1 Introduction

The active layer is defined as the soil layer overlying the permafrost. This active layer thaws in summer and freezes in winter (Qin et al., 2014 ); and along with a cover of vegetation and snow, it acts as a "buffer layer" for the permafrost and as the zone of energy and water exchange between the permafrost and the atmosphere (Luo et al., 2014a ,b). It is the most thermodynamically active rock or soil unit; and it has profound effects on many terrestrial processes, such as the surface-energy balance, eco-environmental stabilities, and complex hydrothermal coupling (Li et al., 2002 ). Characteristics of the active layer include the freeze-thaw process, hydrothermal regimes, and physical properties of the soil, which are key factors in mediating between climate and the permafrost. Variation in temperature and thickness of the active layer can greatly affect arctic ecosystems, geologic environments, and hydrologic processes (Liang et al., 2007 ; Luo et al., 2014 a ,b).

Some local environmental factors can significantly affect the hydrothermal processes of soils in the active layer: such as temperature, precipitation, material composition of the soil, moisture content, and canopies (mainly snow cover and vegetation) (Zhao et al., 2000 ; Liu and Wang, 2016). Consequently, there are large variations in the freeze-thaw characteristics of the active layer in spatiotemporal differentiation because a small change in any effect factor may lead to large changes in the active layer (Li et al., 2012 ; Luo et al., 2014a ,b). Hydrothermal processes in the active layer reflect drying, wetting, and thermal conditions of the soil, which play important roles in the earth-atmosphere, water-energy system in permafrost regions (Woo and Winter, 1993; Carey and Woo, 1999; Zhao et al., 2000 ; Wu et al., 2003 ). The active layer is regarded as the lower interface of the arctic ecosystem and is the zone of energy exchange between the atmosphere and the permafrost. This exchange occurs primarily through dynamic changes in water and heat in the active layer (Zhao et al., 2000 ). Hydrothermal exchange in the active layer is also the key factor in preserving the stability of an arctic ecosystem. Furthermore, the distinct water-conservation function of the permafrost and its arctic ecosystem plays a significant role in stabilizing the water cycle and river runoff. Changes in the active layer can result in soil-moisture retention; can directly affect soil-heat transport, moisture distribution, and stability of the ecosystem; and can generate feedback to climate warming (Zhang et al., 2003 ). Changes in the active layer also generate feedback to the effect factors, which has been studied extensively by scientists worldwide due to effects on climate, ecology, the environment, and engineering properties of permafrost (Zhao et al., 2000 ).

There have been several studies of hydrothermal processes in and characteristics of the active layer in China. A series of surveys and investigations of the hydrothermal process in the active layer has been performed on the Tibetan Plateau (Yang et al., 2000 ; Zhao et al., 2000 , 2008; Wu et al., 2003 , 2015; Pang et al., 2006 ; Wu and Zhang, 2010; Li et al., 2012 ; Wang et al., 2012 ; Zhang et al., 2013 ; Luo et al., 2014a ; Jiao et al., 2014 ). However, understanding is still incomplete regarding freeze-thaw processes in the active layer in Northeast China, which holds the third-largest region of permafrost in China. This scarcity of information is particularly relevant to Nanweng'he River National Natural Reserve; also, there was diversity of vegetation forms in the study area, which is an ideal area to study the freeze-thaw processes of soil in the active layer. There, in the Nanweng'he River National Natural Reserve, we collected soil-temperature and water-content data in the active layer from 2011 to 2014 at two observation sites set up by the State Key Laboratory of Frozen Soil Engineering of the Chinese Academy of Sciences. We used the data to study the freeze-thaw process and variations in soil temperature and water content in the active layer near the Nanweng'he River and analyzed the profound effects of water content and vegetation on the freeze-thaw processes. This article is intended to provide reliable information regarding environmental changes at the Nanweng'he River National Natural Reserve.

2 Study area and methodology 2.1 Study area

The Nanweng'he River National Natural Reserve (125°07′55″E–125°50′05″E, 51°05′07″N–51°39′24″N) is located at the southeastern margin of the Yilehuli'shan Mountains, the intersection of the Da and XiaoXing'anling (Hinggan) mountains (Figure 1). It measures 72.7 km north-to-south by 57.3 km east-to-west, i.e., an area of 22.95 km²; and it spans an elevation range of 393–803 m.

