Accumulation of Na⁺ in Cotton Field under Mulched Drip Irrigation of Brackish Water in Arid Areas

Abstract

In arid areas, the disordered accumulation of Na⁺ in cotton fields under mulched drip irrigation seriously affects the normal growth and development of cotton. To reveal the process and mechanism of Na⁺ accumulation, in situ field tests were conducted in typical cotton planting bases in the south of Xinjiang, China. Eight experimental scenarios were set up to use brackish and fresh water for drip irrigation under mulch during the whole growth period of cotton according to the drip irrigation system. By monitoring the weather, groundwater level, and soil moisture before and after irrigation, and testing the soil salinity and Na⁺ concentration, the temporal and spatial distribution and transport characteristics of Na⁺ were compared and studied. The results showed that: (1) owing to the strong spatial variation of soil permeability, soil water and salt dynamics exhibited significant differences after irrigation in all experimental scenarios in the study area, especially at a depth of 0–60 cm. (2) The Na⁺ profile exhibited a “thin waist”, whose depth depended on the climate and lithology of the soil; however, this phenomenon was not observed in the salt profile. (3) The accumulation of Na⁺ in cotton fields is an extremely complex dynamic process that is influenced by both natural and human activities. This process is controlled by the dynamic characteristics of soil water and salt, but it is different from the distribution of soil salt in time and space. This study provides necessary technical support for the formulation of drip irrigation systems under brackish water film, and provides a scientific basis for fine agricultural planting and water resources management in arid areas.

1. Introduction

High concentrations of NaCl have a toxic effect on cotton seedlings, which can be caused by ion toxicity and osmotic stress [1]. When Na⁺ reaches 1000 mg kg⁻¹, the cotton seed emergence rate is only 60%, the rate of diseased and dead seedlings is more than 80%, and the biomass is significantly reduced [2]. When the concentration of Na is lower than 500 mg kg⁻¹, the growth of cotton seedling is significantly promoted [3]. Because plants absorb and take away relatively little sodium, approximately 60–80% of the sodium applied to the soil remains in the soil [4,5]. The Cl⁻, Ca²⁺, and Na⁺ contents in cotton plants increase during the growing period, and more salt ions are transported from the roots [6] to the aboveground parts and accumulate in the stems and leaves [7], while less salt ions accumulate in the buds and bolls [8].

Arid areas in China account for 1/3 of the country’s land area [9], and the saline-alkali soil is widely distributed [10]. Soil and secondary salinization of irrigated soil are the main obstacles to agricultural development in arid areas [11,12]. Moreover, they are important factors affecting the stability of oasis ecosystems [13]. The interior basins in the arid area of Xinjiang have less precipitation, strong evaporation, and saline parent material, which are natural physical conditions promoting soil salinization [14]. While water-saving irrigation is widely conducted in arid areas, attention should be paid to protecting the soil environment to prevent the accumulation of salt, especially Na⁺, which is also an essential precondition for sophisticated farming and water management [15].

Drip irrigation under film [16] is a composite water-saving irrigation technology [17] that can directly improve the soil water-salt environment in crop root zones [18,19], and impacts physiological characteristics such as crop emergence, root distribution [3], and aboveground growth, thus improving crop growth, development, and yield [20]. In the past 20 years, numerous studies have been conducted on the characteristics of soil water and salt movement under drip irrigation under film. Wang et al. [5] believed that using drip irrigation under film would allow soil salt to migrate to the wetting front edge with the movement of water and be redistributed in the three-dimensional soil [21], resulting in an obvious salt-accumulating and desalting area in the soil profile and strong accumulation of intermembrane salts [22]. Li et al. [3] proposed that in drip irrigation under saline plastic film, salt distribution is circular, and the salt between the films is higher than that within the film. Su et al. studied the redistribution of soil salt and found that after irrigation [23], soil salt showed a bidirectional migration trend from the lower to the upper layer and from inside to between membranes [24,25,26]. However, whether soil salt accumulation under drip irrigation under a membrane is equivalent to Na⁺ accumulation, and whether the process of Na⁺ accumulation is consistent with soil salt accumulation, remains poorly understood. Despite these factors being directly related to the growth and yield of cotton, few studies have investigated the relationship between Na⁺ accumulation and soil water and salt in brackish water drip irrigation under film.

