Methane （CH4） is regarded as the second most important greenhouse gas after carbon dioxide （CO2）. Under the warming and wetting of the climate， the Qinghai-Tibet Plateau may become a potential carbon source. To achieve the 1.5 ℃ and 2 ℃ temperature control targets of the Paris Agreement， it is necessary to accurately estimate the remaining carbon emission space in the future. Therefore， an accurate understanding of the source and sink functions， spatial-temporal changes and mechanisms of atmospheric CH4 in the Qinghai-Tibet Plateau is of great practical significance to predict and cope with warming and help the government make scientific decisions on energy conservation and emission reduction. This paper summarizes the existing research progress from the following three aspects： observation method of atmospheric methane， source and sink of methane， and the spatial-temporal variation of the atmospheric CH4. The results show that the observations of atmospheric CH4 on the Qinghai-Tibet Plateau are mainly ground-based observations and satellite remote sensing， lacking space-based observations， and AIRS has the best data quality among satellite products； Methane of the Qinghai-Tibet Plateau is mainly from natural sources， the main sources are wetlands， lakes and animal livestock， geological activities， vegetation and permafrost are the uncertain CH4 sources. The sink of CH4 is mainly tropospheric hydroxyl radical and alpine meadow； The seasonal distribution of CH4 on the Qinghai-Tibet Plateau has a single-peak in summer， which is synchronized with the seasonal pattern of the Asian summer monsoon； The average rising rate of CH4 concentration on the Qinghai-Tibet Plateau is about 5~8 ng·g-1·a-1， which is greater than that in the surrounding areas； The high value of CH4 concentration near the surface of Qinghai-Tibet Plateau appears in the middle part， and decreases gradually from the ground to the tropopause， but the decrease in the east and north of the plateau is greater than that in the southwest. In the future， three dimensional continuous observation of atmospheric CH4 should be strengthened， satellite inversion algorithm and source sink analytical model should also be improved. In addition， we need to accurately quantify the spatial-temporal change process of atmospheric CH4 in the Qinghai-Tibet Plateau and reveal its change mechanism to provide a scientific basis for the future efficient emission reduction policies.
According to 73 snow samples collected in the southeast and southwest of Tibetan Plateau in January， 2021， the characteristics of hydrogen and oxygen stable isotopes（δD and δ18O） fractionation in dry season and the influence of water vapor migration on the chemical composition change of snow profile were revealed by measuring the stable isotopes of hydrogen and oxygen and soluble inorganic ions in snow， and the relationship between stable isotopes of hydrogen and oxygen and climate and the composition and source of soluble inorganic ions were discussed. The results show that the local meteoric waterline of snowpack in the whole study area is δD=7.86δ18O+11.8 （R2=0.95）， which is close to the Lhasa winter meteoric waterline， and the slope and intercept of the meteoric waterline in the southeast are slightly lower than those in the southwest. δD and δ18O fluctuates from -178.11‰ to -68.07‰ and -23.80‰ to -9.61‰， respectively， and the d-excess values fluctuate from 11.03‰ to 23.49‰， showing low values of δD and δ18O in winter， and high d-excess values. The surface layer is relatively enriched in heavy isotopes， and the isotope values are higher than those of the lower snow samples， and the water vapor migration inside the snow makes the slope of the relationship between δD and δ18O different. The concentration sequence of the main soluble inorganic ions is Ca2+>SO42->Na+>NO3->Cl->K+>Mg2+>NH4+， of which Ca2+ （42.47%）， SO42- （23.53%） accounted for the largest proportion of cation and anion， respectively， and the average concentration of ions in the southeast was higher than that in the southwest. The results of principal component analysis show that terrigenous sources are the main source of ions in snow， and NH4+ and some NO3- are related to human activities. The backward air mass trajectory shows that the source of water vapor is related to the water vapor transport controlled by the upper-altitude westerly circulation， and most of the ions are terrestrial mineral dust carried by westerly winds in winter.
Water accumulation associated with water migration is closely related with the ice segregation， but their coupling relationship is still unclear. To decoupling the relationship of water accumulation and ice segregation， herein， the dynamics of water migration and ice segregation during the freezing and thawing of different soil types under different water supplying conditions have been investigated based on pore water pressure measurement and layer-scanning technique. Results showed that apparent water accumulation near the freezing front during the freezing of silty clay and loess tested here， but there exist differences in modes. During loess freezing under closed system， no ice segregation was observed， the pore water pressure increased， and there existed apparent liquid water accumulation during the early stage of freezing； while during the freezing of silty clay， there existed ice segregation， the pore water pressure decreased， and no apparent liquid water accumulation occurred during the early stage of freezing. The results implied that there exist two modes of water accumulation near the freezing front during soil freezing： one is the water accumulation induced by water pressure gradient induced by pore ice which results in water flowing from the frozen zone and unfrozen zone to the location near the freezing front； the other is the water accumulation induced by cryo-suction of segregation ice which results in the water flowing from the unfrozen zone to the location near the freezing front. Notably， the contribution from each mode associated with water accumulation of soil freezing depends on whether the ice segregation exists. As no ice segregation forms， water accumulation induced water pressure gradient predominates during the early stage of freezing. As there exists ice segregation during freezing， water accumulation induced cryo-suction predominates during the later stage of freezing. Investigating on different modes of water accumulations will be helpful for the exploring the mechanisms of freeze-thaw diseases and the ground ice in the cold regions.
