Effects Of Warming And Freeze-thaw On Soil CO₂ Emissions And Dissolved Organic Matter In Alpine Peat Wetlands

The aggravation of global warming and associated changes of soil freeze-thaw pattern,has been affect the structure and abundance of functional microbial communities related to soil carbon transformation processes,the quantity and quality of dissolved organic matter(DOM),and alter the flux of soil carbon dioxide(CO₂)emissions,thus affecting the biogeochemical cycling of carbon and other elements.At present,existing studies weren't systematic on DOM in wetland soils.This dissertation has taken Zoige alpine peat wetland as the research project.Through field warming and laboratory freeze-thaw incubation experiments,CO₂ fluxes at the soil-atmosphere interface were measured by static chamber-gas chromatography,and the EEM-PARAFAC model combined with UV-vis absorption spectroscopy and other spectroscopic techniques were used to analyze the response of dissolved organic matter(DOM)composition to warming and freeze-thaw.We further determined the structure of soil microbial community,the abundance of key functional genes and the activity of major enzymes for carbon conversion by molecular biology,so as to elucidate the microbial driving mechanism of soil CO₂ emission by temperature increase and freeze-thaw.The main results showed below.Field open-top chambers(OTC)warming led to warm drying of the soil,and gradient warming(increasing low,medium and high temperatures)increased soil temperature(5 cm below ground)by 0.51,0.77 and 1.46°C on average,and significantly reduced soil water content.The warming treatments promoted soil CO₂ emission by 15.87%,27.64%and38.31%,respectively.Temperature increase increased the amount of soil dissolved organic carbon,and its content increased on average by 7.5%(low temperature),10.60%(medium temperature)and 20.33%(high temperature)compared to the control.The contribution of humic acid-like substances in soil DOM decreased,the aromaticity was weakened,the degree of humification was reduced,the complexity of DOM structure was weakened and the stability was reduced.Laboratory simulations of freeze-thaw induced fragmentation of soil macroaggregates and reduced stability.Daily freeze-thaw and deep freeze-thaw resulted in an average decrease of 71.96%and 69.09%in soil macroaggregates,respectively,while dissolved organic carbon(DOC)content increased by an average of 4.68%and 9.13%,respectively.The destruction of soil structure and microbial mortality caused by freeze-thaw released more available substrates and promoted soil CO₂ emissions.Freeze-thaw caused an increase in the species diversity of microorganisms in the soil and promoted soil polyphenol oxidase,?-1,4 glucosidase and?-1,4 xylosidase activities,but inhibited soil peroxidase activity.In addition,freeze-thaw increased the abundance of hard-to-degrade carbon(cellulose,chitin,and lignin)genes,accelerated the mineralization of hard-to-degrade organic matter,and produced a positive feedback on global warming.Warming has increased CO₂ emissions and microbial richness and activity in alpine peat wetlands limited by low temperatures.In connection with the fact that the actual warming trend on the Tibetan Plateau is higher than the national average,the response and mechanism of this region to warming should receive more attention.