Effects of Biochar and Straw Regulation on Snowmelt Infiltration in Seasonal Frozen Soil Regions of Northeast China

Abstract

In the seasonal frozen soil region of Northeast China, freeze–thaw processes destabilize soil structure and elevate the risk of spring flooding. While biochar and straw are recognized for their ability to enhance soil structure, their regulatory effects on the characteristics of frozen front migration and snowmelt infiltration in this region have not been thoroughly investigated. This study conducted indoor simulation experiments in 2024, establishing three different initial moisture contents (W1: 20%, W2: 15%, W3: 10%) and four distinct regulation measures (CK: blank control, B: 1.0% biochar, J: 0.5% straw, BJ: 1.0% biochar and 0.5% straw) to investigate the influence of various regulation modes on snowmelt water infiltration in freeze–thaw soils. The experimental results indicate that the application of biochar and straw significantly enhances soil aggregate stability, with the BJ treatment increasing small pores by 58.25–60.17% and micropores by 26.69–77.71%. The application of biochar and straw can increase both the migration depth of the soil freezing front and its average migration velocity. An appropriate amount of biochar and straw can enhance the cumulative soil infiltration amount and infiltration rate. Additionally, biochar and straw enhance the relationship between the cumulative soil infiltration amount and the migration characteristics of the freezing front.

1. Introduction

The cold regions at middle and high latitudes globally are typical areas affected by climate change and soil freeze–thaw cycles. The complexity and uniqueness of farmland water and soil environments present significant challenges to agricultural production [1]. In regions with seasonal frozen soils, soils experience multiple freeze–thaw cycles (FTCs), characterized by “daytime thawing and nighttime freezing.” These cycles disrupt the physical structure of the soil and lead to severe water loss, which profoundly impacts spring plowing and sowing activities [2]. The spring snowmelt process replenishes soil moisture, alleviating issues related to water deficits. However, the presence of a frozen soil layer inhibits the infiltration of snowmelt water, causing unabsorbed water to generate surface runoff or perched water, which can result in soil erosion or significant spring flood disasters [3]. Based on the aforementioned issues, it is of significant importance to identify suitable regulatory measures for seasonal frozen soil regions that enhance soil water storage and moisture retention capacity, improve soil permeability, and increase the utilization efficiency of snowmelt water resources. These measures are crucial for ameliorating the farmland water and soil environments in these areas.

Biochar, as a solid product, significantly alters soil characteristics due to its high porosity, specific surface area, and strong adsorption capacity [4]. The distribution of soil pore sizes and agglomerates is altered, leading to a decrease in soil bulk density and an enhancement of total soil porosity (TP) [5]. Additionally, returning straw to the field enhances soil organic matter content, improves the properties of soil aggregates, and increases the soil’s capacity to retain water and nutrients. Straw mulch can also effectively reduce evaporation from the shallow soil layer [6,7]. Furthermore, the combined application of biochar and straw is beneficial in increasing both capillary porosity and aeration pore porosity in the soil. This, along with the porous structure of biochar, enhances the soil’s water-holding capacity and minimizes water loss [7].

Soil water redistribution due to freeze–thaw cycles complicates the problem of soil water infiltration [8,9]. It has been demonstrated that the factors driving the transformation between different phases of soil water and the processes of water migration include soil temperature, water content, pore characteristics, and the location characteristics of freezing fronts [10,11]. Wu [12] studied the dynamics of freezing front movement speed under varying conditions of water content, temperature, and soil type using heat conduction theory. The study found significant influences of water content and temperature on freezing front movement speed, with the uncertainty of freezing front position consistently affecting the infiltration process of snowmelt water. Previous research on soil infiltration has predominantly focused on the quantitative analysis of infiltration mechanisms [13,14] or employed models to predict the effects of soil moisture and rainfall intensity on water infiltration [15]. Recent studies have shifted towards investigating the impact of exogenous biomass materials on soil water infiltration [16,17]. Ibrahim et al. conducted experimental studies that indicated the application of biochar significantly enhances soil water storage and moisture retention capacity, thereby improving soil infiltration capacity [18]. Zuo et al. explored the characteristics of snowmelt water transport under various gradients of biochar application, revealing that biochar application can mitigate the upward movement of deep soil water caused by freeze–thaw cycles [19]. They found that a moderate application of biochar could enhance the efficiency of snowmelt water utilization, while excessive biochar application might yield negative effects [19]. The aforementioned study effectively demonstrates the effects of biochar and straw on enhancing soil structure and water retention capacity, as well as quantitatively analyzing the infiltration characteristics of snowmelt water in cold regions when influenced by exogenous biomass materials. However, it neglects to consider the impact of freezing front migration characteristics in cold region soils under the regulation of biochar and straw on snowmelt water infiltration.

