1. Introduction
Cotton fiber has ideal length, good texture, strong moisture absorption, air permeability, and dye affinity [
1,
2]. Therefore, cotton (
Gossypium Spp.) is an important textile material, as one of the principal economic crops in China [
3]. At present, cotton accounts for more than 40% of all textile raw materials, and this proportion continues to increase [
4]. Xinjiang is the main region of origin of cotton in China, as a result of its unique climatic conditions and vast land resources making it very suitable for the growth of cotton [
5]. Since under-film drip-irrigation technology has been widely implemented in Xinjiang, high-yield cotton cultivation techniques have gradually matured. According to the National Bureau of Statistics, as of 2019, the cotton planting area in Xinjiang was 2.54 × 10
6 hm
2 and the yield was 5.00 × 10
6 t, accounting for 76.08% of the national cotton planting area and 84.94% of the total cotton yield [
6,
7]. However, with the increase in under-film drip irrigation over time, its disadvantages of excessive consumption of soil water and soil resources have become more obvious, and the cotton yield has decreased year by year [
8,
9]. In recent years, in order to increase yield, cotton farmers have continuously increased irrigation water consumption, even exceeding the demand for cotton growth [
10]. Problems such as rampant plant growth, yield and quality decline, low water-use efficiency, and premature senescence of cotton have become increasingly intense, and have become prominent problems affecting the further yield increase of cotton [
11,
12]. Therefore, it is urgent to advance the high-yield cultivation techniques of cotton in Xinjiang to improve the utilization rate of water resources and increase the cotton yield.
Automatic irrigation control systems combine water-saving irrigation technology, computer technology, sensors, and communication technology, enabling monitoring and prediction of soil moisture in real time, depending on the characteristics of crop water demand, and can achieve on-demand irrigation and precision irrigation [
13,
14]. The crisis of water resources in Xinjiang is prominent, and serves as a good foundation for automatic irrigation. The Xinjiang Production and Construction Corps vigorously promote automatic drip-irrigation technology, where the eighth division’s 148 regiments oversee 45 hm
2 of cotton using automatic drip-irrigation technology. Automatic drip-irrigation technology mainly uses soil moisture sensors to control irrigation, which also plays a certain role in improving crop water-use efficiency and yield [
15,
16]. Jones [
17] and Van [
18] considered capacitance- or frequency-domain reflection measurement sensors to be suitable soil moisture sensors for automated irrigation systems in nursery and greenhouse production, due to ease of maintenance, low cost, and reliability. Bacci used tension gauges to detect water potential in flowerpots to adapt water supply to plants’ actual needs, reducing consumption without having a negative impact on plants [
19]. Riber and Yoder used soil water sensors and a weather forecasting device to monitor soil moisture and predict crop transpiration in a real-time fuzzy control irrigation system. The system used changes in climate and soil moisture to control irrigation [
20]. Devitt [
21] developed an intelligent irrigation automation system based on changes in plant transpiration, and achieved good water-saving and yield-increasing effects after local practical application. Nielson [
22] and Yuan [
23] used different water shortage indices of crop canopy temperature as feedback indices to judge the water shortage status of crops, so as to accurately find the threshold of irrigation time and achieve more accurate irrigation. Yuan [
24] designed an intelligent irrigation system based on GPRS + ZigBee wireless networking technology, which could adjust and control the amount of irrigation according to the changes in light intensity, environmental humidity, and soil temperature, so as to ensure the balance and stability of the irrigated ecological environment. Scholars have studied large-scale farmland irrigation computer control systems with multiple communication forms and remote control irrigation and fertilization, which can be commonly used in farmland, orchards, and other green spaces [
25,
26]. Advanced irrigation technology can improve the irrigation water-use efficiency and the yield of cotton, and achieve the efficient utilization of water resources, which is the inevitable trend of the development of high-yield cultivation techniques in Xinjiang.
