Ions Transport in Seasonal Frozen Farmland Soil and Its Effect on Soil Salinization Chemical Properties

Abstract

The salinization of farmland soil is exacerbated during the freeze–thaw (FT) process, endangering agricultural production. The change of soil salt ions results in the formation and development of soil salinization. The objectives of this study were to investigate the migration characteristics of salt ions during the FT process, identify the effects of inconsistencies in ions transport on the development of soil salinization chemical properties. A six-month field observation was conducted from November 2020 to April 2021 in the Hetao Irrigation District, China, a typical seasonally frozen soil area affected by salinization. Soil salt ions, soil moisture content (SMC), soil temperature, and pH were measured. Soil salt content (SSC), sodium adsorption ratio (SAR) were calculated. The ions accumulated in the frozen soil layer during the freezing period in the order of Cl⁻ > Mg²⁺ > Ca²⁺ > Na⁺ > SO₄²⁻, and accumulated in the topsoil during the thawing period in the order of Cl⁻ > Na⁺ > Mg²⁺ > Ca²⁺ > SO₄²⁻, while the change in HCO₃⁻ was mostly the opposite. The FT process changed the main salt anions from sulfate to chloride. After the FT process, the topsoil was endangered by high salinization, excess Cl⁻ toxicity, and a potential alkalization threat. This study has great guiding significance for the management and control of soil salinization before spring sowing in saline areas.

1. Introduction

Soil salinization and alkalization is an urgent environmental issue [1,2] that leads to the decrease of crop yields [3,4] and endangers agricultural production. The worldwide area of saline-alkali farmland is about 9.54 million km² [5], of which one-third is located in northern China [6]. These saline farmlands are mostly affected by seasonal freeze–thaw (FT) process [7] that are accompanied by significant soil temperature variations [8], water phase changes between liquid water and ice [9], and water migration [10]. The precipitation, dissolution, adsorption, desorption, and migration of salt ions in soil are affected by changes in soil temperature and soil liquid water content [11], and are the key reasons for soil salinization [12]. Therefore, it is important to conduct field experiments to study the effect of FT process on soil ions transport and the influence of these processes on the development of soil salinization.

FT process can significantly influence water and salt migration in soil [13,14]. During the soil freezing process, the temperature gradient drives soil water and salt from the unfrozen layer to the frozen soil layer [15], causing the increase of soil moisture and salt content in the frozen layer [16]. Ice crystals form in frozen soil, and salt is separated out of the frozen water and enriched in the remaining unfrozen water [17]. Slower freezing rates can increase the exclusion of salt from frozen soil water, resulting in a steeper concentration gradient between the frozen soil layer and the unfrozen layer, which may drive salt migration towards the unfrozen layers [18]. During the thawing process, intense surface evaporation greatly reduces water in thawed soil and promotes salt accumulation in surface soil [19,20]. The residual frozen soil can restrain the evaporation and salt migration from the lower layer, and slow or reduce the infiltration of water and salt from the upper thawed layer [7]. However, the above studies mostly focused on the migration of the total salt in soil, which is actually a comprehensive process involving the movement of various soluble ions. For example, Chuvilin [21] observed that the freezing process induced the accumulation of light metal ions (such as sodium, calcium, and magnesium) in frozen soil, while the contents of copper and zinc had no significant change in the frozen layer. The experimental results of Wang et al. [22] showed that the Na⁺, Cl⁻, and SO₄²⁻ storage increments in the upper 0–1 m soil layer were identical to that of the soil total salt, but the storage of Na⁺, Cl⁻, and SO₄²⁻ decreased with the increasing storage of HCO₃⁻. However, these previous studies have mainly focused on changes in various soil soluble ion contents and their distribution characteristics, while the migration characteristics, driving factors, and influencing mechanisms of salt ions during soil freezing and thawing processes remain unclear.

At present, the migration characters of various salt ions and their influencing factors are mostly studied during the processes of soil leaching and evaporation. For example, Zhao et al. [23] conducted a soil column leaching experiment and found that the variation of Na⁺ content was similar to that of Cl⁻; stronger than that of Ca²⁺, Mg²⁺, SO₄²⁻; and negatively correlated with HCO₃⁻. In addition, Kong et al. [24] reported that the decreasing order of salt ion capacity during the leaching process was as follows: Na⁺ > K⁺ > Ca²⁺ > Mg²⁺ and HCO₃⁻ > SO₄²⁻ > CO₃²⁻. The inconsistencies in the migration of various ions in these studies are due to the combined effects of their physicochemical properties and the accompanying chemical reactions during the movement of the soil solution, such as the ionic charge [25], ionic radius [26], relative molecular/atomic mass [27], hydrolysis–complexation, precipitation–dissolution [28], adsorption–desorption, and ion exchange [29]. In addition, migration inconsistencies are intensified by environmental factors, such as the soil texture [30], the chemical composition of groundwater [31], and the land use types [32]. However, factors controlling the migration characters of salt ions during soil FT processes are significantly different. For example, the decrease of temperature during the soil freezing process can significantly reduce the solubility of chemical compounds such as Na₂SO₄ and MgSO₄ [33]. Salts crystallize and precipitate out of the soil solution more easily as the liquid water content decreases [34,35], thus reducing the amount of freely migrating ions in the soil. However, the upward movement of soil water induced by low temperatures [36] can weaken this ionic-decrease effect, and make the movement of ions in the FT process more complex. Thus, it is of great importance to investigate how temperature affects ion migration during the FT process.

Differences in the migration of soil ions can significantly alter the salt chemical composition [37], further affecting soil salinization and alkalization [38]. For example, the pH of soil usually exceeds 10 when the CO₃²⁻ and HCO₃⁻ contents are high [39,40], and higher levels of Na⁺ and lower levels of Ca²⁺/Mg²⁺ significantly increase the soil sodium adsorption ratio (SAR), thereby increasing soil alkalinity [41]. An imbalance of the soluble ion composition will also cause obvious single ion toxicity in soil structure and plants [42,43]. Thus, investigating the influence of FT processes on changes in salt chemical composition is of great significance to the guiding of spring irrigation and crop planting.