The different aspects lead to great differences in the processes of water， heat， and energy balance， which further affect the soil environment and the growth of alpine plants. Based on an experimental study with eight aspects （abbreviated as octagonal platform） of Huashixia frozen soil observation base on the Qinghai-Tibet Plateau， the influence of aspects on soil environments and alpine vegetation growth was studied. The results showed that： （1） After 6 years， the soil temperature of each slope near the surface （10 cm and 30 cm depth） from high to low was as follows： south > southeast > southwest > west > east > northwest > northeast > north， that is， the relatively sunny slope （east， southeast， south and southwest） was higher than the relatively shady slope （west， northwest， north and northeast）. However， there was no significant difference in soil water content between the sunny slope and the shady slope at the depth of 0~30 cm. （2） The growth trend of aboveground vegetation （including plant height， coverage and aboveground biomass） on sunny slope was better than that on shady slope. The growth trend of underground vegetation （including root depth and underground biomass） on sunny slope was worse than that on shady slope. （3） In the depth of 0~10 cm， the content of soil organic carbon and total nitrogen on sunny slope was higher than that on shady slope. However， the content of total phosphorus on sunny slope was lower than that on shady slope. There was no significant difference in total potassium and available nutrients among different slopes （P>0.05）. In general， the effect of temperature on vegetation growth and nutrient distribution is significant in alpine regions， and these findings provides an important reference for vegetation restoration and energy balance research in different aspects.
In recent years， more and more attention has been paid to the problem of the cryosphere changes on the Tibetan Plateau， and it has gradually become a hot issue for scholars. Known as the “water tower of Asia”， the Tibetan Plateau is the source of many major rivers in Asia. Under the combined influence of climate change and human activities， water resources on the Tibetan Plateau have undergone profound changes， especially soil water， as an important component of water resources， which plays an important role in regulating vegetation and crop growth， rainfall and runoff. However， global warming leads to the degradation of permafrost and seasonally frozen soil， which affects the original water cycle process and the spatial and temporal pattern of water resources by changing the properties of soil water storage and water transport. In the Tibetan Plateau， where there are few data， it is difficult to directly study the soil water cycle process under freezing-thawing by using original data. Therefore， it is an important means to simulate the variation characteristics of soil water and temperature under freezing-thawing in seasonally frozen soil regions of the Tibetan Plateau by using coupling model of soil water and heat. Aiming at the key problem of the difference of soil temperature and moisture characteristics in typical seasonally frozen soil regions under different meteorological conditions， this paper simulated the characteristics of soil moisture and temperature change in Maqu， Naqu （Nagqu） and Shiquanhe from 2017 to 2018 by using SHAW （Simultaneous Heat and Water） model and three soil moisture characteristic curve models. The simulation effect and variation characteristics of soil moisture and temperature under different meteorological conditions were analyzed， and the influence of soil moisture characteristic curve model on the simulation effect was studied. The results show that SHAW model can well simulate the temporal variation and vertical distribution of soil temperature and moisture under different meteorological conditions. The simulation effect of soil temperature is better than that of soil moisture. The average NSE， R2 and RMSE of soil temperature are 0.88， 0.96 and 2.20 ℃， respectively. The mean NSE， R2 and RMSE of soil moisture are 0.60， 0.72 and 0.03 m3·m-3， respectively. In terms of different meteorological conditions， the simulation effect of soil temperature in relatively dry region was significantly better than that in humid region， while the simulation effect of soil water in relatively humid region was significantly better than that in arid region. From different depths in soil， the simulation effect of soil temperature decreases gradually with the increase of depth， while the simulation effect of soil moisture in the middle and lower layers is better than that in the surface layer. From the view of different soil moisture characteristic curve models， different soil water characteristic curve models have no significant effect on soil temperature simulation effect， but there are significant differences in soil moisture simulation effect. In addition， there are great differences and uncertainties in simulating soil temperature and moisture in different freezing-thawing stages. With the increasing trend of climate warming， permafrost and seasonally frozen soil on the Tibetan Plateau may continue to degrade， may change the current water resources pattern， resulting in frequent extreme weather events. Therefore， from the perspective of numerical simulation， this paper verified the applicability of soil moisture and heat coupling model in soil temperature and moisture simulation under different meteorological conditions， revealed the influence of precipitation and temperature on soil temperature and moisture simulation at different depths in seasonally frozen soil regions， and analyzed the differences in simulation effects of different soil moisture characteristic curve models. The results provide reference for the study of soil water resources variation under freezing-thawing conditions.
Analysis and assessment of landslide susceptibility using 3S technology and mathematical statistical model is still the current hot topics. It is very important for selection of unit and factor and model to the landslide susceptibility assessment reasonably. Tianshui City is selected as the study area which is in the typical regions of Longzhong ecologically vulnerable area. Based on the weights-of-evidence approach （WOE） and two kinds of assessment units （grid-unit， GU， slope-unit， SU） and also two resolutions （30 m， 10 m）， a total of four different data sets （GU30， SU30， GU10， SU10） is established. Then quantitative analysis and contrast assessment division of the loess landslide susceptibility in the study area though selected four kinds of thirteen factors. The results show that stratum lithology， altitude， relief amplitude， roughness and slope are the key control factors of landslide development in the area. The geological factors and topographic factors control and determine the formation and development of the landslides in study area. The susceptibility value of stratum lithology is the highest value. Statistical analysis of susceptibility division is carried out according to the point and area of landslide respectively. The landslide ratio shows that the division based on the slope-unit is higher than that of the grid- unit. Landslide susceptibility area in highest division based on the slope-unit and 10m high resolution is accounted for the highest. Therefore， it has higher precision in landslide susceptibility analysis and assessment that using the slope-unit and high resolution data. The landslide susceptibility result is consistent with the actual landslide development in the study area. The division results can also be used for the landslide prevention and control and national spatial planning in the study area.
