Effects Of Simulated Warming On Litter Microbial Community Structure And Extracellular Enzyme Activities In An Alpine Timberline Of Western Sichuan

The decomposition of litter is a key step in the circulation of nutrients between the aboveground and underground parts of the forest ecosystem.Microorganisms are the main decomposers in the litter decomposition process.They are directly involved in litter organic carbon decomposition,carbon mineralization,conversion of greenhouse gases between litter and the atmosphere,and nutrient cycling processes to control the rate of litter decomposition.Microorganisms convert organic macromolecules in litter into small inorganic molecules for the absorption of animals and plants by producing extracellular enzymes related to nutrient cycling.Therefore,the number of microorganisms,structural characteristics and enzyme activity characteristics during litter decomposition are extremely important for the rate of litter decomposition.Microbial and enzyme activities are mainly controlled by environmental factors,litter substrate quality and so on.Global warming and tree types will have a significant impact on microbial and enzyme activities.At present,the research on the response of microorganisms and enzyme activities to global warming is increasing day by day,but most of these studies are focused on forest soils,and the results are quite different.Will climate warming affect microbial,extracellular enzymes,ecoenzymatic stoichiometry and microbial relative nutrient limitation in the process of forest litter decomposition in an alpine timberline ecotone of western Sichuan?There is still a lack of relevant research in this area.Therefore,in this experiment,an open-top chamber(OTC)was used to simulate the temperature increase,and the experiment was carried out in an alpine timberline ecotone of western Sichuan.The dominant species Abies fargesii and Sorbus rufopilosa was selected as the research object,systematically studied the decomposition rate,microbial biomass carbon and nitrogen,microbial content(bacteria,fungi,gram-negative bacteria,gram-positive bacteria,total microbial biomass)and their community structure characteristics,enzyme activity and their stoichiometric response to warming.The results showed that:1.After two years of decomposition,the mass loss of the litter of Sorbus rufopilosa reached 58.02%-59.02%,and the mass loss of Abies fargesii reached 30.11%-30.61%.Simulated warming had no significant impact on litter decomposition rate and mass loss.Season and tree species had a significant effect on the litter decomposition rate.Litter decomposition rate and mass loss of Sorbus rufopilosa in the same sampling period were higher than those of Abies fargesii.2.Simulated warming had no significant effect on the microbial biomass carbon and nitrogen and microbial community structure of the two tree species litter.But there was an obvious seasonal effect on the microbial biomass and microbial community structure during litter decomposition.The microbial biomass nitrogen,fungi,gram-positive bacteria,and fungi/bacteria in the growing season of Sorbus rufopilosa were higher than those in the non-growing season,while gram-negative bacteria,gram-negative bacteria/gram-positive bacteria were lower than non-growth season.Microbial biomass nitrogen,fungi,gram-negative bacteria,fungi/bacteria and gram-negative bacteria/gram-positive bacteria in the growing season of Abies fargesii were higher than non-growth season,while gram-positive bacteria was lower than the non-growing season.Tree species significantly affected the microbial community structure during litter decomposition.Microbial biomass carbon and nitrogen,total microbial biomass,fungi,bacteria,gram-negative bacteria,gram-negative bacteria/gram-positive bacteria of Sorbus rufopilosa were higher than the litter of Abies fargesii,while gram-positive bacteria was lower than that of Abies fargesii.During the decomposition of litter,the microbial community structure was regulated by the litter substrate quality and environmental factors,the microbial content(bacteria,fungi,gram-negative bacteria,gram-positive bacteria,total microbial biomass)and biomass carbon and nitrogen were significantly positively correlated with N,N/P,moisture content and positively correlated with C/N.There was no significant difference in fungi/bacteria between Sorbus rufopilosa and Abies fargesii in different sampling periods,while gram-negative/gram-positive bacteria of Sorbus rufopilosa was higher than that of Abies fargesii.3.Simulated warming had no significant effect on the enzyme activities of the two tree species.There was a clear seasonal effect of enzyme activities during litter decomposition.The acid phosphatase,?-glucosidase,cellobiohydrolase,leucine aminopeptidase and 1,4-?-N-acetylglucosaminidase of Sorbus rufopilosa showed a trend of increasing first and then decreasing.The content of peroxidase and polyphenol oxidase in growing season was higher than that in non-growing season.The tree species significantly affected the enzyme activity characteristics during litter decomposition.The enzyme activities of Sorbus rufopilosa were higher than that of Abies fargesii.The enzyme activity was significantly positively correlated with N and water content and significantly negatively correlated with C/N.4.Simulated warming had no significant effect on the ecoenzymatic stoichiometry and microbial relative nutrient limitation of the two tree species litter.Simulated warming had not changed the microbial nutrient demand status of the litter decomposition in an alpine timberline ecotone of western Sichuan.At the end of the first non-growing season and at the end of the second growing season,the microbial relative carbon limit and C:N_EEA in Abies fargesii was higher than that of Sorbus rufopilosa,while the N:P_EEA and microbial relative nitrogen and phosphorus limits were lower than that of Sorbus rufopilosa.C.P_EEA of Sorbus rufopilosa was higher in different decomposition periods.Sorbus rufopilosa was restricted by relative nitrogen in the early stage of litter decomposition,and as the decomposition proceeded,the nitrogen restriction shifted to phosphorus restriction.Abies fargesii was restricted by phosphorus in the early stage of litter decomposition,and then changed to be restricted by nitrogen as the decomposition proceeded,and finally changed to restricted by phosphorus.In summary,simulated warming had no significant effect on the microbial and extracellular enzyme activities of the two tree species,and the microbial and extracellular enzyme activities during the litter decomposition process show a significant seasonal effect.The tree species type will significantly affect the litter decomposition process.In the same decomposition period,the microbial biomass carbon and nitrogen,total microbial biomass,fungi,bacteria,gram-negative bacteria,gram-negative bacteria/gram-positive bacteria and enzyme activities in Sorbus rufopilosa were higher than Abies fargesii.Total nitrogen content,C/N,and moisture content were the main factors that regulating the activity of microorganisms and enzymes during the decomposition of litter.The results are of great significance for understanding the biochemical cycle process of forest litter under the background of global warming.