Response Of Methanotrophic Community To Soil Degradation In Alpine Meadow Of Northwestern Sichuan

Alpine meadow has significant ecological and economic value,while grassland degradation is a major challenges facing northwestern Sichuan.The main grassland type in northwestern Sichuan is alpine meadow.However,the grassland degradation has a significant impact on soil properties,plant communities and biogeochemical cycle.Methane（CH₄）is a greenhouse gas,and its atmospheric concentration affects the greenhouse effects.Terrestrial soil is a methane sink,which is mainly absorbed by aerobic methane-oxidizing bacteria in.These bacteria are crucial in slowing down the greenhouse effect by oxidizing CH₄into CO₂.Understanding the effects of grassland degradation on aerobic methane-oxidizing bacterial community and diversity is important for elucidating the structure and function of ecosystems and restoring degraded grassland.However,the effects of alpine meadow degradation on the diversity,community structure and function of methanotrophic bacteria remain unclear.In this study,221 soil samples were collected from northwestern Sichuan.All sampling sites were classified as lightly degraded（LD）,moderately degraded（MD）and severely degraded（HD）according to national and local standards.Pmo A gene amplicon sequencing,real-time PCR,bioinformatics and statistical analysis were used to systematically study the diversity,community composition,network structure characteristics,community assembly processes and key influencing factors of aerobic methane-oxidizing bacteria in alpine meadow soil at different degradation levels in northwestern Sichuan.The results were as follows:（1）Changes in the abundance,community composition and diversity of aerobic methane-oxidizing bacteria at different degradation levels:①There was no significant difference in the abundance of aerobic methane-oxidizing bacteria between light and moderate degradation,but a significant decrease was observed at severe degradation.The key factor affecting the abundance of methanotrophic bacteria was soil C/N ratio.②The compositions of methanotrophs mainly included AOB-rel,TUSC,RA21,USC-g,USC-α,Crenothrix and Mcystisand,and was primarily affected by soil p H.③While there was no significant difference in the alpha diversity of methanotrophic bacterial communities among different degradation levels,a significant difference in beta diversity were observed.The key factors affecting beta diversity were annual average temperature and aridity index.（2）Changes in the co-occurrence network and community assembly processes of methanotrophs at different levels of degradation:①With the increase of degradation the co-occurrence network became more complex yet less coherency.Clustering coefficient,average path length,and modularity represent the microbial coherency that make up the network.The topology structure parameters of methanotrophic network differed significantly among the three degradation levels,and they showed significant differences between mild and moderate degradation,between moderate and severe degradation,and no significant differences between mild and severe degradation.The network complexity（edge and average degree）of methanotrophic bacteria in moderately degraded soil were the lowest.The network complexity of methanotrophic bacteria in severely degraded soil were the highest.The changes of methanotrophic bacteria network topology parameters in lightly and severely degraded soils were predominantly affected by environmental variables,while the changes in moderately degraded soil was mainly affected by biological variables.The variables with the greatest relative contribution were p H（LD）,Shannon index（MD）and C:N（HD）.In contrast to most previous studies,the community assembly processes for methanotrophic bacteria were mainly stochastic.（3）Responses of rare,moderate and abundant taxa of methanotrophic community to degradation in the alpine meadow,and key driving factors:①the common ASVs shared among the three degradation levels were mainly abundant taxa（relative abundance>0.1%）,while the unique ASVs were mainly rare taxa（relative abundance<0.01%）.Only rare taxa showed significant differences in alpha diversity among different degradation degrees.②However,beta diversity of different taxa groups significantly varied among different degradation groups,and the community structure of abundant taxa was most similar to that of the total community.③The assembly of the three methanotrophic subcommunities was dominated by random processes.In summary,this thesis investigated the impact of grassland degradation on the diversity,community composition,network structure characteristics,community assembly processes of methanotrophic bacteria and analyzed the differences and similarities between rare,moderate,and abundant taxa.Key factors affecting the methanotrophic community were also identified.The findings of this study enhance our understanding of the structure and functions of soil microorganisms in alpine meadow,and help us to predict the responses of methanotrophic bacteria to degradation.The results also provided a theoretical basis for mitigating methane emissions,and scientific and technological support for the protection and restoration of degraded grassland.