The cryosphere is one of the climate system component which plays an important role in the global biogeochemical mercury cycling. Anthropogenic mercury has been transported to remote cold and high regions worldwide through atmospheric circulation， and then transformed into methylmercury in the cryosphere. As one of the highly toxic environmental pollutants， methylmercury can be greatly bioaccumulated and biomagnified through the food web， which potentially poses threats to human and wildlife as well as the global cryospheric environments. In order to gain a full picture of research progress on methylmercury in the cryosphere， our study comprehensively summarized the concentration levels and analytical methods of methylmercury， and biogeochemical processes such as the migration， transformation and fate of methylmercury in various cryospheric environments including glacier， permafrost， snow ice and sea ice. We particularly made the literature review of microbial mercury methylation and evaluated the risk of methylmercury exposure to human and wildlife in the cryosphere. Meanwhile， we focus on distribution， behavior， and environmental effects of methylmercury in the cryosphere against the backdrop of climate change， which is essential for assessing the exposure risk of methylmercury to humans and wildlife. Perspectives of methylmercury researches in the cryosphere have also been highlighted in this review， though there is existing a knowledge gap of biogeochemical methylmercury cycling in the low temperature environments which merits further study.
Based on the daily precipitation data of 16 national meteorological observation stations on the northern slope of Tianshan Mountains from 2000 to 2020 （September to April the following year）， 28 blizzard weather processes were screened out. Then NCEP/NCAR reanalysis data and HYSPLIT model were used to simulate backward tracking of the water vapor during the snowstorm， analysis of the circulation background of the snowstorm process on the northern slope of the Tianshan Mountains， as well as the main sources and transport of water vapor and its contribution to the snowstorm. The research showed that the snowstorm area in the northern slope of the Tianshan Mountains was located on the right side of the axis of the southwest jet at 300 hPa high， the southwest airflow in front of the West Siberia trough at 500 hPa， the front convergence of the exit area of ?the southwest jet at low level at 700 hPa， and the convergence area of water vapor flux divergence and the overlap near the ground cold front area. The water vapor affecting the blizzard on the northern slope of the Tianshan Mountains mainly came from the Mediterranean Sea， the Black Sea and its vicinity， Southwest Asia， Central Asia， the Atlantic Ocean and its coasts， as well as the 850 hPa water vapor in Europe and northern Xinjiang. The water vapor from North America and other places had a relatively small contribution to the blizzard； after each water vapor source reaches the key area with the westerly airflow， under suitable circulation conditions. It mainly entered the blizzard area along the westward （southwest） and the northwest paths. But there were some differences between the layers. Based on the above characteristics，the structure of the source and transport of water vapor in the snowstorm process on the northern slope of the Tianshan Mountains was established and the characteristics of vapor transport at various heights were revealed.
Snow is an important part of the cryosphere and plays an important role in the hydrological cycle and energy balance. Study of the spatiotemporal characteristics of snow cover and its change is the prerequisite for analyzing the formation， distribution and variation of runoff from mountains in inland river basins. In this study， we selected the upper reaches of the Taolai River basin of Qilian Mountains as the study area， used downscaling methods to obtain high-resolution snow depth data， and adopted methods of spatial statistics， sensitivity analysis and contribution separations to quantify snow cover distribution and variation influenced by terrain and the regional climate during the time period from 2002 to 2018. Results showed that basin early average snow depth ranged from 0 cm to 2.5 cm， with variation from -0.19 cm·a-1 to 0.06 cm·a-1. The area of snow depth reduction during the study period accounted for 68.30% of the total area. It was found that the snow depth increase more with altitude and less with the increase of slope. Variation of snow depth increased below 2 500 m a.s.l. and decreased above 2 500 m a.s.l. As the slope increases， it first increases and then decreases； the snow depth of each aspect decreases， especially in the northwest orientation. The sensitivity of snow depth to air temperature and solar radiation were found negative in general， while that of the precipitation was found positive. The precipitation in high-altitude areas has a relatively large contribution to the snow depth variation， while in the valley areas， the contribution of temperature to snow cover is more significant. This work provides an example for the study of snow dynamics in the upper reaches of inland river watersheds， and benefits model simulation and prediction of mountain runoff and regional water management.
