Research on the distribution of soil water, heat, salt and their response mechanisms under freezing conditions

Soil & Tillage Research 196 (2020) 104486

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Research on the distribution of soil water, heat, salt and their response mechanisms under freezing conditions

T

Ren-jie Houa,b,1, Tian-xiao Lia,b,1, Qiang Fua,b,c,*, Dong Liua,b, Mo Lia,b, Zhao-qiang Zhoua, Jia-wen Yana, Shuo Zhanga a

School of Water Conservancy and Civil Engineering, Northeast Agricultural University, Harbin, Heilongjiang 150030, China Key Laboratory of Effective Utilization of Agricultural Water Resources of Ministry of Agriculture, Northeast Agricultural University, Harbin, Heilongjiang 150030, China c Heilongjiang Provincial Key Laboratory of Water Resources and Water Conservancy Engineering in Cold Region, Northeast Agricultural University, Harbin, Heilongjiang 150030, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Unidirectional freezing Soil water, heat, and salt redistribution Response relationship

In seasonally frozen soil compartments, freezing affects hydrothermal cycles and solute transport. In order to effectively identify the variation characteristics of soil temperature, moisture and salinity during freezing and the interaction among them, a laboratory unidirectional freezing experiment was set up. During the experiment, three initial gravimetric moisture contents (N1, 18.84 %; N2, 23.43 %; N3, 28.78 %), three freezing temperatures (N4, −5°C; N5, −10°C; N6, −15°C), and three soil bulk densities (N2, 1.33 g⋅cm−3; N5, 1.45 g⋅cm−3; N7, 1.58 g⋅cm−3) were established. The spatial distribution characteristics of the temperature, moisture and salinity in the soil columns were analysed, on the basis of these results, soil water, heat, and salt response relationship functions were constructed by using three indicators, i.e., the soil temperature difference, water flux and salt flux. The study results indicated that as the moisture content increased, the water migration ability increased. In addition, as the freezing temperature decreased and the soil bulk density increased, the rapid freezing of soil inhibited water migration. Furthermore, the movement of water carried a certain amount of salt, and in the soil columns with freezing temperatures of -10 and -15°C, the total soil salt migration decreased by 10.05 and 16.41 mg·cm-2, respectively, compared with the soil column with a freezing temperature of -5°C. The response relationship among the soil water, heat, and salt improved with an increase in the soil moisture content, weakened with a decrease in the freezing temperature and an increase in the soil bulk density, simultaneously, the RMSE between the simulated and measured values of the soil water, heat, and salt response function reduced. In addition, at the spatial scale, the response relationship among soil water, heat, and salt was most significant at the critical soil freezing depth. These study results elucidate the migration and diffusion of soil water, heat, and salt in cold regions and their associated interaction mechanisms, thereby providing guidance for scientific and effective measures for soil improvement, improving the soil ecological environment and ensuring stable increases in grain production.

1. Introduction Soil freezing is a very complex process accompanied by physical, chemical, and mechanical phenomena that include mainly soil heat transfer, water migration, and salt accumulation (Bing et al., 2015; Qi et al., 2018; Júnior et al., 2006). In the composite water, heat, and salt system of frozen soil, the migration of moisture influences the thermal characteristic parameters of the soil and the diffusion of soil solutes, resulting in the redistribution of heat (Wu et al., 2015; Masum et al.,

2016; Hou et al., 2019a). In turn, soil temperature gradients influence soil moisture migration and changes in water characteristic parameters (Nagare et al., 2012; Chang et al., 2014; Fu et al., 2018). During the soil freezing process, the phase of liquid water within soil changes, forming a freezing front under the influence of temperatures below 0℃. Especially when the soil moisture content is high, under the driving force of the potential energy difference, the liquid water in the soil undergoes a significant diffusion phenomenon, which in turn carries the salt ions in the soil to migrate toward the frozen front. Furthermore, the soil water

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Corresponding author at: School of Water Conservancy and Civil Engineering, Northeast Agricultural University, No.600 Changjiang Road, Harbin, Heilongjiang 150030, China. E-mail address: [email protected] (Q. Fu). 1 These authors contributed to the work equally and should be regarded as co-first authors. https://doi.org/10.1016/j.still.2019.104486 Received 20 August 2018; Received in revised form 11 September 2019; Accepted 21 October 2019 0167-1987/ © 2019 Elsevier B.V. All rights reserved.

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Table 1 Soil physical characteristic parameters of the studied area. Soil depth (cm)

0 10 20 30 40

Soil bulk density (g∙cm3)

1.41 1.45 1.51 1.53 1.55

Soil texture Sand (%)

Silt (%)

Clay (%)

23.6 21.8 20.6 19.8 18.3

34.6 33.8 33.5 32.5 32.1

41.8 44.4 45.9 47.7 49.5

Saturated water content (%)

Initial moisture content (%)

Field water holding capacity (%)

46.51 45.23 44.98 45.17 44.23

23.98 24.51 24.89 25.12 25.76

34.25 33.89 33.55 34.28 33.07

surface. Accordingly, snow plays a very important role in regulating soil moisture and salt (Rusuli et al., 2015). Most of the above studies focused on the effects of regulatory measures on the variations in soil water, heat, and salt under freeze-thaw conditions. However, the research on the mechanism of water, heat and salt transport in frozen soil under different initial conditions and the interaction among them are relatively deficient. In this study, the sandy loam of the Songnen Plain of Northeast China is taken as the research object. An indoor soil freezing experiment is conducted to explore the effects of different initial moisture contents, freezing temperatures, and soil bulk density values on the migration of soil water, heat, and salt. The purpose of this work is to describe the transport phenomena and response relationship among the soil water, heat and salt of frozen soil. This study provides a theoretical reference for understanding the mechanisms of soil water, heat, and salt transport in cold regions, formulating irrigation systems rationally, and effectively preventing soil salinization.

