the relative abundance of Ascomycota with strong stress resistance significantly decreased with BSC development,whereas the relative abundance of Basidiomycota with lignin degradation ability significantly increased.(2)With BSC development,the keystone taxa in both bacterial and fungal networks changed,and the network structure became more complex,,indicating a strengthened pattern in microbial interactions.In the early stage of BSC development,oligotrophic groups with strong resistance could alleviate environmental pressure and prevent soil and wind erosion by promoting the cementation of soil particles and increasing the stability of the soil surface.In the later stage of BSC development,copiotrophic groups with degradation ability and autotrophic groups could obtain more nutrients by promoting the fixation of carbon and nitrogen and the decomposition of litter,thus promoting the material cycle of the ecosystem.With BSC development,the microbial network structure became more complex and the community became more stable,playing a greater role in the circulation of ecosystem materials and the resistance to environmental interference.More inter-community competition in bacterial communities and less inter-community competition in fungal communities have been observed with BSC development,indicating that bacteria might play a more active role in the stability of the community than fungi.Competitions between bacterial and fungal communities were increased with BSC development.(3)With BSC development,the microbial nutrient cycle was strengthened,and the role of microbes in the process of carbon and nitrogen cycle was enhanced.With BSC development,the relative abundance of functional genes related to metabolism significantly increased,promoting the microbial nutrient circulation.The relative abundance of carbon fixation genes and hardly carbon degradation genes significantly increased with BSC development,indicating that the carbon fixation and hardly carbon degradation ability of microbes improved.With BSC development,the relative abundance of the genes involved in nitrification,denitrification,assimilation nitrate reduction and dissimilation nitrate reduction significantly increased and microbial nitrogen fixation ability was improved.Bacteria played important roles in the carbon and nitrogen cycle in different developmental stages of BSCs,whereas fungi played important roles in the later stages of BSC development.(4)Microbial diversity and community structure of BSCs were similar between different shrub communities under the same environmental conditions,but the difference of community structure gradually increased with the development of BSCs.Microbial community diversity of different developmental stages of BSCs was no significantly difference between the Artemisia ordosica community and Sabina vulgaris community in Mu Us Sandy Land.With BSC development,bacterial community diversity in both two shrub communities significantly increased.Microbial community structure was similar between the two shrub communities,but the proportion of endemic species gradually increased and the community composition gradually diverged with BSC development.The relative abundance of certain microbial taxa in BSCs was significantly different between Artemisia ordosica community and Sabina vulgaris community.Physicochemical factors of BSCs were considered as the major factors for the differences in the bacterial community structure of BSCs between two shrub communities,among which the nutrients played a critical role.Vegetation factors were considered as the major factors for the differences in the fungal community structure of BSCs between two shrub communities,among which the aboveground biomass of shrubs played a critical role.(5)On the regional scale,bacteria in the BSCs were more sensitive to environmental changes than fungi,and the distribution rules and construction mechanisms of microbial species composition and functional composition of BSCs were different.Bacterial diversity of BSCs in Mu Us Sandy Land and Gonghe Basin sandy land was significantly higher than that in Gurbantunggut Desert,whereas the fungal diversity of BSCs in Mu Us Sandy Land was significantly higher than that in Gurbantunggut Desert and Gonghe Basin sandy land.Bacterial diversity was mainly affected by latitude and annual average precipitation and fungal diversity was mainly affected by the physicochemical properties of BSCs.Microbial species composition of BSCs were significantly different among three sandy areas.In particular,Proteobacteria was the dominant bacterial phylum of BSCs in all sandy areas,with the highest relative abundance in the BSCs in Mu Us Sandy Land.Ascomycetes was the dominant fungal phylum of BSCs in all sandy areas,with the highest relative abundance in the BSCs in Gurbantunggut Desert.At the regional scale,microbial species composition of BSCs was mainly affected by geographical distance and had obvious distance-attenuation distribution,which fitted to the neutral theory and was best explained by dispersal limitation.By contrast,microbial functional structure of BSCs was not significantly changed at the regional scale.Microbial functional composition of BSCs was similar among three sandy areas and was mainly affected by the physicochemical properties of BSCs,which fitted to the niche theory and was best explained by environmental selection.