In this study, the experiment of drip irrigation under film, including fresh and brackish water irrigation, was conducted in a typical cotton planting base in Xinjiang, China, with a total of eight irrigation scenarios. During the whole growth period of cotton, weather, groundwater level, and soil water content were monitored, soil salinity and Na⁺ concentration were measured, and the dynamics of soil moisture and salinity and Na⁺ under different irrigation systems were analyzed to reveal the accumulation process of Na⁺ and its mechanism.

2. Materials and Methods

2.1. Overview of the Study Region

The test area was located in the alluvial plain of the Kongque River at the edge of the Tarim Basin at the southern foot of the Tianshan Mountains. It belongs to Xinier Town, Weli County, near the Xinier Reservoir, 22 km from Korla City and 25 km from Yuli County. The Ku-Ruo highway (National Highway 218) passes near the study area, with an elevation of 897–902 m. The Kunchi River alluvial fan and plain have good soil and water conditions, which are suitable for the survival and development of organisms. The Kunchi River alluvial fan is influenced by the Sinier structure uplift, which causes the river to deviate from east to west, uplifting the Sinier area. The study area is an old irrigation area. After years of operation, the land blocks, canals, roads, and forest belts have taken shape, and the land is relatively flat. The terrain slopes from northeast to southwest, with the ground slope between 1/2900 and 1/1000. The pH of irrigated soil is greater than 8.0.

2.2. Arrangement of Irrigation

The experimental field was sown on 23 April 2018, with four rows around one film (Figure 1). The spacing between two drip irrigation belts was 150 cm, that of plant drip heads was 30 cm, that of plants was 10 cm, that of narrow rows was 20 cm, that of wide rows was 50 cm, and that of inter-membranous rows was 40 cm. The drip tape was laid in the middle of the wide row. The cotton belongs to Xinjiang long staple cotton, malvaceous cotton species. The cotton variety tested was Xinluzhong 21. Groundwater was used for saline irrigation (total dissolved solids are shortened to TDS, which is approximately 3 g·L⁻¹ g·L⁻¹), and surface water was used for fresh water (TDS was approximately 1 g·L⁻¹). Base fertilizer: urea, 225 kg·hm⁻²; ammonium phosphate bibasic, 375 kg·hm⁻²; 45% potassium sulfate, 300 kg·hm⁻²; farmyard manure, 15 m³ ·hm⁻². Irrigation began on June 22, and multiple irrigation scenarios were set. The standard scenario involved 10 times of irrigation (7 d between two irrigation intervals), 15 times of high-frequency irrigation (3.5 d between two irrigation intervals), and 5 times of low-frequency irrigation (10.5 d between two irrigation intervals). An amount of 75 kg·hm⁻² urea was applied at the bud and early flowering boll stages, and 45 kg·hm⁻² at the late flowering boll stage.

Eight irrigation scenarios were set up in the field experiment:, II, III, IV, VI, VII, VIII, and IX (the V was bare land). This study only considered the fourth irrigation process—which was in the middle stage of irrigation, and the physical property indexes of the cotton field were in the average state, with strong representative significance—as an example to dynamically depict the water, salt, and Na⁺ of the cotton field. The fourth irrigation quota was determined according to the water demand characteristics and irrigation quota of the cotton growing period (

Table 1. The fourth irrigation quota of different irrigation scenarios.

Irrigation Scenario	I	II	III	IV	VI	VII	VIII	IX
Irrigation program	5250S	3750S	4500S	3000S	3000S(L)	4500S(H)	3000S(H)	3750(50F50S)
Amount of irrigation/mm	61.4S	43.9S	52.7S	35.1S	0	26.4S	17.6S	43.9F

). In Table 1, S stands for salty water irrigation, F for freshwater irrigation, H for high frequency irrigation (twice the number of standard irrigation and 0.5 times the amount of irrigation), L for low frequency irrigation (the fourth irrigation wheel empty, that is, the amount of irrigation is 0), 5250S for salty water irrigation of 5250 m³·hm⁻², and 50F50S refers to the scenario where the first and last five irrigations used freshwater and salty water, respectively.

