Analysis of Soil Moisture, Temperature, and Salinity in Cotton Field under Non-Mulched Drip Irrigation in South Xinjiang

Abstract

The mulch film residues in cotton fields in south Xinjiang have caused serious harm to the soil environment and ecological security in the oasis areas. Non-mulched planting provides an alternative approach to this problem. In this experiment, irrigation was provided on the basis of the reference crop evapotranspiration (ET₀). Two layouts of drip tapes (1T4R—one tape for four rows; 2T4R—two tapes for four rows) were applied to the non-mulched, drip-irrigated cotton fields in south Xinjiang, and their impacts on soil water–heat–salt dynamic changes and the water consumption and yield of cotton were compared and analyzed. The experiment shows that the 2T4R layout provided an excellent soil water–salt environment for cotton growth and yield formation. Soil temperature decreased by 0.8 °C and drip irrigation belt input increased by CNY1200·hm⁻². However, a higher profit derived from the 2T4R layout could compensate for the increased expenditure. The results show that cotton cultivation using non-mulched drip irrigation instead of mulched drip irrigation can potentially alleviate soil environmental and ecological security problems caused by plastic mulch residues in oasis areas. Although cotton yield was reduced by about 15%, water and nitrogen strategies and other field management could be adjusted to compensate for the disadvantages.

1. Introduction

The Xinjiang region has sufficient sunshine, sparse precipitation, and strong evaporation, which are conducive to crop agriculture, and the efficiency of agricultural production depends entirely on the available water resources [1,2]. In order to ensure a positive trajectory in agricultural production in Xinjiang, mulched drip irrigation has improved rapidly. With its beneficial effects of water saving, soil moisture conservation, and salt suppression, this technology has been widely applied in cotton cultivation [3,4]. However, the long-term and continuous utilization of this type of cultivation significantly increases the problem of mulch film residues [5]. The mulch films not only harm the soil environment and ecological security in the oasis areas in Xinjiang but also present a serious threat to the sustainable development of regional agricultural production [6]. Studies have shown that the amount of mulch residues in agricultural fields in Xinjiang is about 158.4 kg·hm⁻² [7] (with total amount exceeding 5.0 × 10⁵ t [8]) and is increasing dramatically at an annual rate of 15.69 kg·hm⁻² [9]. The Aksu region is seriously polluted, with average residual mulch above 275.63 kg·hm⁻² [10]. The long-term accumulation of residual mulch can damage soil structure, impede soil water transport, and reduce soil nutrient content. As the amount of residual mulch increases, the unit weight of soil decreases. Consequently, the porosity increases [11], reducing the water-retention capacity of the soil [12]. After the removal of residual mulch, the water-balancing time in the 0–20 cm soil layer is reduced by 45–50% [13]. In addition, residual mulch contamination reduces seed germination and affects crop growth and development, thus reducing the crop yield [14]. In areas with average mulch residue of 261.1 ± 117.8 kg·hm⁻², the seed-germination rate decreased by 5.1%, while the rotten seed and root rates increased by 1.7% and 1.58% on average, respectively, compared to areas without residual films [15]. When the amount of soil residual mulch was higher than 210 kg·hm⁻², the cotton yield was reduced by 16.9–21.6% [16]. Therefore, how to effectively curb the expansion of pollution caused by residual mulch films in cotton production has become a major scientific and practical issue that needs to be urgently addressed in Xinjiang [17].

Considering the long-term soil environmental problems, attempts to cultivate cotton using drip irrigation without mulch have provided an alternative approach and key technical support for solving the problem of mulch film residues in cotton fields in southern Xinjiang [14]. However, as the cotton cultivation mode changes from mulching cultivation to non-mulching cultivation, the risk of cotton yield reduction increases. The microenvironment of farmland will also be fundamentally changed, and consequently requires corresponding changes in the water-supply pattern and irrigation system. At present, cotton under mulched drip irrigation in Xinjiang is usually cultivated in a configuration of wide (20 cm) and narrow (40 cm) rows with multiple rows under one film. The layout of drip irrigation tapes has a significant impact on the dynamics of soil water and salinity and cotton-root distribution [18], but there are regional differences in the layout of drip tapes. In north Xinjiang, the capillary tape under the mulch film is arranged as one film, one tape, and four rows, which is more beneficial for reducing the soil salinity of cotton-root zones than the layout of one film, two tapes, and four rows [19]. In south Xinjiang, the capillary tape layout of one film, two tapes, and four rows forms desalination zones suitable for cotton growth in the main root layer and the salt stress is relatively small, but the layout of one film, one tape, and four rows produces long horizontal and vertical migration distances for soil salt, and as a result the cotton plants in the outer rows are in the salt-accumulation zone and cotton growth is affected by salt stress [20]. If the drip tape spacing is too large, the cotton in the inner rows and outer rows grows unevenly and the cotton rhizomes become small. Reducing the drip tape spacing can make the cotton grow evenly and the rhizomes large [21]. In respect of different soil types, Liu et al. [22] proposed that the appropriate layout modes of drip tapes under mulch film for sandy and clay soils are one film–one tape–two rows and one film–one tape–four rows, respectively. When the soil is sandy, there is no significant difference in the movement of water to the midpoint of the drip irrigation line at drip tape intervals of 0.91 m or 1.82 m, and the differences in cotton yield and irrigation water use efficiency are also small [23].

However, the change in cotton cultivation from mulching to non-mulching increases the risk of cotton yield reduction. The soil microenvironment of the farmland may also be fundamentally changed. Consequently, farmland water management measures also need to be adjusted accordingly. Firstly, under drip irrigation conditions, ground evaporation is significantly reduced due to the barrier effect of the mulch. Thus, cotton water consumption is significantly reduced. Mulching also has a significant effect on soil water infiltration and redistribution. A relatively independent water transport and absorption unit is usually formed under the same mulch. In contrast, without mulching, soil evaporation will increase significantly in the early stages of cotton growth. Hydrothermal conditions of soils will also significantly change. Therefore, the water consumption process in cotton fields is significantly different from that under mulched drip irrigation. Secondly, salt control is an important element of farmland irrigation management in Xinjiang, especially in southern Xinjiang. Under non-mulching cultivation, strong surface evaporation and serious surface salt aggregation occur during the cotton reproductive period (especially the early reproductive period). Existing irrigation salt washing and salt control measures cannot meet the needs of the non-mulching cotton cultivation mode. Salt washing and control patterns of cotton fields need to be accordingly changed based on the changes in the water and soil environment of farmland under the new cultivation mode.

