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Article

Freeze–Thaw-Driven Dynamics of Soil Water–Salt and Nitrogen: Effects and Implications for Irrigation Management in the Hetao Irrigation District

by
Weili Ge
1,2,
Jiaqi Jiang
1,
Chunli Su
1,*,
Xianjun Xie
1,
Qing Zhang
2,
Chunming Zhang
3,
Yanlong Li
2,
Xin Li
2,
Jiajia Song
2 and
Yinchun Su
2
1
School of Environmental Studies, China University of Geosciences, Wuhan 430078, China
2
Geological Survey Academy of Inner Mongolia Autonomous Region, Huhhot 011020, China
3
Inner Mongolia Geo-Environmental Ecology Technology Co., Ltd., Hohhot 010020, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(20), 2991; https://doi.org/10.3390/w17202991
Submission received: 12 September 2025 / Revised: 11 October 2025 / Accepted: 14 October 2025 / Published: 16 October 2025
(This article belongs to the Section Soil and Water)
Browse Figures
Versions Notes

Abstract

This study investigated the mechanisms of soil water–salt and nitrogen transport and optimal strategies under freeze–thaw (F-T) cycles in the salinized farmlands of the Hetao Irrigation District. A combined approach of field monitoring and laboratory simulation, utilizing both undisturbed and repacked soil columns subjected to 0–15 F-T cycles and five irrigation treatments, was employed to analyze the spatiotemporal dynamics in Gleyic Solonchaks. The results demonstrated that freeze–thaw processes play an important role in salt migration in surface soil layers, driving salt redistribution through phase changes of soil moisture. Increased freeze–thaw cycles reduced surface soil moisture content while promoting upward salt accumulation, salt dynamics exhibited pronounced spatial heterogeneity and irrigation source dependency, and the surface layer exhibited lower salinity levels after irrigation compared to pre-irrigation levels. These cycles also enhanced short-term soil nitrogen transformation and facilitated inorganic nitrogen accumulation. Different irrigation regimes exhibited a significant impact on the dynamics of water–salt and nitrogen in soil, with low-salinity treatment (S2) and moderate-nitrogen irrigation (N2) effectively reducing surface salt accumulation while improving nitrogen utilization efficiency (moderate-nitrogen irrigation exhibited higher mineralization rates, which facilitated the release of inorganic nitrogen from soil). This study reveals the synergistic transport mechanisms of water–salt and nitrogen under freeze–thaw driving forces and provides a scientific basis and practical pathway for sustainable agricultural management in cold arid irrigation districts.

Graphical Abstract

1. Introduction

Soil salinization has become a critical environmental issue that constrains agricultural production, threatens ecological security, and hinders regional sustainable development [1,2]. In cold and arid regions, unique climatic conditions (low temperatures, aridity, and intense evaporation) and geological structures have shaped distinct seasonal freeze–thaw-induced salinization zones, leading to salt accumulation at the soil surface and declining cropland productivity [3,4]. Soil salinity is a key factor influencing soil fertility. Salts in soil can induce soil compaction, inhibit nitrogen transformation, and reduce nitrogen fertilizer efficiency [5,6]. The nitrogen cycle exhibits high sensitivity to the dual stress of freeze–thaw and salinity: its microbially driven transformation processes (mineralization, nitrification, and denitrification) are susceptible to structural disruption and redox fluctuations induced by freeze–thaw cycles [7,8], while high salinity specifically inhibits nitrification [9]. Simultaneously, as the primary limiting nutrient for local crop production, nitrogen availability and use efficiency are directly linked to agricultural sustainability [10], and it possesses management advantages for loss control through irrigation regulation. Compared to other nutrients such as soil organic carbon, potassium, and phosphorus, nitrogen responds more sensitively and directly to irrigation management, making it the most operational indicator for evaluating freeze–thaw effects and irrigation optimization.
As a core driver of vadose zone material transport in cold arid regions, freeze–thaw cycles reshape the soil water potential field through water-ice phase transitions, profoundly governing the spatiotemporal patterns of water–salt–nitrogen migration. Repeated freeze–thaw cycles may exacerbate soil salt accumulation [11]. Water–salt migration manifests as capillary-driven upward salt transport during freezing (forming surface salt accumulation) and evaporation-induced secondary salinization during thawing [12], with salt dynamics regulated by initial salinity gradients, co-saturation points, and ice lens distribution, resulting in heterogeneous salt accumulation or leaching [13,14]. However, nitrogen migration and transformation exhibit more complex biogeochemical coupling mechanisms. Freeze–thaw alternations destroy soil microstructure to release colloid-bound nitrogen [15,16]. Seasonal freeze–thaw cycles have been reported to change the forms of soil nitrogen and enhance plant nutrient absorption, as they can stimulate the mineralization of organic nitrogen, resulting in increased concentrations of NH4+-N and NO3−-N, which are readily absorbed by plants [17,18]. Some scholars have conducted extensive research on soil freeze–thaw cycles and proposed some foundational theories and diverse ideas concerning the migration of water, salt, and nitrogen in the soil during these cycles [14,19]. Notably, both water–salt and nitrogen migration processes are constrained by factors (such as soil aggregate stability, freeze–thaw frequency, temperature, and irrigation water quality), exhibiting complex spatiotemporal heterogeneity [20]. However, current understanding of multiphase interfacial water–salt–nitrogen migration and its response mechanisms to freeze–thaw cycles remain limited, necessitating further exploration through multiscale observations and freeze–thaw simulation experiments.
Hetao Irrigation District is located in the west of Inner Mongolia Autonomous Region, Northern China, with a typical cold and arid climate, where agriculture heavily relies on the Yellow River water for irrigation purposes. In the past few decades, flood irrigation has been predominantly adopted for agricultural practices, which has caused severe soil secondary salinization [21]. Long-term flood irrigation and underdeveloped drainage systems have elevated the groundwater level, and intensive evaporation subsequently results in the upward migration of moisture and salt in the vadose zone, culminating in high salinity in the surface soil [22,23,24]. Periodic irrigation has been demonstrated to facilitate the downward migration of salt in soil to a certain extent [25,26]. In the study area, large-scale flood irrigation was carried out in early winter to leach soil salinity. Typical seasonal freeze–thaw cycles can significantly change soil dynamics [21]. The interplay between seasonal freeze–thaw cycles and daily water-heat fluctuations complicate the fate of the salinization process and nitrogen migration in the soil. Research indicates that the accumulation of total organic carbon (TOC) and total nitrogen (TN) in the soil during freezing periods is significantly correlated to water and salt accumulation, which are influenced by gradients in temperature and water potential [22]. Attempts have been made to prevent secondary salinization of soil, including lowering groundwater levels [27]. However, research on the impacts of seasonal freeze–thaw on the migration of salt and nitrogen remains limited, especially understanding of the long-term effects of flood irrigation combined with freeze–thaw processes on soil ecology. Previous research has predominantly focused on the isolated aspects of water, salt, or nitrogen migration under freeze–thaw conditions, with insufficient attention to their interrelated dynamics in areas prone to salinization. In particular, the effectiveness of irrigation for salt leaching and its impacts on soil ecology have been scarcely studied in salinized regions.
Based on the above, this study proposes the following hypotheses: freeze–thaw cycles drive the upward migration of water and salts as well as the transformation of nitrogen forms (H1); irrigation water quality can affect salt accumulation and nitrogen use efficiency (H2); and freeze–thaw–irrigation interactions jointly regulate the transport processes of water, salt, and nitrogen (H3). In order to fill these gaps and verify the research hypothesis, this study aims to elaborate the effects of freeze–thaw cycles and irrigation regimes on soil salt and nitrogen migration in an area prone to soil salinization through experiments. The main objectives included (1) Identifying the migration patterns of water–salt–nitrogen in the vadose zone under freeze–thaw cycles; (2) investigating the effects of different irrigation conditions (including varying salinity levels and nitrogen application rates) on water–salt and nitrogen migration during freeze–thaw processes; and (3) unravelling the mechanism of soil salinity and nitrogen transport under freeze–thaw cycles and further exploring optimal irrigation strategies for agricultural practices. The findings are expected to offer valuable insights into the ecological processes occurring within saline soil areas during seasonal freeze–thaw cycles, serving as guidance for evidence-based irrigation and ecological environmental protection in the study area.