In this study, we conducted a series of experiments to investigate the characteristics of snowmelt water infiltration in freeze–thawed soils, based on the principles of freeze–thaw deformation, the functional properties of biochar and straw, and the natural conditions of snowmelt water infiltration in seasonal permafrost zones. This article aims to explore the regulation modes of biochar and straw during the freeze–thaw process: (1) characteristics of changes in soil physical structure; (2) characteristics of changes in migration of freezing fronts; (3) laws of changes in the infiltration characteristics of snowmelt water; (4) mechanisms of the influence of changes in the migration characteristics of freezing fronts on the infiltration of snowmelt water, as well as the establishment of a response relationship between the cumulative amount of infiltration and the characteristics of the migration of freezing fronts. The results of the study can provide theoretical references for the rational use of biochar and straw resources and improve the utilization efficiency of meltwater resources.

2. Materials and Methods

2.1. Test Material and Device

The test soil was taken in 2024 from Acheng District, Harbin City, Heilongjiang Province, China (127°23′44′ E, 45°50′24′ N), the soil texture is clay loam. Soil samples were taken at a depth of 0–30 cm, the sampling method used a stratified multi-point type, the soil samples were placed in a cool and ventilated place to dry, and residual debris was removed, subjected to crushing and sieving, and then saved for backup. The test straw was crushed from corn stover, and the test biochar was made from corn stover by burning under anaerobic conditions at 500–600 °C for 2 h. Both the biochar and straw were passed through a 2 mm fine sieve.

The mechanical composition of soil particles in the experimental materials was determined using a laser particle size analyzer (S3500, Microtrac Inc., Montgomeryville, PA, USA). The soil organic matter content was measured using the potassium dichromate oxidation-external heating method. Suspension mixtures were prepared at soil-to-deionized water ratios of 1:5, and biochar and straw-to-deionized water ratios of 1:10. The pH values of the soil, biochar, and straw were measured separately using a pH meter (PHS-2F, Lichen Technology Co., Shanghai, China). The carbon (C), nitrogen (N), and hydrogen (H) element contents in biochar and straw were determined using an elemental analyzer (2400II, PerkinElmer, Waltham, MA, USA). The ash content of biochar was measured by weighing after combustion in a muffle furnace at 900 °C for 4 h, while the ash content of straw was determined by weighing after combustion in a muffle furnace at 550 °C for 4 h [7]. The basic physicochemical properties of the experimental materials are presented in

Table 1. Physicochemical parameters of test materials.

Parameters	Soil	Parameters	Biochar	Corn Straw
Sand mass fraction (%)	22.84	pH value	9.14 ± 0.10	8.75 ± 0.13
Silt mass fraction (%)	35.89	C mass fraction (%)	69.60 ± 3.12	46.50 ± 2.19
Clay mass fraction (%)	27.27	N mass fraction (%)	1.28 ± 0.13	1.08 ± 0.21
Dry density (g·cm−3)	1.32 ± 0.06	H mass fraction (%)	1.93 ± 0.31	4.13 ± 0.18
Natural moisture content (%)	24.30 ± 0.22	Ash content (%)	25.18 ± 3.96	8.35 ± 3.41
Organic matter content (%)	25.86 ± 0.53	/	/	/
pH value	5.76 ± 0.12	/	/	/

.

2.2. Experimental Scheme

2.2.1. Test Apparatus

The experiment was designed with varying initial water content gradients (W1: 20%, W2: 15%, W3: 10%) and the application rates of regulating materials were established based on recent domestic and international studies regarding the ratios of biochar and straw in soil research and their effects [7,19]. Four treatment groups were implemented: B, the application rate of biochar to soil mass ratio is 1.0%; J, the application rate of straw to soil mass ratio is 0.5%; BJ, which combined a biochar application at 1.0% and a straw application at 0.5%; and the control treatment group, CK. Soil columns for the test were prepared using backfill, designed as follows: the dimensions of the Plexiglas box (PVC) were 20 cm × 20 cm × 65 cm, with the test soil filled to a height of 60 cm, resulting in a total soil weight of 35 kg per box. Soil moisture sensors were installed at 8 cm intervals (CR200, Beijing, China) to accurately explore the migration characteristics of freezing fronts. Additionally, soil temperature chain sensors were installed at 4 cm intervals (PTWD-2A, Liaoning, China), with a data collector recording measurements at 1 min intervals. To efficiently mimic the unidirectional freezing of soil from the upper layer downward and to avoid energy transfer between the lateral and bottom soil and the external environment, a 50 mm thick rubber insulating material (made from rubber and PVC, exhibiting a thermal conductivity of 0.018 W/(m·K)) was encased around the sides and base of the Plexiglas box. The test setup is illustrated in Figure 1. The test soil columns were uniformly compacted and filled in layers to ensure a consistent compaction density. Subsequently, they were transferred to an artificial climate chamber, sealed with plastic film, and cured at room temperature for a duration of 30 days. A total of 8 FTCs were conducted during the experiment, with each freeze–thaw cycle lasting 96 h (48 h of freezing at −20 °C, followed by 48 h of thawing at 10 °C). At the conclusion of the soil freeze–thaw cycle, stratified samples were collected from depths of 15 cm and 30 cm in the soil column using a soil collector. These samples were designated as L1 and L2, respectively, and the relevant indices were measured within 24 h.