Therefore, the feasibility of cotton non-mulched cultivation was investigated by field experiments to alleviate the problems of soil environment and ecological safety in the oasis area caused by plastic film residues. The effects of different drip tape layouts on soil water–heat–salt dynamics, water consumption, and yield of cotton were further studied to increase the cotton yield and the possibility of sustainable production in the hope of helping to cope with soil water–heat–salt changes and cotton yield reduction risks in cotton fields, thus providing technical support for the reasonable layout of field irrigation systems with non-mulched drip irrigation in south Xinjiang. This study is significant for sustainable production of cotton in south Xinjiang and large-scale promotion of cotton under drip irrigation without mulch.

2. Materials and Methods

2.1. Experimental Site

The experiments were conducted at the experimental irrigation station of Xinjiang Production and Construction Crops, First Division (81°17′56″ E, 40°32′36″ N, 1014 m above sea level) (Figure 1). The experimental station is in a typical inland zone with a dry climate, sparse precipitation, and strong evaporation all year round. The multiyear average of air temperature is 11.3 °C, precipitation 45.7 mm, evaporation 1876.6–2558.9 mm, sunshine duration 2950 h, frost-free period 207 days, and groundwater depth 3.5–5.0 m. The soil at the experiment station has a sandy, loamy texture, with an average bulk density of 1.58 g cm⁻³ and good air permeability, and the 0–80 cm field moisture capacity is 23.8% (gravimetric soil moisture determination).

2.2. Experimental Design

The cotton variety provided for the experiments was the new, very early-maturing cotton variety Zhongmian 619, sown on 22 April 2018 and harvested on 27 October 2018. This experiment adopted a single-factor, randomized block design. There were six blocks in total, each with a size of 45 m × 6 m (length × width), and each treatment was replicated three times. The two configurations of the drip tapes were one tape for four rows (1T4R), and two tapes for four rows (2T4R). The row spacing of cotton was 20 + 40 cm (Figure 2), and the plant spacing was 10 cm.

The FAO proposed the crop irrigation strategy based on crop evapotranspiration [24]. Fontanet [25] and Tan [26] et al. demonstrated the feasibility of irrigation strategies by applying different irrigation quota (25–100% ET) and irrigation frequency. In the irrigation experiment, the necessary information on the crop coefficient (K_C) in cotton fields under non-mulch cultivation method is not available, so ET₀ × K_C cannot be used to confirm the irrigation strategy. Therefore, we designed an irrigation strategy referring to the experimental methods used by Cheng [27] and Fan et al. [28]. At present, the irrigation quota for cotton under mulched drip irrigation in south Xinjiang is about 30 mm [29,30]. The irrigation quota for cotton under non-mulched drip irrigation was increased by 50% to 45 mm (1.0 ET₀) in this study [27]. The experiments started from the cotton seedling stage. The reference evapotranspiration (ET₀) was calculated using the Penman–Monteith formula recommended by FAO-56 (Figure 3), ET₀-P (precipitation) was calculated from daily weather data, and irrigation was implemented when the accumulated value had reached 45 mm (ET₀-P = 45 mm) [14,31]. Irrigation quotas and irrigation dates are shown in Table 1. The drip tapes used were disposable, single-wing, labyrinth drip tapes with a specification of Φ16, a distance of 30 cm between water droppers, a maximum flow rate of 3.0 L h⁻¹, and a working pressure of 0.1 MPa. The specified fertilizer for drip irrigation was applied at 1200 kg hm⁻², and the spraying of pesticides and other agronomic measures were implemented following the local best practice.

2.3. Measurement of Factors and Methods

2.3.1. Soil Moisture Content and Temperature

Soil moisture and temperature were measured by Decagon 5TM sensors and the EM50 (Washington, DC, USA) data loggers recorded data every hour [32,33]. The sensors were distributed in wide and narrow rows and placed at depths of 10, 20, 40, 60, and 80 cm (Figure 1). At the end of each growth stage, the sensors were recalibrated by soil sampling and drying. Table 2 shows the divisions of growth stages.

2.3.2. Soil Salinity

Soil salinity was characterized by the conductivity measurement value. At the end of each growth period, the soil was sampled and dried, and the dried soil sample was crushed. The soil sample was passed through a 1 mm sieve, 20 g was weighed into a flask, and 100 mL distilled water was added. The flask was shaken for 10 min, left to stand for 15 min and filtered to prepare an extract with a water to soil mass ratio of 5:1. The electrical conductance of the extract EC_5:1 was measured by the DDB-303A (Shanghai, China) portable conductivity meter [34].

2.3.3. Cotton Yield

During the cotton harvest, three pickings were conducted according to the opening of bolls. The actual picking yield of each of the three harvests was weighed and the number of bolls and mass per hundred bolls recorded at the same time. The total cotton yield was calculated as in Formula (1) [35]:

(1)$Y=0.01n_p\omega \rho$

where Y is yield (kg·hm⁻²), n_p is number of bolls per plant (pieces·plant⁻¹), ω is mass of a single boll (g), and ρ is planting density (plants·m⁻²).

