Introduction: Permafrost and seasonally frozen soil are widely distributed on the Qinghai-Tibetan Plateau, and the freezing-thawing cycle can lead to frequent phase changes in soil water, which can have important impacts on ecosystems.Methods: To understand the process of soil freezing-thawing and to lay the foundation for grassland ecosystems to cope with complex climate change, this study analyzed and investigated the hydrothermal data of Xainza Station on the Northern Tibet from November 2019 to October 2021.Results and Discussion: The results showed that the fluctuation of soil temperature showed a cyclical variation similar to a sine (cosine) curve; the deep soil temperature change was not as drastic as that of the shallow soil, and the shallow soil had the largest monthly mean temperature in September and the smallest monthly mean temperature in January. The soil water content curve was U-shaped; with increased soil depth, the maximum and minimum values of soil water content had a certain lag compared to that of the shallow soil. The daily freezing-thawing of the soil lasted 179 and 198 days and the freezing-thawing process can be roughly divided into the initial freezing period (November), the stable freezing period (December-early February), the early ablation period (mid-February to March), and the later ablation period (March-end of April), except for the latter period when the average temperature of the soil increased with the increase in depth. The trend of water content change with depth at all stages of freezing-thawing was consistent, and negative soil temperature was one of the key factors affecting soil moisture. This study is important for further understanding of hydrothermal coupling and the mechanism of the soil freezing-thawing process.

The soil freeze-thaw phenomenon is one of the most outstanding characteristics of the soil in Heilongjiang Province. Quantitative analysis of the characteristics of changes in key variables of the soil freeze-thaw processes is of great scientific importance for understanding climate change, as well as ecological and hydrological processes. Based on the daily surface temperature and air temperature data in Heilongjiang Province for the past 50 years, the spatial-temporal distribution characteristics of key variables and their correlations with air temperature and latitude in the freeze-thaw process of soil were analyzed using linear regression, the Mann-Kendall test, the local thin disk smooth spline function interpolation method, and correlation analysis; additionally, the spatial-temporal distribution of key variables and the changes in the surface temperature during the freeze-thaw process are discussed under different vegetation types. The results show that there is a trend of delayed freezing and early melting of key variables of the soil freeze-thaw process from north to south. From 1971 to 2019 a, the freezing start date (FSD) was delayed at a rate of 1.66 d/10 a, the freezing end date (FED) advanced at a rate of 3.17 d/10 a, and the freezing days (FD) were shortened at a rate of 4.79 d/10 a; with each 1 degrees C increase in temperature, the FSD was delayed by about 1.6 d, the FED was advanced by about 3 d, and the FD was shortened by about 4.6 d; with each 1 degrees increase in latitude, the FSD was delayed by about 2.6 d, the FED was advanced by about 2.8 d, and the FD was shortened by about 5.6 d. The spatial variation in key variables of the soil freeze-thaw process under the same vegetation cover was closely related to latitude and altitude, where the lower the latitude and altitude, the more obvious the variation trend; among them, the interannual variation trend of key variables of soil freeze-thaw under meadow cover was the most obvious, which varied by 9.65, 16.86, and 26.51 d, respectively. In addition, the trends of ground temperature under different vegetation types were generally consistent, with the longest period of unstable freeze-thaw and the shortest period of stable freeze in coniferous forests, compared to the shortest period of unstable freeze-thaw and the longest period of stable freeze in meadows. The results of the study are important for our understanding of soil freeze-thaw processes and changes in Heilongjiang Province, as well as the evolution of high-latitude permafrost; they also promote further exploration of the impact of soil freeze-thaw on agricultural production and climate change.

Ground temperature plays a significant role in the interaction between the land surface and atmosphere on the Tibetan Plateau (TP). Under the background of temperature warming, the TP has witnessed an accelerated warming trend in frozen ground temperature, an increasing active layer thickness, and the melting of underground ice. Based on high-resolution ground temperature data observed from 1997 to 2012 on the northern TP, the trend of ground temperature at each observation site and its response to climate change were analyzed. The results showed that while the ground temperature at different soil depths showed a strong warming trend over the observation period, the warming in winter is more significant than that in summer. The warming rate of daily minimum ground temperature was greater than that of daily maximum ground temperature at the TTH and MS3608 sites. During the study period, thawing occurred earlier, whereas freezing happened later, resulting in shortened freezing season and a thinner frozen layer at the BJ site. And a zero-curtain effect develops when the soil begins to thaw or freeze in spring and autumn. From 1997 to 2012, the average summer air temperature and precipitation in summer and winter from six meteorological stations along the Qinghai-Tibet highway also demonstrated an increasing trend, with a more significant temperature increase in winter than in summer. The ground temperature showed an obvious response to air temperature warming, but the trend varied significantly with soil depths due to soil heterogeneity.