2.2.2. Experimental Design

To better simulate the unpressured infiltration process of soil during the spring snowmelt period, we selected the artificial precipitation method for the infiltration test. The snowmelt equivalent was established based on typical annual average snow depth data for Heilongjiang Province, China, from 2016 to 2020, as provided by the National Meteorological Science Data Center, and was subsequently converted into the corresponding water volume. During the experiment, unit hourly precipitation was employed to simulate the daily scale snowmelt water infiltration process. A sprayer was utilized to evenly distribute the water over the soil surface, with water change data recorded every 10 min, and the total duration of the experiment was 12 h. The initial temperature of the test was set at 0 °C, while the final temperature reached 10 °C. The temperature change was automatically regulated by an artificial climate chamber. Based on pre-tests and previous studies [20,21], we assumed that the soil temperature of −0.5 °C indicates the position of the freezing front, which reflects changes in the location of this front. Additionally, considering that the soil transitions from a frozen to a non-frozen state, the frozen layer inhibits soil evaporation. Given the short duration of the test and the minimal evaporation [22], soil evaporation was not considered in this experiment. Three replicate trials were conducted for each treatment.

2.3. Research Methodology

Soil water-stable agglomerate distribution characteristics determined by wet sieve method [23], and four indicators of soil agglomerate stability, mean weight diameter MWD (mm), geometric mean diameter GMD (mm), >0.25 mm water-stable agglomerate content WR > 0.25 (%), and percentage of agglomerate destruction PAD (%), were selected to evaluate the stability of soil agglomerates [24].

$$\mathrm{MWD}=\sum _{\mathrm{i}=1}^{\mathrm{n}}\overline{{\mathrm{x}}_{\mathrm{i}}}{\mathrm{W}}_{\mathrm{i}}$$

(1)

$$\mathrm{GMD}=\exp \sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{W}}_{\mathrm{i}}\ln \overline{{\mathrm{x}}_{\mathrm{i}}}$$

(2)

$${\mathrm{WR}}_{0.25}=\left[1-{\displaystyle \frac{{\mathrm{M}}_{\mathrm{x}<0.25}}{{\mathrm{M}}_{\mathrm{T}}}}\right]$$

(3)

$$\mathrm{PAD}={\displaystyle \frac{{\mathrm{DR}}_{0.25}-{\mathrm{WR}}_{0.25}}{{\mathrm{DR}}_{0.25}}} \times 100\%$$

(4)

where $\overline{{\mathrm{x}}_{\mathrm{i}}}$ represents the mean diameter of the i-th grain level (in millimeters), Wi indicates the percentage of agglomerates that correspond to $\overline{{\mathrm{x}}_{\mathrm{i}}}$ (%), M_x<0.25 refers to the weight of agglomerates smaller than 0.25 mm (in grams), M_T denotes the total weight of the agglomerate (in grams), DR_0.25 signifies the fraction of mechanically stable agglomerates larger than 0.25 mm as determined by dry sieving (%), and x_max is the largest particle size of the agglomerates (in millimeters).

The soil moisture characteristic curves for various treatments were acquired utilizing a high-speed refrigerated centrifuge (CR21GIII, Hitachi, Tokyo, Japan), and the soil pore size distribution was calculated indirectly using the soil moisture characteristic curves. The relationship between suction h and pore diameter d can be expressed as

$$\mathrm{h}=4\mathsf{\sigma }/\mathrm{d}$$

(5)

where σ is the coefficient of surface tension of water, and generally at room temperature is 75 × 10⁻⁵ N·cm⁻¹. In relation to the suction range examined in this study, the corresponding pore size was categorized into six segments based on size: <0.3 μm (very micro pore), ≥0.3–5 μm (micro pore), ≥5–30 μm (small pore), ≥30–75 μm (medium pore), ≥75–100 μm (large pore), >100 μm (soil void) for analysis.