It is well known that crop growth is affected not only by the soil environment, but also by the farmland microclimate environment. Farmland microclimate is derived from the balance of matter and energy between soil–crop–atmosphere systems [
27]. A reasonable farmland microclimate can regulate the temperature and humidity of the environment, improve light-use efficiency, and prevent wind and sand [
28]. It can also effectively regulate crop photosynthesis and material conversion, and has a positive impact on crops’ physiological growth and yield improvement [
29]. As for influencing factors of the farmland microclimate, previous studies have mainly focused on crop planting density, intercropping mode, planting mode, and coating film types; however, there have been relatively few studies on irrigation methods [
30,
31]. Irrigation is an important part of agricultural production; it changes the soil environment and, thus, the farmland microclimate environment. Therefore, this study discusses the effects of different irrigation methods on cotton’s physiological and growth indices, yield, and farmland microclimate, thus providing valuable information to boost yields by selecting appropriate irrigation techniques in similar areas.
4. Discussion
Plant height, leaf area, and stem thickness are important traits of crop growth and important indicators for evaluating crop growth [
34,
35]. Cotton plant height directly affects cotton density and light utilization. Cotton stem thickness has an important impact on crop nutrient absorption and migration [
36]. The effect of cotton leaf area on green leaf coverage and light-use efficiency is significant, and it is also one of the important indices to measure early onset of cotton [
37]. With the rapid decrease in water supply and the increase in crop water demand, people are more and more interested in precise irrigation technology to improve water productivity [
38,
39]. Wang [
40] pointed out that a reasonable irrigation threshold can improve the growth of cotton, which is similar to the conclusion of this study. The soil environment and climate are constantly changing, and irrigation systems that can meet crops’ water demand should also be available, and more accurate information irrigation is needed to achieve this purpose. Automatic drip irrigation as a technology to obtain irrigation information is more precise than the general artificial irrigation technology, and the differences in each growth index between the three treatments were most obvious in the flowering stage. The flowering period is a sensitive period for cotton with respect to watering [
41]. The amount of irrigation must be kept within a reasonable range. Insufficient irrigation will lead to the decrease in the number of cotton bolls and a decrease in cotton yield. Excessive irrigation will also lead to delays to the cotton growth period, and the cotton bolls will not produce cotton, which will also reduce its yield [
42]. This study showed that the plant height, leaf area, and stem diameter of T3 at the flowering and boll-opening stages were significantly higher than those of the T2 and T1 treatments due to advanced water and fertilizer management techniques, and the cotton yield was significantly increased. It can be seen that automatic drip irrigation under mulch can make reasonable irrigation decisions in the irrigation-sensitive period of cotton by collecting information on the soil and atmospheric environment, so as to improve crop growth and yield.
The photosynthesis, growth, and development of crops depend on their genetic characteristics to a large extent, but the external environment also has a significant impact on them [
43]. The construction of a reasonable farmland microclimate environment can promote the growth of crops and the improvement of photosynthetic capacity [
44], which is also one of the important factors for the improvement of cotton yield. Academics [
45,
46] found that the fertilizer level of rice was inversely proportional to the air temperature within the population, but positively associated with the relative humidity. This is consistent with the results of this study. The air temperature of the T3 treatment was significantly lower than that of the T2 and T1 treatments (
p < 0.05), while the relative humidity of the T3 treatment was significantly higher than that of the T2 and T1 treatments (
p < 0.05). After the seedling stage, the soil temperature and air temperature were both negatively correlated with yield, although relative humidity was not. This demonstrates that the technology of automatic drip irrigation under mulch can provide a colder and wetter environment for crops. Under automatic irrigation, the growth and the vegetation coverage rate of crops are better than those attained when using on-film irrigation or under-film drip irrigation. Thus, the automatic irrigation method has a good shielding effect against the light radiation, which not only improves the light-utilization efficiency, but also protects the soil from direct light. In addition, with frequent irrigation at the beginning of the flowering period, the soil moisture content is always higher, the soil heat capacity is increased, and the soil temperature is noticeably decreased. In this study, the on-film irrigation had the lowest planting density and the worst growth potential. Although the planting density of the under-film drip irrigation was the same as that with automation, the growth potential was not as good as that of automation. Therefore, different irrigation technologies regulate the farmland microclimate through their planting density, and affect the growth potential of crops. This shows that the physiological growth of crops and the microclimate of farmland are mutually connected.