This paper conducted a six-month field observation during the FT process from November 2020 to April 2021 in Hetao irrigation district, a typical seasonal frozen soil district in China. The contents of soil soluble ions, soil moisture content (SMC), soil temperature, and soil pH values were measured, and the soil salt content (SSC), soil SAR were calculated. The aims of this paper were to (1) identify the migration characteristics of different salt ions and reveal the chemical mechanisms of the ions transport difference during the FT process; (2) reveal the effect of ions transport inconsistencies during the FT process on the development of soil salt chemical composition; and (3) assess how changes in the salt chemical composition affect soil alkalization properties.

2. Materials and Methods

2.1. Study Site and Soil Sampling

The sampling site was located at the Yonglian experimental station (108°00′35″ E, 41°04′15″ N) of the Hetao Irrigation District, in the west of the Inner Mongolia Autonomous Region, China (Figure 1). The study area has a typical arid continental climate. The annual precipitation is 137–214 mm, the annual evaporation is 1993–2373 mm, and the annual average temperature is between 6 and 8 °C. Soils begin freezing in the second half of November, freeze to a depth of about 0.7–0.8 m, and completely thaw in late April of the following year. The duration of frozen ground is about 180 days.

2.2. Experiment Design

The experiments were conducted from November 2020 to April 2021. A 20 m × 10 m saline farmland planted with sunflower was selected as the experiment field. Considering the spatial variability of soil salinity, moisture and ion content in the experimental field, a pre-experiment was conducted. The test field was divided into 32 test plots (2.5 m × 2.5 m), and the SSC, SMC, soil texture, and ion content of each plot were texted. Three plots with similar SSC, SMC, soil texture, and ion content were selected as replicated to conduct the following FT tests, as shown in Figure 1d. The average value of the three sampling points was used as the analysis data.

Soil was oven-dried at 105 °C for 8 h to determine the soil water content, then sieved with a 2-mm sieve to the measure soil particles using a Microtrac S3500 laser particle size analyzer (Microtrac Inc., Largo, FL, USA). The soil was classified as a silt loam according to the USDA system [44]. Soil bulk density was also determined by using a steel ring (diameter: 5 cm, height: 5 cm) to sample soil and then dividing oven dried sample mass by ring volume. The cation exchange capacity (CEC) was determined by sodium acetate method. The basic soil properties at the beginning of the experiment in the 0–80 cm soil layer are listed in

Table 1. Soil basic physicochemical properties before the experiment.

Parameters	Soil Layer (cm)
	0–5	5–10	10–20	20–30	30–40	40–60	60–80
Soil salt content (g 100 g−1)	0.29 ± 0.05	0.28 ± 0.06	0.23 ± 0.05	0.28 ± 0.04	0.27 ± 0.02	0.21 ± 0.03	0.20 ± 0.04
Electric conductivity (µS·cm−1)	681 ± 110	479 ± 96	406 ± 53	505 ± 98	662 ± 84	495 ± 97	541 ± 110
Soil moisture Content (%)	16.78 ± 2.14	17.98 ± 0.53	19.41 ± 0.94	20.21 ± 1.82	20.27 ± 1.38	19.72 ± 2.51	19.39 ± 2.17
Na+ (meq 100 g−1)	2.00 ± 0.40	1.69 ± 0.29	1.37 ± 0.11	1.75 ± 0.26	1.86 ± 0.11	1.26 ± 0.22	1.25 ± 0.21
Ca2+(meq 100 g−1)	1.06 ± 0.24	0.90 ± 0.12	0.85 ± 0.05	0.98 ± 0.10	1.00 ± 0.10	0.66 ± 0.04	0.74 ± 0.03
Mg2+(meq 100 g−1)	1.14 ± 0.50	1.50 ± 0.19	0.92 ± 0.12	1.04 ± 0.22	1.08 ± 0.26	1.12 ± 0.06	0.82 ± 0.04
SO42– (meq 100 g−1)	2.00 ± 0.31	2.24 ± 0.31	1.56 ± 0.28	1.90 ± 0.11	2.04 ± 0.08	1.64 ± 0.14	1.52 ± 0.12
Cl− (meq 100 g−1)	1.84 ± 0.18	1.40 ± 0.30	1.12 ± 0.18	1.82 ± 0.03	1.60 ± 0.21	1.07 ± 0.22	0.90 ± 0.24
HCO3− (meq 100 g−1)	0.85 ± 0.13	0.87 ± 0.12	0.88 ± 0.23	0.87 ± 0.16	0.67 ± 0.07	0.59 ± 0.07	0.67 ± 0.13
Soil bulk density (g cm−3)	1.42 ± 0.06	1.40 ± 0.03	1.41 ± 0.07	1.38 ± 0.05	1.35 ± 0.03	1.42 ± 0.09	1.45 ± 0.05
CEC (meq 100 g−1)	7.22 ± 0.32	7.35 ± 0.22	7.63 ± 0.37	–	–	–	–
Sand (%)	29.02 ± 0.11	27.29 ± 1.60	28.00 ± 0.00	–	–	–	–
Silt (%)	66.57 ± 0.27	68.04 ± 2.12	67.24 ± 0.24	–	–	–	–
Clay (%)	4.41 ± 0.38	4.67 ± 0.52	4.76 ± 0.24	–	–	–	–
Soil texture	Silt loam	Silt loam	Silt loam	-–	-–	-–	-–

Note: Data are expressed as mean ± S.D. – indicated that the index was not texted.

.