Evapotranspiration （ET） plays a key role in land surface-atmosphere hydrological and energy interactions， and shows strong tempo-spatial variabilities in high mountainous area. Accurate estimates of ET are important for researches in hydrological processes in high mountainous area. The latest version of Community Land Model （CLM）， CLM5.0 is a state-of-the-art land surface model to date. The performance of CLM5.0 in simulating ET was evaluated against in situ observations at five in situ sites in the upper reaches of Heihe river watershed， a typical alpine mountainous area. Results show that ET simulated by CLM5.0 is reliable with R values of 0.601~0.839， RSR values of 0.964~1.145， and BIAS values of -1.220~0.597 mm·d-1. It indicates that CLM5.0 performs well in catching temporal trend of observed ET， but still with a certain underestimation. The BIAS values range between -0.904~-0.367 mm·d-1 during the non-growth period， and range between -2.094~-0.794 mm·d-1 during the growth period， which indicates that the underestimation of the simulated value is mainly derived from the simulation of the growth period. Meanwhile， R values range between 0.299~0.651 and RSR values range between 1.135~1.332 on alpine meadow， whereas R value is 0.209 and RSR value is 1.450 on alpine grassland. Thus， the performance of CLM5.0 in simulating ET is better on meadow than on grassland. R values range between 0.605~0.840 and RSR values range between 0.252~1.193 during the daytime， whereas R values range between 0.344~0.651 and RSR values range between 0.482~2.966 during the night， by comparison， the performance of CLM5.0 in simulating ET is better during the daytime than night. These conclusions can provide a scientific basis for the application and improvement of CLM5.0.
As a sensor and amplifier of global climate change， the hydrological process of the Qinghai-Tibet Plateau is a hot topic at present. Among them， evapotranspiration is the most difficult component in the hydrological cycle of the Qinghai-Tibet Plateau. Based on the observation data of vorticity system， Lysimeter and meteorological observation system in the Fenghuoshan area in the hinterland of the Qinghai-Tibet Plateau， the actual evapotranspiration in the growing season of the Fenghuoshan area in 2019 was evaluated by means of vorticity correlation method， lysimeter measurement and FAO56 Penman-Monteith formula. The results showed that： in Fenghuoshan area of Qinghai-Tibet Plateau， the crop coefficients of early， middle and late growing season were 0.93， 1.11 and 1.14， respectively； the actual evapotranspiration values obtained by the three methods were basically similar. The evapotranspiration in growing season was （495.00±21.69） mm， which was greater than 377.89 mm of precipitation in the same period； The results showed that the daily mean of evapotranspiration in Fenghuoshan area was （2.70±0.12） mm in the growing season， and the daily mean value of evapotranspiration in different stages of the growing season in Fenghuoshan area was （3.03±0.10） mm> （2.49±0.12） mm> and （2.23±0.18） mm， respectively. The calculated crop coefficients and evapotranspiration at different growth stages provide basic data for evapotranspiration observation and simulation in similar areas of the Qinghai-Tibet Plateau.
During the soil freeze-thaw process， near-surface soil has a significant impact on the land-air exchange and releases a large amount of CH4 and CO2， which further intensifies the process of climate warming and has a significant impact on the environment and human activities. The frozen ground of Northeast China is a typical high-latitude permafrost， which is located in the southern margin of Eurasia. It is mainly composed of Xing’an-Baikal permafrost. The permafrost layer is relatively thin， which is extremely sensitive to temperature rise and external changes. In the context of climate warming， the permafrost degradation in Northeast China has been significant in the past 40 years. The permafrost degradation is mainly manifested by the rise of ground temperature， which leads to changes in the freeze-thaw cycle， namely， the start time of surface freezing is delayed， the end time of thawing is advanced， and the duration of freezing is shortened. In addition， it will affect the global and regional gas-energy exchange， vegetation growth， agricultural production， surface runoff， carbon cycle and terrestrial ecosystems. Remote sensing technology has the characteristics of large detection range， less limited by ground conditions and fast data acquisition， providing an effective technical means for continuous monitoring of soil freeze-thaw cycles at large and regional scales. Although visible infrared remote sensing has high spatial resolution， frozen soil is usually developed underground， while visible and infrared sensors cannot penetrate the surface， and are seriously affected by clouds， and the temporal resolution is low. Therefore， it has great limitations to distinguish freeze-thaw using visible infrared remote sensing. However， microwave remote sensing has a long wavelength and is not affected by sunlight and clouds， so it can penetrate the soil to obtain information within a certain depth of underground. In addition， microwave remote sensing is very sensitive to the dielectric changes between soil freeze-thaw states. When near-surface soil undergoes freeze-thaw cycle， the dielectric constant in soil changes， resulting in the change of bright temperature of passive microwave， and then the near-surface soil freeze-thaw state is identified. At present， passive microwave remote sensing has become an effective method to monitor global and regional near-surface soil freeze-thaw cycles. The surface freeze-thaw status discrimination algorithms based on microwave data mainly include double index algorithm， discriminant function algorithm， decision tree algorithm and seasonal threshold algorithm， among which the double index algorithm and the freeze-thaw discriminant function algorithm are widely used. Discriminant index algorithms election algorithm 36.5 GHz vertical polarization brightness temperature （
Freezing and thawing index is not only of great significance to the study of frozen soil， but also a useful index to reflect climate change. The daily temperature observation values of 11 national meteorological stations in the Qilian Mountains were used to calculate the annual air and ground surface freezing and thawing indices from 1961 to 2014， and the statistical and distribution characteristics of these indices were analyzed. The temporal and spatial variation trends of the annual freezing and thawing indices were analyzed by nonparametric Mann-Kendall test， Sen’s slope estimation method and correlation analysis method. The results showed that in the past 54 years， the freezing index had a significant downward trend， and the thawing index had a significant upward trend. The annual average air freezing index， air thawing index， ground surface freezing index， and ground surface thawing index were roughly distributed between 994.3 ℃·d and 1 540.9 ℃·d， 1 828.2 ℃·d and 2 376.6 ℃·d， 744.7 ℃·d and 1 287.3 ℃·d， 2 706.0 ℃·d and 3 542.6 ℃·d， respectively. The climatic tendency rates were -6.5 ℃·d·a-1， 6.5 ℃·d·a-1， -7.7 ℃·d·a-1， and 9.1 ℃·d·a-1， respectively. From northwest to southeast， the freezing index showed the distribution characteristics of high in the middle and gradually decreasing in the east-west direction， while the thawing index was opposite； in addition to altitude and latitude， freezing and thawing index was also affected by slope aspect， surrounding topography， snow depth and human activities. The abrupt change point of freezing and thawing index time series occurred in 1994—1995， which corresponded to the abrupt change of air temperature； after the abrupt change point， the growth rates of the air and ground surface thawing indices and the decline rate of the ground surface freezing index increased， while the decline rate of the air freezing index decreased. The change rate of the ground surface freezing and thawing index was greater than that of the air freezing and thawing index during the whole study period， showing that the change of ground surface temperature was more sensitive to global warming. In addition， there was a strong linear relationship between freezing and thawing index and annual average air temperature and ground surface temperature， and the proportion of annual thawing index in the composition of annual average ground surface temperature was larger than that of the freezing index. The research results have reference significance for understanding the climate and frozen soil changes in the Qilian Mountains， further calculating the changes of frozen soil parameters and harnessing the ecological environment in the Qilian Mountains.