and salt accumulate in the soil surface, resulting in secondary salinization of the soil (Xue et al., 2017; Wang et al., 2013; Wu et al., 2013). Salt accumulation changes the soil ecological environment and threatens the sustainable development of agriculture in arid areas (Luo et al., 2008; Zhang et al., 2016). Therefore, it is of great theoretical and practical importance to conduct unidirectional soil column freezing tests in the laboratory on the mechanisms of the migration of soil water, heat, and salt under freezing conditions. Numerous researchers have conducted relevant experimental studies on the characteristics of water, heat, and salt migration in frozen soil. In the process of soil freezing, both the structure of the soil and the shape of the ice lens change, thereby affecting the spatial distribution of the soil potential energy (Nassar et al., 2000). Water enrichment occurs at the depth at which the soil is frozen; at this time, the salt migration effect is the most significant (Krishnaiah and Singh, 2003; Hou et al., 2019b). In addition, the salt ions in the frozen soil is continuously deposited, accompanied by a small amount of unfrozen water in the soil, and continues to migrate upward along the channel between the soil particles. And thus, the salt in the soil gradually migrates towards the surface (Hansson and Lundin, 2006; Zheng et al., 2009). Based on the research on the redistribution characteristics of water, heat, and salt in freeze-thaw soil, more and more scholars have committed to the management and restoration of agricultural soil environment. Askri et al. (2010) adopted measures for the irrigation of saline-alkali soil in autumn; through these measures, the salt in the soil was leached, and thus, the surface soil salinization was weakened during the next year's sowing date. In addition, combined with the transport mechanisms of soil water, heat, and salt and their comprehensive influencing factors, Li et al. (2010) and Wang et al. (2015) used the simultaneous heat and water (SHAW) and HYDRUS-2D models to simulate the transport patterns of soil water, heat, and salt, thereby providing a scientific basis for soil water and salt management and the sustainable development of irrigation areas. The freeze-thaw cycle affects the spatial distribution of soil water and heat and leads to the movement of salt, while the soil surface cover regulates the relationship between water and heat transfer to some extent (Musa et al., 2016); therefore, an increasing number of scholars are paying more attention to the effects of the surface cover on regulating the migration and diffusion of salt in soil during freeze-thaw processes. Flerchinger et al. (2003) explored this subject and found that different stubble covers affect the transfer of water and heat through the soil surface to varying degrees. In addition, prediction models of water and heat transmission under different coverage conditions have been established to provide better management decisions for treating the cover of overwintering farmland in northern regions with seasonally frozen soil. Zhou et al. (2011) conducted freeze-thaw tests on soils under four different vegetation types in the Changbai Mountains and discussed the varying effects of the freeze-thaw cycle on soluble salts in soils. Wang et al. (2012) found that the plastic film was a medium layer with low permeability, and it weakened the redistribution of soil water and salt. Maurer and Bowling (2014) proposed that the water and heat variations in soil during freeze-thaw processes are strongly affected by snow cover; leaching via the melting of snow moves salt from the surface soil downward, reducing the risk of soil salt accumulating at the

2. Materials and methods 2.1. Test soil columns Considering the significance and accuracy of the test results, the soil used in the experiment was taken from the Xiangyang Farm in the hinterland of the Songnen Plain in Northeast China. The soil in the study area is mainly sandy loam soil, and it is the main agricultural soil resource in northeastern China. When the soil was extracted from the field, 10 points were arranged across a plot of 50 × 50 m, and the soil was taken from depths of 0–40 cm at each point. Then, the soil was evenly mixed. Before performing the experiment, the soil physical characteristics of the 0–40 cm soil layers in the area were tested, and the details are shown in Table 1. 2.2. Test equipment The soil column was unidirectionally frozen in an artificial climate chamber with the length, width and height were 5, 2.6 and 2.9 m respectively, and an area of 13 m2. The indoor temperature of the artificial climate chamber varied approximately between -15 °C and 40 °C. The indoor soil unidirectional freezing device was composed mainly of a capacity bucket, an insulation board, a refrigeration compressor, a water supply system, a soil temperature sensor and a permafrost device. A schematic diagram of the test device is shown in Fig. 1. During the experiment, the artificial climate chamber was cooled using a refrigeration compressor. An electric fan was installed at the top of the climate chamber to ensure that the air was uniformly cooled from top to bottom; the cooling temperature error was ± 0.5 °C. The soil column was welded to the capacity bucket, which had a thickness of 2 mm and dimensions of 60⋅60⋅100 cm. To avoid the loss of soil and water, glass glue was applied to the corner of the capacity bucket. To ensure the soil was unidirectionally frozen, an XPS insulation board with a thickness of 5 cm was attached to the sidewall and bottom of soil column, this kind of insulation board was a rigid foam plastic board which was compressed by polystyrene and other auxiliary materials 2

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gravimetric soil moisture content and salt content. Between them, the former (liquid water + solid water) was determined through the drying method. For the latter, the soil salt content is determined by summing the concentrations of various ions. 2.3. Test programme The sandy loam soil was dried, crushed, sifted (2 mm) and used in soil columns with different initial moisture contents, freezing temperatures and soil bulk densities. According to the influencing factors involved in this research and the gradient levels of the corresponding indicators, the test programme covered three influencing factors and three levels. Furthermore, to explore the influences of different factors on the characteristics of soil water, heat and salt migration, in the design of the experimental scheme, two factors were controlled to remain unchanged while varying the level of the third factor. In addition, to ensure the reliability and representativeness of the research results, 7 groups of test soil columns were established, and the best level combinations were constructed to represent three different initial moisture contents, three different freezing temperatures and three different bulk densities (Zhang and Yang, 2013; He and wang, 2016). The specific designed soil columns are shown in Table 2, in which N1, N2, and N3 represent the test groups with different initial soil moisture contents, N4, N5, and N6 represent different freezing temperature test groups, and N2, N5, and N7 represent different soil bulk density test groups. The preparation methods of the soil columns were as follows. (1) Different initial moisture contents: according to the average soil moisture content in the Songnen Plain, the gravimetric moisture contents of the soil columns were set to 18.84 %, 23.43 % and 28.78 %. To ensure the uniformity of the moisture content throughout each soil layer, the mixed soil was sealed for a specific amount of time. (2) Different freezing temperatures: the moisture of these soil columns was 23.43 %, and they were frozen in the artificial climate chamber at -5 °C, −10 °C, and −15 °C. Under normal circumstances, after the compressor was turned on, the artificial climate chamber reached the target temperature within 1 h. In view of this relatively short time, the effect of artificial climate chamber cooling process on the variation of soil water, heat and salt can be neglected. (3) Different soil bulk densities: the bulk densities of the soil columns were set to 1.33, 1.45, and 1.58 g⋅ cm−3 by using the plate compaction method. To ensure a uniform soil density, the soil was compacted layer by layer during the production of the soil column. As is well known, with an increase in the thickness of the frozen soil layer, the effect of soil energy transfer decreases, and the rate at which the frozen front moves downward gradually diminishes. Hence, combined with the dimensions of the soil column and to ensure a significant difference in the freezing depth among the soil columns, the freezing time of the soil column was set to 14 days.

