Differences Of Soil Function And Nitrogen-Fixing Microorganisms And Their Influencing Factors Above And Below Typical Alpine Treeline

Biological nitrogen fixation is the main source of nitrogen in natural ecosystems,the abundance and community composition of nitrogen-fixing microorganisms plays an important role in soil nitrogen supply and soil fertility maintenance.Soil respiration and soil extracellular enzymes are important components of soil function,they play a key role in maintaining ecosystem stability.The alpine treeline serves as a transition zone between forest and shrub（or grassland）ecosystems,extremely sensitive to global climate change.To reveal the differences between nitrogen-fixing microorganisms and soil functions at the upper and lower limits of alpine treeline and their regulatory factors,it is great significance to predict the response of biogeochemical cycles to future climate change.In this paper,the transition zone of treeline between sunny slope of Baima Snow Mountain and the shady and sunny slopes of the Sejila Mountain are taken as the research object,soil respiration,extracellular enzyme activity,community diversity and structure of nitrogen-fixing microorganisms were analyzed and compared at the upper and lower limits of treeline,the main regulatory factors are also explored.Specific research results are as follows:（1）Affected by soil temperature（ST）,soil moisture content（SM）,total nitrogen（TN）and total carbon（TC）,the difference of soil function between the upper and lower limits of treeline was not consistent in the sunny slopes of Baima Snow Mountain and Sejila Mountain.Soil respiration and most extracellular enzymes at the lower limit of the treeline were significantly higher than the upper limit of the treeline;there was no significant difference on the sunny slope of Sejila mountain or the lower limit of treeline was lower than the upper limit of treeline.Soil respiration and extracellular enzyme activities on the sunny slope of Baima Snow Mountain were positively correlated with ST,SM,TN and TC,there was a significant negative correlation between soil urease（SU）and β-glucosidase（BG）with soil p H on the sunny slope of Sejira Mountain,soil basal respiration（SBR）,phosphatase（PHOS）and β-xylosidase（XYL）were positively correlated with SM,SU,soil sucrase（SSC）and XYL were positively correlated with ST,while β-1,4-n-acetylglucosaminidase（NAG）was negatively correlated with ST,SBR,soil induced respiration（SIR）,SSC and α-1,4-glucosidase（AG）were positively correlated with TN,SBR,PHOS and AG were positively correlated with TC.（2）Nitrogen-fixing microorganisms and nitrogen-cycling enzymes showed the same trend in the shady slope and sunny slope of Sejila Mountain,but the main influencing factors were different in different slope direction.The abundance of nitrogen cycling enzyme NAG and nif H genes at the lower limit of the treeline was higher than that at the upper limit of the treeline,and they were mainly affected by NO₃^--N and C/N on the shady slope of Sejila Mountain,the sunny slope was mainly affected by TN,TC and NH₄⁺-N.At the same time,the results of RDA analysis showed that NH₄⁺-N,C/N,SM and ST were the main factors causing the difference between the shady slope and the sunny slope of Sejila Mountain.（3）The dominant azotobacter genera had little difference between the upper and lower limits of treeline,and there was a big difference between different slope direction.Proteobacteria was the dominant phylum in this study area,the dominant genera were Nitrospirillum,Paraburkholderia and Geotalea,they all belong to Proteobacteria.Correlation analysis showed that C/N,NH₄⁺-N,NO₃^--N and SM significantly affected most dominant azotobacter genera,however,the correlation results between different azotobacter genera and soil properties were very different.In conclusion,the difference between the upper and lower limits of the treeline may not had a direct effect on nitrogen-fixing microorganisms and soil function,but different soil temperatures,different soil moisture content and differences in soil nutrients caused by different vegetation types were important regulators of nitrogen fixing microorganisms and soil function.This study provided new knowledge for the study of nitrogen-fixing microbial diversity and soil ecosystem function in alpine treeline ecosystem,it provided a scientific basis for the evaluation of alpine soil ecological function under climate change.