Nanweng'he River National Natural Reserve contains the largest cold-temperate wetland ecosystem in China. There, the permafrost and periglacial phenomena generate a distinct cold-wet ecosystem of forests and swamps in the southeastern Da Xing'anling Mountains. The wetland in the reserve consists mostly of swamp, including moss bog, shrub swamp (swamp with at least 30% shrub cover), meadow bog, and forest bog (bog with at least 20% tree cover; Lin, 2010).

The study area is located in the headwater region of the Nanweng'he River, a tributary of the Nenjiang River, and has a cold-temperate climate. According to 1982 data from the Heilongjiang hydrological union station (Wang and Gong, 2009), at that time, the mean annual air temperature in the area was −3.0 °C, the annual precipitation was 400–600 mm, and the stable snow cover was 0.3–1.0 m. Based on observations, in 2012 the mean annual air temperature was −1.8 °C; and the annual precipitation was 879 mm. Compared with 1982 measurements, air temperature increased by 1.2 °C; and precipitation also increased.

Due to constraints imposed by the wetland topography, permafrost is distributed as large discontinuous patches. Compared with neighboring areas at similar latitudes (Guo et al., 1981 ; Zhou et al., 2000 ), permafrost is colder and thicker. Within this region, approximately one-third of the area is underlain by permafrost. In September 2011, two sites for observing the active layer were selected, based on the distinct ground vegetation in the study area. These observation sites were one located in wetland, where the main vegetation was Carex tato, and the second located in a shrub cover less than 100 m away. The observation sites were set up in a permafrost region with an active layer between 70 and 140 cm thick.

2.2 Data acquisition and analysis

On the basis of soil texture and ground vegetation, the thermal sensors for temperature and the CS616 probes for soil-moisture content were installed at depths of 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, and 180 cm at the monitoring sites, where the thermistor cables, consisting of a string of temperature sensors with a precision of ±0.05 °C, were developed by the State Key Laboratory of Frozen Soil Engineering (SKLFSE), Lanzhou, China. The CS616 probes for measuring soil-moisture content were made by the Campbell Scientific Inc., USA, with an accuracy of ±0.05% and a resolution of 0.1%. The CR3000-XT data loggers were programmed to automatically collect temperature and unfrozen water content of soil in the active layer. Measurements for all active-layer parameters were taken at 1-hour intervals.

In this study, as in some earlier similar work (Osterkamp and Romanovsky, 1997; Romanovsky and Osterkamp, 1997; Mutter and Phillips, 2012), the freeze-thaw process in the active layer was divided into three periods of spring-summer thawing, autumn-winter freezing, and winter freeze-up. Zero-curtain is the phenomenon in which the soil temperature remains close to 0 °C (a range between −0.1 °C and 0.1 °C in accordance with the precision of the soil-temperature sensors) for an extended period of time (Muller, 1947). Here, dates for the beginning of thawing or freezing were taken as the dates when the mean daily soil temperature was above or below 0 °C, respectively (Muller, 1947). Soil-water content was defined as the percentage of liquid water in the soil. In the following analysis, the soil-water content during the thawing period is the total soil-water content. For soil in the frozen state, the soil-water content is defined as the liquid-water content, excluding the ice content.

3 Study results: freezing and thawing process of the permafrost active layer

During the process of the freeze-thaw cycle, the entire active layer undergoes cooling, initial freezing until complete, further cooling, warming, initial thawing until complete, and further warming. During these stages, the distribution and transport of energy and water vary in the active layer. As in most permafrost regions (Zhao et al., 2000 ), the freezing process of the active layer at the monitoring sites begins upward from the permafrost table. Provided that 0 °C isotherm is the freezing point of soil water in the study area, as shown in Figures 2a and 3a, the freezing process began to develop gradually from the permafrost table upward in late September and early October from 2011 to 2014.

Stable freezing began from the ground surface within the following 10 days and developed rapidly downward. Bidirectional freezing processes then began to develop and lasted until middle October, when the entire active layer became frozen.