2.3.4. Cotton Water Consumption

The water consumption (ET) of cotton was calculated by water balance, following Equation (2) [36]:

(2)$ET=\Delta S+P+I+G+R_0+D_P$

where ET is evaporative water consumption (mm) of the cotton field in a certain period, ∆S is changed amount of soil moisture content in the 0.8 m deep soil layer at the beginning of the period compared to the end of the period (mm), P is effective rainfall recharge obtained by the cotton field (mm), I is irrigation water supply to the cotton field in a certain period (mm), G is groundwater recharge (mm), R₀ is surface runoff (mm), and D_P is deep seepage loss during the period (mm). The terrain of the experiment field was flat, and there was no surface runoff. The groundwater level was deep (3.5–5.0 m), so the groundwater recharge obtained by the cotton field was negligible. The irrigation method of the experiment field was drip irrigation, so the deep seepage loss can also be ignored [28].

2.3.5. Water Use Efficiency

The water use efficiency values WUE_ET and WUE_I were calculated by Formulae (3) and (4), respectively:

(3)$WU{E}_{ET}=Y/ET$

(4)$WU{E}_I=Y/I$

where ET represents actual water consumption (mm).

2.3.6. Data Processing

Data processing was performed using Microsoft Excel (2010). Figures were drawn by Origin (2017). Statistical analyses were performed using DPS (16.05), and analysis of variance and treatment differences were calculated using Duncan’s new multiple-range method (α = 0.05).

3. Results

3.1. Dynamic Changes in Soil Moisture with Different Drip Tape Layouts

Under different planting conditions, the differences in the range and distribution of soil moisture have a great impact on the moisture absorption and growth of crops [37]. The dynamic changes in soil moisture at 0–80 cm depths in the two drip tape layouts are shown in Figure 4. The cycle of dynamic changes in soil moisture was consistent with the irrigation cycle. The soil moisture content of the wide rows in the 1T4R layout was higher than that of the narrow rows, while the soil moisture content of the narrow rows in the 2T4R layout was higher than that of the wide rows. In general, the soil moisture content of wide and narrow rows in 1T4R was lower than that in 2T4R. When the drip tape layout was 1T4R, the 0–20 cm soil moisture content of the narrow rows was higher than that of the wide rows, while the soil moisture below 20 cm of the wide rows was higher. The soil moisture content of wide and narrow rows fluctuated greatly at a depth of 0–40 cm. While the soil moisture content decreased rapidly, the soil moisture storage capacity was low, and an obvious “dry zone” was formed in the short term. The fluctuations in soil moisture content gradually diminished at a depth of 40–60 cm. At 60–80 cm depth, the soil moisture content was relatively stable with small fluctuations and a large “wet zone” formed. When the drip tape layout was 2T4R, the 0–20 cm soil moisture content of the wide rows was higher than that of the narrow rows, but was lower in the 20–80 cm zone. The soil moisture content of wide and narrow rows had relatively large fluctuations between the depths of 0 and 20 cm, which was an active layer with relatively low soil moisture content, and an obvious “dry zone” formed. The variation in soil moisture content between 20 and 40 cm was relatively small, as a subactive layer. The soil moisture content between 40 and 80 cm was relatively stable, and this was a stable layer where the relatively high soil moisture content formed a “wet zone.” At soil depth of 0–40 cm, soil moisture levels close to the dry zone based on the type of soil reported can be a problem for the crop in the early phases with low root development.

3.2. Dynamic Changes in Soil Salinity in Different Drip Tape Layouts

The dynamic changes in soil salinity in different drip tape layouts are shown in Figure 5. Before irrigation, the soil salinity of the fields with the two drip tape layouts were all relatively high. After irrigation, salinity decreased significantly (p < 0.05), and at the seedling stage had decreased by 20.38% (1T4R wide), 20.55% (1T4R narrow), 23.42% (2T4R wide), and 16.35% (2T4R narrow) compared with the original value. After entering the blooming and boll-setting stage, a resalinization phenomenon appeared, and at 0–80 cm depth salinity gradually increased. In general, the drip tape layout of 2T4R had a better leaching effect on salt concentration than the 1T4R layout. The layouts of drip tape had different effects on the distribution of salt in the soil profile. When the drip tape layout was 1T4R, the 0–40 cm salinity of the wide and narrow rows of the cotton field was relatively low, while below 40 cm it was relatively high. During the entire cotton growth period, the 0–80 cm salinity of the wide rows was lower than that of the narrow rows. When the drip tape layout was 2T4R, the 0–40 cm salinity of the wide and narrow rows of the cotton field was low, and the salinity gradually increased at depths below 40 cm. The salinity of the 2T4R layout were therefore generally lower than those of the 1T4R layout across the row profiles.

3.3. Impacts of Different Drip Tape Layouts on Soil Temperature

Correct soil temperature is closely related to cotton growth and crop yield [38]. The changes in the outside air temperature and the average soil temperature of the two drip tape layouts are shown in Figure 6. The soil temperature is greatly affected by the air temperature and changes commensurately. At the seedling stage, the soil temperature of the two drip tape layouts was not significantly different. After the bud stage, the average soil temperature in the main root zone of the cotton field using the 1T4R layout was 0.8 °C higher than in the 2T4R layout, but the profile of the two tape types was similar throughout the soil depths, increasing and then decreasing.

The dynamic changes in soil temperature in the wide and narrow rows of cotton fields using two drip tape layouts are shown in Figure 7. When the drip tape layout was 1T4R in the cotton seedling stage, the 0–40 cm soil temperature of the narrow rows was lower than that of the wide rows. After entering the cotton bud stage, the 0–40 cm soil temperature of the narrow rows was higher using the 1T4R configuration, while the 40–80 cm soil temperature of the narrow rows was higher than that of the wide rows. When the drip tape layout was 2T4R in the cotton seedling stage, the 0–20 cm and 60–80 cm soil temperature of the narrow rows was higher than that of the wide rows, while the 20–60 cm soil temperature of the wide rows was higher. After entering the bud stage, the 60–80 cm soil temperature of the wide rows was higher. Until the early stage of cotton blooming and boll setting, the soil temperature of the two drip tape layouts decreased with the increase in soil depth, while in the later stage of cotton blooming and boll setting, it increased with the increase in soil depth.