2.3. Meteorological and Water Table Monitoring Arrangement

Continuous observation of field meteorological data was performed by self-recording small weather station WATCHDOG, with an observation interval of 5 min. The observation items included rainfall, temperature, solar downward short-wave radiation, wind speed, wind direction, relative humidity, and dew point, and the potential evapotranspiration of reference crops was calculated automatically according to the measured meteorological data.

The monitoring of groundwater change characteristics included the monitoring of water level and TDS. A water level observation well was set in the test area, and the continuous variation of the groundwater level was observed by DIVER, a self-recording water level meter, at an interval of 0.5 h.

2.4. Soil Physical Properties

The soil particles of the cotton field were analyzed, and soil samples were collected from the cotton field section and tested by sieving and hydrometer in the laboratory. The soil was classified using the USDA soil particle classification method. To determine the dry bulk density of soil, vertical profiles were dug in the standard treatment, samples were taken at different depths with a ring cutter with a known volume (392.5 cm³), and the dry bulk density of soil at each depth was determined by drying method [27]. The soil in the test site was divided into four layers according to particle analysis and dry bulk density, as shown in

Table 2. Soil physical properties of the cotton field.

Depth/cm	Percentage of Particle Size/%			Name	DRY DENSITY
	Sand	Silt	Clay
0~20	79.167	14.167	5.833	loamy sand	1.54
20~40	75.000	17.667	5.667	Sandy loam	1.62
40~100	77.667	16.667	3.500	loamy sand	1.43
100~170	87.500	9.167	0.667	sand	1.69

.

2.5. Methods for Monitoring and Testing Soil Water and Salt

Owing to the great interference of natural and man-made influences on field measurements, soil moisture content was determined using the soil drying method and a neutron soil moisture tester, and the mutual correction was made to obtain more accurate field data. Additionally, the soil drying method was used to calibrate the neutron meter in the field, and the moisture content of the 0–30 cm soil layer was measured to compensate for the measuring error of the neutron meter.

Soil samples were collected once before the fourth irrigation and once after the dive level of the cotton field increased significantly (from 2.01 m to 1.83 m) 24 h after irrigation. Soil profiles were sampled from the wide row (WR), narrow row (NR), and inter-membranous (IM) in the eight irrigation scenarios, with sampling depths of 10 cm, 20 cm, 30 cm, 40 cm, 50 cm, 60 cm, 80 cm, and 100 cm. To reduce the sampling error caused by spatial variation in the scene, two groups of repeated samples were taken in the WR, NR, and IM at the corresponding depths, and the three groups of samples were fully mixed and packaged in aluminum boxes for laboratory testing.

Tests included soil moisture content (θw) and total dissolved solids (TDS). TDS measurement included eight types of Canion content: Ca and Mg ions were titrated using EDTA, and Cl ions using silver nitrate. K and Na ions were measured using a flame photometer, and carbonate and bicarbonate by the double indicator (phenolphthalein and methyl orange) method. Sulfate was titrated indirectly using EDTA, and TDS was the sum of the eight ions [28].

3. Results

3.1. Water Distribution in Cotton Field Profile under Different Irrigation Scenarios

Soil volumetric water content was measured before and after the fourth irrigation, and the distribution maps of soil volumetric water content in 24 sections in eight scenarios with WR, NR, and IM were obtained before and after irrigation (Figure 2, in which each scenario was ranked by decreasing irrigation quota).

The increase in soil moisture content in the profile after irrigation decreased significantly with the decrease in irrigation quota (Table 1), especially when it was above 35.1 mm. Before and after irrigation, the largest variation of soil moisture content occurred in the shallow layer (0–30 cm), and the maximum variation reached 14.3% (Figure 2II). Except for scenarios VI, VII, and VIII, the irrigation rate of the other five scenarios was below 1 m. Therefore, the irrigation rate of 35.1 mm (IV) can make soil water travel downwards 1 m, while an irrigation rate of 26.4 mm (VII) cannot.