Dams are important infrastructure with specific functions in reservoirs， widely used for flood control， irrigation， water supply， sediment control， improving ecological environment and solving uneven distribution of water resources， etc. More than 87 000 dams have been built in China in the last decade， of which more than 60% are located in cold regions. Due to the complex operating environment， reservoir dams in cold regions face the problems of frequent freezing disasters and many disaster-causing factors. Those problems seriously affect the normal operation of dams， increase the security risk and add the cost of renovation and maintenance. In order to effectively prevent the occurrence of dam frost damage and improve the level of dam risk management， a frost damage risk analysis method of reservoir dam in cold regions based on T-S fuzzy fault tree theory was proposed. The T-S fuzzy fault tree uses fuzzy numbers to describe the degree of failure and the probability of failure and replace the logic gates in the traditional fault tree with T-S fuzzy gates. The method not only can systematically sort out the dam frost damage failure events and perform reliability and safety calculations by logical deduction， but also takes into account the uncertainty and fuzziness of the probability of failure and solving the problem of the traditional fault tree based on the two-state assumption and relying on a large amount of failure probability data. In this paper， a T-S fuzzy fault tree was established with dam uneven deformation， dam leakage intensification， and panel frost damage as subordinate events. Then the main risk factors were analyzed by calculating the importance of the bottom event. At the same time， in order to exclude to the interference of subjective factors on the evaluation results of specific engineering risks， this paper tries to combine mechanical calculation and risk analysis to evaluate the actual engineering. The method combines frost swelling mechanics analysis and T-S fuzzy fault tree to calculate and analyze the risk of frost damage faced by the dam in Hongqipao Reservoir. Firstly， the relevant theories and methods of engineering mechanics and permafrost mechanics were used to analyze the frost damage problem faced by the dam in Hongqipao Reservoir. Then the calculation results were transformed into the bottom event fault degree substituted into the T-S fuzzy fault tree. Finally， the fuzzy possibility of the top event of frost risk was calculated and analyzed by the frost risk analysis process established in this paper. The study found that in the process of freezing risk control and decision making of reservoir dams in cold regions， the focus should be on repeated freezing-thawing action， wind and wave erosion， water level fluctuation and ice accumulation in the reservoir area， quality defects of the panel and dam filling， and insufficient anti-seepage and thermal insulation measures. It was also found that the frost damage risk of the dam in Hongqipao Reservoir is relatively high， and extra attention should be paid to the control and treatment of frost damage risk during operation and maintenance， and frost damage risk identification and maintenance based on the results of the importance of each bottom event analysis. The results of application show that the proposed method can scientifically analyze the risk of dam freezing damage and determine the key risk factors. This work can provide technical support for the risk identification， management and decision-making of freezing damage to reservoir dams in cold regions， and further it is useful for the design， construction， operation， maintenance and frost damage prevention of reservoir dam.
Microencapsulated phase change material （PCM） is a material that can affect temperature changes by transforming its morphology. In order to study the effect of these materials on frost heave of subgrade soils， unidirectional frost heave tests were carried out on the ordinary coarse-grained soils and microencapsulated PCM coarse-grained soils mixed with different contents of 5%， 8% and 10%. The results show that compared with the ordinary coarse-grained soils， the temperature changes of the coarse-grained soil mixed the microencapsulated PCM can be delayed， and the final temperature of these soils also shows a higher value. Meanwhile， it can affect the development of the soil sample freezing depth and reduce the maximum freezing depth value of microencapsulated PCM coarse-grained soils. The water migration ability of coarse-grained soils are weaken， thus resulting in reducing the water replenishment amount and the final water content of the soil samples. The incorporation of microencapsulated PCM can mitigate the development of coarse-grained soil frost heave， and the frost heave amount and frost heave rate were both reduced to a certain extent. Comparing the frost heave test results of coarse-grained soil with 5%， 8% and 10% contents of microencapsulated PCM， it was found that the higher microencapsulated PCM content may show better improvement effects in terms of affecting the temperature， moisture and frost heave of coarse-grained soils. Therefore， the incorporation of microencapsulated PCM into the foundation soil of high-speed railway in cold regions has certain engineering significance for improving the occurrence of subgrade frost heave.