The infiltration test was conducted using a one-way jet sprayer with a nozzle outlet hole for the movable orifice plate. Initially, the soil was frozen, preventing water from flowing through the frozen layer, which resulted in the retention of precipitation in the surface layer of the soil. The change in the depth of the surface layer was recorded at 2 min intervals at the beginning of the test, while the change in water volume was recorded at 10 min intervals during the middle and later stages. Once the water had completely infiltrated the soil, the required time and infiltration amount were recorded, and the next precipitation experiment was conducted until the end of the experiment. The total precipitation amount from each time period represents the accumulated infiltration amount of the soil.

2.4. Statistical Methods

Data analysis was conducted using SPSS (IBM, SPSS Statistics 26, Armonk, NY, USA). One-way ANOVA and Least Significant Difference (LSD) methods were employed to assess differences and significance among treatments, with a significance level set at p < 0.05. Various lowercase letters were employed to represent significant differences among the treatments. The Shapiro–Wilk test was conducted to assess the normality of the distribution of the data. The correlation between the characteristics of soil physical structure, freezing front migration, and snowmelt infiltration was estimated using Pearson’s correlation coefficient. Graphical plotting was conducted using Origin 2024.

3. Results

3.1. Effects of Different Modes of Regulation on Soil Physical Indexes During Freeze–Thaw Cycles Process

3.1.1. Characterization of Changes in Soil Aggregate Stability

The variation characteristics of soil agglomerate stability indicators after different regulation treatments are shown in Figure 2. GMD, MWD, and WR > 0.25 increased and PAD decreased under B, J, and BJ treatments compared to CK. GMD increased by 20.06–50.57%, MWD increased by 18.75–43.77%, WR > 0.25 increased by 36.76–79.41%, and PAD decreased by 12.46–25.81% under W1, where soil agglomerate stability was higher under BJ treatment. However, only treatment B in W2 and W3 showed significant changes in each indicator (p < 0.05), which indicated that the application of biochar was the main factor in improving the stability of soil aggregates with low water content. Moreover, as the initial moisture level of the soil reduced, the stability of the soil aggregates also diminished. The reduction in GMD by 14.28–28.57%, 16.67–25.11%, and 33.33–40.28% under treatments B, J, and BJ (Figure 2a), the application of biochar mitigates to some extent the potential for destabilization of soil aggregates. In addition, GMD, MWD, and WR > 0.25 gradually increased with increasing soil depth, and the change trend was consistent with the L1 layer. Based on the analysis presented, when biochar and straw were applied together, the soil exhibited greater agglomerate stability in conditions of relatively high soil water content (W1, W2). In contrast, the use of biochar alone was more effective in conditions of low soil water content (W3).

3.1.2. Characterization of Soil Pore Space Changes

The soil pore size distribution under different treatments is shown in Figure 3. The application of biochar and straw reduced the proportion of soil voids (>100 μm) and very micro pore (<0.3 μm) and increased the proportion of small pore sizes (5–30 μm) and micro pore sizes (0.3–5 μm). B, J, and BJ treatments under W1 reduced soil voids by 38.51%, 41.28% and 53.44%, reduced very micro pore size by 15.24%, 10.47%, and 15.30%, increased small pore size by 58.25%, 0.01%, and 60.17%, and increased micro pore size by 26.69%, 54.32%, and 77.71%, respectively, compared to CK, this indicates that the combined application of biochar and straw significantly affected the soil pore size distribution. However, the magnitude of change in large and medium pore sizes increased in the B treatments under W2 and W3, driving the transformation of pore sizes from large to small in W2 and W3, indirectly enhancing the water conductivity of the soil. In addition, with the increase in soil depth, the change trend of L2 layer is approximately the same as that of L1 layer, but the change in very small pore size decreases and the change in small pore size and micro pore size increases. In addition, as the initial water content of the soil decreased, the change trend of each pore size was basically the same and the change amplitude decreased; however, the medium pore size of W3 showed an increasing trend with an increase of 24.03% in treatment B (Figure 3c).

3.2. Characteristics of Changes in Freezing Fronts Under Different Modes of Regulation During the Freeze–Thaw Cycle Process