2. Materials and Methods

2.1. Study Area

The study area was located in Hanggin Rear Banner in the middle of Bayannur City, Inner Mongolia Autonomous Region, Northern China (Figure 1). It is characterized by a continental arid and semi-arid climate, with an average annual precipitation of 136.5 mm concentrated mainly from July to September. The average annual evaporation rate can be as high as 2306.5 mm [23]. Soil freezing commences in mid- to late-November, with the freezing depth reaching approximately 0.7–1.3 m within the Hetao irrigation area [28,29]. Thawing starts in mid- to late-April of the following year, resulting in a freezing period exceeding 150 days.
The Yellow River flows through the southern edge of the Hetao Irrigation District and serves as the main source of surface water. The area of salinized cultivated land is 394,000 hm2, accounting for 68.65% of the total cultivated land area in the study area [30]. The primary crops cultivated are Zea mays and Helianthus annuus. Compared to the spring, soil salinity is slightly higher in the summer and lower but more spatially distributed during autumn. Saline soil predominantly occurs along the main channel of the Yellow River, extending towards the main drainage channel and encircling the natural lakes. Salt accumulation is more pronounced in low-lying areas and less severe on slopes. The degree of soil salinization in the study area varies significantly, with salt content ranging from 0.02% to 4.27%. The study area has historically employed flood irrigation as a method to conserve water, sustain soil moisture, and facilitate salt leaching. Large-scale flood irrigation, known as autumn watering, is usually conducted from mid-October to mid-November and results in an elevated groundwater table before freezing, at depths ranging from 0.5 to 1.5 m [31].

2.2. Methodology

The experimental design was structured to systematically test the three core hypotheses proposed above, H1, H2, and H3.

2.2.1. Sample Collection and Analysis

A test site was established for the freeze–thaw simulation experiments in Yongfeng Village, Manhui Town, Hanggin Rear Banner (107.182° E, 40.993° N) (Figure 1). The topography of the test site is flat, with relief variation within 0.5 m. It spans an area of 9080 m2 (113.5 m × 80 m) with an average elevation of 1034 m. To investigate the influence of seasonal freeze–thaw on soil properties (addressing H1), soil samples were collected from the vadose zone in September 2022 (autumn, pre-winter freeze–thaw period) and April 2023 (spring, post-winter freeze–thaw period), for comparative studies with winter soil samples that underwent freeze–thaw cycles (simulated using collected undisturbed soil columns under controlled freeze–thaw conditions). Samples were collected using the 5-point method in the test site, with a thin layer of surface soil (approximately 1–2 cm) along with the weed vegetation removed. A soil sampling auger was used to collect samples at intervals of 20 cm down to the depth of the shallow groundwater table (reaching depths of 100 cm in autumn and 140 cm in spring). Some freshly collected samples were separated into subsamples for measuring soil pH, electrical conductivity (EC), moisture content, and NH4+-N and NO3−-N contents. The remaining soil samples were air-dried indoors, with a small portion passing through a 100-mesh sieve used to determine TN and TOC. The majority of the soil samples were sifted through a 10-mesh sieve to analyze salinity and to subsequently prepare soil extracts and artificial soil columns. Seven sets of undisturbed soil columns (with three replicates per set) were also collected from the test site (Gleyic Solonchaks) using polyvinyl chloride (PVC) pipes, with a diameter of 11 cm and a height of 50 cm (considering the freezing depth reaching approximately 0.7 m within the study area, the unsaturated zone affected by freeze–thaw cycles is relatively shallow) on September 2022 to simulate soil properties during the winter.
Soil water content was determined gravimetrically by drying the samples at 105 °C [32]. Soil pH and EC were measured following leaching at a soil–water ratio of 1:5 by using a portable multi-parameter meter (HQ40D, Hach Company, Loveland, CO, USA). Recognizing its significant correlation with salinity, EC1:5 was employed as a substitute to describe changes in soil salinity [33]. The soil salinity content (g/kg) can be calculated from the measured EC1:5 (μs/cm) using the linear regression formula of the experimental data: the soil salinity content (g/kg) = (EC1:5/1000) × 3.7657 − 0.2405 (to serve the verification of H1 and H2). Potassium chloride solution was employed to extract NH4+-N and NO3−-N from the soil samples, and their concentrations were subsequently determined by Nessler’s reagent spectrophotometry and ultraviolet spectrophotometry, respectively [34]. Notably, the adsorption of NO3−-N on clay and other mineral particles is typically less than 6%, and was considered negligible [35]. The TN and TOC contents were quantified using a Multi N/C 3100 Total Organic Carbon Analyzer (Analytik Jena AG, Jena, Germany) and the C/N ratio was calculated from the measured TOC and TN values (C/N = TOC/TN). The specific formula for calculating the soil nitrogen mineralization rate is as follows: nitrogen mineralization rate = (inorganic nitrogen content after freeze–thaw cycles − inorganic nitrogen content before freeze–thaw cycles)/number of freeze–thaw cycles.