Fig. 1. Schematic diagram of the indoor soil unidirectional freezing device.

under heating conditions. The thermal conductivity of the XPS insulation board was 0.028 W·m−1 K−1, and it provided superior heat insulation. The average groundwater depth throughout the Songnen Plain was 6.85 m over the last five years, and the position of the phreatic layer was relatively low. Therefore, the recharge capacity of the groundwater to the shallow soil was relatively low, and the recharge of the moisture content in the 0–80 cm soil layers originated mainly from the lower soil layers. In this experiment, the fixed-head water supply method was used for water replenishment. The water supply system consisted of a Mariotte bottle and a glass conduit connected to the atmosphere. During the test, the water level in the Mariotte bottle is controlled by adjusting the position of the glass conduit, and ensure that free water was provided to the bottom of the soil column, thereby simulating the supply of water from the lower soil to the 0–80 cm soil layer. The recharge water was a Na2SO4 solution, which prevented the migration of soil water driven by differences in the soil water solution concentration. In addition, the water in the Markov bottle was replenished every 3 h. The buried depths of the soil temperature sensors (Beijingqudao, CR200) were 10, 20, 30, 40, 50, 60, 70, and 80 cm. The soil temperature data were automatically acquired by a data collector, and the data were recorded at a frequency of 1 recording per h. A set of permafrost devices (Jinzhoulicheng, LQX-DT) was installed in the soil column to monitor the change in the soil freezing depth, and the freezing depth data were manually recorded at a frequency of 1 recording every 6 h. Simultaneously, during the soil freezing process, the soil in each soil layer was regularly accessed through soil drilling to measure the

2.4. Research methods 2.4.1. Soil temperature difference Based on the temperature data monitored by the soil temperature

Table 2 Freezing test scheme settings. Soil colume number

N1 N2 N3 N4 N5 N6 N7

Soil salt content (%)

0.245 0.245 0.245 0.245 0.245 0.245 0.245

Soil texture Sand (%)

Silt (%)

Clay (%)

35.6 34.5 36.1 35.5 34.7 34.8 35.1

41.8 41.6 40.4 41.6 40.2 40.6 40.8

22.6 23.9 23.5 22.9 25.1 24.6 24.1

Initial gravimetric moisture content (%)

Soil dry density (g∙cm−3)

Freezing temperature (°C)

Freezing time (d)

18.84 23.43 28.78 23.43 23.43 23.43 23.43

1.33 1.33 1.33 1.45 1.45 1.45 1.58

−10 −10 −10 −5 −10 −15 −10

14 14 14 14 14 14 14

3

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Si j − Si j − 1 = Qi j − Qi −j 1

sensors, the difference method was used to measure the temperature difference in each soil layer at different times:

ΔTi j = Ti j + 1 − Ti j

is the salt flux at the lower boundary of the i-th layer in Formula: a period between j-1 and j time nodes, mg·cm−2·d-1. The upward direction is positive, and the boundary salt migration at the upper soil layer is Q0.

(1)

Formula: Tij is the temperature of the i-th layer in the j-th time node. 2.4.2. Soil water migration During the freezing process, the soil moisture content was measured by using the drying method, after which the water storage capacity in each soil layer was calculated as follows:

Wi j = hi × γi × ωi j × 10/100

2.5. Evaluation method In this study, based on the temperature difference, water migration and salt migration of different soil layers, the response function of the soil water, heat and salt was constructed by using DRS data analysis software. At the same time, the root mean square error (RMSE), NashSutcliffe efficiency coefficient (NSE) and coefficient of determination (R2) were selected to verify the effectiveness and accuracy of the model (Herbst et al., 2008).

(2)

Wij

Formula: is the water storage capacity of the i-th layer in the j-th time node, mm; hi is the thickness of the i-th soil layer, cm; ©i is the bulk density of the i-th soil layer, g/cm3; ωij is the gravimetric moisture content of the i-th layer in the j-th time node, %; i = 1,2,3…m; and j = 1,2,3…n. Therefore, the change in soil water storage between two time nodes is expressed as follows:

ΔWi j = Wi j − Wi j − 1

NSE = 1 − n

R2 =

n ∑i = 1

(yi − y¯)2

∧

(yi − y¯)2

(9) n

=1−

∧

∑i = 1 (yi − y )2 n ∑i = 1

(yi − y¯)2

(10)

Formula: yi is the measured value; yˆ is the simulated value; and y¯ is the mean value. In the evaluation results, the RMSE represents an overall estimate of the deviation between a simulated value and a measured value: the smaller the RMSE value is, the better the simulation results are. The NSE reflects the coincidence between a simulated value and a measured value; when the data value is close to 1, the coincidence is good, whereas the coincidence is relatively poor when the data value is close to 0.

