Soil Microbial Network Complexity Predicts Ecosystem Multifunctionality Of Alpine Meadows Along An Elevation Gradient On The Tibetan Plateau

The relationship of soil microbial communities and ecosystem functioning is a central subject in ecology.Existing studies typically focused on the role of univariate diversity or composition metrics,but there is still a gap in research involving changes in the co-occurrence network structure of soil microbial communities and their impact on ecosystem versatility.Here,we assessed the relationships between soil microbial communities（fungal and bacterial community）and ecosystem multifunctionality through intensive sampling along a multi-elevation gradient in alpine meadows on the western slope of the Mila Mountain in the Tibetan Plateau.The main findings are as follows:（1）With the increase of altitude,the ecological functions of soil nutrients（SOC、TN、NH4⁺-N、NO3^--N、STP、SAP）and the contents of extracellular enzymes related to soil C,N,and P cycles were significantly weakened.Reduced carbon mineralization,N2O emissions and the resistance to potential fungal plant pathogens significantly increased（P<0.05）.The ecosystem multifunctionality（the average multifunctionality index,principal component multifunctionality index,and multi-threshold multifunctionality index）all increased with altitude and decrease.Soil environmental factors are significantly affected by elevation.With the increase of altitude,soil moisture increased and soil p H changed from alkaline to acidic;soil Al³⁺ion content increased,while Ca²⁺ion content decreased;soil clay content decreased（P<0.05）.（2）The diversity,composition and co-occurrence network structure of bacterial and fungal communities all showed an altitudinal distribution pattern.The bacterial and fungal diversity（the species diversity,Shannon index and Chao 1 index）decreased significantly with increasing altitude（P<0.05）.In the bacterial community,Acidobacteria and Chloroflexi showed an increasing trend;Proteobacteria,Actinobacteria,Bacteroidetes and Firmicutes showed a decreasing trend;Verrucomicrobia generally showed a hump pattern.Basidiomycota in the fungal community showed an increasing trend;Ascomycota and Mortierellomycota showed a decreasing trend in general.Both bacterial and fungal co-occurrence networks are scale-free,small-world and modular,and positive correlations are dominant in the above networks.The co-occurrence network complexity（number of nodes,number of edges,average node neighborhood,edge density）decreased with increasing altitude（P<0.05）.In addition,the species diversity,Shannon index and Chao 1index of bacterial and fungal communities were significantly positively correlated with the co-occurrence network complexity of the corresponding communities（P<0.05）.（3）Although significant positive relationships between microbial diversity and multifunctionality were detected（P<0.05）,this positive correlation could not be maintained,when environmental factors were controlled.Regardless of whether environmental factors are considered,the valid positive relationships can always be found between microbial network complexity and ecosystem multifunctionality（P<0.05）.Variance partitioning analysis（VPA）model and random forest（RF）model also showed that microbial network complexity was more closely related to ecosystem multifunctionality.Multiple regression models（MRM）showed that when both bacterial and fungal network complexity were considered,network complexity was more explanatory of ecosystem multifunctionality.Structural equation modeling（SEM）models suggest that bacterial network complexity and fungal network complexity can jointly promote ecosystem multifunctionality.After considering the effects of climate factors,soil factors and microbial diversity on ecosystem multifunctionality,the valid positive relationships can always be found between soil bacterial and fungal network complexity with multifunctionality.The impact of bacterial and fungal community diversity on ecosystem multifunctionality is indirectly driven by the corresponding community network complexity.In conclusion,network complexity positively drives ecosystem multifunctionality,and when both bacterial and fungal network complexities are considered,network complexity is a better predictor of ecosystem multifunctionality.The driving effect of bacterial and fungal diversity on ecosystem multifunctionality is mainly achieved indirectly through the corresponding community network complexity.Therefore,microbial network complexity can play a key driving role in the relationship between biodiversity and ecosystem function,and the application of this indicator can contribute to a comprehensive understanding of how soil microbial communities maintain ecosystem function in the natural environment.