Microencapsulated phase change material （mPCM） possess an excellent heat storage capacity， which can mitigate temperature fluctuation of soil during freezing/thawing in seasonal frozen ground. Two types of mPCM with butyl stearate， paraffin as the core material （B-PCM and P-PCM， respectively） and SiO2 as shell material are evaluated using Scanning electron microscopy， X-ray photoelectron spectroscopy， Fourier transform infra-red spectroscopy and Thermal analysis， the effects of 4%， 8%， and 10% of mPCM on the frost heave characteristics of the silty clay in Lanzhou are studied through the freeze-thaw process and frost heave tests. The results show that the latent heat released by mPCM in the phase change process provides the soil with a thermal insulation effect， and the elapsed time required for the soil containing mPCM with 10% addition to drop from -10 ℃ to 10 ℃ is extended by 71.8%. The frost heave deformation， freezing depth and water content change rate of the soil decrease with the increase of mPCM concentrations； and the differences in core-to-shell ratio between B-PCM and P-PCM leads to different trends in the extent of water migration after frost heave. In general， the positive effect of mPCM on the soil is manifested in improving the thermal insulation performance of the soil， inhibiting frost heave deformation， etc. However， adding more than 8% mPCM will increase the frost heave of the superficial layer of the soil， which is related to the water absorption of the SiO2 shell material.
In order to study the deformation law and strength characteristics of frozen sodium sulfate silty clay， negative temperature triaxial shear tests were carried out on silty clay with different sodium sulfate contents in the temperature range of -2， -6， -10 ℃ under 1 MPa confining pressure. Based on the experimental data， the tangential modulus of frozen sodium sulfate silty clay was calculated， the nonlinear relationship between axial strain and deviational stress was studied， the p-q plane strength criterion of sample was derived， and the strength formula of frozen sodium sulfate silty clay was proposed. The modified Duncan-Chang constitutive model of frozen salt-bearing silt with parameters was established， and the relevant parameters were fitted to verify the accuracy and applicability of the model. The results show that the deviational stress of the sample tends to be stable at the later stage of strain hardening， and the tangential modulus E of the sample at negative temperature decreases first， then increases and then decreases with the increase of salt content. The tangential modulus E has changed in a small variation range and p-q shows an obvious linear relationship. In the temperature range of 0 ℃ to 15 ℃， the modified Duncan-Chang constitutive model has good prediction effect on the strength of frozen sodium sulfate silty clay with salt content less than 2.5%.
Special attention should be paid to the frost heave due to silt’s high capillary water content in permafrost engineering. The mechanism of lime improvement is not clear on frost resistance in silt areas. A series of one-dimensional frost heave tests to the frost heaving characteristics of lime-modified silt were carried out in an open system. The effects of different lime content on frost-heaving characteristics of silt were studied. The results show that the freezing depth of soil samples first increases and then decreases with the increase of ash content， the final frost-heave capacity and additional water amount decrease， and the frost-heaving ratio decreases first and then increases. The indexes exhibit different change rules， and the frost-heaving characteristics are complex. Under certain conditions， increasing the ash content can reduce the freezing depth and frost heave amount， while the frost heave rate of the lime-modified silt is always higher than that of plain soil. Therefore， it is necessary to comprehensively evaluate the improvement effect of lime on the frost resistance of silty soil by combining the three indexes of freezing depth， frost heave amount， and frost heave rate instead of a single indicator in specific projects.
As the main body of the terrestrial ecosystem， forest soil plays an irreplaceable role in the global carbon and nitrogen cycles. Under natural conditions， the distribution of forest soil organic carbon and available nitrogen is controlled by factors such as climate and vegetation. Climate usually affects soil water and heat conditions and the distribution patterns of vegetation. Vegetation affects soil carbon and nitrogen content through its own growth and litter decomposition. However， due to the significant regional variation in response to climate warming， limited field observations and large spatial heterogeneity， the understanding of soil organic carbon and available nitrogen content and spatial distribution patterns in the deep soil of different types of permafrost zones remains largely uncertain. At the same time， in the past century， the temperature in the Greater Hinggan Mountains has experienced a warming of more than 1 ℃， and the frozen soil has degraded from continuous permafrost zone to discontinuous permafrost zone， sporadic permafrost zone or island permafrost zone. At present， there is still a lack of research on the spatial distribution characteristics and influencing factors of soil organic carbon and available nitrogen in different types of permafrost zones in the Huma River basin. Therefore， this paper selects three types of permafrost zones in the Huma River basin （discontinuous permafrost zones， sporadic permafrost zones and island permafrost zones）. Based on the spatio-temporal transformation method， we explored the spatial variation characteristics of forest soil organic carbon and available nitrogen in the process of permafrost degradation and revealed the main controlling factors and relative contributions of forest soil organic carbon and available nitrogen in the watershed. In this study， forest soil was selected as the research object. In September 2020， 16 soil profiles with a depth of 0~100 cm were selected for sample collection in discontinuous permafrost zones， sporadic permafrost zones and island permafrost zones in the Huma River basin. The soil samples were collected vertically downward from the surface into 5 layers， 0~20 cm， 20~40 cm， 40~60 cm， 60~80 cm and 80~100 cm in sequence. Three replicates of soil samples were collected at different sides of the same depth of the profile， and a total of 240 soil samples were collected. Each sampling point records basic information such as elevation， longitude， latitude， and dominant species of above-ground and surface vegetation. The effects of environmental variables such as elevation， climate， permafrost zone type and vegetation type on forest soil organic carbon and available nitrogen content were discussed based on the structural equation model （SEM）. The results showed that the contents of soil organic carbon and nitrate nitrogen in the discontinuous permafrost zone were higher than those in sporadic permafrost zone and island permafrost zone， while soil ammonium nitrogen content in the sporadic permafrost zone was higher than that in island permafrost zone and discontinuous permafrost zone. In the vertical profile， the contents of soil organic carbon and available nitrogen contents tended to decrease with increasing soil depth， and there was a significant negative correlation between soil organic carbon and available nitrogen （P<0.05）. The structural equation model clarified that vegetation type and mean annual temperature were the main controlling factor for soil nitrate nitrogen content， and mean annual precipitation had the weakest effect on soil organic carbon content； permafrost zone type and vegetation type were the main controlling factors for soil ammonium nitrogen and nitrate nitrogen content. This study is helpful to understand the distribution patterns and main controlling factors of forest soil organic carbon and available nitrogen in different types of permafrost zones in the Huma River basin， and can provide certain data support for the accurate simulation and estimation of forest soil carbon and nitrogen storage in the watershed in the future.