3.4. Impacts of Drip Tape Layout on Water Consumption of Cotton

The water consumption of the two drip tape layouts is shown in Table 3. In the cotton seedling stage, the plants were small and the air temperature was low. The average daily water consumption of cotton was around 3.2 mm. At the cotton bud stage, as the air temperature gradually increased, the cotton plants entered a period of vigorous growth, and the average daily water consumption reached a maximum of 4.6 mm. After entering the blooming and boll-setting stage, the average daily water consumption of cotton slightly decreased to around 3.8 mm. In contrast, throughout the entire cotton growth period, the 2T4R drip tape layout had higher water consumption. However, the difference was not statistically significant. The water consumption of cotton throughout the entire cotton growth period in the 1T4R layout was 488.87 mm, while that of the 2T4R layout was 514.04 mm.

3.5. Impacts of Different Drip Tape Layouts on Cotton Yield and Irrigation Water Use Efficiency

The cotton yield and water use efficiency can reflect the impacts of different drip tape layouts on cotton growth and water consumption. The number of bolls per plant, the mass of a single boll, and the yield of seed cotton with the 2T4R layout were all higher than those of the 1T4R layout, by 2.73%, 0.87%, and 3.54%, respectively (Table 4). There was no difference in any of the cotton yield and water use efficiency response variables in response to the two drip tape configurations. This is to be expected, as the less water applied, the lower the productive yield, which indicates the high sensitivity of the cotton to the available water. Undoubtedly, there were moments of hydric stress that affected the crop, as documented in [29].

4. Discussion

In terms of methods for real-time irrigation decisions, they are mainly categorized into four types worldwide: based on evapotranspiration (ET) calculation and water balance, soil moisture, crop moisture, and models [39]. The ET and water balance-based irrigation decision method [24] is easier and more general to implement. It has been used in the CROPWAT [40] developed by the USDA FAO on the smartphone software Smart Irrigation Apps [41]. In addition, irrigation decisions need to be combined with precision irrigation systems to form precise irrigation decision and control systems. Thus, the decision effectiveness and the irrigation system benefit can be truly reflected [42]. Currently, precision irrigation systems are mainly studied in three aspects: (1) analyzing the relationship between environment and crop water requirements [43]; (2) conducting simulation experiments using crop irrigation models [44]; and (3) using expert knowledge to obtain irrigation rules [45]. Since cotton cultivation with non-mulched drip irrigation is still in the exploration stage, the ET value remains unclear. Therefore, in this paper, ET₀-P was used to guide irrigation [14,27,31]. ET₀ is strongly influenced by temperature and rainfall, increasing with temperature and decreasing with rainfall (Figure 3).

Planting and irrigation methods can cause significant differences in soil moisture, salinity, and temperature, affecting cotton growth and yield. For cotton planting under non-mulched drip irrigation, the layout of drip tapes affects the dynamic changes of soil water, temperature and salinity. The spatial dynamic changes in soil moisture content in cotton fields with 1T4R and 2T4R drip tape layouts were relatively consistent. They were characterized by a long cotton seedling period, relatively low air temperature, low soil evaporation, low transpiration of cotton plants, and small water consumption [46]. Therefore, the soil in the cotton seedling stage maintained high moisture content and the layout of drip tapes had little impact on the soil moisture content. As cotton entered the rapid growth phase, the air temperature concomitantly increased, as did soil evaporation, in the absence of mulch film covering. Simultaneously, water consumption of cotton plants increased in the later period of cotton blooming and boll setting, with gradual decrease in air temperature, soil evaporation, and water consumption, as well as soil moisture, such that such the moisture content of the soil rebounded. For the drip tape layout of 1T4R, the fluctuation of 0–40 cm soil moisture content was large, and it was easy to form a “dry zone” in the short term [47]. The fluctuation of 40–60 cm soil moisture content was relatively small, and the 60–80 cm soil moisture content remained stable, forming a stable layer of water recharge [20,48], and a “wet zone” with relatively high moisture appeared. For the drip tape layout of 2T4R, the soil depth of 0–20 cm was a “dry zone” of water activity, the soil moisture change was relatively small between 20–40 cm, and the 40–80 cm depth was a “wet zone” with relatively stable soil moisture content. Since the moisture of the soil surface was greatly affected by cotton transpiration, its fluctuation was large, while the soil moisture in the stable layer was less affected, and moisture accumulation formed at a depth of 80 cm. Meanwhile, affected by the layout of drip tapes and the duration of irrigation, a “narrow and deep” wet zone was formed for the 1T4R layout, and a “broad and shallow” wet zone was formed for the 2T4R layout [49,50]

Before irrigation at the seedling stage, the soil evaporation was strong due to the effect of no mulch film coverage, and the salinity value of the 0–80 cm deep soil layer was relatively high, but as times of irrigation increased, 0–80 cm soil salinity gradually decreased and the salt diffused horizontally and vertically [20]. By the cotton-blooming and boll-setting stage, cotton was affected by both air temperature and evapotranspiration, and soil resalinization occurred. The 2T4R drip tape layout was more beneficial for the leaching of soil salt than the 1T4R layout. This is mainly because the 2T4R layout of drip tapes formed two desalination zones in the cotton main root layer, which widened the salt-leaching areas [20,51]. When the drip tape layout was 1T4R in the cotton seedling stage and bud stage, the 0–20 cm soil salinity of the narrow rows was lower than that of the wide rows, while it was higher than that of the wide rows after entering the blooming and boll-setting stage. Since the 1T4R drip tapes were distributed in the wide rows, the 20–60 cm moisture content of the wide rows was higher than that of the narrow rows, and the salinity was relatively low, but the salinity of the wide rows increased below 60 cm. When the drip tape layout was 2T4R, the 0–20 cm salinity of the narrow rows was lower than that of the wide rows. Since the 2T4R drip tapes were distributed in the narrow rows, the water accumulated in the wide rows, the salt was carried to the wide rows, and the salinity was therefore higher.