(4)

Formula: is the water flux at the lower boundary of the i-th layer in a period between j-1 and j time nodes, mm·d−1. The upward direction is positive, and the boundary water migration at the upper soil layer is q0. 2.4.3. Soil salt migration In order to effectively measure the characteristics of salt transport in soil, eight main salt ions were selected as research objects, including K+, Na+, Ca2+, Mg2+, SO42−, Cl-, CO32− and HCO3-. These eight ions were usually present in the soil in a free state and were easily resolved from the solid soil, and then diffused with moisture migration (Elsenousy et al., 2015; Lin et al., 2019). In the study, the sum of eight salt ions were taken as the soil salt content. Among these ions, the CO32− and HCO3- in the soil were determined by double indicator neutralization titration (Rout et al., 2015), Cl- was determined by silver nitrate titration (Yang et al., 2016), SO42− was determined by EDTA indirect titration (Chen and Duan, 2015), K+ and Na+ were measured by flame photometry, and Ca2+ and Mg2+ were measured by atomic absorption spectrometry (Cadaret et al., 2016). After measuring the soil salinity in each soil layer, the salt storage of the i-th layer soil at the j-th time node was calculated as follows:

3. Results and analysis 3.1. Characteristics of the temperature variation in frozen soil In this study, the temperature of each vertical soil profile at the end of the freezing period was plotted, as shown in Fig. 2. In the N1 soil column, the surface soil temperature was -9.4 °C. Moreover, as the soil depth increased, the soil temperature gradually increased; in the 10, 20, and 30 cm soil layers, the soil temperatures were -7.8, -6.3, and -4.1 °C, respectively, and the temperature difference between the surface soil layer and the deepest (80 cm) soil layer was 12.5 °C. In the N2 and N3 soil columns, the surface soil temperatures changed to -9.0 and -8.7 °C, respectively, and the temperature differences between the surface soil and the 80 cm soil layer were 13.2 and 14.1 °C. With an increase in the initial soil moisture content, the temperature difference between the surface (10 cm) and deepest (80 cm) soil layers gradually increased. With a decrease in the freezing temperature, the effect of freezing on the vertical temperature profile of the soil was significant, and the range of the temperature change gradually increased. A comparative analysis of Fig. 2(b), (e) and (g) showed that the surface temperatures of the N5 and N7 soil columns were reduced by 0.3 and 0.5 °C, respectively, relative to that of the N2 soil column. Furthermore, with an increase in soil depth, this reduction trend became gradually significant. Moreover, the temperature differences between the surface (10 cm) and deepest (80 cm) soil layers in the N5 and N7 soil columns decreased by 1.3 and 2.1 °C, respectively, compared with the temperature difference in the N2 soil column. During the soil freezing process, a transition zone exists between the frozen and unfrozen areas of the soil, and this transition area is usually used as the boundary of the soil freezing depth; therefore, the depth of

(5)

Formula: Sij −2

is the salt storage of the i-th soil layer in the j-th time node, mg·cm ; Cij is the salt content of the i-th soil layer in the j-th time node, %; i = 1,2,3…m; and j = 1,2,3…n. Therefore, the change in soil salt storage between two time nodes is written as follows:

ΔSi j = Si j − Si j − 1

∧

∑i = 1 (yi − y )2

∑i = 1 (y − y¯)2 n ∑i = 1

(8)

n n

(3)

qij

Si j = hi × γi × Ci j × 1000/100

∧

n

∑i = 1 (yi − y )2

RMSE =

Formula: ΔWij is the change in soil water storage in the i-th layer during the j-th period (between the j-th time node and the j-1-th time node), mm. In this study, considering the water balance of each soil layer under one-dimensional soil conditions, the increase in the water storage in a soil layer should be equal to the difference in the inflow volume:

Wi j − Wi j − 1 = qi j − qi −j 1

(7)

Qij

(6)

Formula: ΔSij is the change in soil salt storage in the i-th layer during the j-th period (between the j-th sampling and the j-1-th time node), g. Similarly, considering the salt balance of each soil layer under onedimensional soil conditions, the increase in salt storage in a given soil layer should be equal to the difference in the inflow volume: 4

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Fig. 2. Spatial distribution of soil column temperatures under different initial treatments (a, b and c represent initial moisture content of 18.84 %, 23.43 % and 28.78 %, respectively, d, e and f represent freezing temperature of −5, −10 −15℃, respectively, b, e and g represent bulk densities of 1.33, 1.45, and 1.58 g⋅ cm−3, respectively) at the end of the freezing period.

moisture contents of the vertical soil profile at the end of the freezing stage were 32.21 % and 38.78 %, respectively, which increased by 9.17 % and 10.35 %, respectively, compared with the initial value. These increases in the initial moisture content provided an adequate water supply for soil water migration, resulting in an increase in the soil moisture at the freezing depth. At the end of the freezing period, the maximum soil moisture content of the vertical soil profile of the N4 soil column was 36.64 %, and the depth of the maximum moisture content was between 30 and 40 cm. With a decrease in the freezing temperature, the maximum moisture contents in the vertical profiles of the N5 and N6 soil columns were 32.94 % and 31.66 %, respectively, both of which showed a decreasing trend, and the soil moisture concentrations in these layers gradually decreased. Similarly, a comparative analysis of the N2, N5, and N7 soil columns showed that the soil water migration ability decreased as the soil bulk density increased, and the soil water accumulation layer also showed a decreasing trend. On the basis of the spatial distribution of the soil moisture content, the variability of water storage in different soil layers was further explored, and the specific results are shown in Table 3. First, according to the size of the capacity bucket and the thickness of the insulation board, the volume of the test soil column was determined to be 2 × 105 cm3 (50 × 50 × 80 cm). In the N1 soil column, the change in soil water storage in the surface (0∼10 cm) soil layer was 1.81 mm, while the soil water storage in the 10∼20 cm, 20∼30 cm and 30∼40 cm soil layers increased by 1.24, 2.14 and 3.33 mm, respectively, compared with the 0∼10 cm soil layer; evidently, the soil water storage capacity also showed an increasing trend. In the 50∼60 cm layer, the soil water

this transition zone is called the critical depth in this study. Combined with the soil freezing depth data and soil temperature change characteristics in the vertical profile, the critical depth of soil freezing in the N1 soil column was 51.3 cm. With an increase in the soil moisture content, the soil freezing range decreased, and the soil freezing critical depths in the N2 and N3 soil columns increased by 8.6 and 17.5 cm, respectively, compared with that in the N1 soil column. Comparative analyses of the N4, N5 and N6 soil columns showed that the freezing depth gradually increased with a decrease in the freezing temperature, and the critical layer depths in the N5 and N6 soil columns were reduced by 16.6 and 20.3 cm, respectively, compared with that in the N4 soil column, showing a gradually decreasing trend. Similarly, for the N2, N5 and N7 soil columns, with an increase in the soil bulk density, the heat exchange between soil and atmospheric environment strengthened, and the depth of the frozen critical layer showed a decreasing trend.