2.2.2. Freeze–Thaw Simulation Experiments

1.
Undisturbed soil column experiments for freeze–thaw cycles
This experiment was designed to directly test the driving effect of freeze–thaw cycles on the migration and transformation of water, salt, and nitrogen (H1). These undisturbed soil columns were placed in a high–low temperature test chamber for freeze–thaw simulation experiments (Figure 2a). These experiments involved six sets of distinct undisturbed soil columns (with three replicates per set) subjected to varying numbers (0, 1, 3, 5, 7, and 15) of freeze–thaw cycles, considering the characteristics of the multi-year average temperature variations in the study area, where each cycle consisted of a freezing phase at −20 °C for a duration of 12 h, followed by a thawing phase at 20 °C for another 12 h. The 0 cycles column was never frozen. A seventh set of the undisturbed soil column remained completely frozen for 12 h. To monitor temperature dynamics, temperature probes were inserted at depths of 5, 15, 25, 35, and 45 cm in each undisturbed soil column.
2.
Artificial soil column experiments for freeze–thaw and irrigation practices
Based on the results of the undisturbed soil column experiments, some artificial soil column experiments were designed to validate the individual effects of irrigation water salinity and nitrogen application rate (H2), as well as their interactive effects with freeze–thaw cycles (H3). The artificial soil columns (packed using field-collected soil from the experimental site) were used to simulate irrigation under two variables: salinity levels and nitrogen application rates, for the verification of H2, two simulated irrigation water with different values of salinity (groundwater irrigation: EC = 3010 μs/cm, nitrogen application 225 kg/hm2, designated as S1; and Yellow River irrigation: EC = 697 μs/cm, nitrogen application 225 kg/hm2, designated as S2), for the verification of H2 and H3, and three simulated irrigation with different rates of nitrogen fertilizer application (high: 300 kg/hm2, EC = 0.05 μs/cm, designated as N1; medium: 225 kg/hm2, EC = 0.05 μs/cm, designated as N2; and low: 150 kg/hm2, EC = 0.05 μs/cm, designated as N3; pure water was used as the medium), aiming to investigate how irrigation water quality (salinity) and nitrogen input influence salt and nitrogen migration processes and mechanisms in saline soils under freeze–thaw conditions. To better replicate field conditions, the artificial soil columns had a plexiglass structure with a diameter of 6 cm and a height of 62 cm, and the cyclic temperature was maintained in the range from −15 to 5 °C (freezing at −15 °C for 12 h followed by thawing at 5 °C for 12 h, constituting one freeze–thaw cycle). To enable detailed observation of parameter variations, the number of temperature probes was increased, with sensors installed at depths of 5 cm, 10 cm, 15 cm, 20 cm, 25 cm, 30 cm, 35 cm, and 40 cm. Freeze–thaw cycles were applied 5 times (0, 1, 3, 7, and 15 cycles). The freeze–thaw simulation experiment devices and treatments are illustrated in Figure 2b,c.
In this study, five irrigation treatments simulating autumn irrigation were considered (Figure 2c), including two simulated irrigation water types with different salinity values (S1, S2) and three simulated irrigation types with varying rates of nitrogen fertilizer application (N1, N2, and N3). Considering previous research [23].and real-world agronomic practices, the irrigation water amount was set at 100 mL and the artificial soil columns were left to stabilize for 1 day after infiltration with irrigation water. Sodium chloride was used as the experimental salt and large-particle urea (containing N: 46.4%, commonly used by local farmers) as the experimental fertilizer. After the undisturbed soil columns were subjected to 3, 7, or 15 freeze–thaw cycles and the artificial soil columns were treated with five different irrigation, samples of the surface soil (5–10 cm) of the soil columns were collected to explore the effects of freeze–thaw cycles, irrigation water quality (salt content), and nitrogen application rates on ammonia nitrogen adsorption/desorption. The specific experimental procedures align with the operational guidelines proposed by Duan et al. [36].

2.2.3. Data and Statistical Analysis

All indoor freeze–thaw simulation experiments were performed with three replicates to ensure the reliability and reproducibility of the results. Each treatment group (varying in irrigation water salinity, nitrogen application rate, and number of freeze–thaw cycles) consisted of three independent soil columns. A completely randomized design was adopted for sample grouping, treatment application, and experimental sequence to minimize systematic error and procedural bias. Data were organized and visualized using Excel and Origin, while statistical analysis was conducted using SPSS 25.

3. Results and Discussions

3.1. Effects of Seasonal Freeze–Thaw Processes on Soil Properties

The discrete soil samples taken in the autumn and winter of 2022 and spring of 2023 were used to explore changes in soil physicochemical properties across different seasons. Overall, during the three seasons, soil moisture content exhibited fluctuating variations with increasing soil depth (Figure 3). The pH of soil samples ranged between 8.63 and 9.53, exhibiting strong alkalinity. It is known that this alkaline condition will influence the composition and activity of nitrogen-cycling microorganisms, particularly in high-pH environments, which is more favorable for ammonia-oxidizing bacteria (AOB) than archaea (AOA) [37,38]. The soil salinity in the surface layer (0–20 cm) was as high as 4.27–6.27 g/kg, while soil salinity at 20–120 cm depths showed minimal fluctuation with increasing soil depth. According to soil salinity classification, the soil in the study area can be categorized as weakly saline (soil salinity: 1.15–3.37 g/kg) and moderately saline (soil salinity: 3.37–6.69 g/kg). The surface layer (0–20 cm) demonstrated moderate salinization intensity and inorganic nitrogen (NH4+-N and NO3−-N) contents, serving as a salt and nitrogen accumulation zone, while inorganic nitrogen and salinity levels remained relatively stable in deeper soil layers.
Specifically, soil moisture profiles exhibited fluctuations with increasing soil depth, but soil water content increased near the groundwater table (Figure 3). Compared to autumn, soil moisture content rose in winter, and snowmelt contributed to further increases in soil water content in spring. Before autumn irrigation, surface soil (0–20 cm) showed elevated soil salinity, which gradually decreased with depth (Figure 3a,b). Due to winter irrigation for salt leaching, the surface soil salinity (0–20 cm) in spring became lower than in autumn, while soil salinity increased at 20–80 cm depths. The EC values in spring increased by 20.88%, 231.10%, and 64.03% compared to autumn at 20–40 cm, 40–60 cm, and 60–80 cm depths, respectively, likely due to salt migration toward the surface with water movement [39]. After the soil experienced freezing and thawing, the distributions of water and salt within the soil profile were more uniform. This homogenization effect can be attributed to the freeze–thaw-induced soil pores restructure and enhanced connectivity of water transport channels [40].
The content of NH4+-N and NO3−-N in 0–20 cm soil was relatively high in autumn, measuring 7.43 mg/kg and 2.62 mg/kg, respectively. Following the freeze–thaw processes, these levels decreased to 0.55 mg/kg and 0.50 mg/kg in spring, while inorganic nitrogen content at other soil depths showed minimal variation. Therefore, the analysis of soil physicochemical properties during the three periods revealed that freezing may facilitate the release of NH4+-N and NO3−-N in soil. However, after undergoing a seasonal freeze–thaw cycle, the inorganic nitrogen content in the soil exhibited a relative decrease. The significant reduction in inorganic nitrogen in the surface soil suggests enhanced microbial nitrogen transformation activities during freeze–thaw periods, especially the nitrification and denitrification processes facilitated by the physical release of organically bound nitrogen in the disrupted soil aggregates [41].

