Soil Water-heat Process And Nitrogen Transformation During Freezing And Thawing Period In Wetland Of Momoge

Snow cover and freezing-thawing cycle are common phenomena in winter in seasonal frozen areas.It has been believed that global winter climate change can indirectly lead to decrease the depth and duration of snow cover.Such change may have some impacts on microclimate and soil physical chemical properties,and have a key impact on the soil nitrogen cycle further.We took Momoge wetland in western Jilin province,northeast China as the research area.The water-heat change,soil physical chemical properties and nitrogen turnover were analyzed by field observation experiment and laboratory experiment.In order to provide basic data for further research on wetland eco-hydrology and nutrient management under the background of future climate change.1.We observed the change of soil temperature and soil moisture in the different vegetation coverage,and analyzed the synergistic effect of snow cover and vegetation coverage of water-heat process.The results show that the continuity of frozen and thaw process of Momoge wetland soil was 150 days in the nature condition.Vegetation coverage was more effective in the preservation of soil temperature and water than bare land.The thawing and freezing started-time of soil were later and the duration of freezing were longer with low humid grassy marshland and steppe meadow than bare land,the thawing started-time of low humid grassy marshland soil was 1-14 days later than bare land and steppe meadow soil,and the influence of vegetation coverage on soil temperature is more dominant in melting process than in freezing process.Due to thermal insulation effect of the saline soils in bare land?0-10cm?,there is no significant difference in thawing and freezing started-time between bare land and vegetation coverage land.Vegetation coverage results in rising the annual minimum soil temperature in low humid grassy marshland and steppe meadow.Vegetation coverage help to the soil hold water and increase the infiltration rate of snowmelt water.Snowmelt water utilization is higher in low humid grassy marshland,after the frozen soil fully thawed,the soil layer of 0-30cm in which the water content increased significantly was low humid grassy marshland,followed by steppe meadow,bare land,the increase in water content in the 0-30cm soil layer with low humid grassy marshland,steppe meadow,bare land was 0.19,-0.01,-0.5.Using regression analysis to analyze the relationship between soil temperature and air temperature,it was found that soil temperature has a strong correlation with air temperature,and the response of soil temperature to air temperature in the same study area decreases with increasing soil depth.It shows that the air temperature has greater influence on bare land soil than vegetation coverage soil.The land-atmosphere energy exchanges with altered vegetation coverage were observed.The dying degree of response of soil temperature to air temperature was bare land,steppe meadow,low humid grassy marshland.2.In the laboratory study,we evaluated the effect of different snow conditions and freeze-thaw cycles on soil physical chemical properties,including water-stable aggregates,NO₃^--N?NH₄⁺-N?organic matter and so on,which are often influenced by freeze-thaw event from typical wetland soil.We also evaluated the effect of our manipulations on the gross rates of nitrification,denitrification by using the Barometric Process Separation System?BaPS?.The results show that moisture content is the key factor affecting soil water stable aggregates,>2mm SWA content gradually showed an increasing trend with the increased freeze-thaw cycles at the low water content of 0S and 1S treatments,the breaking effect of aggregates is smaller than that of aggregating.<0.053mm SWA content in 20cm snow cover of increased snow cover increased with the increase of freeze-thaw cycles,and the effect of freeze-thaw promoted the splitting decomposition of the aggregates.NO₃^--N content were decreased with increased snow cover and following freeze-thaw cycles,and NH₄⁺-N content were increased.Due to the infiltration of snowmelt water,the content of nitrate nitrogen in soil leachate in 16th freeze-thaw cycles under 10cm and 20cm snow cover increased 1.58 times and 2.28times than that in the second freeze-thaw cycles,respectively.Freeze-thaw process can promote the transformation of soil nitrogen in a short period of time,but its effect may be weakened with the continuous freeze-thaw cycles.And the soil denitrification rate increased with increased snow cover depth.Snowmelt water can partially dissolve and take away nitrogen accumulated in soil,the risk and effect of soil N losses should be pay more attention during the winter of snow cover and freeze-thaw cycles in the future.