A R T I C L E I N F O

Handling Editor - Dr. B.E. Clothier

Keywords:

Vertical tube irrigation Jujube yield

Irrigation water use efficiency Soil water content

A B S T R A C T

Water shortage restricts the development of jujube plantation in southern Xinjiang, and the application of water–saving irrigation technology is the key to solve this problem. This study investigated the response of irrigation water use efficiency to various irrigation technologies (vertical tube irrigation and surface drip irri- gation) and jujube yield and quality, and to choose a suitable irrigation method for Hui–jujube trees planting. The field experiment was conducted over 2 years. Vertical tube irrigation was implemented with three levels of

water pressure by using water pressure heads 0.8, 1.0, and 1.2 m. Additionally, surface drip irrigation was used as a control, and four treatments were used in total. The results indicated that vertical tube irrigation helped save

approximately 37–70% of water, and its water use efficiency was 1.4–4.3 times that of surface drip irrigation. Compared with Hui–jujube trees watered by drip irrigation, those watered by vertical tube irrigation had slightly higher yield and decreased fruit cracking (25–83%). Moreover, the sugar–acid ratio increased significantly (P <

0.05). Besides, the continuous water supply used in vertical tube irrigation allowed the maintenance of relatively stable SWC in the main root layer (20–60 cm), can provided a good soil–water–air environment for jujube growth. Vertical tube irrigation saved water primarily because it sends water directly to the main root layer

through the emitters buried in the soil, reducing the evaporation of water in the upper soil layer. The irrigation water amount of vertical tube irrigation was proportional to the water supply pressure head. Water use efficiency was the highest under the lowest water supply pressure (water pressure head 0.8 m). The present findings provide a scientific basis for the application of vertical tube irrigation in arid areas.

1. Introduction

China is the world’s largest producer of the jujube, with yields constituting 98% of the world total, reaching 8.78 million tons in 2018 (Wang et al., 2020). The largest land area devoted to jujube trees in

China is in southern Xinjiang, where the jujube has become a major local commercial crops (Bai et al., 2020; Yu et al., 2020). This region has a temperate continental climate, with less rainfall and high evaporation rates. The utilization efficiency of natural precipitation during the pro- duction of jujube trees is very low and is predominantly dependent on irrigation. Irrigation techniques and management methods affect the yield and water use of jujube trees.

Compared with surface irrigation, drip irrigation, which can be divided into surface drip irrigation and underground drip irrigation (according to the method by which the dripper is buried), helps save more water (Martínez and Reca, 2014). Studies have proved that both

surface drip irrigation and subsurface drip irrigation can increase crop yields (Yazar and Sesveren, 2017; Aydinsakir et al., 2021). Subsurface drip irrigation can reduce the amount of water produced by evaporation (Phogat et al., 2016; Roy et al., 2019) and has a higher water use effi- ciency than surface drip irrigation (Martínez-gimeno et al., 2018a, 2018b; Çetin and Kara, 2019). Research on the water requirement rules of crops in different growth stages of crops has demonstrated that the amount of irrigation water affects both water use efficiency and crop yield. For example, when the grapes were regulated deficit irrigation, and the amount of irrigation is appropriately reduced, the quality of the fruit is higher (Yang et al., 2020). Nevertheless, the fruit swelling period in the jujube reduces the amount of irrigation water and reduces crop transpiration, resulting in reduced yield (Feng et al., 2017). In an experiment of surge-root irrigated of jujube trees, Dai et al. (2019) found that water use efficiency and yield depend mainly on two factors, irri- gation level and nitrogen content.

* Correspondence to: 5 South Jinhua Road, Xi’an 710048, China.

E-mail address: baidan@xaut.edu.cn (D. Bai).

https://doi.org/10.1016/j.agwat.2021.106875

Received 22 October 2020; Received in revised form 15 March 2021; Accepted 17 March 2021

Available online 26 March 2021

0378-3774/© 2021 Elsevier B.V. All rights reserved.