According to the meteorological data, collected by the meteorological station set 100 m away from the experimental field, six sampling time (2 November 2020, 9 December 2020, 10 January 2021, 22 February 2021, 19 March 2021, and 19 April 2021) were selected. At each sampling time, three soil cores were randomly sampled from each plot using artificial soil drilling, and soil was collected from the core of each soil pillar at depths of 0–5 cm, 5–10 cm, 10–20 cm, 20–30 cm, 30–40 cm, 40–60 cm, and 60–80 cm. Soil temperature was measured by seven auto sensors (WT0T1, Wang Yun Shan Information Technology Co., Ltd., Fujian, China) embedded in the soil profile at the depths of 2.5 cm, 7.5 cm, 15 cm, 25 cm, 35 cm, 50 cm, and 70 cm. The data were collected at 1-h intervals by a matched temperature data collector. The frozen and thawing depths were recorded according to the sampling situation.

2.3. Soil Sampling and Chemical Analysis

After the soil sampling, soil samples were transported to Wuhan University for analysis. Soil samples were divided into two portions. One portion was oven-dried at 105 °C for 8 h to determine the soil water content. The remaining portion was air-dried and sieved with a 2-mm sieve to make soil extract solutions (soil:water = 1:5) to determine the soil chemical properties. The pH of soil extract solutions (soil:water = 1:5) was measured using a pH meter (DZS-706 Multi-Parameter Analyzer, Hunan Lichen Instrument Technology, Hunan, China). The concentrations of CO₃²⁻ and HCO₃⁻ were tested by the double indicator-neutralization titration method; the concentration of Cl⁻ was tested by direct titration with silver nitrate; the concentrations of Ca²⁺ and Mg²⁺ were tested by direct titrations with EDTA; SO₄²⁻ was tested by the indirect EDTA titration method; and the concentrations of K⁺ and Na⁺ were tested by spectrophotometer (FP640, Shanghai Precision Scientific Instrument Co., Ltd., Shanghai, China).

The soil salt content (SSC) was calculated according to the sum of eight dominant soluble ions. The soil salinization grade was classified into five grades according to the SSC: none salinization soil (SSC < 0.2 g/100 g), mild salinization soil (0.2 ≤ SSC < 0.4 g/100 g), moderate salinization soil (0.4 ≤  SSC < 0.6 g/100 g), severe salinization soil (0.6 ≤  SSC < 1.0 g/100 g), salinized soil (SSC ≥ 1.0 g/100 g) [45,46].

The SAR of the soil solution can be calculated as follows [47]:

$$SAR=N{a}^{+}/{[(C{a}^{2+}+M{g}^{2+})/2]}^{1/2}$$

(1)

2.4. Estimation of Amount of Crystalline Sodium Sulfate

During the freezing and thawing process, the decrease of soil temperature led to the formation of ice crystals and reduced the soil liquid water content. Assuming that salt is completely excluded from ice crystals and exists only in soil liquid water, sodium sulfate with lower solubility will preferentially precipitate from soil liquid water to form salt crystals, as they are limited by the solubility of compounds. Because the solubility of sodium sulfate changes very slowly with temperature when the temperature decreases below 0 °C [48], this study selects the solubility of salt at 0 °C for calculation. The maximum concentration of sodium sulfate that can be dissolved in soil liquid water is calculated as follows:

$$c={\theta }_u\times \frac{m_s}{M}=a{\left|T\right|}^{-b}\times \frac{m_s}{M}$$

(2)

$${\theta }_u=a{\left|T\right|}^{-b}$$

(3)

where c is the maximum concentration of the sodium sulfate (mmol/100 g); ${\theta }_u$ is the liquid water content (%), and a = 0.114 and b = 0.208, which are parameters related to the initial SMC and soil texture, respectively, estimated based on the empirical parameters proposed by Wu et al. (2015) [49]; m_s is the solubility of the sodium sulfate, 4.9 (g); M is the relative molecular mass of Na₂SO₄·10H₂O, 322; T is the soil temperature (°C).

Then the amount of sodium sulfate salts crystals is estimated as follows:

$$C=N-c$$

(4)

where N is the molar concentration of sodium sulfate, based on the measured data (mmol/100 g); c is the maximum concentration of sodium sulfate that can be dissolved in soil solution (mmol/100 g).

2.5. Statistical Analysis

SPSS 24.0 and Origin 2021 software were used for data processing, graphing, and tabulation. Each data point was summarized by calculating the average value and standard deviation (S.D). The least significant difference (LSD) method was used to test the significance of the differences in soil salt ions at different times and in different soil layer. A significance level of p = 0.05 was set. The Shapiro-Wilk test was adopted to verify the normal distribution of the data. The Spearman correlation analysis was conducted to determine the relationship between the dominant soil soluble ions (Cl⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, Na⁺, K⁺, Mg²⁺, and Ca²⁺) and the soil properties (SMC, SSC, pH, SAR). The correlation coefficient matrix was drawn using MATLAB.

3. Results

3.1. Changes in Temperature and SMC during the FT Process

The meteorological conditions during the test period are shown in Figure 2a. During the experimental period, the only rainfall was a 1.11 mm rainfall event that occurred in November. Figure 2b shows the temperature change of the soil profile. This paper determined that the soil was frozen when the soil temperature stabilized below 0 °C. According to the air temperature and soil freezing state, six sampling dates were set on 2 November 2020, 9 December 2020, 10 January 2021, 22 February 2021, 19 March 2021, and 19 April 2021 (T₁–T₆, respectively), and the FT process was divided into five periods. During the initial freezing period (T₁–T₂), soil froze during the night and thawed during the day, and the frozen soil depth was 30 cm at the end of this period. During the stable freezing period (T₂–T₃), soil was stably frozen (T_max < 0 °C), the minimum air temperature reached −21.53 °C, and the maximum freezing depth reached 75 cm. During the early unstable thawing period (T₃–T₄), T_max > 0 °C, T_min < 0 °C; however, there was no obvious thawing layer on the soil surface, because T_ave < 0 °C. During the late unstable thawing period (T₄–T₅), T_ave > 0 °C, and the topsoil was gradually thawed. Up to T₅, the thawed depth was 31 cm. From T₅–T₆, a stable thawing period was observed (T_min > 0 °C most of the time). By T₆, the soil had completely thawed. Taking T₄ as the dividing point, in the T₁–T₄ stage, the temperature gradient was positive and the soil temperature gradually increased with the increase of depth; in the T₄–T₆ stage, the temperature gradient was negative, and the soil temperature gradually decreased with the increase of depth.