Ground temperature plays a significant role in the interaction between the land surface and atmosphere on the Tibetan Plateau (TP). Under the background of temperature warming, the TP has witnessed an accelerated warming trend in frozen ground temperature, an increasing active layer thickness, and the melting of underground ice. Based on high-resolution ground temperature data observed from 1997 to 2012 on the northern TP, the trend of ground temperature at each observation site and its response to climate change were analyzed. The results showed that while the ground temperature at different soil depths showed a strong warming trend over the observation period, the warming in winter is more significant than that in summer. The warming rate of daily minimum ground temperature was greater than that of daily maximum ground temperature at the TTH and MS3608 sites. During the study period, thawing occurred earlier, whereas freezing happened later, resulting in shortened freezing season and a thinner frozen layer at the BJ site. And a zero-curtain effect develops when the soil begins to thaw or freeze in spring and autumn. From 1997 to 2012, the average summer air temperature and precipitation in summer and winter from six meteorological stations along the Qinghai-Tibet highway also demonstrated an increasing trend, with a more significant temperature increase in winter than in summer. The ground temperature showed an obvious response to air temperature warming, but the trend varied significantly with soil depths due to soil heterogeneity.

We present a method to characterize soil moisture freeze-thaw events and freezing/melting point depression using permittivity and temperature measurements, readily available from in situ sources. In cold regions soil freeze-thaw processes play a critical role in the surface energy and water balance, with implications ranging from agricultural yields to natural disasters. Although monitoring of the soil moisture phase state is of critical importance, there is an inability to interpret soil moisture instrumentation in frozen conditions. To address this gap, we investigated the freeze-thaw response of a widely used soil moisture probe, the HydraProbe, in the laboratory. Soil freezing curves (SFCs) and soil thawing curves (STCs) were identified using the relationship between soil permittivity and temperature. The permittivity SFC/STC was fit using a logistic growth model to estimate the freezing/melting point depression (T-f/m) and its spread (s). Laboratory results showed that the fitting routine requires permittivity changes greater than 3.8 to provide robust estimates and suggested that a temperature bias is inherent in horizontally placed HydraProbes. We tested the method using field measurements collected over the last 7 years from the Environment and Climate Change Canada and the University of Guelph's Kenaston Soil Moisture Network in Saskatchewan, Canada. By dividing the time series into freeze-thaw events and then into individual transitions, the permittivity SFC/STC was identified. The freezing and melting point depression for the network was estimated as T-f/m = - 0.35 +/- 0.2,with T-f = - 0.41 +/- 0.22 degrees C and T-m = - 0.29 +/- 0.16 degrees C, respectively.

Thaw and liquid precipitation retard cooling of snow cover and soil surface and so may be a factor of heating. This slows down the soil freezing due to more active freezing of the wet snow, and, thus, promotes cooling and re-cooling of the soil. However, there are a number of factors which intensify the soil freezing after thaw. With thaw, the thickness of the snow cover decreases, and its density increases. In addition, after freezing wet snow improves the contact between the ice crystals, which increases the hardness and thermal conductivity of the snow As a result, after the thaw, the thermal protection ability of the snow decreases, and this can accelerate freezing of the soil. The dynamics of snow accumulation in Russia is considered in the paper. Using data obtained in the Western Svalbard, we demonstrate the increase in the number of thaws and liquid precipitation and influence of them on the snow cover and soil freezing. The influence of thaw on the growth of thermal resistance of snow cover is also considered. Calculations have shown that in the absence of a thaw, the depth of soil freezing is 1.26 m. With a thaw lasting 10 days, which begins on the 40th day from the start of soil freezing, the depth of freezing is reduced down to 1.2 m without considering changes in snow cover. When taking into account changes in the thermal resistance of snow cover, the depth of soil freezing by the end of the cold period increases up to 1.32 cm. With a thaw in the mid-winter, i.e. on the 70th day, the depth of freezing decreases down to 1.22 m, that is smaller than the depth of freezing without thaw This scenario is in accordance with changes in snow accumulation dynamics under the present-day climate, as in many areas most of the solid precipitation falls in the first half of the cold period. As a result, for a period after a thaw the smaller volume of snow will be deposited, and this will retard increasing in thermal resistance of the snow cover.