3.2. Distribution Pattern of Water–Salt–Nitrogen Under Simulated Freeze–Thaw Cycles

The undisturbed soil columns sampled from the field were subjected to controlled freeze–thaw cycles to investigate the migration patterns of water, salt, and nitrogen within the soil profile. Experimental results demonstrated that the soil moisture content at 0–10 cm depth exhibited consistent regular fluctuations (Figure 4a). Notably, while a transient increase in moisture content was observed following the fifth freeze–thaw cycle, a progressive decrease in moisture content was subsequently recorded with increasing number of freeze–thaw cycles. The changes in soil moisture content below 10 cm were less pronounced but followed a similar trend to that of the surface layer (Figure 4a). As illustrated in Figure 4b, the salinity of the top 10 cm of soil initially increased after a single freeze–thaw cycle, but this response gradually weakened after successive (up to five) cycles, eventually leading to a decrease in salinity. Notably, the soil salinity then increased after seven cycles and reached its maximum after 15 cycles, exhibiting a 2.25-fold increase compared to values under 0 freeze–thaw cycles; this signifies that the impacts of freeze–thaw on salinity in the soil may be periodic. This periodic pattern likely reflects the dynamic balance between salt exclusion during ice formation and salt concentration in unfrozen water films, which are strongly regulated by soil clay mineralogy and specific surface area [42].
The salinity in the 10–20 cm soil layer exhibited little variation and was similar after the completion of each cycle. Below 20 cm depth, the salinity in the soil demonstrated an overall trend of first decreasing and then increasing with increasing soil depth. Nevertheless, the changes during the freeze–thaw cycles were similar to those noted for the surface layer, with the highest soil salinity observed after 15 freeze–thaw cycles. Considering both moisture content and salinity, the response of the 0–10 cm soil layer to the freeze–thaw cycle was the most obvious. In general, as the number of freeze–thaw cycles increased, the moisture content decreased and the salinity increased, possibly attributed to the evaporation of surface soil moisture.
Figure 4c demonstrates that freeze–thaw cycles have a significant influence on soil NH4+-N content. Under unfrozen conditions, NH4+-N content exhibited fluctuating characteristics along the soil profile, initially decreasing, then peaking at 7.12 mg/kg in the 30–40 cm layer, before declining again. This fluctuation may correlate with soil organic matter distribution, as this horizon potentially accumulates highly humified organic matter. Previous studies have indicated that dissolved organic matter in the Hetao Basin primarily consists of terrestrial humic-like components, and the degradation of these low-molecular-weight humic substances promotes NH4+-N release and heavy metal mobilization [43]. Compared to surface soils, the 30–40 cm layer features lower oxygen levels, where anaerobic microbial metabolism utilizes such organic carbon sources, thereby enhancing NH4+-N accumulation. The accumulation of NH4+-N in the 30–40 cm soil layer may be further enhanced by the adsorption of ammonium ions on the surface of clay minerals (particularly 2:1 phyllosilicates such as illite and vermiculite), which protects them from nitrification under low-oxygen conditions [44]. After one freeze–thaw cycle, NH4+-N content generally decreased across most layers but increased in the 20–30 cm layer. This spatial heterogeneity reflects influences from soil environmental factors (e.g., oxygen availability and temperature) [45]. Surface soils with better aeration preferentially promote nitrification under aerobic conditions, converting NH4+-N to NO3−-N. This nitrification process is primarily mediated by ammonia-oxidizing bacteria (AOB) and archaea (AOA), whose relative contributions vary with soil depth and environmental conditions [46].
Microbial activity also exhibited stratified responses to temperature fluctuations: surface microbial activity was significantly inhibited by freeze–thaw cycles, while the relatively stable thermal regime in the middle layer (20–30 cm) sustained ammonification [3]. During soil thawing, the surface layer melted first, allowing meltwater to transport NH4+-N from the surface to the middle layer, resulting in elevated NH4+-N content at 20–30 cm. Following 15 freeze–thaw cycles, NH4+-N content showed a decreasing trend across all soil layers, indicating that prolonged freeze–thaw processes inhibit nitrogen transformation by reducing microbial metabolic efficiency [6].
The distribution pattern of NO3−-N in response to freeze–thaw processes differed from that of NH4+-N. Under unfrozen conditions, the highest NO3−-N content occurred in the surface layer (0–10 cm), while deeper soil layers (30–50 cm) exhibited stabilized levels (Figure 4d). This pattern may be attributed to aerobic surface conditions promoting nitrification and oxygen-depleted deeper environments inducing denitrification losses, resulting in lower NO3−-N content at depth [47]. After a single freeze–thaw cycle, the NO3−-N content in the 0–10 cm layer increased to 5.96 mg/kg. During the freeze–thaw cycles, water undergoes repeated freezing and thawing. Upon freezing, water forms ice crystals that may encapsulate NO3−-N, which is gradually released into soil solutions during thawing. Freezing temporarily suppresses microbial activity, but surface soils with superior aeration, sufficient oxygen supply, and abundant organic matter and nutrients may facilitate faster microbial recovery during thawing, leading to NO3−-N accumulation [3,48]. Conversely, deeper soils experience minimal temperature fluctuations and limited organic/nutrient availability, constraining microbial activity and leaving NO3−-N levels largely unaffected. As freeze–thaw frequency increases, microbial adaptation to environmental changes enhances NO3−-N migration or transformation, resulting in declining NO3−-N content in surface soil and gradual reductions in deeper layers. This observation aligns with findings by Koponen et al. [8] in Finnish agricultural ecosystems, where microbial communities adapted to low-temperature stress. Freeze–thaw processes reduce soil oxygen levels, stimulating anaerobic microbial activity and favoring denitrification. Recent research by Nie et al. [2] further confirmed that NO3−-N decreases with increasing initial soil moisture, and high-frequency freeze–thaw cycles enhance NO3−-N consumption.