In late March of the following year, daily freezing and thawing appeared now and then, and gradually changed to daily cycles from the beginning of April. The active layer began to thaw downward in late April and early May until reaching a maximum depth in late August to early September (Figures 2a and 3a).

3.1 Autumn-winter freezing stage

After the active layer thawed to its maximum depth, it began to freeze upward, thereby initiating the freezing process until the entire layer became frozen (Figures 2a and 3a), which is defined as the autumn-winter freezing stage of the active layer. The process could be divided into two periods, i.e., the unidirectional freezing period and the "zero curtain" period. The zero-curtain is thought to be a stage of warming and cooling down to the maximum thaw depth instead of an immediate initiation of the freezing process. The unidirectional freezing stage is from the date when the active layer begins to freeze from the permafrost table upward to the date when the stable frozen ground surface is formed.

The ground vegetation differed between the two monitoring sites; and therefore, the start dates of unidirectional freezing also differed. At the wetland site, unidirectional freezing started downward from late September until the ground surface froze completely on October 6; then the downward freezing in the autumn zero-curtain period started from the ground surface, and the upward and downward freezing fronts closed up on October 13 (Figure 3a). At the shrub-cover site, the unidirectional freezing started from the bottom of the active layer in early October. The downward freezing in the autumn zero-curtain period started from the ground surface on October 13, and then the upward and downward freezing fronts closed upon October 19 (Figure 2a).

During the unidirectional freezing process, the active layer is a system open to the air; the mass exchange happens between the active layer and atmosphere in daytime or part of daytime. The daily heat exchanges between the atmosphere and the upper parts of the active layer are in a relative balance state even though the active layer absorbs heat in daytime from and releases heat at night to the atmosphere. In this stage, the temperature in the middle and upper active layer was higher than that in the bottom; the temperature gradient was low and continued to decrease with time. At the bottom of the active layer, moisture was migrating from the thawed part toward the freezing front, driven by the temperature gradient and freezing there, and has the trend of continuously downward migration accompanied by upward movement of the freezing front. Heat is transferring from the thawed part to the frozen part. In the unfrozen part of the active layer, heat is transferred on the advection flow, driven by advection of water vapor; and it gradually becomes the main way of heat transfer. Heat transferred by conductive heat flow accounts for a small amount of the total heat (Figures 2b and 3b).

After the steady freezing of the ground surface, the zero-curtain stage started with the characteristics of bidirectional freezing in the active layer. In this stage, the active layer is a closed system; mass-exchange channels with the atmosphere are closed by the surface frozen soil. The temperature in the active layer was higher in the middle part and decreased both upward and downward from there. The temperature of the thawed layer between the upper and lower freezing fronts was approximately 0 °C or a little above 0 °C.

After the stable surface freezing formed, the upper freezing front moved downward rapidly; and the lower one moved upward slowly. Simultaneously, moisture contained in the thawed part in the middle of the active layer was migrating to both the upper and lower freezing fronts and then freezing. By which, heat was transferred from the middle part to both of the freezing fronts—similarly during the moisture migration and freezing process and by which the temperature gradient in the active layer became gentler and gentler. After a period, freezing terminated; and the entire layer became completely frozen except for a very small amount of moisture that moved upward, driven by the temperature gradient.

3.2 Winter freeze-up stage

Winter freeze-up stage begins when the freezing process of the active layer finishes, and it lasts approximately 200 days. Based on the temperature variations, this stage can be subdivided into the winter-cooling stage and the spring-warming stage (Zhao et al., 2000 ).

After the freezing process of the active layer finished, the active layer in the completely frozen state started to undergo rapid, continuous cooling until late January of the following year (Figures 2a and 3a). This process is referred to as the winter-cooling process, in which the temperature of the active layer is lower in its upper part and higher in its lower part; and the temperature of the active layer increases with increasing depth, thereby creating an upward temperature gradient. With further cooling, the temperature gradient gradually increased with upward heat transfer and heat emissions from the ground to the atmosphere, which resulted in a ground temperature that continually decreased until reaching a minimum. Accordingly, the temperature throughout the layer also decreased until reaching a minimum.