The distribution of SMC at the depth of 0–80 cm is shown in Figure 3. The SMC gradually increased from 15.46% to 23.15% with the increase of soil depth before freezing (T₁), and the average SMC was 21.74%. During the FT process, the soil moisture accumulated during the freezing process and decreased during the thawing process. For example, the SMC of the frozen soil layers (0–30 cm) at T₂ increased by 77.29% compared with the initial value (T₁) (p < 0.05), while the SMC of the unfrozen soil layers (30–80 cm) had no significant differences between T₁ and T₂ (p > 0.05); at T₃, the SMC of the newly frozen soil layers (30–80 cm) increased by 24.86% compared with T₂ (p < 0.05) while the SMC of old frozen soil layers (0–30 cm) had no significant differences between T₂ and T₃ (p > 0.05). This finding indicated that during the freezing process, water mainly accumulated in the newly frozen layer. During the T₄–T₅ period, the SMC of the thawed layer (0–30 cm) decreased by 33.20% (p < 0.05), and the unthawed layer (30–80 cm) was relatively stable; during the T₅–T₆ period, the SMC showed a downward trend, and the soil moisture declines of the surface (0–5 cm) and deep (30–80 cm) layers were more than 30% (p < 0.05). After the entire FT process (T₆), the soil moisture, which was 23.38% on average and gradually increased from 15.91% to 25.64% with the increase of soil depth, did not change significantly compared with that at T₁ (SMC_ave = 23.15%)

3.2. Changes in SSC and Ion Content during the FT Process

3.2.1. Change in SSC during the FT Process

The profiles of SSC at the depth of 0–80 cm are shown in Figure 4. Before freezing, the SSC of the surface layer (0–5 cm) was 0.300 g 100 g⁻¹, and the average SSC of the section (0–80 cm) was 0.239 g 100 g⁻¹, which was categorized as mild saline soil. The SSC value in different soil layers had no significant differences. During the freezing period, the most obvious changes in soil salt appeared in the 10–20 cm soil layer during T₁–T₂ (p < 0.05). During the thawing periods, the average SSC values of 0–5 cm significantly increased by 147.25% during T₄–T₆ (p < 0.05). After the entire FT process, although the average SSC of 0–80 cm was 0.268 g 100 g⁻¹, which was categorized as slightly saline soil, the SSC of the topsoil (0–5 cm) reached 1.145 g 100 g⁻¹, with an increase of 298.61% over the initial situation, which was categorized as salinized soil. Regarding the vertical distribution, the SSC of the topsoil (0–5 cm) was 4.48 times that of the 5–80 cm soil and decreased with the soil depth.

3.2.2. Changes in Salt Ion Content during the FT Process

The changes in the soil ion contents in different soil layers during the FT process are shown in Figure 5. The variation trends of five major soluble ions (Cl⁻, Na⁺, SO₄²⁻, Mg²⁺, and Ca²⁺) in different soil layers were similar (Figure 5a–e), with only differences in change values. During the freezing period, these five ions moved upward and accumulated in the frozen layer (0–30 cm) and decreased in the unfrozen layer (30–80 cm) in the early freezing period (T₁–T₂). These five ions then migrated downward, which caused their concentrations to decrease in the 0–5 cm soil layer and increase in the newly frozen layer (30–80 cm) during the stable freezing period (T₂–T₃). But the variations of these ions during the freezing period were not significant (p > 0.05). During the thawing period (T₃–T₆), the contents of Cl⁻, Na⁺, SO₄²⁻, Mg²⁺, and Ca²⁺ in the 0–5 cm soil layer continued to increase, especially in the T₄–T₅ period, during which these ions increased by 148.37%, 136.07%, 113.91%, 110.99%, and 94.44%, respectively, showing explosive increases (p < 0.05). After the FT process (T₆), the contents of Cl⁻, Mg²⁺, Ca²⁺, Na⁺, and SO₄²⁻ in the 0–5 cm soil layer increased by 563.41%, 396.52%, 342.97%, 321.39%, and 276.74%, respectively, compared to the initial value (T₁). In the 5–80 cm soil layer, the concentrations of these five soluble ions did not change significantly.

The changes of HCO₃⁻ content in different soil layers are shown in Figure 5f. At the early stage of freezing, the HCO₃⁻ concentration significantly increased by 71.92% in the 30–80 cm unfrozen layer, where other ions remained stable. During stages T₂–T₃, HCO₃⁻ increased by 39.92% in the 0–5 cm soil layer, and decreased by 15.37% in the 30–80 cm soil layer, but the variations were not significant (p > 0.05). During thawing period (T₃–T₆), HCO₃⁻ significantly decreased by 51.43% in the 0–5 cm soil layer (p < 0.05) where other ions significantly increased. After freezing and thawing, the HCO₃⁻ content in the 0–5 cm soil layer decreased by 30.64% compared with the initial content.

As a result, the FT process greatly altered the distribution pattern of different ions (Figure 6). Before freezing, the spatial variation coefficients of each ion were between 0.16–0.27, indicating that the ion distribution was relatively uniform between different soil layers. However, the spatial variation coefficients of most ions (except for HCO₃⁻) increased to 0.89–1.31 following the FT process, which was mostly induced by the obvious increase of ions in the 0–5 cm soil layer.

3.2.3. Migration Inconsistencies in Soil Salt Ions during the FT Process

The storage change rates of five ions (in addition to HCO₃⁻) at the 0–5 cm soil layer during different periods are shown in Figure 7a. In the whole FT period, Cl⁻ and Na⁺ were the ions with the largest storage change rate, followed by SO₄²⁻, Mg²⁺, and Ca²⁺.