3.3. Water–Salt–Nitrogen Dynamics Driven by Irrigation in Freeze–Thaw Cycling Systems

3.3.1. Freeze-Thaw Driven Water and Salt Transport

The freeze–thaw cycles and irrigation regimes significantly governed water and salt transport processes in soil systems. When irrigated with simulated groundwater (S1), soil moisture content increased across all depths compared to pre-irrigation levels, particularly in surface layers. After 1 day of incubation, water infiltration reached approximately 15 cm depth, with the majority accumulating between 5 and 10 cm (Figure 5a). Gravity-driven water infiltration markedly elevated moisture content in the 5–10 cm soil layer, while limited hydrological variations occurred below 10 cm. Notably, the contents of surface soil moisture decreased significantly after seven freeze–thaw cycles, potentially attributable to preferential flow pathways created by ice lens formation during phase transitions [49]. Repeated freeze–thaw cycles induced soil structural reorganization through frost heave destruction of original pore structure, coupled with the development of continuous water transport pathways via oriented colloidal particle arrangement during thawing [5,50]. This observation aligns with findings from Hou et al. [51] in permafrost regions of Northeast China, where seasonal freeze–thaw cycles enhanced snowmelt infiltration through altering pore structure. In contrast, the moisture content of the Yellow River water irrigation system (S2) initially exhibited a reduction pattern from surface soil to deep layers (Figure 5d). Surface soil moisture (0–20 cm) displayed a stepwise decrease with increasing freeze–thaw cycle frequency (cycle-1 > cycle-7 > cycle-15), closely associated with freeze–thaw-induced alterations in soil matrix potential gradients. Freeze–thaw cycles-mediated ice crystallization and melt processes caused particle displacement and soil structure reshaping. Initial freeze–thaw cycles (e.g., cycle-1) reduced macropores in the soil system while increasing micropores, temporarily enhancing water retention. However, progressive structural damage from repeated freeze–thaw cycles (7–15 cycles) diminished pore connectivity, disrupted matrix potential gradients, and ultimately reduced water retention capacity [52,53].
Salt dynamics exhibited pronounced spatial heterogeneity and irrigation source dependency. Under both S1 and S2 treatments, the surface layer (0–20 cm) exhibited lower salinity levels after irrigation compared to pre-irrigation levels, indicating the salt-suppressing effect of irrigation (Figure 5b,e). Following multiple freeze–thaw cycles, soil salinity demonstrated an increasing trend with depth, attributable to enhanced ion leaching effects during freeze–thaw processes. The repulsive forces generated by ice crystal growth drove dissolved salts towards unfrozen zones, while the fracture networks formed through repeated freeze–thaw cycles facilitated downward salt migration [54]. The differential salt migration patterns between S1 and S2 treatments can be partially explained by the influence of salt concentration on soil hydraulic properties and microbial activity, as high salinity affects both soil structure stability and microbial community composition [55].
Notably, the salinity in soil layers below 20 cm exceeded pre-irrigation levels, with particularly significant salt accumulation in deep soil layers after freeze–thaw cycles in the S1 treatment. This change trend may be influenced by the coupling of evaporation and crystallization processes. On one hand, freeze–thaw cycles disrupt soil aggregates, increasing the surface area available for evaporation, while the high-concentration ionic salts in the S1 simulated solution promote preferential salt crystallization and precipitation during water evaporation [56]. On the other hand, ice lens formation during repeated freeze–thaw cycles creates preferential flow pathways, enhancing salt migration with water flow, ultimately leading to increased salinity in deeper soil layers [57]. Additionally, a significant negative correlation between moisture and salinity was observed in this study (Figure 5c,f), further validating the role of evaporation in salt accumulation. Comparatively, the S2 treatment demonstrated higher desalination efficiency than S1 after multiple freeze–thaw cycles, suggesting that brackish water irrigation facilitates soil salinity reduction under freeze–thaw conditions. Some studies have confirmed that brackish water irrigation can effectively decrease salt contents, enhance salt leaching, and mitigate soil salinization risks in typical salinized regions (such as Xinjiang and Inner Mongolia) [28,58].