3.2. Characteristics of the soil moisture redistribution in frozen soil In frozen soil, the spatial distribution of soil moisture in different periods was plotted, as shown in Fig. 3. In the N1 soil column, at 48 h, water accumulated between the 10 and 20 cm soil layers. With an increase in the freezing time, the soil freezing front gradually extended downwards, and at the end of the freezing period (336 h), the maximum moisture content of the soil vertical profile was 26.54 %. Furthermore, the depth of the maximum moisture content was in agreement with the freezing depth, and the maximum value was increased by 7.80 % relative to the initial value. In the N2 and N3 soil columns, the maximum 5

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Fig. 3. Spatial distribution of the soil quality moisture content under different initial treatments (a, b and c represent initial moisture content of 18.84 %, 23.43 % and 28.78 %, respectively, d, e and f represent freezing temperature of −5, −10 −15℃, respectively, b, e and g represent bulk densities of 1.33, 1.45, and 1.58 g⋅ cm−3, respectively) in different periods.

water migration in the N2 and N3 soil columns also showed an increasing trend relative to the N1 soil column. In the N1 soil column, the total amount of soil water transferred to the upper layer was 22.68 mm, while the total amount of soil water loss in the lower layer was 10.89 mm, and the difference between the two was 11.79 mm. The water supply of the difference part comes from the external water supply, i.e., the water supply in the lower layer. In addition, in the N2 and N3 soil columns, the differences between the total amount of soil

storage capacity decreased, and with an increase in soil depth, the change in soil water storage gradually diminished. An accumulative analysis showed that the total water migration in each soil layer of the N1 soil column was 31.57 mm. In the N2 and N3 soil columns, the changes in soil water storage between the 0 and 10 cm soil layers increased by 39.78 % and 51.38 %, respectively, compared with that in the N1 soil column. With an increase in soil depth, the change in water storage showed different degrees of increase. Furthermore, the total soil

Table 3 Variation characteristics of water storage in soil layers under different treatment conditions. Soil colume

N1 N2 N3 N4 N5 N6 N7

Changes in soil water storage through cross section of soil column in different depth ranges (mm)

Total migration (mm)

0∼10 cm

10∼20 cm

20∼30 cm

30∼40 cm

40∼50 cm

50∼60 cm

60∼70 cm

70∼80 cm

1.81 2.53 2.74 2.85 2.33 1.99 2.13

3.05 4.90 5.48 5.30 3.75 2.66 3.39

3.95 6.97 7.94 8.86 7.02 3.42 3.71

5.14 10.50 11.18 10.66 9.43 3.82 4.17

8.73 −6.46 −7.82 −6.57 −5.54 5.03 5.41

−5.82 −4.14 −5.28 −4.64 −4.22 −5.82 −5.33

−3.42 −3.45 −4.10 −3.94 −3.93 −3.58 −4.23

−1.65 −3.26 −4.49 −2.66 −2.23 −1.93 −2.70

6

31.57 42.21 49.03 45.48 38.45 28.25 31.07

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water transferred to the upper layer and the total amount of soil water loss in the lower layer were 7.59 and 5.65 mm, respectively, indicating that the recharge from the lower layer to the 0–80 cm soil layer decreased with an increase in the initial soil moisture content. In the N4, N5 and N6 soil columns, the water storage in the vertical section of the soil also generally showed an increase in the shallow layers and a decrease in the deep layers. First, in the N4 soil column, the change in water storage over the surface (0–10 cm) soil layer was 2.85 mm. As the freezing temperature decreased, the changes in the water storage of the N5 and N6 soil columns in the same soil layer decreased by 18.25 % and 30.17 %, respectively, compared with that of the N4 soil column, indicating that the soil moisture migration ability gradually decreased as the temperature decreased. Additionally, the total soil water migration in the N5 and N6 soil columns decreased by 15.46 % and 37.88 %, respectively, compared with that in the N4 soil column. The difference between the total amount of soil water transferred to the upper layer and the total amount of soil water loss in the lower layer gradually decreased, indicating that the amount of soil water recharged to the 0–80 cm soil layer gradually decreased. Similarly, with an increase in the soil bulk density, the change in water storage over each soil layer showed a decreasing trend.

storage of the N2 and N3 soil columns in the 0∼10 cm layer increased by 19.96 % and 34.86 %, respectively, compared with that of the N1 soil column. With an increase in soil depth, the change in the salt content of each soil layer increased by different degrees with respect to the N1 soil column. Accumulative statistics showed that the total amount of salt migration in the N2 and N3 soil columns increased by 22.94 % and 44.21 %, respectively, compared with that in the N1 soil column. In the N4 soil column, the storage capacity at the surface (0∼10 cm) soil layer was 12.64 mg·cm−2, and as the soil freezing temperature decreased, the salt storage of the N5 and N6 soil columns decreased by 17.09 % and 25.16 %, respectively, compared with that of the N4 soil column. Therefore, the accumulation of soil salt near the surface was weakened. Furthermore, the total amount of soil salt migration also decreased. Similarly, the total salt migration values in the N7 and N5 soil columns were 14.26 % and 27.54 % lower, respectively, than that in the N2 soil column, indicating that the spatial diffusion of salt also exhibited a weakening trend as the soil bulk density increased.