Northeast China with seasonally frozen soil is quite sensitive to global warming. The changes in soil freezing and thawing processes initiated by global warming could alter the hydrological cycle of agricultural fields. A pairedplot experiment was conducted in frozen agricultural soils in Northeast China to examine the impacts of simulated warming on soil freezing and thawing processes and on soybean production. Infrared radiators were used to simulate global warming, rising surface soil temperature (5 cm depth) by 2.86 degrees C. We showed that, artificial warming caused the freeze duration shortened by 22 days, and the thaw duration shortened by 17 days resulting in the mean duration of soil freezing-thawing significantly shortened by 39 days and the maximum frost depth reduced by about 40 cm. Simulated warming had no significant effect on the average annual freezethaw cycle frequency. Warming induced a larger water accumulation in the 0-100 cm soil layer during 2014-2015 soil freezing period. In the dry year of 2015, warming did not significantly affect surface soil moisture during period from sowing date to VC (soybean cotyledon) date. Thus, warming-induced an increase in soybean yield in the dry year may be attributable to the positive effect of enhanced soil temperature on soybean growth (above-ground dry matter accumulation) and consequent on soybean production. In the wet year of 2014, warming decreased surface soil moisture from sowing date to the date of VC stage because warming advanced the soil thaw-end date in 20-60 cm layer by 15 days. This decline in surface soil water availability may potentially offset the positive effects of increased soil temperature on soybean yield, thus warming effects on soybean production was neutral in the wet year. Our findings highlight the potential role of seasonally soil freezing and thawing dynamics in regulating soybean to global warming and suggested that warming effects on soil water dynamics during soil freezing and thawing periods, and subsequent on the surface soil water availability at the early vegetative stage and soybean production were associated with the hydrological year. We conclude that under current precipitation patterns, the no response of soil surface water availability to warming during early vegetative growth, coupled with warming-mediated increases in soil temperature, might improve soybean production during dry years in Northeast China.

Numerical models of permafrost evolution in porous media typically rely upon a smooth continuous relation between pore ice saturation and sub-freezing temperature, rather than the abrupt phase change that occurs in pure media. Soil scientists have known for decades that this function, known as the soil freezing curve (SFC), is related to the soil water characteristic curve (SWCC) for unfrozen soils due to the analogous capillary and sorptive effects experienced during both soil freezing and drying. Herein we demonstrate that other factors beyond the SFC-SWCC relationship can influence the potential range over which pore water phase change occurs. In particular, we provide a theoretical extension for the functional form of the SFC based upon the presence of spatial heterogeneity in both soil thermal conductivity and the freezing point depression of water. We infer the functional form of the SFC from many abrupt-interface 1-D numerical simulations of heterogeneous systems with prescribed statistical distributions of water and soil properties. The proposed SFC paradigm extension has the appealing features that it (1) is determinable from measurable soil and water properties, (2) collapses into an abrupt phase transition for homogeneous media, (3) describes a wide range of heterogeneity within a single functional expression, and (4) replicates the observed hysteretic behavior of freeze-thaw cycles in soils.

Frozen soil is predicted to change in the boreal areas with climate warming. We studied growth, longevity and mortality of fine roots at different levels of frozen soil in winter followed by a delayed soil thawing in spring in a 47-year-old stand of Picea abies (L. Karst.) in the boreal zone. The treatments, repeated over two winters, were: (i) natural insulating snow accumulation and melting (CTRL), (ii) snow removed during winter (OPEN), and (iii) as OPEN in winter but soil thaw delayed by insulation at the top of the forest floor (FROST). Short and long roots were monitored at different depths by minirhizotron imaging at one-month intervals from May to October in the 2 years during and 2 years after the treatments, to assess standing length (SSL), production volume (SPV) and mortality. A survival function estimate was calculated according to the nonparametric maximum likelihood estimate for interval censored data, and the mean and median root longevities were calculated as with a Kaplan-Meier estimate. CTRL and OPEN did not differ for SSL and SPV but they differed in FROST where compensatory growth occurred in the follow-up seasons. The mean longevity ranged from 276 to 305 days for short roots and from 425 to 464 days for long roots, being higher in OPEN than CTRL and FROST, and higher in the deeper soil layers than near the soil surface. The mean and median longevities were largely the same for short roots but the means were 80-100 days higher for long roots. We conclude that the winters with deep soil freezing are not detrimental for fine roots of Norway spruce, insofar as soil thawing will not prolong the growing season. The longer lifetime in OPEN suggests declining carbon flux into the soil following winters with deeply frozen soil. (C) 2013 Elsevier B.V. All rights reserved.

Recently, there has been a revival in the development of models simulating coupled heat and water transport in cold regions. These models represent significant advances in our ability to simulate the sensitivity of permafrost environments to future climate change. However, there are considerable differences in model formulations arising from the diverse backgrounds of researchers and practitioners in this field. The variability in existing model formulations warrants a review and synthesis of the underlying theory to demonstrate the implicit assumptions and limitations of a particular approach. This contribution examines various forms of the Clapeyron equation, the relationship between the soil moisture curve and soil freezing curve, and processes for developing soil freezing curves and hydraulic conductivity models for partially frozen soils. Where applicable, results from recent laboratory tests are presented to demonstrate the validity of existing theoretical formulations. Identified variations in model formulations form the basis for briefly comparing and contrasting existing models. Several unresolved questions are addressed to highlight the need for further research in this rapidly expanding field. (C) 2013 Elsevier Ltd. All rights reserved.