However, the storage change rates of ions were induced by the combined effect of ion mobility and ion content in different soil layers. Therefore, the ratio of the ion increases (II) in the 0–5 cm soil layer to the ion storage (IS) in the 5–80 cm soil layer during the previous stage (II_(0–5)/IS_(5–80)) was used as an index to evaluate the ion migration ability when the ions moved upward (T₁–T₂, T₃–T₄, T₄–T₅, and T₅–T₆). The index was changed to the ratio of the reduction (ID) of the ion storage in the 0–5 cm soil layer to the ion storage in the previous stage in the 0–5 cm layer (II_(0–5)/IS_(0–5)) when the ion storage in the 0–5 cm soil layer decreased T₂–T₃. Ions with larger ratio had stronger migration ability. The migration abilities of different ions (IR_(0–5)/IR_{(5–80 or 0–5)}) are shown in Figure 7b, and rank in order as Cl⁻ > Mg²⁺/Ca²⁺/Na⁺ and SO₄²⁻. Cl⁻ was always the ions with the strongest migration ability, while SO₄²⁻ with weakest migration ability (expect P₂–P₃). The order of cations varied in different periods; for example, the order of cations was Mg²⁺ > Ca²⁺ > Na⁺ in the T₁–T₂ and T₅–T₆ periods, and Na⁺ > Mg²⁺ > Ca²⁺ in the T₄–T₅ period.

3.3. Change in Soil Salinization during the FT Process

The proportions of soil cations and anions are shown in

Table 2. Soil anion and cation composition.

Time	Soil Layer (cm)	Proportion of Anion Equivalent Concentration (%)			Proportion of Cation Equivalent Concentration (%)
		HCO3−	SO42−	Cl−	Ca2+	Mg2+	K+	Na+
T1	0–5	18.63	40.95	40.42	19.06	20.54	4.55	55.85
	5–10	19.94	48.02	32.04	17.34	28.96	3.38	50.32
	10–20	25.37	42.30	32.34	19.86	23.13	4.31	52.69
	20–30	19.50	40.77	39.74	19.50	20.69	5.85	53.96
	30–40	15.95	45.75	38.30	19.78	21.47	1.85	56.89
	40–60	18.45	47.99	33.57	17.27	29.68	1.92	51.14
	60–80	22.45	47.69	29.86	20.75	23.05	2.24	53.96
T3	0–5	17.29	36.91	45.79	20.53	26.35	2.71	50.41
	5–10	18.16	43.11	38.73	20.30	21.09	3.47	55.15
	10–20	18.55	39.01	42.44	17.75	19.04	2.75	60.46
	20–30	15.65	41.61	42.74	17.23	16.56	2.54	63.67
	30–40	17.82	41.31	40.87	17.88	14.57	2.33	65.21
	40–60	24.92	40.06	35.02	19.46	11.68	1.86	67.00
	60–80	49.27	31.83	18.90	25.74	7.59	1.40	65.27
T6	0–5	3.06	33.42	63.52	19.62	23.71	1.965	54.71
	5–10	16.23	39.20	44.56	21.43	18.27	3.154	57.15
	10–20	20.63	33.21	46.17	23.39	15.70	2.930	57.98
	20–30	16.99	40.43	42.58	20.21	17.66	1.896	60.24
	30–40	19.58	37.23	43.20	21.52	13.64	1.312	63.53
	40–60	22.62	35.75	41.63	20.45	13.29	1.003	65.25
	60–80	31.89	31.45	36.66	20.81	12.14	0.685	66.36

Note: T₁ represents the first sampling, soil was not frozen; T₃ represents the third sampling, frozen soil depth reached the maximum; T₆ represents the sixth sampling, soil was thawed at 0–80 cm.

. Initially, the order of cation ions in each layer of soil was Na⁺ > Mg²⁺ > Ca²⁺ > K⁺, accounting for 50.32–56.89%, 20.69–29.68%, 17.27–20.75%, and 1.85–5.85% of the total amount of cations, respectively. The order of anions in the soil was SO₄²⁻ > Cl⁻ > HCO₃⁻, accounting for 40.77–48.02%, 29.86–40.42%, and 15.85–25.37% of the total anions, respectively. After the soil freezing process, the proportions of Cl⁻ and Mg²⁺ in 0–5 cm soil layer were increased to 45.79% and 26.35%, respectively, while the proportions of SO₄²⁻ and Na⁺ were decreased to 36.91% and 50.41%, respectively. After the soil FT process, the order of anions in each soil layer changed to Cl⁻ > SO₄²⁻ > HCO₃⁻. With the increase of depth, the proportion of HCO₃⁻ gradually increased from 3.14% to 31.89%, while the proportion of Cl⁻ gradually decreased from 63.52% to 36.66%, and the proportion of SO₄²⁻ was relatively stabilized between 31% and 40%. The FT process also changed the composition of cations in soil. As the depth increased, the proportion of Na⁺ increased from 54.71% to 66.36%, the proportion of Mg²⁺ decreased from 23.71% to 12.14%, and the proportion of Ca²⁺ stabilized between 20% and 25%.

3.4. Changes in Soil Alkalization Parameters during the FT Process

The changes in the soil SAR and pH values during the FT process are shown in

Table 3. Soil SAR and pH values during the freeze–thaw process.

