Effects And Mechanisms Of Warming And Freeze-thaw Cycles On Soil CH₄ Fluxes In Permafrost Region Of The Great Hing’an Mountains

As a sensitive area responding to climate change,the northern high-latitude permafrost regions have experienced rapid climate warming in recent years,with a warming rate almost twice the global average.Climate warming can induce the degradation of permafrost and accelerate the decomposition of organic matter by soil microorganisms,thus releasing greenhouse gases and forming a positive feedback effect between permafrost carbon and climate warming.As an important carbonic greenhouse gas,CH₄ has the warming potential of 25-30 times than CO₂ on a century scale,it contributes approximately 20%of global warming.The changes in CH₄ flux are crucial for the direction and intensity of terrestrial carbon-climate feedback.A series of ecological processes caused by climate warming such as the variations in soil hydrothermal dynamics,changes in soil freeze-thaw environment,succession of vegetation community and degradation of permafrost can affect soil CH₄ flux,resulting in significant uncertainty in CH₄ budget in cold regions.Although extensive researches about soil CH₄ flux had been carried out both domestically and internationally,climate,vegetation,soil physicochemical properties,and microbial communities can all affect soil CH₄ flux.The main factors affecting CH₄ production and oxidation process in different regions are still unclear.Therefore,to accurately estimate the feedback effect of permafrost respond to climate change under climate warming still need to furtherly explore the dynamics of soil CH₄ flux and its potential regulatory mechanism in permafrost regions.In this study,the research object was forest-wetland ecotone in the permafrost region of the Great Hing’an Mountains.Based on field in-situ monitoring and open top chamber（OTC）warming experiments,the spatiotemporal variation characteristics of soil CH₄ flux and its response mechanism to warming were analyzed;Simulate changes in freeze-thaw environments to elucidate the impact mechanism of freeze-thaw amplitude and frequency on soil CH₄ flux;Through aerobic and anaerobic incubation experiments at different temperature gradients,the response mechanism of CH₄ production potential and oxidation potential in active layer and permafrost layer to warming were clarified.The main results are as follows:（1）There was a significant seasonal variation in soil CH₄ flux in the forest-wetland ecotone of the Great Hing’an Mountains,with soil CH₄ flux significantly higher during the growing season than in the thawing period and the freezing period.During the observation period,wetland patches were the source of soil CH₄ emission,while forest patches only exhibited soil CH₄ emission source during the growing season.The dominant methanogens included acetoclastic Methanosarcina and hydrogenotrophic Methanobacterium and Methanocella.The dominant methanotrophs included both type I Methylobacter and type II Methylocystis and Methylosinus.The alpha diversity of methanogens increased with month,and which in wetland patches was higher than in forest patches.However,the alpha diversity of methanotrophs was not sensitive to seasonal and habitat changes,and the season and habitat types had significant effects on the beta diversity of methanogens and methanotrophs.The dependence of soil CH₄ flux on soil temperature and moisture varied seasonally.There was a positive correlation between soil CH₄ flux and temperature and moisture during spring thawing period and autumn freezing period,while during the peak growing season,soil CH₄ flux was linearly negatively correlated with temperature and moisture.The contribution of soil physicochemical factors,methanogens and methanotrophs on soil CH₄ flux was 24.27%,35.46%and 20.87%,respectively.Among them,soil NH₄⁺-N content and water content were important non-biological factors driving soil CH₄ flux.The relative abundance of soil type II methanotroph Methylosinus and the beta diversity of methanogens and methanotrophs were important biological factors changing soil CH₄ flux.（2）Situ OTC warming increased soil temperature by 1.75,1.03,0.80 and 0.62°C at the depths of 10,20,60 and 100 cm,respectively,and significantly increased soil NH₄⁺-N contents,but did not significantly change the relative abundance,diversity and community structure of soil bacterial community.OTC warming reduced the contribution of soil temperature and moisture on soil CH₄ flux,but enhanced their combined contribution.During the observation period,the average soil CH₄ flux was13.61±1.08 nmol m^-2 s^-1.OTC warming significantly promoted soil CH₄ emissions with the average flux of 35.51±2.90 nmol m^-2 s^-1.OTC warming treatment increased the sensitivity of soil CH₄ flux to deep soil temperature.In addition,soil SOC,NH₄⁺-N contents,bacterial community composition and diversity were significantly positively correlated with soil CH₄ flux.In summary,warming or thawing of deep soil in permafrost regions may further promote soil CH₄ emissions by increasing the utilization of microbial substrates.（3）Freeze-thaw cycles stimulated the release of available substrates such as DOC,NH₄⁺-N and NO₃^--N,and reduced the alpha diversity of soil bacteria.Freeze-thaw cycles increased soil CH₄ emissions in forest and wetland patches,and even changed the source and sink patterns of soil CH₄ in forest patches.The soil CH₄ emission rate was higher under high freeze-thaw amplitude,but the increase gradually decreased with freeze-thaw frequency.Under the influence of freeze-thaw cycles,soil CH₄ emission rate was mainly affected by the relative abundance of Proteobacteria and soil inorganic nitrogen contents.Therefore,the estimation of CH₄ flux in permafrost regions by climate-carbon feedback model should fully consider the complex freeze-thaw cycle process under climate warming.（4）The SOC,DOC contents and soil temperature in the active layer of forest-wetland ecotone were significantly higher than those in the permafrost layer in permafrost regions of the Great Hing’an Mountains.The abundance of mcr A was higher in the permafrost layer of forest patches,while the highest abundance of pmo A was found in the surface organic layer.There was no significant difference in the abundance of mcr A and pmo A among soil layers in wetland patches.The alpha diversity of methanogens in the deep mineral layer was higher than that in the surface organic layer and permafrost layer in forest and wetland patches,while the alpha diversity of methanotrophs increased with soil depth in forest patches and decreased in wetland patches.Soil CH₄ production potential of permafrost layer was higher than that of active layer,while the CH₄ oxidation potential of active layer soil was significantly higher than that in permafrost layer.Overall,soil CH₄ production and oxidation potential of wetland patches were significantly higher than those in forest patches,and warming significantly increased the CH₄ production and oxidation potential of two patches.Soil physicochemical properties and microbial characteristics could explain 81%and 93%of the changes in CH₄ production and oxidation potential.Among them,the abundance of soil mcr A and the diversity of methanogen had direct positive impacts on soil CH₄production potential.Soil p H and soil carbon and nitrogen substrates were the main indirect factors affecting soil CH₄ production potential;the abundance of soil pmo A and water content had direct positive effects on soil CH₄ oxidation potential,while the community structure of soil methanotroph and p H level had direct negative effects on soil CH₄ oxidation potential.In summary,habitat（patch types）had an indirect impact on soil CH₄ production and oxidation potential by affecting soil carbon and nitrogen substrates,water content,electrical conductivity and p H level,while soil depth had an indirect impact on soil CH₄ production and oxidation potential through changing soil temperature,water content,carbon substrate and electrical conductivity.