The overall soil moisture content of the WR and IM was lower before irrigation, while the water content of the full section of the NR was relatively higher under each scenario, which was caused by the intense evaporation in the arid area. The fourth irrigation was in the middle of cotton growth; the upper film of the WR was broken by the cotton rhizomes and lost the ability to conserve moisture. Therefore, strong evaporation and water absorption of cotton roots in arid areas lasting for 7 d before irrigation caused the WR and IM to lose a large amount of water, and the water content decreased rapidly. Soil moisture evaporated less only with NR under the cover of cotton stems and leaves. After irrigation, with the decrease in irrigation quota, the soil moisture content of WR, NR, and IM gradually became the same, except for the WR in scenario IV. The reason is that the water penetrated below 1 m, indicating that the wide-row soil in this scenario had strong permeability and poor water storage capacity, which is representative of soil spatial variation.

From the longitudinal section of each scenario, the difference of soil moisture content between 0 and 60 cm was large, and the difference between 60 and 100 cm was relatively small. Especially in the situations where the irrigation quota was less than 43.9 mm, the soil moisture content of the 60–100 cm layer after irrigation was less than that before irrigation, such as that at 80 cm in the ninth scenario. This was due to the combined effect of the root system absorbing water upward and gravity infiltration downward, resulting in a decrease in 4% of water in NR after irrigation at 80 cm in this scenario. In arid inland basins, evaporation is large and precipitation is scarce, so irrigation amount causes significant differences in soil water content at the same depth in adjacent scenarios.

3.2. TDS Distribution of Cotton Field Profile under Different Irrigation Scenarios

Soil samples were taken before and after the fourth irrigation to measure TDS, and the distribution maps of soil TDS in 24 sections across the WR, NR, and IM in the eight scenarios were obtained before and after irrigation (Figure 3, in which the plots were ranked according to the gradual decrease in the total salt content of irrigation).

From the irrigation quota of the eight scenarios, the section TDS of scenarios I, II, and IX decreased (0–1 m) after irrigation, whereas the TDS of the remaining five scenarios increased (0–1 m) to varying degrees. This was because the irrigation quota of scenarios I, II, and IX were all over 40 mm, the irrigation process was equivalent to salt washing, and the TDS of the section was significantly reduced after irrigation, effectively protecting the root system of cotton. However, the TDS of scenario III (irrigation quota 52.3 mm) was slightly increased, which was due to the poor soil permeability at the sampling site in this scenario and the accumulation of salt in brackish irrigation water at 0–60 cm depth of soil. In contrast, due to the higher sand content, the permeability of scenario IV was better, and the TDS of the section after irrigation was higher than that before irrigation, which was basically the same as that of brackish water irrigation (3–5 g·L⁻¹).

Similar to the soil moisture content, the TDS of the NR before irrigation was relatively higher under each scenario. There are two main reasons for this phenomenon. First, it was caused by intense evaporation in arid areas. Salt travels with water, and more salt accumulates in the NR where water stays. Second, it was determined by drip irrigation itself. Drip irrigation forms an ellipsoid moist body [29], which is the main boundary for salt accumulation [8]. In particular, the farther the lateral distance is from the drip head, the weaker the water infiltration capacity is, and thus salt accumulates. After irrigation, the TDS of WR, NR, and IM in scenario I with the largest irrigation quota tended to be the same (this is because the irrigation quota exceeded 60 mm, resulting in the salt-washing effect), and salt accumulation of different degrees was formed in the WR in the other scenarios, the highest occurring at the depth of 10 cm in scenario VIII, reaching 35 g·L⁻¹. NR and IM salt accumulation were not evident in any scenarios immediately after irrigation.