	Soil Layer (cm)	T1	T2	T3	T4	T5	T6
SAR	0–5	3.76 ± 0.57 bA	4.19 ± 0.71 bA	3.85 ± 1.1 bAB	4.21 ± 1.54 bAB	6.27 ± 1.39 aA	6.51 ± 1.47 aA
	5–10	3.03 ± 0.61 aA	3.47 ± 0.34 aAB	3.80 ± 1.42 aAB	3.04 ± 0.73 aC	4.16 ± 1.43 aB	3.91 ± 1.08 aB
	10–20	2.90 ± 0.93 aA	3.03 ± 0.93 aB	4.30 ± 0.81 aAB	3.23 ± 0.39 aBC	4.12 ± 1.43 aB	3.78 ± 0.79 aB
	20–30	3.43 ± 1.28 bA	3.21 ± 0.33 bAB	4.89 ± 0.89 aA	3.40 ± 0.66 abBC	4.01 ± 1.4 abB	4.01 ± 0.96 abB
	30–40	3.58 ± 1.38 bA	3.57 ± 0.46 bAB	4.82 ± 0.57 aA	4.30 ± 0.47 abAB	4.36 ± 1.94 abB	4.18 ± 0.48 abB
	40–60	2.62 ± 0.69 cA	3.41 ± 0.58 bcAB	4.44 ± 0.61 abAB	4.69 ± 0.88 aA	3.73 ± 1.81 abcB	4.06 ± 0.78 abB
	60–80	2.77 ± 1.18 aA	2.84 ± 0.60 aB	3.21 ± 0.53 aB	3.23 ± 0.61 aBC	3.49 ± 1.9 aB	3.79 ± 0.73 aB
pH	0–5	6.98 ± 0.23 cA	7.48 ± 0.28 bC	8.07 ± 0.45 aA	7.95 ± 0.04 aB	8.04 ± 0.07 aB	8.27 ± 0.06 aA
	5–10	7.04 ± 0.32 cA	7.61 ± 0.32 bBC	8.12 ± 0.25 aA	8.01 ± 0.12 abB	8.29 ± 0.25 aAB	8.44 ± 0.06 aA
	10–20	7.17 ± 0.17 dA	7.72 ± 0.32 cABC	8.11 ± 0.10 bA	8.00 ± 0.27 bcB	8.30 ± 0.18 abAB	8.54 ± 0.15 aA
	20–30	7.42 ± 0.42 cA	7.85 ± 0.14 bABC	8.01 ± 0.22 bA	7.93 ± 0.20 bB	8.27 ± 0.11 abAB	8.58 ± 0.18 aA
	30–40	7.36 ± 0.33 cA	7.93 ± 0.15 aB	8.00 ± 0.24 bA	7.95 ± 0.26 bB	8.31 ± 0.03 aAB	8.55 ± 0.20 aA
	40–60	7.30 ± 0.15 cA	8.01 ± 0.17 bAB	8.07 ± 0.28 abA	8.19 ± 0.18 abAB	8.49 ± 0.27 aA	8.51 ± 0.34 aA
	60–80	7.39 ± 0.16 cA	8.06 ± 0.08 bA	8.12 ± 0.23 bA	8.44 ± 0.33 abA	8.59 ± 0.25 aA	8.39 ± 0.26 aB

Note: Data are expressed as mean ± S.D; Different lowercase letters indicate significant differences between different times (p < 0.05). Different capital letters indicate significant differences between different soil layer (p < 0.05).

.

Before soil freezing, the SARs of different soil layers varied from 2.62 to 3.76, with an average of 3.00. During the freezing process, SAR values significantly increased in the frozen soil layer (20–40 cm) (p < 0.05). At the stable freezing stage (T₃), SAR values in the 20–30 cm soil layer reached 4.89, or increase of 48.34% relative to T₁. During the thawing process (T₄–T₆), the SAR values of the 0–5 cm soil layer increased by 69.42% (p < 0.05), but slightly decreased in the 10–80 cm soil layer (p > 0.05). At the end of the thawing process (T₆), the SAR values of the 0–5 cm soil layer were 6.52, or increases of 71.73% relative to the initial values before freezing (p < 0.05).

Initially, the soil pH increased from 6.98 to 7.42 with the increase of soil depth, and the average pH was 7.29. During the initial freezing period, the soil pH value significantly increased in all soil layers (p < 0.05), reaching 7.71 in the frozen soil layers and 8.02 in the unfrozen layers on average. During the thawing period, the soil pH value increased slowly, but the differences were not significant (p > 0.05). At the end of FT process, the soil pH in different layers ranged from 8.27 to 8.58, and the average pH value was 8.48. FT process significantly increased the soil pH in all soil layers.

4. Discussion

4.1. Chemical Mechanisms of the Differences in Salt ion Migration during the FT Process

During the FT process, various ions were continuously redistributed along the vertical soil profile. This redistribution not only changed the amount of ions in different soil layers, but also altered the distribution patterns of ions. For example, most ions decreased with the soil depth after the FT process (except for HCO₃⁻) (Figure 6), while ion distribution was relatively uniform before the FT process (Figure 6, Table 2). This was because the temperature gradient during the freezing process and the strong evaporation during the thawing process caused the soil water to carry a large number of ions, resulting in their migration to the surface layer. The ions then decreased in the subsoil and accumulated in the topsoil (0–5 cm).

During different FT periods, the changes in the contents of Cl⁻, Mg²⁺, Ca²⁺, Na⁺, and SO₄²⁻ were similar, but the change ratios were distinctly different (Figure 7). For example, the five ions accumulated in the order of Cl⁻ > Mg²⁺ > Ca²⁺ > Na⁺ > SO₄²⁻ during the initial freezing period (T₁–T₂); Cl⁻ > Na⁺ > Mg²⁺ > Ca²⁺ > SO₄²⁻ during the late unstable thawing period (T₄–T₅); and Cl⁻ > Mg²⁺ > Ca²⁺ > Na⁺ > SO₄²⁻ during the stable freezing period (T₅–T₆) (Figure 7). The FT process mainly affected the migration order of Na⁺, which was related to the formation of Na₂SO₄·10H₂O. The reasons were that the soluble salt were mainly existed as Na₂SO₄ and NaCl in the soil, because the proportion of Na⁺, Mg²⁺, Ca²⁺ to cations were about 50–67% and 21–29, 17–20%, and the proportion of SO₄²⁻, Cl⁻, HCO₃⁻ to anions were 40.77–48.02%, 29.86–40.42%, and 15.85–25.37%, respectively (Table 2). During the freezing period, the decrease of soil temperature led to the formation of ice crystals and reduced the soil liquid water content. Ions were separated out of the frozen water and enriched in the remaining unfrozen water, increasing the ion concentration of the solution. The solubility of Na₂SO₄ was much lower than chloride, so more Na₂SO₄·10H₂O was formed and then crystallized and precipitated out of the soil solution (

Table 4. Soil precipitation of Na₂SO₄·10H₂O (mmol/100 g) during the freeze–thaw process.

