Effects Of Simulated Freezing-thawing Processes On Carbon And Nitrogen Gas Exchanges In Shallow Soil Of Permafrost Wetland

According to the fifth assessment report of the Intergovernmental Panel on Climate Change?IPCC?,with the increase of global average surface temperature,the permafrost area at high latitudes of the northern hemisphere will be greatly reduced.And it is expected that by the end of the 21st century,the permafrost area in the region will be reduced by 37%-81%compared with that in the end of the 20^th century.With the degradation of permafrost,the water,heat and freeze-thawing conditions of frozen soil will change drastically,and the freeze-thawing cycle of soil will accelerate the process of soil carbon and nitrogen turnover and carbon and nitrogen gas emissions in early spring,thus producing positive feedback on global climate change.In order to find out the influence of climatic factors?interannual variation of rainfall and soil freeze-thawing cycle?on carbon and nitrogen exchange processes in typical terrestrial ecosystems of permafrost regions,this paper takes the peat mire in the permafrost region on the north slope of Great Hing'an Mountains as the research object.In order to avoid the disturbance that field observation on fragile frozen soil environment during the freeze-thawing period,the large-scale soil column?diameter:15cm;height:60 cm?was combined with the indoor simulation experiment,static chamber-gas chromatography method and dynamic chamber-chemiluminescence method were used to quantify the exchange fluxes of variable components?methane,carbon dioxide and nitrous oxide?and strongly variable component gas?nitric oxide?at the terrestrial-atmosphere interface,respectively.In order to establish a dynamic chamber method observation standard to measure the fluxes of strongly variable components,reveal the influence of the freeze-thawing cycle on the exchange fluxes of carbon and nitrogen gases,and explore the net greenhouse gas budget status in the peat wetland ecosystem during the freeze-thawing period as well as the influence degrees of annual change of precipitation on the above-mentioned freeze-thawing effect.Therefore,the treatments with 30,80 and 130 mm precipitation?R30,R80 and R130?was set to simulate the precipitation during the freezing period?from October to April of the next year?in dry,normal and rain-rich years,respectively,where each treatment contained three spatial replicates,and all treatments consisted of nine soil samples.The temperature changes of three freeze-thawing cycles were simulated the trend of spring air temperature in permafrost regions,after the three-time freeze-thawing cycle processes,the soil physical and chemical properties of all soil samples were determined by destructive sampling method.The main findings of this paper are as follows:1.Effects of freeze-thaw cycles on CH₄,CO₂ and N₂O exchange flux and net greenhouse gas budget in peat swamps?1?Impacts of freeze-thaw process on the exchange of greenhouse gases?CH₄,CO₂and N₂O?in peat swampsThe exchange fluxes of CO₂ and N₂O in the whole culture period showed obvious freeze-thaw effect,that is,the soil thawing process promoted the emission of CO₂ and N₂O,but the promoting effect gradually weakened with the increase of the number of freeze-thaw cycles and the consumption of the soil substrate.The exchange fluxes of CO₂ and CH₄ showed sensitivity to temperature.With the increase of temperature,the total respiration rate of the marsh wetland ecosystem increased significantly?p<0.001?,and gradually shifted from atmospheric CH₄ absorption to CH₄ emission.The freeze-thaw period is a critical period for N₂O emissions from marsh wetlands.To accurately characterize the annual cumulative emissions and emission characteristics of permafrost ecosystems,it needs to be special paid to the interannual variability of freeze-thaw emissions and freeze-thaw effects.?2?Source and sink intensity of CH₄,CO₂ and N₂O in peat swamp during freeze-thaw periodDuring the freeze-thaw period,the marsh wetland is mainly characterized by the absorption of atmospheric CH₄ rather than the strong emission source during the growing season,and the sinking intensity of the freeze-thaw period is at least two orders of magnitude lower than that of the growing season.The swamp wetland during the freeze-thaw period is a source of net CO₂ emissions,and the swamp wetland in the freeze-thaw period is generally a N₂O emission source.As the number of freeze-thaw cycles increases,it was converted from weak N₂O emission sources to weak N₂O absorption sinks at the treatment of the 30 mm and 80 mm precipitation,N₂O emission source intensity gradually weakened at the treatment of 130 mm precipitation.?3?The impact of interannual variation of precipitation on the freeze-thaw effect and the net greenhouse gas budget:The total respiratory rate of the ecosystem during the freeze-thaw period dominates the net income of greenhouse gases.Therefore,the swamp wetlands in the different precipitation years showed a positive radiative forcing effect(calculated by the CO₂equivalent of the 100-year time scale,and the NGHGB treated by R30,R80 and R130were 4678.2±615.5,2955.8±258.9 and 1951.1±317.3 kg CO_2-eq ha^-1,respectively).Compared with the dry and normal years,the freeze-thaw process and temperature increase during the rainy season stimulated the N₂O and CH₄ emissions,making the freeze-thaw period a critical period for annual cumulative emissions,but the CO₂emission flux decreased,and the total net greenhouse gas budget was only 2/3 of the regular year and 2/5 of the dry year.Therefore,during the rainfall year the positive raining forcing effect was weakened and the soil carbon pool was accumulated.2.Effects of freeze-thaw cycles on NO exchange flux in peat swamps?1?Dynamic chamber observation standard for the determination of nitric oxide?NO?flux:The equilibrium state means that the NO concentration in the dynamic chamber does not change with the time of the cover box,in other words,the gas concentration in the dynamic chamber reaches a stable state,balance time?T?as the cover box to stable gas concentration?equilibrium?time.Obtaining the equilibrium state and balancing time T depends on three factors:the flow rate?Q?of the input synthesis air,the NO emission flux level(F_NO),and the reaction rate of NO and O₃.In the equilibrium state,the closer the NO concentration in the tank is to the ambient atmosphere,the better the difference between the concentration of NO in the tank and the synthetic air can be detected,and the difference value is the smaller the better.The shorter the equilibrium time T is better,T depends on the relative size of Q and F_NO.The larger the F_NO will have the longer the balancing time,and the larger the Q will have the shorter the balancing time.Controlling Q in the same direction with F_NO to maintain the optimal equilibrium time range?3⁶ min?.What's more,the selection of carrier gas,the pressure difference between inside and outside the chamber??2 Pa?,and the accurate recording of equilibrium state time and concentration data would be also standardized.?2?Effect of freeze-thaw process on NO gas exchange in marsh wetland and source/sink intensity of NOThe exchange flux of NO showed obvious freeze-thaw effect,that is,the soil thawing process promoted the discharge.As the number of freeze-thaw cycles increased and the main microbial process transformed,the promoting effect was weakened at first and then enhanced.The exchange flux of NO showed sensitivity to temperature,as the temperature increased,the NO flux level of the marsh wetland increased exponentially?p<0.001?.During the freeze-thaw period,the marsh wetland shows a net NO emission source.In the future,observations during the freeze-thaw period should be strengthened to more accurately quantify the cumulative emission characteristics and levels of NO gas in the permafrost ecosystem.?3?The effect of interannual variation of precipitation on NO freeze-thaw effectThe NO emission level during the freeze-thaw period of the precipitation years in marsh wetland is higher than that of the dry and normal years.The increase of precipitation during the rainy year had an effect on soil moisture conditions,soil aggregates,substrates?NH₄⁺?and microbial processes?nitrification and denitrification?,which promoted the significant emission of NO in the wetland during the freeze-thaw period?p<0.05?,but the effect difference of the dry and normal freeze-thaw processes on NO emissions is not obvious.