The variation characteristics of frozen front migration in soil under different regulation treatments are illustrated in Figure 4. During the top-down thawing process, treatments B, J, and BJ resulted in an increased migration depth of the soil frozen front by 19.44% to 53.66%, 5.56% to 70.02%, and 30.03% to 78.05%, respectively, compared to the control (CK). Treatment J significantly influenced W1, while treatment BJ exhibited notable effects on W2 and W3. This indicates that the application of biochar and straw promotes the transformation of solid soil ice into liquid water and increases the migration depth of soil freezing fronts. Furthermore, as the initial soil water content decreased, the trend of change was consistent across treatments B, J, and BJ, with the greatest increase observed at 15% initial soil water content for B and BJ treatments, and at 20% initial soil water content for the J treatment. The application of biochar and straw significantly enhanced the mean migration velocity of the freezing front. Specifically, for W1BJ, the mean migration velocity increased from 0.26 cm/h to 0.37 cm/h, while for W2BJ, it rose from 0.34 cm/h to 0.35 cm/h. Additionally, as the initial water content of the soil decreased, the average migration rate of freezing fronts exhibited a similar trend to that of the migration depth; however, the variation amplitude decreased. All treatments in W3 showed a decrease, with reductions of 11.36%, 9.09%, and 15.91% for treatments B, J, and BJ, respectively. This indicates that the combined application of biochar and straw under low moisture content (W3) conditions inhibited the migration of freezing fronts.

During the bottom-up thawing of the soil (Figure 4d–f), W1 and W2 exhibit a similar trend. Treatments B, J, and BJ increase the average migration rate of the freezing front by 12.40% to 80.00%, 40.00% to 85.74%, and 28.57% to 53.33%, respectively. Additionally, they enhance the migration depth of the freezing front by 73.33% to 120.59%, 76.47% to 193.33%, and 73.33% to 73.52%. Notably, the maximum migration depth of the freezing front was observed in the W1J treatment (Figure 4d) and the W1B treatment (Figure 4f), indicating that the application of biochar and straw significantly influenced the migration characteristics of the bottom freezing front. Conversely, when the initial water content of the soil was reduced to W3, the average migration rate of the freezing front increased by only 12.50% for treatment B, while the migration depth decreased by 4.46%. In contrast, for treatments J and BJ, the average migration velocity of the frozen fronts decreased, whereas the migration depth increased.

3.3. Effects of Different Modes of Regulation on Snowmelt Water Infiltration During Freeze–Thaw Cycles Process

3.3.1. Characteristics of Soil Cumulative Infiltration with Time

The variation in cumulative soil infiltration over time for different regulation modes is illustrated in Figure 5. The application of biochar and straw significantly enhanced cumulative soil infiltration in treatments W1 and W2. Compared to the control group (CK), treatments B, J, and BJ increased cumulative soil infiltration by 12.02% to 19.99%, 16.11% to 23.22%, and 29.03% to 35.25%, respectively. Notably, the overall trends in treatments B and J were similar over the same time scale, with treatment BJ exhibiting the highest cumulative infiltration. Furthermore, as the initial soil water content decreased, cumulative soil infiltration also diminished across all treatments, although the rate of change was less pronounced. Interestingly, the W3BJ treatment appeared to inhibit the infiltration process of the snowmelt water fraction during the first 0–7 h, demonstrating a trend of W3CK < W3B < W3J < W3BJ. However, the cumulative infiltration in W3BJ exhibited a smooth increase, peaking around the 8th hour, while W3CK, W3B, and W3J continued to rise over time.

3.3.2. Characteristics of Soil Infiltration Rate Variation

The variation curves of soil infiltration rates over time under different regulation modes are illustrated in Figure 6. The soil infiltration rate generally exhibits an increasing trend followed by a decrease, with the peak infiltration rate for each treatment occurring earlier as the initial soil water content decreases. Compared to the control (CK), treatments B, J, and BJ enhanced peak soil infiltration rates by 7.71% to 38.93%, 19.67% to 38.40%, and 28.95% to 58.97%, respectively, with treatment BJ demonstrating a significant increase in peak soil infiltration rates. Furthermore, the effects of treatments BJ and J diminished as the initial water content of the soil decreased, resulting in reductions in peak soil infiltration rates of 8.72% and 20.37%, respectively; in contrast, treatment B exhibited an increase of 47.18%. Research indicates that biochar positively influences the infiltration process in soils with low water content, surpassing the effects of treatments J and BJ. This enhancement is likely due to the application of biochar, which improves the connectivity of soil pores, thereby facilitating the mobility of soil water and promoting a faster infiltration process [24,25]. Additionally, around 450 min, the soil infiltration rate gradually stabilizes, following the trend W1 > W2 > W3. This indicates that the infiltration of snowmelt water is influenced by various factors, including initial water content, pore connectivity, and agglomerate distribution. Notably, each curve displays a minor peak following the highest peak, with the occurrence of this minor peak tending to happen earlier as the initial soil moisture content decreases. The frequency of the minor peak’s occurrence increases with lower initial soil moisture levels.