Soil Layer (cm)	T1	T2	T3	T4	T5	T6
0–5	-	1.312	0.824	-	-	-
5–10	-	0.148	0.327	-	-	-
10–20	-	0.348	-	-	-	-
20–30	-	-	0.195	-	-	-
30–40	-	-	-	-	-	-
40–60	-	-	-	-	-	-
60–80	-	-	-	-	-	-

Note: Stage T₁–T₃ represent the freezing process; and stages T₄–T₆ represent the thawing process.

). Besides, the solubility of Na₂SO₄ decreased rapidly with decreasing temperature, while the solubility of other salt such as CaSO₄ varies little with temperature [33,50]. These effects further decreased the Na⁺ and SO₄²⁻ ions in the soil solution that could migrate with soil water (Table 4). During the initial thawing period (T₄–T₅), the soil temperature rose above 0 °C and the increased liquid water from melting ice crystals led to the dissolution of the precipitation of Na₂SO₄·10H₂O in the thawed soil layer (Table 4). Under these conditions, the mobility of Na⁺ was strongly improved because it had a lower affinity with soil colloids due to the smaller ionic radius and a lower positive charge compared with Ca²⁺ and Mg²⁺ [27]. This finding was similar to the findings of Guo and Liu et al. [51], who suggested that the mobilization order of cations was Na⁺ > Mg²⁺ > Ca²⁺ in saline ice during the soil melting process. However, as the thawing process progressed further, the mobility of Na⁺ was relatively decreased, which was related to the cation exchange processes. As shown in

Table 5. Percentage of CEC in total soluble cations in 0–5 cm soil layer during the freeze–thaw process.

	T1	T2	T3	T4	T5	T6
CEC (meq 100 g−1)	7.12	8.44	8.65	8.93	9.26	9.79
Total soluble cations (meq 100 g−1)	4.27	7.85	6.63	7.49	16.84	18.41
Percentage of CEC to total soluble cations (%)	166.57	94.72	130.38	119.28	55.00	53.18

Note: Stages T₁–T₃ represent the freezing process; and stages T₄–T₆ represent the thawing process. CEC, cation exchange capacity; total soluble cations, sum of Na⁺, K⁺, Ca²⁺, Mg²⁺.

, the CEC of soil accounted for more than 53.18% of the total soluble cation, so. the exchange capacity of Na⁺ would be enhanced due to the significant accumulation of Na⁺ [28], which converted partially soluble Na⁺ into exchangeable Na and increased the soluble Ca²⁺ and Mg²⁺ contents. Therefore, the change of Na⁺ content was not only determined by the migration of free Na⁺, but was also strongly affected by several chemical reactions, such as precipitation–dissolution, cation adsorption, and cation exchange. Soil temperature, which controlled the occurrence and intensity of these chemical processes, was the controlling factor of the Na⁺ migration order during the soil FT process.

Cl⁻ and SO₄²⁻ were the ions with the largest and smallest migration order, respectively, during the entire FT process (Figure 7). According to the correlation analysis, the change of Cl⁻ content was strongly positively correlated with soil moisture, indicating that the movement of Cl⁻ was mainly affected by soil water and less affected by other chemical reactions. For example, Cl⁻ migrated more freely compared with cations because Cl⁻ was repelled by soil colloids owing to its negative charge [52]. Compared with SO₄²⁻ in frozen soil, the main chlorides, such as NaCl, MgCl₂, and CaCl₂, did not crystallize and precipitate as their solubility did not change significantly with decreasing temperature [33]. Compared with SO₄²⁻ in thawed soil, Cl⁻ did not participate in the precipitation–dissolution of CaSO₄, and it was subject to greater electrostatic repulsion force due to soil colloids because of the smaller half valence shell of the hydrated ions [53].

The change of HCO₃⁻ content differed from the patterns exhibited by Cl⁻, Mg²⁺, Ca²⁺, Na⁺, and SO₄²⁻. For example, the content of HCO₃⁻ decreased when other ions accumulated in the 0–5 cm soil layer during the T₁–T₂ and T₄–T₅ periods, and the content of HCO₃⁻ increased when other ion contents reduced in the 30–80 cm soil layer during T₁–T₂ and in the 0–5 cm soil layer during T₂–T₃. According to the correlation analysis (Figure 8), the concentration of HCO₃⁻ was negatively correlated with Ca²⁺ (p < 0.05), followed by SO₄²⁻, while had no significant correlation with soil moisture and other ions. This phenomenon may be partly attributed to the precipitation–dissolution of CaCO₃ [27].

4.2. Effects of Ion Migration Differences on Soil Salt Composition

The redistribution process of ions induced by the FT process greatly adjusted the ion composition distribution patterns, which is a more effective indicator to measure the impact of salinity on crop growth than the soil total salt according to the theory of single ion toxicity [54,55]. After the FT process, in the 0–80 cm soil layer, as soil depth increased, the proportions of Cl⁻ and Mg²⁺ gradually decreased, the proportions of Na⁺ and HCO₃⁻ gradually increased, and the proportions of SO₄²⁻ and Ca²⁺ remained relatively stable (Table 2), inducing the main salinization type to shift from sulfate-chloride to soda-sulfate-chloride (Table 2). The differences in distribution patterns of different ions were determined by the migration abilities of the ions. Ions with stronger mobility tended to be distributed in the upper soil layer, the ions with moderate mobility tended to be evenly distributed along the soil profile, and the ions with low mobility tended to accumulate in the subsoil layer.

