Characteristics Of Soil Water And Heat Under Freezing And Thawing Cycles In The Permafrost Regions Of The Tibetan Plateau

As the birthplace of many rivers in Asia,the Qinghai-Tibet Plateau is an important ecological function service area,a"sensor"for climate change,a leading area for carbon balance,and rare germplasm resources preservation base.The permafrost on the Qinghai-Tibet Plateau has experienced significant degradation due to global warming in the past few decades,and the degradation of permafrost on the Qinghai-Tibet Plateau will have a strong impact on soil environment,alpine ecology and hydrological processes in cold regions.Freezing and thawing processes and soil hydrothermal dynamics have profound impacts on terrestrial ecosystems,hydrological processes,ground-gas-water heat exchange and carbon cycling,climate systems,and engineering in cold regions.In this paper,the Fenghuo Mountain basin in the hinterland of the Qinghai-Tibet Plateau as the research area is used to systematically observe,sample and study the changes in soil quality,meteorological factors,evapotranspiration,soil infiltration,and soil moisture and temperature in the region at different freeze-thaw cycle stages of the active layer.The study of soil water-heat change characteristics under the freeze-thaw cycles in the permafrost region of the Qinghai-Tibet Plateau was carried out,and the following main results were obtained:?1?The minimum data set?MDS?that affects soil quality under the background of alpine grassland vegetation degradation in the permafrost region of the Qinghai-Tibet Plateau:alkaline hydrolysis nitrogen,salinity,total phosphorus,and organic matter,and with the increase of vegetation coverage,the soil quality index?SQI?also shows an increasing trend.When the vegetation coverage is less than 30%,the average SQI is 0.300-0.442,and the vegetation coverage is 30-50%,the average value of SQI is 0.308-0.457,when the vegetation coverage is 50-70%,the average value of SQI is 0.328-0.491,and when the vegetation coverage is more than 70%,the average SQI is 0.327-0.532.Different soil quality indexes were calculated by using linear and non-linear scoring functions showed that the soil quality index based on MDS can express the soil quality under the background of vegetation degradation in more detail.Compared with the other three methods(SQI_L-A,SQI_L-WA,SQI_N-A),MDS-based nonlinear weighted soil quality index(SQI_N-WA)can make more accurate evaluation of the vegetation degradation of alpine grasslands in permafrost regions of the Qinghai-Tibet Plateau.?2?During the different freezing and thawing phases in permafrost regions,the temperature and potential evapotranspiration have the same trend,and both reached the peak during the summer melting period of the soil;and through further analysis of meteorological elements in different freezing and thawing phases of the active layer soil,we found that the contribution rate of each meteorological element to the potential evapotranspiration showed a significant change with the change of the freeze-thaw cycles.The sensitivity coefficient and contribution rate of net radiation and vapor pressure deficit to potential evapotranspiration are high,and the average values of sensitivity coefficients are 0.52 and 0.44,respectively,while the contribution rate of wind speed and air temperature to potential evapotranspiration are low,and the average values of their sensitivity coefficients are 0.08 and 0.01,respectively.?3?The actual evapotranspiration process during the winter cooling period and the spring warming period is mainly sublimation of snow and ice,and is mainly affected by net radiation.There is a range of meteorological elements in the actual evapotranspiration during the summer melting period,when each meteorological element reaches its range of change at the same time,that is,soil heat flux reaches1.44-4.46 MJ·d^-1·m^-2,air temperature reaches 1.09-3.10 ^oC,relative humidity reaches 56.02-63.70%,net radiation reaches 40.34-48.90 MJ·d^-1·m^-2,water vapor deficit reaches 0.24-0.34 kPa and the wind speed reaches 5.67-5.79 m·s^-1,the actual evapotranspiration reaches the maximum value,but under other conditions,the evapotranspiration will decrease slightly.In the autumn freezing period,the amount of condensed water reached the maximum value at 7:00,and the actual evapotranspiration volume increased sharply due to the sudden change of the meteorological elements at 9:00,resulting in a sudden change in the evapotranspiration volume from 7:00 to 9:00,and the process of evapotranspiration occurring during the day and the condensation process occurring during the night.?4?Under the freeze-thaw cycles of the permafrost regions of the Qinghai-Tibet Plateau,there is an obvious interaction between the soil hydrothermal processes in alpine meadows,and at high cover vegetation,the changes of deep soil hydrothermal are more stable than in the low-cover areas,the soil moisture content is the highest at a depth of 20 cm on the surface,and the soil moisture and temperature show a decreasing correlation with air temperature and relative atmospheric humidity,respectively.The change of soil hydrothermal processes at low cover levels will aggravate the occurrence of a series of ecological and environmental problems such as desertification of alpine meadow ecosystems compared to high cover vegetation,that is,the deepening of the temperature change of deep soil will lead to the loss of soil moisture in all layers,especially shallow soil moisture.