Throughout the freezing stage, the entire active layer was frozen; and thus, the moisture content generally remained constant. When the active layer was in the continuous cooling stage, the soil temperature increased with depth. Due to the temperature gradient, the remaining unfrozen water tended to migrate upward; but the amount of migrating moisture was not high, due to the very low ground temperature, which restricted the content and the activity of the unfrozen water.

Beginning in late January and early February, accompanied by the rise of air temperature, spring warming occurred in the active layer. In this stage, both the air and ground temperatures gradually rose, resulting in a slowly decreasing downward temperature gradient. With further warming, the temperature gradient increased; and heat was transferred upward. In late March, the daily freeze-thaw cycles appeared on the ground surface.

In this stage, the temperature gradient gradually decreased; and the evaporation capacity near the surface became stronger and stronger, with the rate of unfrozen water migration decreasing gradually (Figures 2b and 3b). Beginning in late March, daily freeze-thaw occurred on the surface in the absence of snow cover. The soil near the ground surface had thawed, and evaporation became more extensive in the daytime. At night, these soils were frozen, and the moisture migrated upward to the freezing front. This cycle goes round and round, resulting in a significantly decreasing trend of moisture content near the ground surface. Naturally, with a continuous snow cover, however, this daily process is absent; the thawed water of snow would flow to the ground surface, thus resulting in an increase of moisture content in soil near the ground surface.

3.3 Spring-summer thawing stage

The spring-summer thawing stage is a period of time between the days when the active layer begins to thaw downward from the ground surface and when the thawing process reaches its maximum depth.

The air temperature at the monitoring sites was occasionally above 0 °C in late March; then the ground surface's diurnal thaw-freeze cycling appeared at times, and gradually changed to daily cycles from the beginning of April. With further warming, in early May, the thawing started downward from the ground surface. Temperature at the depth of 5 cm became positive on May 5, and the average daily temperature rose above 0 °C. This process lasted until mid to late August, i.e., until the thawing was complete, which generally took more than three months.

During the initial stage of spring-summer thawing, the rising air temperature resulted in gradually increasing ground-surface temperatures and decreasing thermal gradients in the active layer, from the surface down to greater depths. With further warming, the thermal gradient slowly increased, which transferred surface energy downward; and the freezing began from the ground surface and developed downward. During the spring-summer thawing, the ground temperature in the active layer decreased from the ground surface downward; and the active layer was in the state of absorbing heat from the atmosphere and transferring it downward.

During the thawing process, moisture was also migrating primarily downward. Accompanied by the downward movement of the thawing front and driven by gravity, the free water (generated by the melting ice above the thawing front and the snowmelt) just above the thawing front was migrating downward (Figure 2a). The soils between the thawing front and the permafrost table were in a frozen state. Moisture there migrated downward continuously, driven by temperature gradient. Once the active layer was completely thawed, the soil water that exceeded the water-holding capacity of the active layer was all transported down to the permafrost in large amounts.

All three stages, i.e., autumn-winter freezing, winter freeze-up, and spring-summer thawing, constituted a complete annual cycle of freezing and thawing. Based on the previous discussion, we noted a pattern of variations in the moisture content and temperature in the active layer during this annual cycle. The moisture content of the soil during the thawing stage was relatively high. The moisture content varied dramatically due to precipitation, evaporation, and plant transpiration. This range of fluctuation gradually decreased with increasing depth. The soil in a frozen state had significantly lower moisture content, with a lower fluctuation range, compared with soil in the thawing phase. During the periods of freeze-thaw cycling, i.e., autumn-winter freezing and spring-summer thawing, moisture primarily migrated downward in large amounts, whereas during the winter freeze-up stage, moisture generally migrated upward in small amounts.

4 Discussion

Freeze-thaw in the active layer is controlled not only by the climate but also by the topography, soil texture, ground vegetation, and hydrology. Due to these various factors, the freeze-thaw process and hydrothermal properties varied dramatically in terms of the start dates of freezing and thawing, their durations, and the moisture content in the active layer in different areas. Even in neighboring areas, due to the differences in ground vegetation and topography, the hydrothermal conditions differed in the active layer. The discussion below focuses on the effects of moisture and ground vegetation on the freeze-thaw behavior in the active layer.