In the 0–5 cm soil layer, where the salt content was of great significance to the germination and emergence of crops, the total salt and ions were at their highest concentrations after the FT process, and the proportion of Cl⁻ increased from 40.42% to 60.52%, resulting in the type of soil salinization shifting from chloride-sulfate to sulfate-chloride (Table 2). Thus, after the FT process, the topsoil was not only harmed by a high degree of salinization, but was also exposed to the toxicity of excess Cl⁻, which could hinder the absorption of soil nutrients by crops and result in stunted growth [56,57]. The reason for this outcome was discussed above; with the exception of a slight decreasing trend in T₂–T₃, the contents of SO₄²⁻ and Cl⁻ continued to accumulate at the slowest and fastest rates, respectively, leading to the gradual domination of Cl⁻ in terms of total anions. However, the dominant soil cation was always Na⁺ in topsoil, indicating that the FT process had a greater effect on soil anion composition than soil cation composition in this layer.

In this study, the freezing period lasted approximately 92 days (Figure 2), during which the proportion of Na⁺ in the topsoil (0–5 cm) gradually decreased from 55.85% to 50.41% (Table 2), as the cations accumulated in the order of Mg²⁺ > Ca²⁺ > Na⁺ (Figure 7). The thawing period lasted approximately 61 days (Figure 2), during which the proportion of Na⁺ gradually increased from 50.41% to 54.71% (Table 2) because the migration order shifted to Na⁺ > Mg²⁺ > Ca²⁺ (Figure 7). Thus, the freezing period tended to decrease the proportion of Na⁺ in the topsoil, while the thawing period tended to increase the proportion of Na⁺. These contrasting effects offset each other and maintained the stability of the Na⁺ proportion and cation composition in the present study. Previous studies have shown that the soil freezing period is growing shorter and the soil thawing period has been advancing in northern China owing to the gradual increase of winter temperature under climate change [58,59,60]. As discussed above, the migration ability of Na⁺ was inhibited during the freezing process and recovered during the thawing process. Thus, it can be predicted that, under the influence of climate change, more Na⁺ will migrate to the topsoil and the proportion of Na⁺ in the total topsoil cations will increase after the FT process. Because Na⁺ and Cl⁻ are the most harmful ions for crop growth and soil structure [61], northern China faces the risk of ion imbalance during sowing in spring.

4.3. Effects of Ion Migration Differences on Soil Alkalization Parameters

Soil SAR and pH values are important indicators that can be used to judge the soil alkalization degree [62,63]. In this study, the soil FT process induced increases of the soil SAR and pH values, particularly in the 0–5 cm soil layer (Table 3), which changed from having no alkalization risk to slightly alkaline and close to the critical value that is harmful to crops. According to previous research, the change of the soil alkalization index was strongly related to the change of ions [64]. As SAR was calculated from Na⁺, Ca²⁺ and Mg²⁺ as shown in Formula (1), it was significant positively correlated with Na⁺ (p < 0.01) and negatively correlated with Ca²⁺ and Mg²⁺ (p > 0.05) during the FT process (Figure 9). The positive relationship between SAR and Cl⁻ and SO₄²⁻ were the results of their synchronized migration of Na⁺. Therefore, Na⁺ was the key ion influenced the change of soil alkalization level [42].

Increasing pH was mainly caused by the hydrolysis of alkaline carbonate and exchangeable Na⁺ [63]. During the freezing period, pH had a strong positive correlation with HCO₃⁻, while the positive correlation with Na⁺ was lower (Figure 9), indicating that the change of pH was determined by HCO₃⁻ rather than Na⁺, as some Na⁺ was precipitated out of the soil solution in the form of Na₂SO₄·10H₂O crystallization induced by low temperature (Table 4). During the thawing period, the correlation coefficient between pH and HCO₃⁻ decreased to 0.61 (p < 0.01), and that between pH and Na⁺ increased to 0.39 (p < 0.05), indicating that the change of pH was affected by both HCO₃⁻ and Na⁺. For example, in the thawed soil layer, the complete dissolution of Na₂SO₄·10H₂O induced by the rising soil temperature increased the free Na⁺ in soil solution, resulting in an increase of the content of exchangeable Na [28], which in turn increased the soil pH. However, as discussed in 4.1, the HCO₃⁻ content was negatively correlated with Na⁺. Thus, the explosive accumulation of Na⁺ was accompanied by a decrease in the HCO₃⁻ content, which tended to decrease the soil pH value. These two different effects caused the soil pH to shift in opposite directions and finally led to a stable pH in the thawed soil layer.

5. Conclusions

Based on a field observation conducted from November 2020 to April 2021, the ions transport characteristics during the freeze–thaw (FT) process were investigated, and their effects on the evolution of soil salinization chemical properties were evaluated.

(1)

During the FT process, the dynamics of ions showed that Cl⁻, Mg²⁺, Ca²⁺, Na⁺, and SO₄²⁻ accumulated in the frozen soil during the freezing period, and gathered in the topsoil (0–5 cm) during the thawing period, while the change of HCO₃⁻ content was mostly opposite to the changes in these ions.

(2)

Cl⁻ and SO₄²⁻ exhibited the strongest and weakest migration ability, respectively. The migration ability of cations was in the order of Mg²⁺ > Ca²⁺ > Na⁺ during the freezing period and Na⁺ > Mg²⁺ > Ca²⁺ during the thawing period because the mobility of Na⁺ was restrained by the negative soil temperature due to the formation and precipitation of Na₂SO₄·10H₂O.

(3)

As the result of the ions migration inconsistencies, the main salt anions changed from sulfate to chloride. The soil alkalization degree of 0–5 cm soil increased from no alkalization risk to slightly alkaline mainly due to the accumulation of Na⁺. The topsoil in saline areas was endangered by high salinization, excess Cl⁻ toxicity, and a potential alkalization threat after the FT process.