From the longitudinal perspective of each scenario profile, with the changes of irrigation amount and total salt content, the spatial distribution of TDS in each scenario profile was substantially different, but the difference of TDS at a depth of 0–60 cm was still large, and that at 60–100 cm was relatively small. As shown in Figure 3, the salinity of irrigation quantity above 35.1 mm was significantly reduced below 60 cm (except for scenario III, where it was all reduced to the TDS level of irrigation water), indicating that an irrigation quantity of 35.1 mm could meet the salt-washing effect of 1 m, even though the permeability of scenario III was poor. At depths below 60 cm, TDS decreased by 10–15 g·L⁻¹. When the irrigation volume was less than 35.1 mm, the salt was retained and accumulated at a depth of less than 60 cm with the water, which was representative of scenario VIII. As scenario VI was not watered, “zero flux of salt” was also formed at the zero flux of soil water at 50 cm due to the salt flow with water. The salt in the area above 50 cm increased with time, and the salt water below 50 cm migrated downward. Additionally, the salt accumulated, resulting in the lowest salinity at the zero-flux surface. If the main growth part of the root zone was exactly 50 cm, scenario VI without irrigation was more conducive to the growth and development of cotton; however, 50 cm is not a fixed value. It is determined by factors such as climate and soil structure.

3.3. Na⁺ Distribution in Cotton Field Profiles under Different Irrigation Scenarios

Soil samples were taken before and after the fourth irrigation to measure the Na⁺ content, and the distribution maps of soil Na⁺ before and after irrigation were obtained for 24 profiles in the eight scenarios with WR, NR, and IM (Figure 4, in which each scenario was ranked according to the decreasing irrigation amount). In view of the short time from irrigation to sampling, the absorption of Na⁺ and K⁺ by cotton in this process was ignored in Figure 4, and only the migration process of salt in soil was considered.

According to the irrigation quota of the eight scenarios, the Na⁺ content of scenarios I and II decreased (0–1 m) after irrigation, similar to the TDS of soil profile, which was also caused by the effect of salt washing caused by the larger irrigation quota. However, it should also be noted that the irrigation quota of scenarios IV and VIII was small, but the decrease in Na⁺ content was large, which is opposite to their TDS changes (Figure 3). In scenario IX, freshwater was consumed and the Na⁺ content decreased somewhat, but the decrease was not as significant as in the above four scenarios. The overall change of Na⁺ content in the other scenarios was not obvious.

The Na⁺ content in the eight scenarios was consistent with the TDS of the soil profile. The Na⁺ content of NR before irrigation was relatively higher than that of WR and IM under each scenario, reaching more than 0.4 g·L⁻¹ in each scenario and up to 1.2 g·L⁻¹ in scenario IX, which is unfavorable to most of the growing period of cotton, and irrigation of freshwater is urgent. Furthermore, we noted that after irrigation, the content of Na⁺ in WR was low, with the most typical situation being that of scenarios II, IV, and IX, all of which decreased to below 0.2 g·L⁻¹ and had no negative effect on the normal growth of cotton in each growth period. However, this was not consistent with the TDS of the soil profile, which had no significant difference between WR, NR, and membrane after irrigation.

From the longitudinal aspect of each scenario section, with the decrease in irrigation amount, the salting effect of each scenario section was gradually weakened. The Na⁺ content of sections I, II, and III decreased to different degrees within 1 m after irrigation, as well as when the depth of scenario IV was above 80 cm. However, after irrigation, the Na⁺ content of VII and VIII decreased significantly only when the depth was above 50 cm. In addition, we noted that in scenario VI without irrigation, the Na⁺ content increased significantly at the depth of 80–100 cm compared with that before and after irrigation, that is, 10 d after the last irrigation. It should also be noted that the minimum variation of Na⁺ content in all scenarios before and after irrigation was approximately 50 cm deep, including WR, NR, and IM. Therefore, the variation of Na⁺ content at 45–55 cm depth was small before and after irrigation and on the surface, resulting in a “thin waist” phenomenon in the Na⁺ profile, which should be related to climate, soil lithology, and the growth depth of the cotton root zone.