3.3.3. Characterization of Changes in Soil Profile Water Content

The temporal variations in soil liquid water content in the 0–30 cm layer, under different regulation modes and initial water contents, are illustrated in Figure 7. The period from 0 to 8 h represents the soil freezing stage, while the period from 8 to 20 h corresponds to the soil thawing stage, which also encompasses the duration of the soil infiltration test. The water content of each soil profile in the B, J, and BJ treatments exhibited a general trend of gradual decline, interspersed with fluctuations, followed by another slow decline. During the soil freezing period, the treatments demonstrated an overall slow decreasing trend, with variation diminishing as soil depth increased. In the soil melting period, the soil water content experienced significant changes, increasing by 5.33% to 9.50%, 6.28% to 32.89%, and 10.33% to 25.66% in treatments B, J, and BJ, respectively, compared to CK. Furthermore, the magnitude of changes in J and BJ treatments decreased significantly with lower initial soil water content, indicating that the application of biochar slowed the migration and diffusion of infiltrated water. Following the infiltration test, a comparison with pre-infiltration soil moisture content revealed that the moisture content of the W1 soil profile decreased overall by 1.06% to 14.95%, with the W1J treatment exhibiting the most significant reduction. This suggests that the application of straw can facilitate the transformation between solid and liquid water within the soil, thereby enhancing the downward movement of water. Conversely, the liquid moisture content of the W2 soil increased by 27.28% to 60.19%, with the W2B treatment demonstrating the highest increase. Similarly, the liquid moisture content of the W3 soil rose by 25.84% to 80.55%, with the W3B treatment showing the largest increase. These results indirectly indicate that the application of biochar possesses water-retention properties, which, to some extent, inhibit the transfer and diffusion of soil moisture.

3.4. Analysis of Factors Influencing Soil Infiltration Characteristics

3.4.1. Correlation Analysis

The factors influencing soil infiltration characteristics are presented in

Table 2. Analysis of the relationship of the characteristics of soil infiltration with the soil structure and parameters of the freezing.

Characteristic Parameter	Soil Cumulative Infiltration	Soil Infiltration Rate
L1-MWD (mm)	0.231	0.352
L1-WR > 0.25 (mm)	0.581 *	0.008
L1-Soil void (>100 μm)	0.334	−0.349
L1-middle pore (0.3–30 μm)	0.632 *	−0.499
L2-MWD (mm)	0.328	0.289
L2-WR > 0.25 (mm)	0.392	−0.061
L2-Soil void (>100 μm)	0.320	−0.315
L2- middle pore (0.3–30 μm)	0.590 *	−0.479
V-upper (cm/min)	0.647 *	−0.598 *
V-lower (cm/min)	0.706 *	−0.640 *
D-upper (cm)	0.654 *	−0.609 *
D-lower (cm)	0.546	−0.577 *

Notes: ‘V’ denotes the average migration velocity of freezing fronts; ‘D’ denotes the depth of freezing front migration; ‘*’ indicates significant differences (p < 0.05) between soil physical structure characteristics, freezing front migration characteristics with infiltration characteristics.

. The average migration velocity of the freezing front exhibits a significant positive correlation (p < 0.25) is positively correlated with soil infiltration, with correlation coefficients of 0.581 and 0.392. The analysis of different regulation modes reveals that the migration characteristics of the freezing front and the content of macroaggregates significantly affect soil infiltration characteristics, while MWD and soil voids do not show significant correlations with soil infiltration characteristics.

3.4.2. Response of Cumulative Infiltration to Freezing Fronts

The relationship between cumulative soil infiltration and the depth of frozen front migration under different regulation treatments is illustrated in Figure 8. This study establishes a functional relationship between these two parameters, achieving curve fitting accuracy ranging from 0.834 to 0.997. The fitting accuracy observed for the B, J, and BJ treatments was generally superior to that of the CK treatment. In particular, the pattern of curve alterations across the various treatments was nearly identical, with cumulative soil infiltration presenting a quadratic function that diminishes as the depth of the frozen front migration increases. As soil water content increases, the trend observed in the BJ treatment becomes more pronounced, following the order BJ > J > B > CK. This indicates that the combined application of biochar and straw enhances the relationship between cumulative soil infiltration at higher water content and the depth of the freezing front migration. The change in cumulative infiltration of W3 soil shifted from CK > J > BJ > B to B > BJ > J > CK, indicating an increase in the sensitivity of W3B. This implies that using biochar by itself may improve the connection between the cumulative infiltration in soil with low moisture content and the movement depth of the frozen front.