Air temperature and drip tape layouts have an impact on soil temperature. The change in soil temperature is the process of absorbing or releasing energy with the change in solar radiation and air temperature [35], and the soil temperature varies concomitantly. When the drip tape layout was 1T4R, the 0–40 cm soil temperature of the narrow rows in the cotton seedling stage was lower than that of the wide rows. After entering the cotton bud stage, the 0–80 cm soil temperature of the narrow rows was higher than that of the wide rows. This is because the exposure of cotton seedlings to solar radiation in wide-row soil was higher than that of the narrow-row soil and the soil temperature was commensurately higher. At the bud stage, the effect of air temperature was ameliorated and irrigation itself was responsible for lowering soil temperature. Therefore, the 0–80 cm soil temperature of the narrow rows was higher than that of the wide rows. When the drip tape layout was 2T4R in cotton seedling stage, the 0–20 cm and 60–80 cm soil temperature of the narrow rows was higher than that of the wide rows, while the 20–60 cm soil temperature of the wide rows was higher. After entering the bud stage, the 60–80 cm soil temperature of the wide rows was higher. The increase in soil moisture content leads to an increase in soil-specific heat capacity, which affects soil warming and is not beneficial for water uptake by crop roots [50,52,53]. In general, in the early stages of cotton growth, the soil temperature of the two drip tape layouts was not much different, but with growth, the soil temperature of the 1T4R layout was higher than that of the 2T4R layout.

The water consumption with the growth process of cotton under non-mulched drip irrigation was characterized as follows. The intensity of daily water consumption was low in the cotton seedling stage. In the bud stage, the cotton plants grew faster, and the intensity of daily water consumption increased significantly, reaching a maximum. In the blooming and boll-setting stage, the intensity of daily water consumption slightly decreased. This is slightly different from the study by Wu et al. [48], because the sowing time of cotton under non-mulched drip irrigation was late and the seedling period was longer, resulting in an overall delay of the growth period. In general, the water consumption during the cotton growth period of the 1T4R layout was slightly less than that of the 2T4R layout—488.87 mm and 514.04 mm, respectively—but the difference was not statistically significant. From the analysis of cotton yield composition, seed cotton yield and water use efficiency, the layout of 2T4R was better than 1T4R, but the differences between the two were not statistically significant. Compared to the 1T4R treatment, the 2T4R treatment showed an increase in the cost of drip irrigation tape and the cotton yield benefits by 1200 and CNY1749.5·hm⁻², respectively. Thus, a total of CNY549.5·hm⁻² can still be achieved in the 2T4R treatment after the increased input cost is factored in. In addition, with the development in drip irrigation tape production technology, tougher drip irrigation tape can facilitate manual/mechanical recycling and further mitigate soil pollution by plastic particles.

Compared with existing studies on mulched drip irrigation, it was found that the shift from mulching to non-mulching cotton cultivation increased evapotranspiration and soil salinity, reduced soil moisture and temperature, and decreased cotton yield by about 15% [54]. However, increasing the irrigation quota could reduce the yield difference between the two cultivation modes [14,54]. Although there are several obstacles to the large-scale extension of the non-mulched drip irrigation mode, the cultivation mode with non-mulched drip irrigation is a feasible means to maintain sustainable cotton production and reduce mulch pollution in Xinjiang. In addition, this study only investigated the effect of drip irrigation belt arrangement on soil water–heat–salt conditions and cotton yields under drip irrigation without mulch conditions. The feasibility of cotton cultivation with non-mulched drip irrigation was verified. The appropriate drip irrigation belt arrangement was determined. Further studies will be focused on improving cotton yields and irrigation efficiency of cotton under drip irrigation without mulch.

5. Conclusions

The soil moisture content of the 2T4R layout was greater than that of the 1T4R layout, forming a “broad and shallow” wet zone, which was beneficial for the growth of cotton. Moreover, two desalination zones were formed in the main root zone of the cotton, and the desalination effect was better in the 2T4R. However, the average soil temperature in the main root zone of the 1T4R layout was 0.8 °C higher than that of the 2T4R layout. The cotton yield composition, seed cotton yield, and water use efficiency of the 2T4R layout were slightly higher the 1T4R layout. To sum up, soil moisture under 2T4R layout was higher than that in the 1T4R, forming a “broad and shallow” wet zone, two desalination zones of 2T4R were formed in the root zone of cotton, and the 2T4R layout was suitable for the drip-irrigated cotton without mulch in south Xinjiang. Therefore, we recommend farmers use the 2T4R layout for filmless cotton planting.

Author Contributions

Conceptualization, X.W.; methodology, Y.G.; software, H.C.; formal analysis, H.W.; investigation, H.W. and H.C.; data curation, H.W. and F.J.; writing—original draft preparation, H.W.; writing—review and editing, Y.G.; funding acquisition, X.W., H.W. and Y.G. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We are thankful to Weixiong Huang (orcid.org/0000-0002-1704-5550) for his help in English editing for the paper.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Location of the experimental area and field sampling.

Figure 2. Schematic diagram of cotton planting method and drip irrigation belt layout (cm).

Figure 3. Meteorological data for 2018.

Figure 4. Dynamic changes in soil moisture under different drip irrigation treatment conditions in 2018a. (a) 1T4R wide, (b) 1T4R narrow, (c) 2T4R wide, (d) 2T4R narrow.

Figure 5. Dynamic changes in soil salinity in different drip irrigation treatment conditions in 2018a. Note: KK is 1T4R wide; KZ is 1T4R narrow; ZK is 2T4R wide; ZZ is 2T4R narrow.

Figure 6. Average temperature of soil compared to air temperature in different drip irrigation treatment conditions in 2018a.

Figure 7. Dynamic changes of soil temperature in different drip irrigation treatment conditions in 2018a. (a) 1T4R wide, (b) 1T4R narrow, (c) 2T4R wide, (d) 2T4R narrow.

Table 1

Irrigation quota and irrigation date.

Irrigation Quota	Irrigation Date
45	22 May 2018
45	31 May 2018
45	10 June 2018
45	23 June 2018
45	5 July 2018
45	14 July 2018
45	24 July 2018
45	6 August 2018
45	16 August 2018
45	31 August 2018

Table 2

Divisions of cotton birth stage.