3.4. Na⁺ Equilibrium and Aggregation in Typical Profiles

The scenario II was selected as the typical profile due to the median irrigation amount. The Na⁺ profile (0–1 m) of the irrigation process during the whole growth period in scenario II was calculated. The equilibrium calculation included Na⁺ in irrigation water, soil water, plant absorption, and groundwater, and there was no precipitation during this growth period. According to the irrigation system of scenario II, the total water volume of 10 irrigation times in the whole growth period was 3750 m³·hm⁻². According to the test results of 10 irrigation water samples, the Na⁺ content was between 0.6944 and 0.7833 g·L⁻¹, and we calculated that the amount of Na⁺ injected per unit area was 281.25 g·m⁻². We calculated the Na⁺ per 0.1 m depth according to the soil water content of 0–1 m NR before irrigation and the Na⁺ content at the corresponding depth. The accumulated Na⁺ in the soil water of 0–1 m NR before irrigation was 16.03 g·m⁻²; after 10 times of irrigation, it was 70.19 g·m⁻². The storage variable was 54.16 g·m⁻², so the total Na⁺ absorbed by plants and infiltrated into groundwater during the whole growth period was 227.09 g·m⁻².

Furthermore, Na⁺ equilibrium analysis of typical profiles showed that the aggregation of Na⁺ profiles at different depths after 10 times of irrigation had significant differences (

Table 3. Na⁺ equilibrium calculation at different depths of typical profiles.

Depth/cm	Na+ Equilibrium/g·m−2
	Before Irrigation	After 10 Times of Irrigation	Stored Variation Value
0~10	1.24	9.01	7.77
10~20	1.80	7.47	5.67
20~30	0.58	6.52	5.94
30~40	2.22	5.22	3.00
40~50	3.08	6.39	3.31
50~60	1.72	6.83	5.11
60~70	0.80	5.78	4.98
70~80	0.63	7.70	7.07
80~90	0.73	7.17	6.44
90~100	3.23	8.09	4.86
Total	16.03	70.19	54.16

). The maximum storage variable was 7.77 g·m⁻², which appeared at the shallow depth of 0–10 cm, and the minimum storage variable was 3.00 g·m⁻², which appeared at the depth of 30–40 cm. The 0–100 cm profile was divided into shallow (0–30 cm), middle (30–70 cm), and deep layers (70–100 cm). Table 3 shows that Na⁺ content increased by 19.38 g·m⁻² at 0–30 cm depth, with the largest increase, while Na⁺ increased by 16.40 g· m⁻² at 30–70 cm depth, with the smallest increase. After 10 times of irrigation with brackish water, the largest increase in Na⁺ accumulation was in shallow soil, which became the salt (sodium) accumulation area. During cotton growth, the root system absorbed more Na⁺, and the middle layer was the main development position of the root zone, resulting in the minimum increase in Na⁺ in the soil, which became the “sodium” removal zone.

4. Discussion

(1)

Influencing factors of Na⁺ accumulation in cotton fields

The accumulation of Na⁺ in cotton fields is an extremely complex dynamic process with the combined action of many factors. The direct influencing factors of this process are soil moisture and salt, and the fundamental reason for their dynamic changes is the combined action of natural and human factors [30,31]. The natural factors include precipitation (intensity, duration, and interval), evaporation, soil physical and chemical properties (soil particle size, structure, and salt content of the parent material), cotton’s ability to absorb Na⁺, and the depth of shallow groundwater level. Human factors include irrigation system [32,33], including irrigation quantity, quality, and frequency [34]. The above factors jointly affect the accumulation of Na⁺ in cotton fields, and each factor plays a different role in different growth stages of cotton and different spatial positions (WR or NR) of cotton fields. This study only focused on the relationship between the spatial dynamic characteristics of Na⁺ accumulation and soil water and salt.

(2)

Na⁺ accumulation and soil water and salt dynamics in cotton fields

Salt flows with water, and water removes salt [35,36]. Soil water and salt changes under drip irrigation under film also follow this basic law [37]. However, due to the particularity of drip irrigation, water infiltration under drip irrigation can be divided into four stages.