3.3.2. Nitrogen Transport

The five irrigation scenarios simulated in this study significantly regulated nitrogen migration and transformation processes. Overall, NH4+-N content in different salinity irrigation treatments (S1, S2) exhibited a “decrease followed by increase” pattern with freeze–thaw cycles (decreasing during cycle 1–7 and rebounding significantly at cycle-15) (Figure 6a,c), while NO3−-N content showed an inverse variation trend (Figure 6b,d). This phase transition effect may originate from freeze–thaw cycle-induced continuous soil contraction–expansion [5], which modulates nitrogen transformation pathways by altering redox conditions in the vadose zone [59,60]. Notably, after 15 freeze–thaw cycles, S1-treated soil exhibited significantly lower NO3−-N content than S2-treated soil, demonstrating dual regulation of salinity and freeze–thaw processes on nitrogen transformation. The stronger inhibition of nitrification in high-salinity treatment (S1) can be attributed to the combined effects of osmotic stress on nitrifying bacteria and the disruption of their cellular metabolism by high ionic strength [61]. Numerous studies indicate that elevated soil salinity inhibits NH4+-N conversion by directly impairing critical transformation steps, while freeze–thaw cycles amplify this inhibition through physical disruption and redox fluctuations, ultimately reducing NO3−-N generation [9]. In the study on saline–alkali soils in the Hetao Irrigation District, Zhou et al. [62]. further confirmed that increased salinity reduces soil nitrification potential while enhancing soil denitrification capacity.
Nitrogen transformation rates calculated based on the contents of nitrogen species (Figure 7) revealed that both salinity treatments predominantly affected NH4+-N content in surface soil (0–5 cm). The mineralization rate peaked after the initial freeze–thaw cycle and subsequently declined with increasing cycle frequency, indicating time-dependent stimulation effects of freeze–thaw on nitrogen transformation, consistent with trends reported by Amador et al. [63]. This change may arise from partially lysed microbial cells under transient freeze–thaw cycles. The release of intracellular substrates from these cells supplies bioavailable carbon to surviving microorganisms, driving metabolic activity and accelerating the organic nitrogen mineralization [7,64]. However, prolonged freeze–thaw cycles progressively reduced microbial biomass and activity, ultimately decreasing mineralization rates [65]. Specifically, the net ammonification rate in S1-treated soil initially decreased, followed by an upward trend, peaking and turning positive at the 15th cycle, while that in S2-treated soil exhibited a continuous decline, ultimately approaching 0 mg/(kg·d) (Figure 7). Nitrification rate variations paralleled ammonification trends, with S1 treatment favoring short-term inorganic nitrogen release. In the 5–20 cm soil layer, the nitrogen transformation rate dynamics followed comparable patterns across depths, albeit with slight variations. Below 20 cm, the transformation rates remained stable, and low-salinity irrigation consistently enhanced rates relative to high-salinity treatments. Overall, differences between saline irrigation practices were marginal, but freeze–thaw cycles mainly affected surface soils compared to deeper strata.
Different nitrogen application rates also significantly influenced the migration and distribution of soil nitrogen. As the frequency of freeze–thaw cycles increased, NH4+-N levels in each soil layer showed a gradual decreasing trend, but exhibited a significant increase after 15 cycles (particularly notable in the 0–25 cm layer) (Figure 8a–c). This change trend might be attributed to the formation of transient aerobic micro-zones during initial ice crystal melting in early freeze–thaw stages, which enhanced the activity of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), accelerating the conversion of NH4+-N to NO3−-N [9]. Concurrently, meltwater transported dissolved NH4+-N to deeper soil layers, contributing to the observed NH4+-N reduction. Following multiple freeze–thaw cycles, repeated phase transitions disrupted soil aggregate structures through ice crystal compression, exposing encapsulated organic matter and accelerating mineralization [66]. This mechanism explains the subsequent rebound in NH4+-N levels after initial decline. The three nitrogen application treatments similarly demonstrated significant impacts on NO3−-N content, with the most pronounced variations observed in the 0–25 cm layer (Figure 8d–f). Under increasing freeze–thaw cycles, medium and high-nitrogen treatments (N2 and N3) showed an initial increase followed by a decrease in NO3−-N content, whereas the low-nitrogen treatment (N1) exhibited a consistent downward trend in soil NO3−-N levels.
Notably, in the 0–5 cm surface soil layer, the total inorganic nitrogen content under N2 and N3 irrigation treatments increased after 15 freeze–thaw cycles. The net nitrogen mineralization rate of N1 fluctuated between −5 and 20 mg/(kg·day), while that of N2 gradually decreased, peaking during the first cycle (Figure 7). This suggests that freeze–thaw conditions initially enhanced soil nitrogen transformation in these irrigation treatments within a short period, but this effect diminished with increasing freeze–thaw frequency. By the 15th cycle, NH4+-N content in the 0–5 cm layer increased across all three treatments, with net ammonification and mineralization rates approaching 0 mg/(kg·day). These results indicate that freeze–thaw cycles promote NH4+-N accumulation in soil, with medium-nitrogen irrigation (N2 group) being more effective in enhancing surface soil mineralization rates and inorganic nitrogen release. In the high-nitrogen treatment group of this study, excessive fertilization and repeated freeze–thaw cycles will induce soil acidification, lowering pH levels. This will suppress urease activity and ammonia-oxidizing bacteria (AOB) abundance, creating a negative feedback loop of “acidification–enzyme inhibition–substrate accumulation”, ultimately weakening the production of NH4+-N [67,68]. In contrast, medium-nitrogen treatment (N2) maintained optimal C/N ratios and moisture availability, providing an ideal metabolic environment for ammonifying microbial communities, thereby sustaining higher mineralization rates.
For the 5–10 cm soil layer, although net mineralization, ammonification, and nitrification rates followed patterns similar to those in the surface layer, the net mineralization rate under medium-nitrogen treatment (N2) remained minimal. Freeze–thaw cycles regulated nitrogen transformation in different ways: N2 primarily enhanced NH4+-N conversion, while N1 (high nitrogen) and N3 (low nitrogen) treatments favored NO3−-N conversion. However, these stimulatory effects diminished progressively with increasing freeze–thaw frequency. In the 10–25 cm subsoil layer, three net nitrogen transformation rates decreased with depth but maintained trends consistent with the surface layer. Both N1 and N3 irrigation treatments moderately promoted NO3−-N conversion. For N1, excessive nitrogen input significantly elevated soil NO3−-N concentrations, providing abundant electron acceptors (nitrate) for denitrifying bacteria. During freeze–thaw cycles, localized anaerobic micro-zones formed by ice crystal compression allowed denitrifying bacteria to preferentially utilize NO3−-N for respiration under hypoxic conditions. In contrast, N1 treatment elevated the soil C/N ratio, stimulating heterotrophic denitrifiers to decompose organic matter for energy. The high C/N environment enhanced dehydrogenase activity, accelerating organic carbon oxidation and denitrification [48,64]. However, prolonged ice encapsulation during repeated freeze–thaw cycles likely weakened microbial resilience to freeze–thaw stress, leading to declining nitrogen transformation rates across all treatments as cycles increased. In soil layers deeper than 25 cm, nitrogen transformation rates exhibited minimal variation, with negligible differences observed among the three irrigation treatments. This may be attributed to the spatial attenuation of freeze–thaw driving forces. Compared to surface layers, deep soil environments and structures are more stable, with incomplete ice lens formation failing to disrupt mineral–organic complexes effectively. Consequently, dissolved nitrogen migration and diffusion were restricted. These findings align with the “depth–decay model of freeze–thaw effects” proposed by Lyu et al. [12], revealing distinct biogeochemical critical depths in soil nitrogen cycling and water dynamics response to freeze–thaw processes in seasonally frozen soil areas [68,69].
Taken together, under freeze–thaw cycles, soil NH4+-N and NO3−-N exhibited significant antagonistic effects. NH4+-N content displayed a fluctuating pattern of initial decrease followed by a subsequent increase with rising freeze–thaw frequency, while NO3−-N showed an inverse dynamic trend. This responsive mechanism primarily stems from freeze–thaw-induced soil structural remodeling and fluctuations in redox conditions. Notably, high-salinity (S1) irrigation treatment significantly suppressed nitrification, resulting in markedly lower NO3−-N content compared to the S2 treatment, highlighting a synergistic inhibitory effect of salinity stress and freeze–thaw-induced physical damage. Further analysis revealed that medium-nitrogen (N2) irrigation most effectively enhanced inorganic nitrogen mineralization rates in surface soil. Although high and low-nitrogen fertilization promoted NO3−-N conversion, their regulatory effects gradually attenuated with prolonged freeze–thaw cycles, indicating that freeze–thaw duration serves as a critical temporal threshold governing nitrogen transformation.