We collected 138 soil samples from 23 sampling sites in the Buqu catchment， the source region of the Yangtze River， China. We examined the distribution pattern of soil organic carbon （SOC）， and further discussed the possible influencing factors of SOC content within 1 m. The results showed SOC contents in the 0~10， 10~20， 20~30， 30~40， 40~50 and 50~100 cm layers were （10.23±4.84）， （10.18±5.19）， （9.34±5.20）， （9.04±4.41）， （8.01±4.74）， （9.40±4.67） g·kg-1， respectively. SOC content decreased with the increasing soil depth （R2=0.511）. There was no significant linear relationship between SOC content and altitude （P>0.05）. With the increasing elevation， SOC content was gradually increased between 4 700 to 5 100 m， while the SOC then gradually declined with elevation from 5 100 to 5 250 m. SOC content was negatively correlated with soil pH value （P<0.01）， but positively correlated with C/N ratio （P<0.01）. The SOC content was also positively correlated with volume water content （P<0.05）， but had no significant correlation with mean annual temperature， mean annual precipitation， total nitrogen， total phosphorus， total kalium， inorganic carbon content， cation exchange capacity， bulk density and clay content （P>0.05）. Therefore， soil pH value， C/N ratio and volumetric water content were the main factors that were closely associated with the SOC content within 1 m depths. The results can provide a useful dataset for studying soil carbon cycle in the Buqu catchment， the source region of the Yangtze River， China.
Nam Co， located in the central and southern part of the Tibetan Plateau， is a unique saline lake in the region. To carry out the study on the composition and diversity of culturable bacterial species in coastal water bodies in the summer season in Nam Co and to reveal the response pattern of bacterial species diversity and community distribution to physicochemical factors will not only be able to provide valuable resources and references for the study of microbial diversity and the development and utilization of special bacterial resources under extreme environment， can provide a theoretical basis for the conservation of lake ecosystems on the Tibetan Plateau. In this study， 20 sampling sites were selected based on lake water distribution combined with geographical differences in coastal areas of Nam Co in the summer of 2020. Water samples were collected with sterile samplers at approximately 15 m from the shore and 50 cm from the water surface， the collected water samples were packed in sterile plastic buckets and brought back immediately to the laboratory to isolate the culturable bacteria inside. The direct coating plate method combined with the dilution coating plate method was used to isolate culturable bacteria from lake water， and the plate four zone streaking method was used to purify the whole bacterial strains， and bacterial strains were stored by freezing in glycerol tubes at -20 °C. The medium used in the experiment was beef extract peptone medium， when isolated the cultivable bacteria strains from water samples， the water added in the medium was Nam Co lake water： sterile water=1∶1， when purified the bacterial strains， all the added water in the medium was sterile water. The identification of bacterial strains was performed by the combination of classical taxonomy and 16S rDNA gene sequence analysis， and relevant statistical software， such as R 4.1.1 and SPSS 20.0， etc.， was used to analyze the community distribution and species diversity of bacteria and their correlation with aquatic physicochemical factors. A total of 681 cultivable bacteria were isolated from 20 sampling sites of coastal water in the summer season in Nam Co， and all strains identified were classified into 43 species in 16 genera， of which the dominant species was Acinetobacter johnsonii and the dominant genus was Acinetobacter， and Acinetobacter johnsonii was a common bacterial species among 20 sampling sites. Among the 20 sampling sites， number 12 had the highest number of species， and it had 18 species； the second is number 4， which had 14 species， and number 11， which had the lowest number of species， which had only six species. The Shannon-Wiener diversity indexes， Simpson diversity indexes， and Pielou evenness indexes showed generally consistent trends among 20 sampling sites， the three indexes synthetically indicated that number 10 had the highest species diversity and the most even distribution， whereas the number 9 had the lowest species diversity and the most uneven distribution. Spearman correlation coefficient showed that both total nitrogen and total phosphorus were the main physicochemical factors affecting the diversity of cultivable bacterial species in water bodies in summer in Nam Co， and there was a significant correlation between total bacterial abundance and total phosphorus （P<0.05）， and the Simpson diversity indexes was also significantly correlated with total nitrogen （P<0.05）. Redundancy analysis results showed that ammonia nitrogen was the main environmental factor affecting the distribution of cultivable bacterial communities in water body during summer in Nam Co （P<0.05）. This study initially revealed the species diversity and community distribution of culturable bacteria in coastal water bodies and their correlations with environmental factors in the summer season in Nam Co， and obtained a relatively rich resource of bacterial strains， in the hope of providing a theoretical basis for the conservation of lake ecosystems in the Tibetan Plateau and the exploitation and utilization of special bacterial resources.
