Effects Of Alpine Meadow Degradation On Soil Nitrogenase Activity And Free-living Diazotrophs

Free-living nitrogen fixation reduces N₂in the air to ammonium nitrogen by nitrogen-fixing microorganisms that can be directly used by other organisms,which is one of the important nitrogen fixation methods in ecosystems.especially in grassland ecosystems.The alpine meadows of the Qinghai-Tibet Plateau,with high altitudes and a humid and cold climate all year round,have been degraded to varying degrees in recent years.The changes of free-living diazotrophic communities and nitrogenase activity and its influencing factors in degraded alpine meadow ecosystems are needed to be further studied.In this study,high-throughput sequencing technology of nifH gene amplicon was used to analyze the diversity and community composition of diazotrophs,and acetylene reduction method was used to measure nitrogenase activity.We studied the spatial differentiation of soil nitrogenase activity and free-living diazotrophic communities in alpine meadows and their influencing factors.The main conclusions are as follows:1 Spatial distribution characteristics and influencing factors of nitrogenase activity in degraded soils of alpine meadows in Northwestern Sichuan:（1）The average nitrogenase activity was the lowest in GZ（Ganzi County）degraded soils of alpine meadows in Northwestern Sichuan,while SD（Seda County）had the highest average nitrogenase activity.（2）The average abundance of nifH genes in SQ（Shiqu County）was the lowest in the degraded soils of alpine meadows in Northwestern Sichuan,and the average abundance of nifH genes in SD was the highest.（3）Diazotrophs had the greatest impact on nitrogenase activity.The abundance of nifH gene and the community structure of diazotrophs had a significant impact on soil nitrogenase activity,and there was a significant positive correlation between the abundance of nifH gene and soil nitrogenase activity;The results of SEM analysis showed that there was a significant negative correlation between the community structure of diazotrophic cummunity and soil nitrogenase activity.（4）Soil physical and chemical properties had an important influence on soil nitrogenase activity.TC had the highest correlation with nitrogenase activity.TP could directly or indirectly affect nitrogenase activity by affecting the community structure of diazotrophs,that is,nitrogenase activity is limited by phosphorus..2 The spatial differentiation and main driving factors of the diversity,community structure and composition of free-living diazotrophs in the degraded soils of alpine meadows in Northwestern Sichuan:（1）SQ（Shiqu County）site had the lowest average alpha diversity of diazotrophs,and SD（Seda County）site had the highest.（2）There were significant differences in the community structure of diazotrophs between different sites.（3）pH was the key factor driving the community characteristics of free-living diazotrophs in degraded soils of alpine meadows in Northwestern Sichuan.There were significant differences in soil diazotrophic community composition and diversity under different pH conditions.（4）The construction of free-living diazotrophic communities in degraded soils of alpine meadows in Northwestern Sichuan was dominated by deterministic process.The diazotrophic communities show phylogenetic clustering along the pH gradient,and the pH changes increased the community phylogenetic turnover of diazotrophs.In addition to pH,AI also had a greater impact on theβ-diversity of the diazotrophic communities.3 Degradation affected nitrogenase activity and diazotrophic community characteristics:（1）The nifH gene abundance and nitrogenase activity in different degraded alpine meadow soils in Northwestern Sichuan decreased according to the aggravation of degradation degree,such as LD（Light degeneration）,MD（Moderate degeneration）,SD（Severe degeneration）.（2）Theα-diversity of soil free-living diazotrophs decreased with the degree of degradation,and the soil diazotrophic community had obvious structural differentiation along the degradation gradient.The dominant groups of diazotrophs in different degraded alpine meadow soils in Northwestern Sichuan included Alphaproteobacteria,Betaproteobacteria,Gammaproteobacteria,Nitrospira,etc.;（3）The deterministic processes dominated the community assembly of diazotrophs in soil under different degradation gradients,and the relative contribution of the deterministic processes decreased as the degradation degree increased.The pH increased with the degradation gradient,and pH was the key factor that caused the differences in the community structure of diazotrophs with different degradation degrees.In conclusion,this study explored the effects of alpine meadow degradation on nitrogenase activity and free-living diazotrophic community,and their key driving factors in Northwestern Sichuan.It also revealed the the importance of biotic and abiotic factors on soil nitrogen fixation in degraded alpine meadows,which provided a reference for nutrient management of alpine meadow soils and soil fertility maintanence in degraded alpine meadows.