The Mechanism And Characteristics Of Soil Microbial Community Under The Altitude Gradient And Warming Experiment In Natural Larix Gmelinii Forests In Cold-temperate Zone

The Greater Khingan Mountains is the largest forest distribution area with the highest latitude in China.The natural forest of Larix gmelinii is the zonal vegetation in the northern of the Greater Khingan Mountains,and is also an important part of the cold temperate coniferous forest ecosystem,exhibiting the pivotal ecological service function.To explore the effects of climate change（altitude gradient and simulated warming）on soil microbial community characteristics（activity,diversity,community composition,function,and community assembly,etc.）,which contribute to evaluating the functions of soil ecosystems in larch natural forests in this region.It is of great significance for a profoundly understanding of the distribution pattern of soil microbial communities in alpine ecosystems.In this work,the larch natural forests located at Mt.Oakley（the highest peak in the north of the Greater Khingan Mountains）,Xianfeng Forest farm,and Nannianghe Forest Farm were selected as research objective.First,based on the natural platform formed by altitudinal gradient,we conducted the patterns of soil bacterial and fungal communities along an altitudinal gradient among different seasons in two contrasting soil layers.It attempts to reveal the main drivers affecting the spatiotemporal variations of soil microbial communities.Subsequently,by the short-term simulated warming experiment（A Reciprocal Translocation Experiment）,further elucidating the responses of soil bacterial and fungal diversity,community composition and interplay between microbial community members as well as community assembly to warming.The main findings are as follows:（1）With the increase of altitude,soil microorganisms had a preference for the production of C-and N-acquiring enzymes,and the stronger nitrogen limitation was observed in high altitude and 10–20 cm soil layers.There were diverse responses of different soil extracellular enzyme activities to the altitude gradient in different seasons.In May,α-glucosidase（a G）activity in both soil layers decreased with increasing altitude,except for the a G,cellobiohydrolase（CBH）and L-leucine aminopeptidase（LAP）,others were increased with increasing altitude.The oxidative enzymes activity（Polyphenol oxidase（PPO）,Peroxidase（POD））were highest in mid-altitude sites.Altitude and season had a significant effect on soil enzyme C:N,C:P and N:P.In the contrast to mid-altitude sites,the higher soil enzyme C:N in 0–10 cm soil layer was found in low-altitude sites among three seasons.Soil temperature（ST）and p H were the main drivers affecting the nine enzyme activities along an altitudinal gradient.In addition,the soil bulk density（BD）,moisture,total phosphorus（TP）and nitrate nitrogen（NO₃^--N）played the critical roles in 10–20cm soil layer in May.BD,moisture and TP mediated the variations of soil extracellular enzymes in September.（2）Altitude had a stronger effect on soil fungal community composition than season and soil depth.Altitude affected significantly the dominant taxa of soil fungi and relative abundance of functional groups.The more complex co-occurrence networks were observed in May and September,and stochastic process dominated the community assembly of soil fungi.Ascomycota and Basidiomycota were the dominant phyla of soil fungi,while Agaricomycetes,Leotiomycetes,and Eurotiomycetes were the dominant classes.Altitude had a significant effect on the relative abundance>5%of soil fungal taxa and functional guilds（except for fungal parasite）.The fungalαdiversity（richness,Shannon and Chao 1）in 0–10 cm soil layer was significantly varied with altitude.Altitude had a stronger effect on soil fungalβdiversity than season and soil depth.Compared to the results in July,the soil fungal networks in both soil layers were more complex in May and September,with the characteristics of higher nodes,links and connectivity among modules.The spatiotemporal succession generated by season and soil depth induced the niches of keystone species and roles of functional guilds in soil fungal community.More importantly,soil fungal community assembly was predominantly controlled by stochastic processes（mainly drift）.Despite all that,soil p H and ST,ammonium nitrogen（NH₄⁺-N）,moisture and dissolved organic nitrogen（DON）,p H and BD were the crucial factors affecting the soil fungal community composition in 0–10 cm and 10–20 cm soil layers in May,July and September,respectively.（3）Season had a stronger effect on soil bacterialβdiversity than altitude and soil depth.Altitude significantly shaped the relative abundance of dominant taxa and functional groups of soil bacteria.The higher modular community was found in deeper soil layer,and the soil bacterial community was governed by deterministic processes.Proteobacteria,Acidobacteria,Actinobacteria and Chloroflexi were the dominant phyla,while Alphaproteobacteria,Acidobacteriia and Actinobacteria were the dominant classes of soil bacterial community.Altitude had a significant effect on dominant phyla and classes.Generally,the soil bacterialαdiversity decreased with an increase of altitude across all seasons.Season had the greatest effect onβ-diversity of soil bacterial communities,followed by altitude.In the0–10 cm soil layer,the co-occurrence network in September possessed the characteristics with lower average degree,lower average clustering coefficient,higher average path distance and modular.In 10–20 cm soil layer,the network in July had lower level of topological feature.In addition,the bacterial networks had higher modular in deeper soil across three seasons.Different from the results of soil fungi,the bacterial communities were dominated by deterministic processes,especially by variable selection.The variation of soil factors significantly affected the soil bacterial community composition.（4）Short-term warming alleviated the carbon-limited status of soil microbes,promoted the relative abundance of dominant taxa and functional groups,and altered bacterial and fungal community diversity and stability of co-occurrence networks.After 1a warming manipulation,the relative abundance of nitrogen fixation and cellulolytic bacteria increased and decreased 19.7%and 63.9%,respectively.The relative abundance of ectomycorrhizal fungi,lichenized and plant saprotroph increased 188.7%,56.2%and 210.1%compared to control treatment,respectively.The climate had a stronger effect on soil microbes than site.The short-term warming increased and decreased the bacterial and fungal diversity,respectively.Compared to the site,climate change had a stronger effect on soil microbialβdiversity.In addition,the co-occurrence networks of soil bacteria and fungi were obviously increased in warming treatment,and short-term warming promoted the stability of soil microbial community,as well as varied the network topological features.Moreover,short-term warming changed the community assembly processes of soil bacteria and fungi.The bacterial community was primarily governed by stochastic and deterministic processes,while fungal community was predominantly controlled by stochastic processes.Warming boosted the variations of three main soil extracellular enzyme activity,and alleviated the carbon limitation of soil microbes when they suffered from the harsh environment.In conclusion,in this study,based on the platform of altitudinal gradient and short-term warming faculty,we exerted various bioinformatic analysis to explore the response of soil bacterial and fungal community characteristics to climate change.Our results confirms that the nitrogen limitation of soil microorganisms was stronger in high-altitude and topsoil,and the variations in extracellular enzyme activities microorganism were mainly regulated by soil temperature and p H.In addition,the effects of altitude gradients and warming on fungal and bacterial community characteristics primarily depended on the combined effects of soil factors on microbial community composition,diversity,microbial interactions,and community assembly processes.Taken together,future climate change will alleviate the carbon-limited and nitrogen-limited states of soil microbes,and promote the stability and complexity of microbial communities.These findings provide basic information for understanding soil ecological functions in larch natural forests.It is necessary to combine multi-omics approaches to illustrate the key ecological functions of soil microbes inhabiting larch forest soil and their roles with tree growth in further research.