4.1 Effect of moisture content on the active layer

Temperature variations and freeze-thaw in the active layer are affected by the soil-moisture content. Soil moisture can affect the hydrothermal properties of the soil during the phases of freezing and thawing (Hinkel et al., 2001 ; Jiao et al., 2014 ). Specifically, the amount of soil moisture can significantly affect the freeze-thaw process and heat distribution in the soil, due to the greater heat capacity of water relative to that of soil (Zhou et al., 2005 ; Jiao et al., 2014 ). For example, in soil with a high moisture content, the temperature increases slowly within a small range. A great deal of heat may be released or absorbed by water during a phase change, and thus freezing soil releases a substantial amount of energy, reduces the temperature gradient in the soil, and slows the downward migration of the freezing front. Conversely, thawing soil absorbs a substantial amount of energy, which also reduces the temperature gradient but slows the downward migrate of the thawing front.

The development of the freeze-thaw process in the soil is also associated with the soil-moisture content, which can slow the freeze-thaw process and substantially affect heat distribution in the soil. As shown in Figures 2 and 3, the soil temperature varied moderately, almost approaching the 0 °C isothermal line during the freezing stage, which became clear where the soil depth exceeded 140 cm. In the freeze-thaw cycle, the effect of the moisture content on the soil-temperature variation was reflected in the slower variation of soil temperature relative to the moisture-content variation in the freezing process (Figures 2 and 3), which became more apparent with increasing moisture content.

4.2 Effect of vegetation on the active layer

Freeze-thaw in the active layer is affected not only by the moisture content but also by ground vegetation. The influences of vegetation on the thermal regimes of permafrost are displayed in several ways. First, by shading, the vegetation canopy reflects and absorbs much of the downward solar radiation and reduces its incidence onto the soil surface (Shur and Jorgenson, 2007; Chasmer et al., 2011 ). The canopy structure and its physiological function change the meteorological conditions in the vegetation, which in turn affects the heat and moisture exchange between the atmosphere and the soil (Pomeroy et al., 1998 ). Second, the vegetative layer also results in a decline of soil-moisture content due to the precipitation interception and high rates of evapotranspiration in summer, which release notable latent heat and reduce the thermal conductivity of organic soils (Yuan et al., 2010 ). Last, it is universally recognized that snow accumulation differs substantially between forested and open environments because of the difference in interception, sublimation, and wind redistribution (Pomeroy et al., 1998 ). All other factors being constant, the influences of vegetation may vary due to differences in cover condition and vegetation type; and these effects are primarily reflected in the differences in the start dates and durations of thawing and freezing. As discussed earlier, vegetation differed between the two monitoring sites, which can further affect the ground temperature, the timing of the freeze-thaw period, the freezing duration, and the division of the freeze-thaw process (Table 1, Figures 4 and 5).

As shown in Table 1, the starting dates of freezing and thawing and the freezing time period in the active layer vary with the vegetation. The shrub-cover site started to freeze in early October, whereas the corresponding date at the wetland site was mid-late September, i.e., approximately 10 days ahead of the shrub-cover site. There were similar differences in the date of thawing. The shrub-cover site started to thaw in early May, whereas the corresponding date at the wetland site was mid-late May, i.e., approximately 15 days later than at the of shrub-cover site. These differences are due to the poor drainage conditions caused by the high vegetation coverage and thick organic humus layer at the wetland site, which generate a "thermal semiconductor" effect. In other words, the organic matter and peat layer moderated warming of the ground by solar radiation in the summer and increased heat dissipation with the increase in heat conductivity after the ground froze, thereby protecting the permafrost. These effects were reflected in the later thawing, earlier freezing, and reduced thickness of the active layer.