Growth Stage	Date	Growing Days (d)
Sowing–emergence	22 to 30 April 2018	8
Emergence stage–seedling stage	1 May 2018 to 29 June 2018	60
Seedling stage–bud stage	30 June to 2 August 2018	33
Bud stage–prophase of boll stage	3 August to 21 August 2018	18
Prophase –later period of boll stage	22 August to 10 September 2018	19
Later period of boll stage–floating period	11 September to 27 October 2018	46

Table 3

Effects of different drip irrigation belt arrangements on cotton water consumption in 2018a.

Treatment	Seedling Stage			Bud Stage			Flower Bell Period
	IA	WC	ADWC	IA	WC	ADWC	IA	WC	ADWC
	(mm)	(mm)	(mm d−1)	(mm)	(mm)	(mm d−1)	(mm)	(mm)	(mm d−1)
1T4R	180	194.58a	3.24a	135	151.82a	4.60a	135	142.48a	3.85a
2T4R	180	208.72a	3.48a	135	159.42a	4.83a	135	145.90a	3.94a

Note: 1. IA is irrigation amount, WC is water consumption, ADWC is average daily water consumption. 2. Different letters in the same column indicate significant differences between treatments (p < 0.05).

Table 4

Effects of different drip irrigation belt layouts on yield and irrigation water use efficiency of cotton in 2018a.

Treatment	BN	BM (g)	Yield (kg hm−2)	TIA (mm)	TWC (mm)	WUEET (kg m−3)	WUEI (kg m−3)
1T4R	4.40a	4.58a	4947.96a	450	488.87a	1.01a	1.10a
2T4R	4.52a	4.62a	5122.91a	450	514.04a	1.00a	1.14a

Note: 1. BN is boll numbers per plant, BM is boll mass, TIA is total irrigation amount, TWC is total water consumption. 2. Different letters in the same column indicate significant differences between treatments (p < 0.05).

References

1. Cao, H.; Wang, H.; Li, Y.; Hamani, A.K.M.; Zhang, N.; Wang, X.; Gao, Y. Evapotranspiration partition and dual crop coefficients in apple orchard with dwarf stocks and dense planting in arid region, Aksu oasis, southern Xinjiang. Agriculture; 2021; 11, 1167. [DOI: https://dx.doi.org/10.3390/agriculture11111167]

2. Wang, X.; Wang, H.; Si, Z.; Gao, Y.; Duan, A. Modelling responses of cotton growth and yield to pre-planting soil moisture with the CROPGRO-Cotton model for a mulched drip irrigation system in the Tarim Basin. Agric. Water Manag.; 2020; 241, 106378. [DOI: https://dx.doi.org/10.1016/j.agwat.2020.106378]

3. Yang, P.; Hu, H.; Tian, F.; Zhang, Z.; Dai, C. Crop coefficient for cotton under plastic mulch and drip irrigation based on eddy covariance observation in an arid area of northwestern China. Agric. Water Manag.; 2016; 171, pp. 21-30. [DOI: https://dx.doi.org/10.1016/j.agwat.2016.03.007]

4. Wu, N.; Yang, C.; Luo, Y.; Sun, L. Estimating evapotranspiration and its components in cotton fields under deficit irrigation conditions. Pol. J. Environ. Stud.; 2018; 28, pp. 393-405. [DOI: https://dx.doi.org/10.15244/pjoes/80895]

5. Wang, D.; Wang, C.; Xu, L.; Bai, T.; Yang, G. Simulating growth and evaluating the regional adaptability of cotton fields with non-film mulching in Xinjiang. Agriculture; 2022; 12, 895. [DOI: https://dx.doi.org/10.3390/agriculture12070895]

6. Dong, H.; Liu, T.; Li, Y.; Liu, H.; Wang, D. Effects of plastic film residue on cotton yield and soil physical and chemical properties in Xinjiang. Trans. CSAE; 2013; 29, pp. 91-99.

7. Zhang, D.; Liu, H.; Hu, W.; Qin, X.; Ma, X.; Yan, C.; Wang, H. The status anddistribution characteristics of residual mulching film in Xinjiang, China. J. Integr. Agric.; 2016; 15, pp. 2639-2646. [DOI: https://dx.doi.org/10.1016/S2095-3119(15)61240-0]

8. Wang, L.; Lin, T.; Yan, C.; Wang, J.; Guo, R.; Yue, L.; Tang, Q. Effects of plastic film residue on evapotranspiration and soil evaporation in cotton field of Xinjiang. Trans. CSAE; 2016; 32, pp. 120-128.

9. He, H.; Wang, Z.; Guo, L.; Zheng, X.; Zhang, J.; Li, W.; Fan, B. Distribution characteristics of residual film over a cotton field under long-term film mulching and drip irrigation in an oasis agroecosystem. Soil Till. Res.; 2018; 180, pp. 194-203. [DOI: https://dx.doi.org/10.1016/j.still.2018.03.013]

10. Hu, C.; Wang, X.; Chen, X.; Tang, X.; Zhao, Y.; Yan, C. Current situation and control strategies of residual film pollution in Xinjiang. Trans. CSAE; 2019; 35, pp. 223-234.

11. Jiang, X.; Liu, W.; Wang, E.; Zhou, T.; Xin, P. Residual plastic mulch fragments effects on soil physical properties and water flow behavior in the Minqin Oasis, northwestern China. Soil Till. Res.; 2017; 166, pp. 100-107. [DOI: https://dx.doi.org/10.1016/j.still.2016.10.011]

12. Wang, Z.; Li, X.; Shi, H.; Xu, P.; Li, H. Water characteristic curve model for soil with residual plastic film. Trans. CSAE; 2016; 32, pp. 103-109.

