Effects Of Freezing-thawing Cycle On Soil Water And Heat And Greenhouse Gas Emission Under Three Grazing Treatments In Xilingol Grassland

As the world third permafrost country,China owns a significant amount of perennial and seasonal frozen soil occupied 75% of land area,which most distributes in the sensitive climate change area such as the Tibetan Plateau and Inner Mongolia Plateau.Ice coverage,soil freezing and melting process will significantly affect the exchanges of water and heat balance characteristics at the soil water-gas interface.Particularly in recent years,the fading acceleration of frozen zone caused by global warming,in addition to the influence on the natural environment from human activities,the hydrological effects of permafrost and its evaluation of greenhouse gas emission is getting more and more attention.This study based on positioning of inner Mongolia grassland ecosystem research station,Chinese academy,selects three ways of land use with different number of years of herding and condition of grazing?UG79-ungrazed site since 1979,UG99-ungrazed site since 1999,HG-heavily grazed site since 1979?in a certain area as the research object.Based on the determination results,discuss the influence of freezing and thawing alternation on soil moisture,temperature and greenhouse gas emission under different grazing condition,therefore providing reliable scientific basis for accurate assessment of the hydrological effect for Inner Mongolia seasonal frozen soil freezing and thawing and N₂ O emission in changing environment?global change and way of land use?.This experiment result indicates that:1.A certain thickness of snow can effectively isolate the effects of temperature,reducing soil temperature decrease.Snow thickness in HG site is far less than the other,the soil temperature change is the most intense and the soil freezing and thawing rate is the fastest.The temperature of shallow soil?0-30cm?in UG79 and UG99 site is significantly higher than it is in HG site,while the temperature change in 100 cm depth tend to accord in three different grazing condition,thus the effects of grazing on deep soil temperature is limited.2.During soil freezing period,the area of clustering moisture is most widely distributed in UG99 site and centralized especially at the area of well-developed pasture root system,which is prove to be most positive to soil moisture conservation in pasture root system.During thawing period,soil moisture level is higher than it was before freezing.In thiscondition,the increasing range of soil surface moisture level in UG99 site is higher than it is in UG79 and HG site,which reaches at 0.19 m3·m-3.The clustering moisture effect after freezing is most obvious,thus providing a favorable moisture condition for the vegetation growth during spring.3.CO₂ emission flux in Xilingol grassland is mainly concentrated in the growing season and the thaw period of non-growing season.Although thawing period contribute little to the annual CO₂ emission,it occupies a significant portion of total CO₂ emission in non-growing season,which account for more than 50% of the emission in non-growing season,thus promoting CO₂ emission on interannual scale.CO₂ emission flux and soil temperature shows significantly positive correlation,but no significant correlation with soil moisture content.4.In soil freezing period?November to February?,different part of the soil layer characterized by weak “sink”,while N₂ O shows a pulsed release in soil thawing period,which has a pivotal role in the annual emission.Soil temperature and moisture content have a relatively complex effect on N₂ O,N₂O emission in study area and soil moisture and temperature have no significant correlation.The accumulation of N₂ O emission flux in non-growing season and freezing and thawing period in HG site is higher than it is in UG79 and UG99,the possible reason is associated with the grazing year and study area sample.