4. Discussion

4.1. Relationship Between Physical Properties of Soils and Infiltration Characteristics of Snowmelt Water

Freeze–thaw cycles disrupt soil structure, thereby affecting the stability of aggregates, which in turn causes dynamic changes in the soil’s infiltration capacity [13]. The application of biochar and straw increases the content of soil macromolecules and enhances soil structural stability [26], as shown in Figure 9, promoting the soil water infiltration process. The content of water-stable aggregates greater than 0.25 mm is significantly and positively correlated with cumulative soil infiltration (p < 0.05), making it an important factor influencing the infiltration of snowmelt water into the soil (Table 2). As the initial soil water content decreases, the combined application effect gradually diminishes. In contrast, the application of biochar in low water content soils proves beneficial in improving the stability of soil aggregates (Figure 2), consequently altering cumulative soil infiltration (Figure 5). In other words, the cumulative soil infiltration from the combined application is highest under conditions of elevated water content. However, under low water content conditions (Figure 5c), it is the application of straw that facilitates greater water infiltration into the soil. This suggests that the influence on the soil infiltration process is not solely determined by the content of soil macroaggregates but may also be affected by the structural differences between biochar and straw. Biochar, as a porous granular material, significantly enhances the soil’s water-holding capacity [27] and inhibits the downward migration of pre-existing soil moisture, resulting in reduced replenishment of soil moisture from snowmelt water. Furthermore, freeze–thaw actions damage soil macroaggregates, leading to the release of stored water within these structures, which in turn affects the transition and migration of soil water phases. Concurrently, the active microbial community within the soil consumes substantial amounts of soil nutrients, thereby diminishing the stability of soil aggregates and impeding soil infiltration [28]. The application of appropriate amounts of biochar and straw can facilitate the enrichment of small soil aggregates and microaggregates into larger aggregates. This process not only improves soil aggregate stability but also enhances the soil’s capacity for water migration.

The application of biochar and straw increases the proportion of small and micropore sizes in the soil (Figure 3), as shown in Figure 9. This enhancement of soil hydraulic conductivity [29] and the reduction in soil voids make it less likely for excess infiltrated water to refreeze along preferred pathways, thereby increasing soil infiltration capacity [30]. Two phenomena may arise from this: (1) The increase in small and micropore sizes reduces the possibility of water vapor conversion, thereby diminishing soil aeration and drainage. This can lead to increased evaporation from the topsoil layer, causing water to accumulate at the soil surface and form thin ice in low-temperature environments, which inhibits water infiltration; (2) The application of biochar and straw affects heat transfer within the soil, increasing the temperature of the surface layer and accelerating the melting process. As the temperature rises, solid ice in soil pores transitions to liquid water, while the large and medium-sized pores have not been promptly restored to their pre-freeze small pore state, thus re-increasing the infiltration capacity of the soil. Additionally, soil moisture is a crucial factor affecting the infiltration process [31]. During soil freezing, varying initial water contents lead to different internal ice contents, resulting in significant differences in delayed water entry into the soil (Figure 5). The trend observed indicates that higher soil water content correlates with lower cumulative infiltration amounts. We conjecture that the primary reason for this lies in internal factors: the presence of numerous ice crystals reduces the cross-sectional area of the soil drainage zone, which directly impacts the flow channels of soil water and inhibits the infiltration of snowmelt water. Furthermore, external interference plays a role: the combined application of biochar and straw introduces a high number of particles that occupy soil pores. In conditions of low soil moisture content, this makes it challenging for water to penetrate past the biochar and straw particles, preventing the aggregation of fine pores into larger ones [32,33].

4.2. Relationship Between Freezing Front Variation and Infiltration Characteristics of Snowmelt Water

The development of the frozen soil layer, influenced by temperature potential changes, varies across different regulatory modes; specifically, the location of the freezing front is a critical factor. The presence of ice crystals within the pores of frozen soil leads to reduced hydraulic conductivity and diminished infiltration capacity [34,35]. During the soil melting process, both air temperature and the soil temperature gradient exhibit step changes with soil depth. As the temperature gradually increases, the freezing front migrates downward, resulting in a gradual thinning of the frozen layer and the downward movement of snowmelt water [36], as shown in Figure 9. The presence of frozen fronts contributes to low soil permeability [37], while the migration of the freezing front fundamentally involves the gradual melting of free water, including gravitational water, film water, and hygroscopic water, within the soil [38].