It is of great scientific significance to understand the distribution characteristics of the bacterioplankton in the plateau lakes and clarify its ecological function of the plateau lake ecosystem. In May 2021， we investigated bacterial community in Nam Co lake samples were analyzed by 16S rDNA high-throughput sequencing technology， and the difference of bacterial community was analyzed by α-diversity index. The interaction among bacterial communities was analyzed by co-occurrence network. The correlation between environmental factors and α-diversity index was measured by Pearson correlation coefficient. Redundancy analysis （RDA） was used to investigate the relationship between physicochemical factors and bacterial community structure. Function of bacterioplankton in Nam Co lake was predicted based on PICRUSTt2. The results showed that bacterial community was mainly composed of Proteobacteria， Bacteroidetes， Actinobacteria， Cyanobacteria and Firmicutes. The relative abundance of Proteobacteria was the highest， mainly including Gamma-proteobacteria and Alpha-proteobacteria. Hydrogenophaga and Algoriphagus are the relative dominant species. The α-diversity index showed that the bacterial community was rich in Nam Co lake. The correlation among nodes of Nam Co lake co-occurrence network is mainly positive. Total dissolved salts （TDS） and salinity （Sal） were the key factors affecting the bacterial community structure. Functions of bacterioplankton in Nam Co lake mainly involved 6 metabolic pathways， such as metabolism， genetic information processing and environmental information processing and 46 sub-functions of membrane transport， amino acid metabolism， carbohydrate metabolism， etc. In conclusion， bacterial community structure in Nam Co lake was different among various sites. At the phylum level， the interaction among bacterial groups was mainly synergistic， and the bacterial community structure in Nam Co lake was the result of multiple factors. This study clarified the composition and function of bacterioplankton in Nam Co lake and its relationship with environmental factors， providing scientific reference for the protection of ecological environment of plateau lakes.
Polychlorinated biphenyls （PCBs） are typical components of persistent organic pollutants （POPs） with long-term retention， bioaccumulation， semi-volatility and high toxicity. PCBs can exist in the environment for a long time and migrate over long distances through a variety of environmental media， which result in serious threaten to environmental safety and human health. Snow is considered an almost ideal depositing medium for assessing atmospheric deposition. Previous studies have shown that PCBs exist widely in the atmosphere， but the concentration of PCBs in snow is extremely low， and the collection of a large number of snow samples brings difficulties to analysis and research. In order to explore the content and distribution characteristics of PCBs in snow， this study first established a method for the determination of 28 PCB congeners in snow based on gas chromatography-quadrupole time-of-flight mass spectrometry （GC-QTOF/MS）. The quantitative analysis of PCBs in snow was achieved with a small sample volume （1~2 L） by optimizing the parameters of high-resolution GC-MS. By comparing the qualitative and quantitative parameters of PCBs under different ion sources， it was finally found that the electron bombardment ion source was optimal， and the emission current was further optimized to improve the sensitivity of the instrument. The results showed that the PCBs standard samples had a good linear relationship within the concentration of 0.05~10 μg·L-1， and the correlation coefficient was in the range of 0.993~0.999. At three spiked levels， the recoveries of PCBs were 60.2%~103.2%， the relative standard deviations were 0.9%~14.0% （n=3）， and the detection limit of the method was 1.01~2.92 pg·L-1. This method can be applied to the analysis of snow samples， which provides a basis for further analysis of the distribution characteristics of pollutants in snow. In addition， through the analysis of PCBs in snow in typical areas of Northeast China， it was found that PCBs in snow were mainly composed of pentachlorobiphenyl and hexachlorobiphenyl， and the PCB monomers with higher content were PCB81， 128， 126 and 169. The study preliminarily concluded that the pollution of PCBs in snow is not only related to the historical production and residues of PCBs， but also related to combustion and thermal industrial processes.
According to the investigation of several roads in Inner Mongolia Autonomous Region， it is found that the sections with collision guardrails have serious snow hazards. Due to the limit of many factors such as snowfall， winds， time and research period， it is difficult to monitor the snow drift on site. Therefore， the computer simulation “digital wind tunnel” is used to select the typical embankment section form in the separated subgrade， establish the subgrade model and collision guardrail model， and quantitatively analyze the characteristics and influence relationship of different collision guardrail setting parameters on the wind speed and flow field of highway section. Through the analysis of the data of the pavement wind speed control points and the overall wind speed cloud map of the pavement， the irregularity of the shape of the anti-collision guardrail hinders the flow of wind-blown snow， resulting in a significant reduction in the pavement wind speed， causing a large number of snow particles to deposit on the pavement， resulting in road snow. Therefore， in areas with frequent wind-blown snow， the separated subgrade section may slow down the slope gradient or use cable guardrail to ensure the safety of the roadside and eliminate or reduce the occurrence of snow damage. Therefore， this paper can provide theoretical basis and reference for the design and maintenance of roadside collision guardrail of highway separated subgrade in snowy areas.