3.3. Characteristics of soil salt diffusion in frozen soil

Based on the values of the temperature difference, water flux and salt flux in different time periods, the response relationship functions of the three in each soil layer were constructed. The relationships among them in the surface (0∼10 cm) soil layer and their functions are shown in Fig. 5 and Table 5. First, in the N1 soil column, as the soil temperature difference and the water flux increased, the soil salt flux gradually increased, indicating a positive correlation among the three parameters. A specific analysis showed that when the soil temperature difference was 1 °C·d−1 and the water flux was 0.08 mm·d−1, the soil salt flux was 0.79 mg·cm2 −1 ·d . Furthermore, when the soil temperature difference was 2 °C·d−1 and the water flux was 0.16 mm·d−1, the soil salt flux was 3.34 mg·cm2 −1 ·d . In the N2 and N3 soil columns, with an increase in the initial soil moisture content, the soil salt flux increased gradually under the same temperature difference and water flux conditions. When the soil temperature difference was 2 °C·d−1 and the water flux was 0.16 mm·d−1, the soil salt flux of the N2 and N3 soil columns increased by 0.66 and 2.01 mg·cm-2·d−1, respectively, compared with that of the N1 soil column. Regarding the N4, N5 and N6 soil columns under different temperature differences and water flux conditions, when the soil temperature difference was 2 °C·d−1 and the soil water flux was 0.16 mm·d−1, the salt flux of the N4, N5 and N6 soil columns were 5.26, 3.59 and 1.83 mg·cm-2·d−1, respectively. With a decrease in the soil freezing temperature, the soil salt flux decreased gradually under the same temperature difference and water flux conditions. Similarly, a comparative analysis of the water, heat, and salt migration patterns of the N2, N5 and N7 soil columns revealed that the soil salt flux showed a gradually decreasing trend with an increasing soil bulk density under the same soil temperature difference and water flux conditions. An overall comparative analysis of the constructed functions illustrated that they all passed the significance test (P < 0.05), and the forms all conformed to a Lorentzian surface, indicating that the diffusion of the soil salinity exhibited a significant response relationship to the spatial variations of water and heat. In the N1 soil column, the RMSE between the measured and simulated values of the soil water, heat, and salt response function was 0.24 mg·cm−2·d-1. With an increase in the soil moisture content, the RMSE values between the measured and simulated values in the N2 and N3 soil columns decreased by 0.03 and 0.05 mg·cm−2·d-1, respectively, compared with that in the N1 soil column, and the simulation effect of the model was gradually enhanced. Furthermore, the NSE between the fitted and measured values of the function also showed an increasing trend, and the similarity between the fitting and measured values increased. In addition, as the initial

3.4. Relationship among the responses of water, heat, and salt transfer in frozen soil

During the soil freezing process, with the migration of liquid water, soil salt experiences diffusion. Furthermore, when water is frozen during the transfer process, salt precipitates from the water and continues to migrate along with the unfrozen water to the surface soil (Geng and Boufadel, 2015). Therefore, salt accumulated in the surface layer of the soil. The distributions of the soil salt content in different soil columns are shown in Fig. 4. First, in the N1 soil column, the salt content of the surface soil in the initial period was 0.246 %, and this percentage gradually increased with an increase in the freezing time; at the end of the freezing period (336 h), the surface salt content was 0.318 %, and the variation in the soil salinity reached 0.072 %. As the depth increased, the soil salt content gradually decreased; the soil salt contents were 0.306 %, 0.283 % and 0.273 % in the 10, 20 and 30 cm soil layers, respectively. In the 50 cm soil layer, the soil salt content at the end of the freezing period was reduced relative to the initial value, indicating that the salinity of the soil below this layer diminished. In the N2 and N3 soil columns, the salt contents of the surface soil at the end of the freezing period were 0.338 % and 0.357 %, respectively. The amplitude of the change in soil salinity gradually increased as the moisture content increased. In the N4 soil column, the salt content had increased to 0.346 % by the end of the freezing period. With a decrease in the freezing temperature, the salt contents in the surface soil at the end of the freezing period in the N5 and N6 soil columns were reduced by 0.016 % and 0.031 %, respectively, compared with the N4 soil column, and the magnitude of the variation in the soil salinity gradually decreased. Similarly, a comparative analysis of the N2, N5, and N7 soil columns showed that as the soil bulk density increased, the soil freezing rate accelerated; furthermore, the intensity of salt activity decreased. The soil salt content in each soil layer can be expressed as N2 > N5 > N7. Based on an analysis of the soil salinity spatial distribution, the variation characteristics of salt storage in different soil layers were further explored, and the specific results are shown in Table 4. In the N1 soil column, the change in salt storage in the surface (0∼10 cm) soil layer was 10.27 mg·cm−2, and the changes in soil salt storage were reduced by 27.36 %, 55.01 % and 70.11 % in the 10∼20 cm, 20∼30 cm and 30∼40 cm soil layers, respectively, compared with the 0∼10 cm soil layer; moreover, the soil salt storage capacity showed an overall increasing trend. When the depth of the soil layer exceeded 50 cm, the salt content of the soil showed a decreasing trend. In the N2 and N3 soil columns, the salt storage change trend over the vertical soil profile was similar to that in the N1 soil column; the changes in the salt 7

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Fig. 4. Spatial distribution of the soil salinity under different initial treatments (a, b and c represent initial moisture content of 18.84 %, 23.43 % and 28.78 %, respectively, d, e and f represent freezing temperature of -5, -10 -15℃, respectively, b, e and g represent bulk densities of 1.33, 1.45, and 1.58 g⋅ cm−3, respectively) in different periods.

moisture content of the soil increased, the R2 of the soil water, heat, and salt response function also showed an improving trend. In the N4, N5 and N6 soil columns, with a decrease in the soil freezing temperature, the RMSE between the measured and simulated values in the N5 and N6 soil columns increased by 0.04 and 0.09 mg·cm−2·d-1, respectively, compared with that in the N4 soil column. Similarly, in comparison with the N2, N5 and N7 soil columns, the RMSE between the measured

and simulated values increased with an increase in the soil bulk density. Moreover, the NSE between the fitted and measured values also diminished, and the interaction relationship of the migration of soil water, heat, and salt was weakened. On the basis of an analysis of the interactions among the soil water, heat, and salt in the surface (0–10 cm) soil layer, the characteristics of the spatial variations were further explored, and the specific

Table 4 Variation characteristics of salt storage in soil layers under different treatment conditions. Soil colume

N1 N2 N3 N4 N5 N6 N7

Changes in soil salt storage through cross section of soil column in different depth ranges (mg·cm−2)

Total migration (mg·cm−2)