The migration depth and average migration rate of freezing fronts significantly affect the variation in cumulative soil infiltration (p < 0.05). The use of biochar and straw positively influences the movement of the freezing front in soils that have elevated moisture levels (Figure 4). This phenomenon occurs because biochar and straw application can suppress soil energy dissipation to some extent. Research by Xiu et al. [39] concluded that biochar enhances soil color, absorbs light and heat, promotes soil energy accumulation, and achieves a ‘warming effect’ at low temperatures. Consequently, the average migration rate of freezing fronts in treatments B, J, and BJ, under the same initial water content conditions, is higher than that of CK. This, in turn, leads to a significant increase in cumulative soil infiltration, which can enhance cumulative soil infiltration by 12.02% to 35.25%. Additionally, the application of biochar and straw fills soil pores with these materials, reducing the original solid ice content. This increase in the average migration rate of the soil freezing front subsequently enhances the soil infiltration rate and accelerates the infiltration process. However, the application of biochar and straw at low soil water content inhibits the migration of freezing fronts, which negatively impacts cumulative soil infiltration (Figure 5c). Notably, the cumulative soil infiltration was significantly reduced under the BJ treatment. We conjecture that the combined application of biochar and straw in low water content soils facilitates the cementation of non-colloids due to their physical properties [32,40]. This combination increases the ratio of water retention (WR) greater than 0.25, causing larger agglomerates to break down into smaller ones. This breakdown blocks the pore space under the influence of internal soil water and soil snowmelt [41], resulting in a slower melting of the freezing layer, a decreased migration rate of the freezing front, and a reduced soil infiltration rate.

The migration characteristics of the freezing front significantly influence the changes in water content within the soil profile. As the freezing front migrates downward more rapidly, the depth of migration increases (Figure 4a), leading to greater variations in soil moisture content (Figure 7a). This phenomenon can be attributed to water infiltration into the soil, which elevates the liquid water content; however, sustained low-intensity water infiltration is more conducive to deeper soil infiltration [42,43]. Consequently, the water content of the soil profile exhibits a tendency to decrease, a process that is facilitated by the application of straw. Conversely, the application of biochar has a different effect, resulting in an increase in soil liquid water content. This may be due to the application of biochar under low water content conditions, which enhances the proportion of small and micropores in the soil [44], allowing for water retention until the critical water content is reached, thus preventing a decrease in soil profile water content. Additionally, the presence of thin substrate ice may obstruct the pores, hindering water infiltration into the deeper soil layers [42]. The cumulative infiltration observed under the biochar treatment was lower. Furthermore, the delay in the migration of the freezing front during the experiment led to a corresponding delay in changes in the soil profile water content (Figure 7), which can be attributed to the freezing of previously water-filled soil pores.

Roy et al. [45] and Du et al. [46] pointed out that high water content leads to a low infiltration rate, and that the infiltration rate decreases more slowly with multiple freeze–thaw cycles in the soil. The results of the current study do not fully align with this finding, as they show that the use of biochar and straw enhances soil infiltration rates to a certain degree. Notably, only the control group (CK) exhibited the phenomenon of high water content coupled with a low infiltration rate. This may be attributed to the disturbance of soil particle distribution following the application of biochar and straw [24,25]. The observed decrease and subsequent increase in infiltration rates may be due to the initiation of macropore flow in the upper layers of the soil, where shallow soil serves as the preferred pathway for infiltration [47]. Furthermore, there may be concurrent infiltration of meltwater from the upper soil layers and infiltration water into deeper layers, indirectly enhancing the volume of water infiltrating into these deeper strata. Based on this study, we suggest that the infiltration of snowmelt water in seasonally frozen soils is influenced by various factors, including exogenous biomass materials, soil physical structure (such as pore size distribution), initial soil moisture content, and the migration characteristics of freezing fronts. While the application of biochar and straw enhances snowmelt water utilization, it is important to note that greater quantities of biochar and straw do not necessarily correlate with higher efficiency in snowmelt water utilization.

5. Conclusions

The application of biochar and straw significantly impacts soil physical properties and the migration characteristics of the freezing front in the seasonal frozen soil region of Northeast China, thereby influencing the infiltration process of snowmelt water. The results indicate that the application of biochar and straw mitigates the damage caused by freeze–thaw cycles (FTCs) to soil physical structure, increases the geometric mean diameter (GMD), mean weight diameter (MWD), and the proportion of water-stable aggregates greater than 0.25 mm (WR > 0.25), thereby enhancing soil aggregate stability. Furthermore, the application of biochar and straw modifies pore distribution by increasing the proportion of small and micropores in the soil, which indirectly improves soil hydraulic conductivity. As the initial soil water content decreases, the combined application effect weakens, while the impact of biochar application becomes more pronounced. Additionally, under high water content conditions, the application of biochar and straw increases cumulative soil infiltration and enhances the infiltration rate; conversely, under low water content conditions, their combined application inhibits the infiltration process of snowmelt water. The application of biochar can strengthen the relationship between cumulative infiltration and the migration characteristics of the freezing front in low-moisture-content soils. Based on soil physical properties and snowmelt water infiltration characteristics, it is recommended to apply a combination of straw and biochar for high-moisture-content soils, while biochar alone should be applied for low-moisture-content soils.