The first stage is initial infiltration, where it is in the shape of a three-dimensional ellipsoid. When the amount of drip irrigation is small or the time of drip irrigation is short, water cannot penetrate to the diving surface (soil water stopped infiltrating at 50 cm depth after irrigation under scenarios VII and VIII in Figure 2), and the ellipsoid cannot be connected into a line, which determines the distribution of salt at the edge of the ellipsoid under drip irrigation (salt stayed at 50 cm depth after irrigation under scenario VIII in Figure 3). Furthermore, at the depth of 20–40 cm, the salt of the NR was greater than that of the WR and IM, in line with the shape of the ellipsoid, and there was a “no water belt” between two adjacent ellipsoids, forming a situation of low salt inside and outside the ellipsoid, and high salt only on the edge of the ellipsoid. The second stage is the flourishing period of infiltration: when the drip irrigation lasts for a long time, or the amount of drip irrigation is large, the water penetrates to the diving level (scenarios I and II in Figure 2), enters the groundwater, and the “no water belt” disappears. The salinity of the irrigation water, soil water, and groundwater is connected (scenarios I and II in Figure 3), showing several ellipsoids connected by linear infiltration, and the salt content is the largest at the intersection of two adjacent ellipsoids. The third stage is the end of infiltration: when drip irrigation continues, the intersection space of adjacent ellipsoids becomes increasingly larger, and the intersection space of salts is completely connected, showing an overall downward infiltration until the end of drip irrigation. The fourth stage is when drip irrigation ends; soil moisture and salt may continue to infiltrate to the diving surface under the action of water potential. However, if soil water cannot seep into the water, the edge of the wetting front is where the salt accumulates.

According to the experimental results, the accumulation of Na⁺ in the cotton field was controlled by the dynamic characteristics of soil water and salt, but a spatio-temporal difference existed between Na⁺ and soil salt distribution. As Na⁺ is a salt component, the four stages of infiltration are basically consistent with soil salinity, but have four differences. First, after irrigation, the Na⁺ content in the WR was lower than that in the NR and IM (scenarios II, IV, and IX were the most prominent in Figure 4), and no significant difference was observed in soil salt in the same plane. The migration rate and distribution characteristics of various salt ions were different in the salt composition (Wang et al., 2007), and Na⁺ was more significantly affected by the position of the drip head and strongly affected by leaching. The second difference was in the first stage of decrease, where Na⁺ decreased significantly compared with salt in the early irrigation period (scenarios IV and VIII in Figure 4). Third, in the fourth stage, after drip irrigation stopped, soil Na⁺ increased near the diving level, which was consistent with the results of Su et al. (2011) that showed that soil salinity changed from the lower to the upper layer after irrigation. Finally, at a certain depth, the variation of Na⁺ content in the plane (including WR, NR, and IM) before and after irrigation was small, showing a “thin waist” in the Na⁺ profile, while this phenomenon did not exist in the salt profile.

5. Conclusions

(1)

Owing to the strong spatial variation of soil permeability, significant differences were observed in soil water and salt dynamics after irrigation in all experimental scenarios in the study area, especially at 0–60 cm depth. In the study area, until post-irrigation monitoring, the irrigation water quota of 35.1 mm could penetrate to less than 1 m to achieve the effect of salt washing in cotton fields, but had the risk of pollution of shallow groundwater.

(2)

At a certain depth in the cotton field, before and after irrigation, the variation amplitude of Na⁺ content in the plane (including WR, NR, and IM) was very small, showing a “thin waist” in the Na⁺ profile, but this phenomenon was not present in the salt profile. The location of the thin waist depends on the climate and soil lithology.

(3)

The accumulation of Na⁺ in cotton fields is an extremely complex dynamic process with multiple factors. This process is controlled by the dynamic characteristics of soil water and salt, but it is different from the distribution of soil salt in time and space. However, the root cause of the dynamic change of soil water and salt is the result of the combined action of natural and human factors.

6. Patents

This section is not mandatory but may be added if there are patents resulting from the work reported in this manuscript.