0∼10 cm

10∼20 cm

20∼30 cm

30∼40 cm

40∼50 cm

50∼60 cm

60∼70 cm

70∼80 cm

10.27 12.32 13.85 12.64 10.48 9.46 8.74

7.46 10.21 10.74 9.86 8.62 7.01 6.48

4.62 7.07 7.82 7.14 6.20 3.86 3.82

3.06 5.94 5.49 2.62 2.97 2.52 3.46

1.30 3.84 −10.34 −10.22 2.21 1.30 1.47

−8.62 −9.84 −8.66 −7.82 −8.61 −8.34 −9.86

−7.45 −4.93 −7.14 −6.15 −7.07 −6.73 −5.43

−5.13 −4.75 −5.05 −4.10 −4.34 −4.92 −3.42

8

47.91 58.90 69.09 60.55 50.50 44.14 42.68

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Fig. 5. Migration characteristics of soil water, heat, and salt under different initial treatments (a, b and c represent initial moisture content of 18.84 %, 23.43 % and 28.78 %, respectively, d, e and f represent freezing temperature of -5, -10 -15℃, respectively, b, e and g represent bulk densities of 1.33, 1.45, and 1.58 g⋅ cm−3, respectively) in the surface (0–10 cm) soil layer.

measured and simulated values decreased relative to that in the N1 soil column, demonstrating that the constructed response functions improved as the moisture content increased, and the relationship among the soil water, heat, and salt became stronger. In the N4, N5 and N6 soil columns, with a decrease in the soil freezing temperature, the relationship among the soil water, heat, and salt gradually weakened, and the RMSE values between the measured and simulated values in space were consistent with those in the above mentioned N1 soil column. Among them, in the N4 soil column, the

characteristics are shown in Table 6. In the N1 soil column, with an increase in soil depth, the RMSE values between the measured and simulated values in the 20, 30, and 40 cm soil layers were 0.23, 0.21, and 0.19 mg·cm−2·d-1, respectively. When the soil depth exceeded 50 cm, the RMSE values between the measured and simulated values gradually increased and showed a tendency to decrease first and then increase in the vertical section. On the other hand, indicated that the relationship was the most significant in the vicinity of the maximum soil freezing depth. In the N2 and N3 soil columns, the overall RMSE between the

Table 5 Response functions of the soil water (x), heat (y), and salt (z) under different treatment conditions in the surface (0–10 cm) soil layer. Soil colume

Response function of soil water, heat, and salt

N1 N2 N3 N4 N5 N6 N7

z z z z z z z

= = = = = = =

4.23/((1 4.61/((1 5.75/((1 5.61/((1 4.23/((1 3.35/((1 3.16/((1

+ + + + + + +

((x ((x ((x ((x ((x ((x ((x

− − − − − − −

0.14)/0.06)∧2) 0.16)/0.07)∧2) 0.17)/0.08)∧2) 0.15)/0.07)∧2) 0.14)/0.06)∧2) 0.13)/0.06)∧2) 0.12)/0.05)∧2)

∗ ∗ ∗ ∗ ∗ ∗ ∗

(1 (1 (1 (1 (1 (1 (1

+ + + + + + +

((y ((y ((y ((y ((y ((y ((y

− − − − − − −

2.41)/1.09)∧2)) 2.39)/1.01)∧2)) 2.23)/0.97)∧2)) 1.93)/0.92)∧2)) 2.31)/1.05)∧2)) 2.93)/1.37)∧2)) 2.57)/1.12)∧2))

9

RMSE

NSE

R2

P

0.24 0.21 0.19 0.23 0.27 0.32 0.31

0.90 0.92 0.95 0.91 0.87 0.85 0.83

0.92 0.93 0.95 0.94 0.91 0.89 0.87

0.0064 0.0041 0.0023 0.0087 0.0134 0.0197 0.0231

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Table 6 Spatial variation characteristics of the soil water, heat, and salt response relationships. Soil Depth (cm)

20 30 40 50 60 70 80

N1

N2 2

N3 2

N4 2

RMSE

R

RMSE

R

RMSE

R

0.23 0.21 0.19 0.17 0.22 0.29 0.33

0.94* 0.95** 0.96** 0.97** 0.94* 0.92* 0.86*

0.19 0.17 0.14 0.20 0.21 0.26 0.29

0.95* 0.97* 0.98** 0.95* 0.93* 0.89* 0.89*

0.18 0.16 0.19 0.21 0.22 0.24 0.30

0.96** 0.99** 0.95* 0.93* 0.92* 0.91* 0.88*

N5 2

N6 2

N7 2

RMSE

R

RMSE

R

RMSE

R

RMSE

R2

0.21 0.15 0.19 0.22 0.23 0.31 0.34

0.95** 0.98** 0.94** 0.92* 0.89* 0.88* 0.85*

0.25 0.22 0.17 0.16 0.20 0.22 0.27

0.93** 0.94* 0.96** 0.97** 0.94* 0.90* 0.88*

0.29 0.27 0.20 0.19 0.17 0.23 0.30

0.90* 0.91* 0.93* 0.95** 0.96** 0.92* 0.87*

0.28 0.23 0.21 0.19 0.18 0.20 0.28

0.89* 0.91* 0.93* 0.95** 0.96* 0.92* 0.89*

Note: "**" represents a significance test with P < 0.01; "*" represents a significance test with P < 0.05.

greatly hindered, and the soil water transport capacity correspondingly decreases (Zeng et al., 2017). In conclusion, increasing the soil bulk density also reduces the hydraulic gradient, and the cumulative effect of the frozen soil water is weakened.

layer with the most significant response relationship among the soil water, heat, and salt appeared in the 30 cm soil layer. In the N5 and N6 soil columns, the depth of the most significant layer was reduced by 20 and 30 cm, respectively, compared to that in the N4 soil column. Similarly, a comparative analysis of the N2, N5, and N7 soil columns showed that the RMSE increased gradually as the soil bulk density increased, and the relationship among the soil water, heat, and salt gradually weakened. In the vertical section of the soil column, the most significant depth of the soil water, heat, and salt response relationship gradually declined, similar to the soil freezing depth.