3.4. Response Mechanism of Water–Salt–Nitrogen Migration to Freeze–Thaw Cycles and Irrigation

In this study, the seasonal freeze–thaw processes and simulation experiments revealed the complex response mechanisms of water–salt–nitrogen migration to freeze–thaw cycles and irrigation practices. Freeze–thaw cycles significantly altered soil moisture transport patterns through ice lens formation and pore structure reorganization. Simulated groundwater (S1) irrigation initially promoted surface moisture enrichment, but repeated freeze–thaw events triggered preferential flow channels, leading to reduced water retention in surface soil. In contrast, the Yellow River water (S2) irrigation system formed a significant moisture gradient decreasing with depth due to altered matrix potential gradients. Salt dynamics showed irrigation source-dependent patterns, where ice crystal exclusion and crack networks drove downward salt migration, while evaporation–crystallization coupling caused salt accumulation in deep layers. High-salinity treatment (S1) showed particularly significant salt accumulation after 15 freeze–thaw cycles, whereas S2 treatment demonstrated better desalination efficiency through enhanced leaching effects. These water–salt migration patterns were validated in field practices across the Black Soil region, Hetao Irrigation District, and Manas River Basin in Xinjiang [70,71,72]. Winter salt-suppressing irrigation using brackish water effectively reduced 0–30 cm soil salinity while decreasing deep salt accumulation compared with freshwater irrigation, effectively mitigating spring salt resurgence.
The freeze–thaw-driven nitrogen transformation process exhibited relative complexity. Soil shrinkage–swelling-induced redox fluctuations caused NH4+-N to first decrease then increase, while salt stress combined with freeze–thaw physical damage synergistically inhibited nitrification, resulting in significantly lower NO3−-N content in high-salinity systems compared to low-salinity ones. Medium-nitrogen irrigation (N2) optimized microbial metabolic environments by maintaining appropriate C/N ratios and moisture conditions, promoting surface inorganic nitrogen release. However, freeze–thaw duration and frequency emerged as critical thresholds regulating nitrogen transformation rates through their impacts on microbial community composition and metabolic activities. Similar trends were observed in typical permafrost regions like Songnen Plain, where after freeze–thaw cycles, the NH4+-N content in 0–20 cm soil increased while NO3−-N decreased significantly compared to unfrozen soil, directly leading to reduced spring basal fertilizer utilization [2]. The medium-nitrogen irrigation pattern showed significantly higher net nitrogen mineralization rates during freeze–thaw periods compared to traditional high-nitrogen treatments, with increased available nitrogen supply during spring sowing. This further demonstrates the significant impacts of freeze–thaw cycles and irrigation methods on nitrogen migration and transformation. Moreover, deep soil exhibited significantly weaker responses to water–salt–nitrogen migration due to attenuated freeze–thaw driving forces and enhanced structural stability, confirming the depth-dependent attenuation of freeze–thaw effects.
These mechanisms collectively reveal how freeze–thaw cycles regulate water–salt–nitrogen migration across spatiotemporal dimensions through soil structure remodeling and coupled biogeochemical processes. The study particularly emphasizes water–salt–nitrogen transport processes in the vadose zone under different irrigation conditions, systematically clarifying how freeze–thaw conditions and irrigation practices synergistically regulate water–salt distribution patterns and nitrogen biogeochemical behaviors. The proposed optimized low-salinity and medium-nitrogen irrigation pattern maintains surface desalination efficiency while preserving soil fertility. These findings advance our understanding of water–salt–nitrogen migration mechanisms under freeze–thaw processes, providing important practical value for optimizing irrigation systems and enhancing saline soil reclamation benefits in the cold semi-arid region. The value of this study lies in providing a scientific optimization scheme for the Yellow River irrigation practice already widely adopted in the study area. Experimental results demonstrate that the low-salinity (Yellow River irrigation) and medium-nitrogen (nitrogen application 225 kg/hm2) strategy significantly improves the regulation of soil water–salt–nitrogen dynamics. This implies that, without major modifications to the existing water sources, better ecological and economic benefits can be achieved merely by adjusting nitrogen fertilizer application. This approach not only reinforces the cost advantages of the current water source structure but also contributes to enhancing long-term soil health by preventing secondary salinization, reducing nitrate leaching, and stabilizing the microbial community, thereby promoting soil organic matter accumulation and aggregate stability. To fully exploit the potential of this strategy, we recommend incorporating a real-time monitoring system during implementation to dynamically adjust irrigation and fertilization management, achieving a transition from “water availability” to “efficient water management.”

4. Conclusions

The dynamics of water, salt, and nitrogen migration in the vadose zone of the Hetao Irrigation District under freeze–thaw cycles were explored using simulation experiments and field monitoring. The findings reveal that the response of the surface soil to freeze–thaw cycles is more pronounced than that of deep soil, with water migrating from the unfrozen zone to the frozen zone, result in a decrease in surface soil moisture content with increasing freeze–thaw cycles. Due to the evaporation of surface soil moisture, salt will migrate upwards with water and eventually accumulate in the surface soil. Salt dynamics exhibited pronounced spatial heterogeneity and irrigation source dependency. Under both S1 and S2 treatments, the surface layer (0–20 cm) exhibited lower salinity levels after irrigation compared to pre-irrigation levels, indicating the salt-suppressing effect of irrigation. After freezing in winter and subsequent thawing in spring, the distribution of water and salt in the soil profile in spring is more uniform than in autumn. Additionally, freeze–thaw cycling facilitates the accumulation of inorganic nitrogen in soil, the release of NH4+-N in the deep soil under reducing conditions, and an increase in NO3−-N content in surface soil under oxidizing conditions.
Irrigation using low-salt and medium-nitrogen water was demonstrated to be the most effective. This strategy is not only agronomically efficient but also economically viable and environmentally sustainable, given the ample supply of Yellow River water and the reduced risk of nutrient overloading. Future adoption should be integrated with local water allocation policies and soil health monitoring systems to ensure long-term resilience of the cropping systems. These findings provide crucial theoretical support for the sustainable utilization of salinized lands in cold regions. The proposed low-salinity and medium-nitrogen irrigation optimization scheme can guide the amelioration of salinized soils in irrigation districts by inhibiting salt accumulation in the surface soil and enhancing nitrogen use efficiency, thereby reducing agricultural non-point source pollution risks.
However, the freeze–thaw experiments were conducted under controlled laboratory conditions, mainly focused on internal vadose zone processes, without simulating dynamic hydraulic connectivity with groundwater. This may not fully capture the complexity of field environments or the completeness of water–salt–nitrogen fluxes. Additionally, future work should improve the experimental design by incorporating variable groundwater level control systems to better simulate natural conditions, combined with field monitoring to enhance validation across different seasons. Integration of microbial genomic and metagenomic analyses is needed to elucidate the mechanisms of nitrogen transformation under freeze–thaw and salinity stress.

