The Patterns And Determinants Of Soil Microbial Diversity In China's Drylands

Drylands play an important role in agricultural and socio-economic development.And drylands as an important component of the terrestrial ecosystems,it covers approximately 41% of the global land surface area and support more than 38% of the global population.Soil microorganisms(i.e.,archaea,bacteria and fungi)are major components of the belowgroud biomes in drylands,which not only drive many important ecosystem functions and processes such as climate regulation,biomass production,carbon sequestration,nutrient cycling,and soil fertility,but also determine the ability of drylands to provide products and services to human society.In recent decades,the impact of global warming on arid ecosystems have received much attention,many studies have shown that climate change has already led to the degradation of arid ecosystems,and these areas are at risk of further expansion in the future.However,the role of soil microorganisms is usually neglected in drylands.Therefore,to clarify the community structure and distribution of soil microorganisms in arid zones and elucidate their functions and how they respond and feed back to climate change is one of the current hot issues in ecological research.Moreover,the drylands of Northern China are extensive(about 53% of the national territory),which including typical temperate grassland and temperate desert,also it is one of the most ecologically fragile and sensitive areas to climate change in the world.Although some scholars have investigated and reported soil microbial diversity and its distribution patterns in China's drylands in recent years,most of them only investigated and analyzed soil microorganisms such as bacteria or fungi in some regions or parts of arid areas in China.However,the investigation and analysis on the diversity and distribution patterns of soil bacteria,fungi and archaea in the whole arid area of Northern China(nearly 5000 km from east to west)is still a vacancy.To fill that gap,we analyzed the community structure and biodiversity of soil microorganisms(archaea,bacteria and fungi),investigated the chang rule of the biodiversity and community spatial pattern along geographic distance and environmental gradient on large scales,further elucidated the biogeographic distribution and ecological driving mechanisms of the dominant soil microbial taxa through large-scale systematic field surveys and sampling in the arid regions of Northern China(across Xinjiang Uygur Autonomous Region,Qinghai Province,Gansu Province,Shanxi Provice,Ningxia Hui Autonomous Region and Inner Mongolia Autonomous Region).The main results were summarized as follows,(1)The Euryarchaeota and Thaumarchaeota are dominant phylum of soil archaea in the drylands of Northern China,of which Haloarchaea is the main class of Euryarchaeota,accounting for 68.22% and 19.9% of the abundance of soil archaea,respectively.The bacteria communities are mainly belonging to Actinobacteria,Proteobacteria,Chloroflexi,Acidobacteria,and Bacteroidetes,accounting for 32.21%,23.83%,10.29%,9.94% and 8.52% of the abundance of soil bacteria,respectively.Soil fungal communities are mainly composed of Acidobacteria and Bacteroidetes,accounting for 77.61% of the total abundance of soil fungi.Some of these dominant soil microbial taxa(e.g.,Thaumarchaeota,Actinobacteria,Proteobacteria and Ascomycota)are widespread taxa in the soil environment,while some(e.g.,Euryarchaeota,Acidobacteria and Bacteroidetes)are specialized taxa adapted to the arid zone environment.In addition,Chloroflexi as photosynthetic bacteria,may have important contributions to ecosystem productivity and carbon cycling processes in the arid zone.(2)We found that alpha diversity of archaea,bacteria and fungi under vegetation canopies was higher than that in bare ground soils(without vegetation cover),these results suggestted that vegetation increased nutrients of soil under canopies through the "fertilizer island effect",which in turn increased the soil microbial diversity.This study also indicated that climate,soil and vegetation factors collectively influenced the alpha diversity of soil archaea,bacteria and fungi in arid areas of Northern China.The correlation between bacterial alpha-diversity and climate factors was weaker than that in archaea and fungi,while vegetation factors explained large part of soil fungal alpha diversity.These results indicated that the vegetation factors were more closely related to soil fungi.Moreover,the relationship between soil archaeal alpha diversity and most environmental predictors is opposite to that of soil bacteria and fungi,the prediction model also indicated that the alpha diversity of soil archaea showed a decreasing trend from west to east,and the alpha diversity of archaea was significantly higher in desert soils than that in grasslands.But,the alpha diversity of soil bacteria and fungi showed the opposite trend of increasing from west to east.Furthermore,comparing with soil bacteria and fungi,soil archaea had different response to environmental gradients,and these results showed that there was obvious ecological niche differentiation for archaea at the regional scale.(3)In the drylands of Northern China,the community composition of soil archaea,bacteria and fungi significantly varied among different arid climate types(arid-semihumid,semi-arid,arid and hyperarid)and major vegetation types(desert and grassland),and the distribution differences were associated with differences in climate,soil and vegetation conditions in different regions.Our results also indicated that several soil archaea [e.g.,Haloarchaea,Thaumarchaeota and Nanohaloarchaeota],bacteria [e.g.,Acidobacteria,Bacteroidetes and Deinococcus-Thermus],and fungi [e.g.,Glomeromycota and Mucoromycotina] exhibited a wide range of responses to different environmental conditions(e.g.,drought,soil p H,vegetation type,etc.),which implying the ecological niche differentiation existed among different taxa of soil archaea,bacteria and fungi.(4)By analysing the beta diversity patterns of soil microrganisms in the drylands of Northern China,our results showed that soil archaeal,bacterial and fungal communities were not randomly distributed with spatial distance,instead all of them showed significant distance-decayed distribution patterns.Moreover,environmental distance differences explained more of the beta diversity patterns of soil archaea,bacteria and fungi than spatial distance differences,indicating that the deterministic selection is manly ecological processes driving the distribution of soil microbial communities in the drylands of Northern China,but the stochastic processes also play a role.Furthermore,the beta diversity of soil microorganisms in the arid zone of China may be mainly derived from species turnover or turnover components,while the contribution of richness differences or nested components is relatively little.(5)To clarify the ecological mechanisms driving biogeographical patterns of Haloarchaea and Thaumarchaeota,the two main soil archaeal groups found in the drylands across Northern China,we further hypothesized that niche differentiation determines their distinct distributions along habitat specialization gradients.Our results demonstrated that Haloarchaea tended to be specialists whereas Thaumarchaeota to be generalists.As hypothesized,Haloarchaea and Thaumarchaeota showed strong niche differentiation associated with two vegetation biomes(i.e.,deserts vs.grasslands)differing in the degree of habitat specialization.The relative abundance and richness of Haloarchaea were higher in deserts due to specialization to relatively high soil salinity and extreme climatic condition,while those of Thaumarchaeota were greater in grassland soils.Our results further indicated a divergence in community assembly processes underlying the segregated distributions of Haloarchaea and Thaumarchaeota,with Haloarchaea being governed deterministically by homogeneous selection while Thaumarchaeota were assembled mostly via stochastic dispersal limitation.Generally,the distribution patterns of dominant groups of soil archaea,bacteria and fungi in drylands of Northern China are not uniform along the environmental gradient,which may be due to the niche differentiation and unique environmental preference.The alpha diversity of soil archaea and fungi showed an opposite trend and significant difference with the change of drought gradient.Soil microbial community composition and diversity were regulated by the interaction of multiple factors such as space,climate,soil and vegetation.The study reveales the geographical distribution pattern of soil microorganisms in arid regions,elucidates the driving mechanism of microbial diversity,provides the data and research basis for further understanding the security and stability of arid ecosystems under the background of global climate change,and has important scientific significance for ecological protection and restoration in arid areas of China.