Figure 4 shows the mean annual temperature at various depths at the shrub-cover site and the wetland site in 2012. At any given depth, the annual average temperature of the wetland site was lower than that of the shrub cover site, indicating a negative correlation between the amount of vegetation coverage and the temperature, other geographic and topographic factors being equal. The temperature of the active layer was lower with increasing vegetation coverage. A similar pattern is evident in Figure 5; the coverage in 2013 after a recovery of the vegetation was greater than in 2012, and the surface (5–10 cm) temperature was lower in 2013 than in 2012. However, at greater depths, the mean annual temperatures were generally identical, implying that the complex effects of vegetation differences on the temperature were concentrated primarily in the shallower soil. Because energy equilibrium can be achieved through heat exchange, such as thermal transfer between the ground and the atmosphere in the lower active layer, the upper portion of the active layer may undergo large temperature changes while the lower portion tends to remain at a uniform temperature.

Vegetation disturbance can also cause changes in the species composition, vegetation coverage, and density and thereby alter certain environmental factors and affect the underlying permafrost and its active layer. The vegetation at the wetland site, for example, was heavily disturbed while setting up the monitoring site in 2011. The vegetation had not recovered completely in 2012, continued recovering in 2013, and was completely recovered in 2014. As shown in Table 1, as the vegetation recovered, the start dates of thawing were delayed by 15 days from one year to the next. In 2014, the start date was delayed until early June, with negligible differences in the thawing process and the start date of freezing. However, the freezing time period was extended; and the thickness of the active layer shrank from 100 cm in 2012 to 70 cm in 2013 (Table 1).

5 Conclusions

The freeze-thaw process and its hydrothermal transport properties in the active layer of the permafrost in the Nanweng'he River area were analyzed. The effects of the moisture content and ground vegetation on these processes have also been discussed. Based on the previous analyses and discussions, it can be concluded as follows:

(1) The thawing in the active layer overlying permafrost is unidirectional, whereas the ground freezing is bidirectional (upward from the bottom of the active layer and downward from the ground surface). Three (shrub-cover site) to four (wetland site) months were required for the thawing to extend down to its maximum depth. The freezing process spanned approximately 20 days. During the annual freeze-thaw cycle, the thawing rate was far less than the freezing rate; and the freezing stage was much shorter than the thawing stage.

(2) The moisture in the active-layer migration behaved differently during the various stages of the freeze-thaw cycle. When the active layer was in the frozen state, unfrozen water generally migrated upward due to the temperature gradient, although in small amounts. During the thawed state, the moisture from the ground ice and surface snowmelt migrated downward by gravity to the upper surface of the permafrost. After complete thawing of the active layer, precipitation also moved downward under the influence of gravity; and therefore, the total amount of moisture transport was relatively large. During a complete annual freeze-thaw cycle of the active layer, soil moisture generally migrated downward and accumulated on the upper surface of the permafrost.

(3) The variations in the moisture content and temperature in the active layer displayed similar trends. The presence of soil moisture significantly affected heat transport in the soil. Water-phase changes moderated the soil-temperature curve to approach the 0 °C isotherm. The presence of soil moisture also reduced the temperature variation during the freezing process, with a lag behind the moisture variation in the soil. This phenomenon became more evident with higher moisture content. In addition, the vegetation exerted a remarkable effect on the ground temperature, the start dates of freezing and thawing, the freezing time period, and the freeze-thaw process.

(4) The development of the freeze-thaw process in the soil is associated with the soil-moisture content, which can slow down the freeze-thaw process and substantially affect heat distribution in the soil. In addition, all other factors being constant, the influences of vegetation may vary due to differences in cover condition and vegetation type; and these effects are primarily reflected in the differences in the starting dates and durations of thawing and freezing. As discussed earlier, vegetation differed between the two monitoring sites, which can further affect the ground temperature, the timing of the freeze-thaw period, the freezing time period, and the division of the freeze-thaw process.

(5) Based on these findings, the presence of wetland exerts the greatest effect on the hydrothermal processes in frozen soil in the permafrost region and, to a certain extent, can control whether permafrost will be present. Therefore, wetland should be a priority in environmental protection, resource development, and engineering construction in the future.

Acknowledgments:

This study was supported by the National Natural Science Foundation of China (Grant No. 41401081) and the State Key Laboratory of Frozen Soils Engineering (Grant Nos. SKLFSE-ZT-41, SKLFSE-ZT-20 and SKLFSE-ZT-12).