4.2. Influencing factors on the cooperative movement of salt and water During the soil freezing process, soil water frequently diffuses, convects and disperses, and these processes are accompanied by the transport of solute (salt). Based on the porous media structure of soil, salinity in frozen soil exists mainly in two forms. In one form, salinity accumulates with water in permafrost, and most of the salinity is stored in the frozen layer (Wang et al., 2009). The other form comprises small salt precipitates that form after water freezes that are dissolved in unfrozen water and continue to migrate to the surface under the action of the water potential gradient (Jin et al., 2013). Therefore, during the soil freezing process, with an increase in the initial soil moisture content, the amount of soil water being recharged to the surface becomes larger, and the salt accompanied by this water also increases. In this study, the relationships between the water flux and salt flux in different soil layers at different times were investigated, and the specific results are shown in Fig. 6. The R2 of the fitting line of the water flux and salt flux in the N1 soil column was 0.87. In N2 and N3 soil columns, the R2 values of these two parameters increased by 0.05 and 0.08, respectively, compared with that in the N1 soil column. Regarding the slope of the fitting line, with an increase in the soil moisture content, the slope values had the following order from highest to lowest: N3 > N2 > N1. In addition, the amount of salt carried by water migration increased gradually, thereby confirming that the response of salt to water gradually increased. On this basis, the effects of different freezing temperatures and soil bulk densities on soil water and salt transport were compared and analysed. An analysis showed that a decrease in the freezing temperature and an increase in the soil bulk density both increased the degree of soil freezing and the migration rate of the frozen front, and a large amount of salt was fixed in situ (Wang et al., 2016), while the salt migration channels were blocked. Thus, the migration and diffusion capacity of soil salt were reduced. However, through this study, we found that the flux of water and salt through a soil interface increased gradually with an increase in soil depth. Moreover, the soil water flux and salt flux reached maximum values at the critical soil freezing depth, and the response of the soil salt to water was enhanced. However, the water and salt flux of each soil interface gradually decreased below the critical depth, and the saltcarrying capacity of the soil water decreased. In other words, under different treatment conditions, the most significant salt response to soil water varied with the freezing depth. In this study, employing the temperature gradient to drive water migration and then carry the effect of salt diffusion, we innovatively constructed a response function among the soil temperature difference, water flux and salt flux and quantitatively expressed the relationship among the soil water, heat and salt. The accuracy of the response function was verified by the

4. Discussion 4.1. Influencing factors on the accumulation of water in frozen soil In the composite system of frozen soil, the potential energy difference caused by the temperature field leads to the continuous movement of unfrozen water in the soil towards the frozen area, and water accumulates within the frozen layer. With an increase in the initial soil moisture content, the freezing rate of soil slows, providing sufficient time for the migration of water. Therefore, the accumulation of soil water in the frozen zone is significant. In the N2 and N3 soil columns, the moisture content of the permafrost region increased by 6.19 % and 20.50 %, respectively, relative to that in the N1 soil column. As Wu et al. (2014) reported, a large amount of water recharge affects the distribution characteristics of soil water and heat and enhances the enrichment of soil water. In addition, based on previous studies, this study found that the layer of soil water concentration is significantly consistent with the freezing depth, and this layer gradually rose with an increase in the initial moisture content. With a decrease in the freezing temperature, the freezing rate of soil will accelerate, and a large amount of liquid water will be fixed in place. Simultaneously, soil freezing results in the clogging of soil pores, which also inhibits the transmission of water. Therefore, the effect of the soil temperature gradient on water migration is weakened. Under the three different freezing temperature treatment conditions investigated in this study, the moisture content of the frozen soil area in the N5 and N6 soil columns decreased by 22.15 % and 29.54 %, respectively, compared with that in the N4 soil column. This agrees with the hypothesis of Wang and Akae (2004) insomuch that differences in the soil temperature are the basic factor promoting water migration and that a significant reduction in the ambient temperature leads to an increase in the degree of phase change of soil water and reduces the migration of water to a certain extent. Moreover, this research proposed that the soil freezing depth increased and that the effect of recharging the upper soil with lower soil moisture weakened as the freezing temperature decreased. In addition, increasing the soil bulk density can ensure a more compact structure. On the one hand, the heat transfer energy of the soil is increased through this process, resulting in an accelerated freezing rate of the soil and a shorter water migration time (Qi and Ma, 2010). On the other hand, soil water transport channels are 10

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Fig. 6. Correlation analysis between the soil water migration and salt migration under different initial treatments (a, b and c represent initial moisture content of 18.84 %, 23.43 % and 28.78 %, respectively, d, e and f represent freezing temperature of -5, -10 -15℃, respectively, b, e and g represent bulk densities of 1.33, 1.45, and 1.58 g⋅ cm−3, respectively).

column, water transport increased under the driving force of the temperature potential energy. Furthermore, the driving effect of the soil water and heat on salt was significant, and the response relationship between them was enhanced. However, with a decrease in the freezing temperature and an increase in the soil bulk density, the response relationship among the soil water, heat and salt decreased. In addition, at the spatial scale, with an increasing soil depth, the water and salt flux of the soil layer increased. When the frozen layer was crossed, the migration gradually weakened, the interaction relationships among these three parameters were most obvious at the soil freezing depth. Based on the above analysis, this study elaborated on the characteristics of the effects of different initial moisture contents, freezing temperatures and bulk densities on the water accumulation and salt diffusion in frozen soil. Soil water, heat, and salt transfer functions were effectively constructed, and the test results, which quantitatively expressed the relationship among these three parameters, were good. However, this study was limited to a laboratory exploration of theory and the response mechanism. In future research, it will be necessary to perform an actual field experiment, apply the tillage regulation model, optimize the model parameters, effectively and accurately simulate the water, heat and salt migration characteristics of farmland soil in cold regions, and provide guidance and reference for effectively inhibiting farmland soil salinization.

RMSE; with an increase in soil depth, the RMSE showed a decreasing trend first and then an increasing trend. We also showed that the effect of soil water and heat on salt transport was strongest at the freezing depth. 5. Conclusions During the soil freezing process, the difference in initial conditions affected the accumulation of water and the diffusion of salt in soil. First, as the initial moisture content of the soil column increased, the soil water transport capacity improved gradually, and the solute diffusion ability in the soil increased. In addition, with a decrease in the soil freezing temperature and an increase in the soil bulk density, the temperature difference between the surface layer and deepest layer of soil increased, and a large amount of water was consolidated in situ. At the same time, the soil porosity decreased, restricting the available soil water transmission channels. The accumulation effect of soil water in the shallow layer and the diffusion effect of salt showed a gradual weakening trend. Soil water, heat and salt had strong interactions in varying transfer processes. Response functions among the soil temperature difference, water flux and salt flux were constructed and passed a significance test (P < 0.05). With an increase in the initial moisture content of the soil 11

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