13. Yin, M.; Li, Y.; Fang, H.; Chen, P. Biodegradable mulching film with an optimum degradation rate improves soil environment and enhances maize growth. Agric. Water Manag.; 2019; 216, pp. 127-137. [DOI: https://dx.doi.org/10.1016/j.agwat.2019.02.004]

14. Wang, H.; Cao, H.; Gao, Y.; Wang, X. The effects of drip-irrigation scheduling without mulching on soil moisture, yield and quality of cotton in southern Xinjiang. J. Irrig. Drain.; 2020; 39, pp. 26-34.

15. Wang, X.; Zhou, Y.; Cao, S.; Liu, X.; Wang, M. Research on the present situation, harm and control countermeasures of mulch residue in cotton fields of Xinjiang corps. Proceedings of the 2010 International Conference on Agricultural Engineering of Modern Agricultural Machineryapplication Seminar Subvenue; Shanghai, China, 18 September 2010; Chinese Society for Agricultural Machinery: Beijing, China, 2010; Volume 4.

16. Jiang, Y.; Zheng, D.; Zhu, Z. Effects of remnant plastic film in soil on growth and yield of cotton. Agro-Environ. Prot.; 2001; 20, pp. 177-179.

17. Liang, R.; Chen, X.; Zhang, B.; Meng, H.; Jiang, P.; Peng, X.; Kan, Z.; Li, W. Problems and countermeasures of recycling methods and resource reuse of residual film in cotton fields of Xinjiang. Trans. CSAE; 2019; 35, pp. 1-13.

18. Liu, M.; Yang, J.; Li, X.; Yu, M.; Wang, J. Effects of irrigation water quality and drip tape arrangement on soil salinity, soil moisture distribution, and cotton yield (Gossypium hirsutum L.) Under Mulched Drip irrigation in Xinjiang, China. J. Inteng. Agric.; 2012; 11, pp. 502-511. [DOI: https://dx.doi.org/10.1016/S2095-3119(12)60036-7]

19. Liu, M.; Yang, J.; Li, X.; Yu, M.; Wang, J. Effects of drip irrigation strategy on cotton root distribution and water use efficiency. Trans. CSAE; 2012; 28, pp. 98-105.

20. Wang, C.; Yang, P.; Yu, Y.; Di, F.; Huang, F. Effect of different pipe arrangements on soil water-salt transport and yield of cotton under mulched drip-irrigation. Agric. Res. Arid Areas; 2016; 34, pp. 38-45.

21. Wang, Y.; Li, M.; Wei, C.; Zhang, J. Effect of drip-lines space on cotton root and plant growing under plastic mulched drip irrigation. J. Irrig. Drain.; 2010; 29, pp. 68-73.

22. Liu, J.; Lv, X.; Wang, D.; Li, Z.; Xu, G. Influence of lateral layout of drip irrigation under plastic mulch on water transmission and effect of increase cotton production. Water Sav. Irrig.; 2004; 6, pp. 23-26.

23. Grabow, G.L.; Huffman, R.L.; Evans, R.O.; Jordan, D.L.; Nuti, R.C. Water distribution from a subsurface drip irrigation system and dripline spacing effect on cotton yield and water use efficiency in a coastal plain soil. Trans. ASABE; 2006; 49, pp. 1823-1835. [DOI: https://dx.doi.org/10.13031/2013.22303]

24. Allen, R.G.; Periera, L.S.; Raes, D. Crop Evapotranspiration. Guidelines for Computing Crop Water Requirement; FAO Irrigation and Drainage Paper No. 56 FAO: Rome, Italy, 1998.

25. Fontanet, M.; Scudiero, E.; Skaggs, T.H.; Fernà ndez-Garcia, D.; Ferrer, F.; Rodrigo, G.; Bellvert, J. Dynamic Management Zones for Irrigation Scheduling. Agric. Water Manag.; 2020; 238, 106207. [DOI: https://dx.doi.org/10.1016/j.agwat.2020.106207]

26. Tan, S.; Wang, Q.; Zhang, J.; Chen, Y.; Shan, Y.; Xu, D. Performance of AquaCrop model for cotton growth simulation under film-mulched drip irrigation in southern Xinjiang, China. Agric. Water Manag.; 2018; 196, pp. 99-113. [DOI: https://dx.doi.org/10.1016/j.agwat.2017.11.001]

27. Cheng, M.; Wang, H.; Fan, J.; Xiang, Y.; Liu, X.; Liao, Z.; Ahmed, E.A.; Zhang, F.; Li, Z. Evaluation of AquaCrop model for greenhouse cherry tomato with plastic film mulch under various water and nitrogen supplies. Agric. Water Manag.; 2022; 274, 107949. [DOI: https://dx.doi.org/10.1016/j.agwat.2022.107949]

28. Fan, K.; Gao, Y.; Wang, X.; Wang, H.; Duan, A. Study on cotton drip irrigation under film mulching in southern Xinjiang based on meteorological information. Agric. Res. Arid Areas; 2019; 37, pp. 83-90.

29. Che, Z.; Wang, J.; Li, J. Effects of water quality, irrigation amount and nitrogen applied on soil salinity and cotton production under mulched drip irrigation in arid Northwest China. Agric. Water Manag.; 2021; 247, 106738. [DOI: https://dx.doi.org/10.1016/j.agwat.2021.106738]

30. Cai, H.; Shao, G.; Zhang, Z. Water demand and irrigation scheduling of drip irrigation for cotton under plastic mulch. J. Hydraul. E; 2002; 11, pp. 119-123.

31. Wang, X.; Jiang, F.; Wang, H. Irrigation Scheduling Optimization of Drip-irrigated without Plastic Film Cotton in South Xinjiang Based on Aqua Crop Model. Trans. CSAM; 2021; 52, pp. 293–301, 335.