Author Contributions

Conceptualization, W.G. and C.S.; methodology, W.G.; software, J.J.; validation, J.S.; formal analysis, Y.L.; investigation, Y.L. and X.L.; resources, X.X. and C.Z.; data curation, Y.S.; writing—original draft preparation, W.G.; writing—review and editing, C.S.; visualization, J.J.; supervision, Q.Z.; project administration, W.G.; funding acquisition, Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Inner Mongolia Autonomous Region Natural Science Youth Fund project (grant number 2023QN04019, The funding agency is the Department of Science and Technology of Inner Mongolia Autonomous Region.) and the Inner Mongolia Autonomous Region Department of Natural Resources project (study on biological restoration and comprehensive development of saline–alkali land in Hetao Irrigation District, Inner Mongolia Autonomous Region).

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

Author Chunming Zhang was employed by Inner Mongolia Geo-Environmental Ecology Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Figure 1. Location of the study area and test site.
Figure 1. Location of the study area and test site.
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Figure 2. Devices and design of freeze–thaw simulation experiments. Among these, a schematic diagram of an undisturbed soil column (a), a schematic diagram of an artificial soil column (b), and a schematic diagram of the artificial soil column experiment and the variables considered (c).
Figure 2. Devices and design of freeze–thaw simulation experiments. Among these, a schematic diagram of an undisturbed soil column (a), a schematic diagram of an artificial soil column (b), and a schematic diagram of the artificial soil column experiment and the variables considered (c).
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Figure 3. Physicochemical properties of soil at different depths across different seasons.
Figure 3. Physicochemical properties of soil at different depths across different seasons.
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Figure 4. Distribution patterns of water–salt–nitrogen in soil under freeze–thaw cycles. (a) Soil moisture distribution; (b) Soil salinity distribution; (c) NH4+-N distribution; (d) NO3−-N distribution.
Figure 4. Distribution patterns of water–salt–nitrogen in soil under freeze–thaw cycles. (a) Soil moisture distribution; (b) Soil salinity distribution; (c) NH4+-N distribution; (d) NO3−-N distribution.
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Figure 5. Effects of irrigation water salinity on soil water–salt dynamics: soil moisture (a,d), salinity (b,e), and water–salt relationships (c,f) in groundwater (S1) and Yellow River water (S2) irrigation treatments.
Figure 5. Effects of irrigation water salinity on soil water–salt dynamics: soil moisture (a,d), salinity (b,e), and water–salt relationships (c,f) in groundwater (S1) and Yellow River water (S2) irrigation treatments.
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Figure 6. Effects of irrigation patterns with different salinity on nitrogen migration. (a) NH4+-N distribution under groundwater irrigation (S1); (b) NO3−-N distribution under groundwater irrigation (S1). (c) NH4+-N distribution under Yellow River water irrigation (S2) (d) NO3−-N profile under Yellow River water irrigation (S2).
Figure 6. Effects of irrigation patterns with different salinity on nitrogen migration. (a) NH4+-N distribution under groundwater irrigation (S1); (b) NO3−-N distribution under groundwater irrigation (S1). (c) NH4+-N distribution under Yellow River water irrigation (S2) (d) NO3−-N profile under Yellow River water irrigation (S2).
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Figure 7. Changes in nitrogen mineralization, ammonification, and nitrification rates of soil at varying depths under different irrigation conditions after freeze–thaw cycles, with the rates for each cycle represented by the mean value in graphs.
Figure 7. Changes in nitrogen mineralization, ammonification, and nitrification rates of soil at varying depths under different irrigation conditions after freeze–thaw cycles, with the rates for each cycle represented by the mean value in graphs.
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Figure 8. Effects of irrigation patterns with different nitrogen application rates on nitrogen migration. (a) NH4+-N distribution under low-nitrogen treatment (N1); (b) NH4+-N distribution under medium-nitrogen treatment (N2); (c) NH4+-N distribution under high-nitrogen treatment (N3); (d) NO3−-N distribution under low-nitrogen treatment (N1); (e) NO3−-N distribution under medium-nitrogen treatment (N2); (f) NO3−-N distribution under high-nitrogen treatment (N3).
Figure 8. Effects of irrigation patterns with different nitrogen application rates on nitrogen migration. (a) NH4+-N distribution under low-nitrogen treatment (N1); (b) NH4+-N distribution under medium-nitrogen treatment (N2); (c) NH4+-N distribution under high-nitrogen treatment (N3); (d) NO3−-N distribution under low-nitrogen treatment (N1); (e) NO3−-N distribution under medium-nitrogen treatment (N2); (f) NO3−-N distribution under high-nitrogen treatment (N3).
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MDPI and ACS Style

Ge, W.; Jiang, J.; Su, C.; Xie, X.; Zhang, Q.; Zhang, C.; Li, Y.; Li, X.; Song, J.; Su, Y. Freeze–Thaw-Driven Dynamics of Soil Water–Salt and Nitrogen: Effects and Implications for Irrigation Management in the Hetao Irrigation District. Water 2025, 17, 2991. https://doi.org/10.3390/w17202991

AMA Style

Ge W, Jiang J, Su C, Xie X, Zhang Q, Zhang C, Li Y, Li X, Song J, Su Y. Freeze–Thaw-Driven Dynamics of Soil Water–Salt and Nitrogen: Effects and Implications for Irrigation Management in the Hetao Irrigation District. Water. 2025; 17(20):2991. https://doi.org/10.3390/w17202991

Chicago/Turabian Style

Ge, Weili, Jiaqi Jiang, Chunli Su, Xianjun Xie, Qing Zhang, Chunming Zhang, Yanlong Li, Xin Li, Jiajia Song, and Yinchun Su. 2025. "Freeze–Thaw-Driven Dynamics of Soil Water–Salt and Nitrogen: Effects and Implications for Irrigation Management in the Hetao Irrigation District" Water 17, no. 20: 2991. https://doi.org/10.3390/w17202991

APA Style

Ge, W., Jiang, J., Su, C., Xie, X., Zhang, Q., Zhang, C., Li, Y., Li, X., Song, J., & Su, Y. (2025). Freeze–Thaw-Driven Dynamics of Soil Water–Salt and Nitrogen: Effects and Implications for Irrigation Management in the Hetao Irrigation District. Water, 17(20), 2991. https://doi.org/10.3390/w17202991

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