Photosynthesis is the basis of all life activities of crops, and the improvement of cotton yield is also based on the improvement of photosynthetic products. The strength of photosynthetic capacity is affected by the external climatic conditions and the growth conditions of crops [
47]; suitable growth environment and conditions can improve the photosynthetic capacity of crops [
48]. The results showed that the net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO
2 concentration of drip irrigation were significantly higher than those of the other two treatments (
p < 0.05). The physiological indices of each treatment were positively correlated with the yield. This study further shows that farmland microclimate, crop physiological growth characteristics, and yield are mutually connected, and that these factors are affected by factors such as fertility, irrigation, and soil. This experiment further highlighted that the effect of T3 on these factors was significantly better than that of T2 and T1.
The crop yield is the point of greatest concern for farmers. If we take the increase in yield as the main basis of our evaluation, the yield-increasing effect of T3 was the most obvious. In arid areas such as Xinjiang, water resources are very scarce [
49]. When increasing cotton production, the shortage of water resources in northern Xinjiang should also be considered. Therefore, it is necessary to consider irrigation strategies more comprehensively. This study found that the yield and iWUE of T3 were significantly higher than those of T1 and T2. T3 is the optimal irrigation strategy to achieve water savings and yield increase.
In order to save water and optimize agronomic factors, international modifications have been introduced that improve the production of perennial tropical crops [
50]. Today, for the irrigation of cotton, bananas, and other crops, farmers mainly use sprinklers (under foliage) and drip irrigation; with respect to the latter, there are crops such as bananas that grow in an inconsistent and random way, deviating from the original planting line, and after several years the plants will no longer be aligned with the drip line, reducing the irrigation efficiency. Additionally, drip irrigation in tropical territories has been found to be a method that saves water and reduces runoff, allowing water to trickle slowly towards plant roots, and improving productivity under certain physical or morphological soil properties [
51,
52]. Today, the global water crisis has seriously affected the development of agriculture [
53]. Drip irrigation is recognized as the most advanced irrigation technology in the world, and has a very wide range of application—especially in Xinjiang, China, where water resources are seriously short, and the drip-irrigation area has reached 3.53 × 10
6 hm
2 [
54]. This provides basic conditions for the implementation of automatic drip irrigation. In addition, the rapid development of the Internet, computer technology, and automation has further promoted the development of smart agriculture and agricultural informatization [
55,
56]. Combined with the findings of this study, automatic drip irrigation has obvious advantages in water saving and yield increase, and is an important measure to achieve the sustainable development of agriculture, with very optimistic prospects for future development.
5. Conclusions
(1) Automatic drip irrigation technology significantly increased relative humidity, and reduced soil and air temperature, providing a good farmland microclimate environment for crop growth.
(2) The growth and photosynthetic capacity of cotton under automatic drip-irrigation technology were significantly higher than under the other two treatments, improving the yield of cotton. The yield of automatic drip irrigation was 5.8% and 73.2% higher than that of drip irrigation and film irrigation, respectively.
(3) The physiological growth indices of cotton under different irrigation methods, along with the farmland microclimate indices, were positively correlated with cotton yield, except for soil and air temperature. Based on the analysis of each index, automatic drip irrigation under mulch is the optimal way to regulate physiological and farmland microclimate indices of cotton growth.
(4) Continuous implementation of automatic irrigation in drip-irrigation application areas is an important measure to achieve water saving and yield increase, and has very broad development prospects.