32. Tian, Z.; Ren, T.; Horton, R.; Heitman, J.L. Estimating soil bulk density with combined commercial soil water content and thermal property sensors. Soil Till. Res.; 2020; 196, 104445. [DOI: https://dx.doi.org/10.1016/j.still.2019.104445]

33. Lo, T.H.; Rudnick, D.R.; Singh, J.; Nakabuye, H.N.; Katimbo, A.; Heeren, D.M.; Ge, Y. Field assessment of interreplicate variability from eight electromagnetic soil moisture sensors. Agric. Water Manag.; 2020; 231, 105984. [DOI: https://dx.doi.org/10.1016/j.agwat.2019.105984]

34. Cui, Y.; Wang, F.; Sun, J.; Han, Q.; Wang, J.; Li, N. Effects of irrigation regimes on the variation of soil water and salt and yield of mechanically harvested cotton in Southern Xinjiang, China. Chin. J. Appl. Ecol.; 2018; 29, pp. 3634-3642.

35. Wang, F.; Sun, J.; Liu, Z.; Ning, H.; Qiang, X.; Shen, X. Effect of different irrigation scheduling on salt distribution and leaching in cotton field. Trans. CSAM; 2013; 44, pp. 120-127.

36. Sandhu, R.; Irmak, S. Assessment of AquaCrop model in simulating maize canopy cover, soil-water, evapotranspiration, yield, and water productivity for different planting dates and densities under irrigated and rainfed conditions. Agric. Water Manag.; 2019; 224, 105753. [DOI: https://dx.doi.org/10.1016/j.agwat.2019.105753]

37. Wang, M.; Lv, T.; He, X.; Cao, Y.; Wang, D. Effects of drip-irrigated planting modes on soil water, temperature, salt and cotton growth. Agric. Res. Arid Areas; 2018; 36, pp. 176-186.

38. Li, J.; Wang, J.; Li, N.; Hao, X.; Shi, X.; Tian, Y.; Shi, F.; Luo, H.; Yang, G. Yield of non-film cotton and the response of temperature, humidity and salt in the root zone to irrigation. Trans. CSAE; 2021; 37, pp. 134-143.

39. Gu, Z.; Qi, Z.; Ma, L.; Gui, D.; Xu, J.; Fang, Q.; Yuan, S.; Feng, G. Development of an irrigation scheduling software based on model predicted crop water stress. Comput. Electron. Agric.; 2017; 143, pp. 208-221. [DOI: https://dx.doi.org/10.1016/j.compag.2017.10.023]

40. Savva, A.P.; Frenken, K. Crop Water Requirements and Irrigation Scheduling; FAO Sub-Regional Office for East and Southern Africa: Harare, Zimbabwe, 2002.

41. Vellidis, G.; Liakos, V.; Andreis, J.H.; Perry, C.D.; Porter, W.M.; Barnes, E.M.; Morgan, K.T.; Fraisse, C.; Migliaccio, K.W. Development and assessment of a smartphone application for irrigation scheduling in cotton. Comput. Electron. Agric.; 2016; 127, pp. 249-259. [DOI: https://dx.doi.org/10.1016/j.compag.2016.06.021]

42. Gu, Z.; Yuan, S.; Qi, Z.; Wang, X.; Cai, B.; Zheng, Z. Real-time precise irrigation scheduling and control system in solar greenhouse based on ET and water balance. Trans. CSAE; 2018; 23, pp. 101-108.

43. Sánchez-Molina, J.A.; Rodríguez, F.; Guzmán, J.L.; Ramírez-Arias, J.A. Water content virtual sensor for tomatoes in coconut coir substrate for irrigation control design. Agric. Water Manag.; 2015; 151, pp. 114-125. [DOI: https://dx.doi.org/10.1016/j.agwat.2014.09.013]

44. Feng, S.; Jiang, J.; Huo, Z.; Zhang, C. Optimization of irrigation scheduling under deficit irrigation with saline water for spring wheat based on SWAP model. Trans. CSAE; 2014; 30, pp. 66-75.

45. Lu, X.; Zhang, L.; Liu, H.; Zhi, C.; Li, J. Design of the networked precision irrigation system for paddy field crops in intelligent agriculture. Trans. CSAE; 2021; 17, pp. 71-81.

46. Liu, M.; Yang, J.; Li, X.; Yu, M.; Wang, J. Effects of irrigation amount and frequency on soil water distribution and water use efficiency in a cotton field under mulched drip irrigation. Chin. J. Appl. Ecol.; 2011; 22, pp. 3203-3210.

47. Wang, X. Effects of Winter-Spring Irrigation on Soil Water-Salt Dynamics and Cotton Growth. Ph.D. Dissertation; Chinese Academy of Agricultural Sciences Dissertation: Beijing, China, 2018.

48. Wu, C.; Hu, S.; Zhao, C. Dynamic change of soil moisture and water consumption of cotton under drip irrigation patterns in Tarim irrigation area. Water Sav. Irrig.; 2016; 2, pp. 14-17.

49. Wang, Y.; Li, M.; Lan, M. Effect of soil wetting pattern on cotton-root distribution and plant growth under plastic mulched drip irrigation in field. Trans. CSAE; 2011; 27, pp. 31-38.

50. Li, D.; Li, M.; Liu, D.; Lv, M.; Jia, Y. Effects of soil wetting pattern on the soil waterthermal environment and cotton root water consumption under mulched drip irrigation. Chin. J. Appl. Ecol.; 2015; 26, pp. 2437-2444.

51. Yang, X.; Dong, X.; Liu, L. Study on transportation of water and salt in soil under two tapes positioning style for drip irrigation under film of Cotton. Water Sav. Irrig.; 2011; 2, pp. 40–42, 45.

52. Li, H.; Xia, Z.; Ma, G. Effects of water content variation on soil temperature process and water exchange. J. Hohai Univ.; 2007; 35, pp. 172-175.

53. Feng, B.; Zhang, Z.; Zhang, J.; Wang, Z. Review of effect of temperature on soil water movement. Adv. Water. Sci.; 2002; 13, pp. 643-648.

54. Wang, H.; Zhao, L.; Gao, Y.; Wang, X.; Cao, H. Study on Irrigation Mode and Water Consumption of Cotton Drip Irrigation Without Film in South Xinjiang. J. Agric. Sci. Technol.; 2021; 23, pp. 153-160.