In recent years, water-efficient surface and subsurface drip irrigation technologies have been applied to forest and fruit planting worldwide (Robles et al., 2016; Martínez-gimeno et al., 2018a, 2018b; Hancioglu et al., 2019). However, these methods are unsuitable to the special climate of Xinjiang. When such technologies are implemented, the same forests in different regions can behave differently, and their normal growth is disrupted. In addition, the clogging problem of subsurface drip irrigation the implementation of this method over large areas is chal- lenging. At this stage, most jujube producers use a combination of flood irrigation and surface drip irrigation. Surface drip irrigation is mainly implemented during the cell expansion period, when the fruit is sensi- tive to water. However, its water-saving effect is not obvious and its water use efficiency is low.

Crop growth an water use efficiency are shaped by the soil-water-air environment (Su and Midmore, 2005; Du et al., 2020). Subsurface drip irrigation results in higher SWC, reducing soil air permeability. This negatively affects microorganism in the roots and soil and in turn may result in lower crop yield (Pendergast et al., 2019). Under aerated irri- gation (Chen et al., 2018), an effective solution to this problem, water and oxygen are transported to the root zone, creating a favorable water and air environment for crop growth (Li et al., 2016). This environment is associated with greater crop yield and water use efficiency (Du et al., 2019; Girsang et al., 2020). However, the uneven water-air transmission in the process leads to the loss of a large number of air bubbles. Thus, the method by which air should be transported to the root zone as micro- bubbles is the key to deciding whether this method can be employed on a large scale (Bhattarai et al., 2015). After the end of drip irrigation, the soil water was rapidly reduced under soil evaporation and plant tran- spiration (Çolak et al., 2017), thereby causing water stress that was unfavorable to crop growth.

Developing new irrigation technologies for jujube trees that can provide a good water and air environment for the main root layer as well as improve water use efficiency and crop yield and quality of is chal- lenging. Vertical tube irrigation is a new type of subsurface irrigation. Laboratory infiltration tests have indicated that continuous irrigation is possible trough automatic adjustment of emitter discharge according to the SWC (Dan et al., 2016a; 2016b). In the present study, vertical tube irrigation and surface drip irrigation were applied to jujube trees, and their water efficiencies and impacts on crop yield and quality were compared. In addition, the water-saving and production-increasing mechanisms of vertical tube irrigation were examined. The findings provide insights into the application of vertical tube irrigation in arid

areas and promote the sustainable development of Xinjiang’s fruit industry.

2. Materials and methods

2.1. Experimental site

The field experiment was conducted at a Hui–jujube orchard (Fig. 1) in Alaer (80◦ 50′ 30′′ E, 40◦ 29′ 23′′ N, altitude: 1025.4 m), Xinjiang,

China. The experimental site, a dry farming area, had a temperate desert climate with abundant sunshine. From April to October, the Hui–jujube growing season, sunshine hours exceed 8 h on average and precipitation is low. The groundwater depth is below 3.5 m. Fig. 2 presents the mean monthly maximum and minimum air temperatures and rainfall amounts

in April to October of 2018 and 2019. Temperatures were the highest in July over these 2 years, corresponding to an average of 25.54 ◦ C and

27.49 ◦ C, respectively. Total rainfall over the same period was 71.5 mm and 93.0 mm, respectively. The maximum monthly rainfall in June 2018 and August 2019 were 36 mm and 39 mm, respectively.

Fig. 2. Monthly mean maximum, minimum and average temperature and rainfall during the 2018–2019 jujube trees growing seasons.

Fig. 1. The location of experimental site.

Soil samples from the first meter of soil were taken at 10 cm in- tervals. The soil samples were assessed using a laser particle size analyzer (Mastersizer 2000, Malvern Panalytical, Malvern, UK). Ac- cording to the classification criteria of the United States Department of Agriculture, silty loam characterized the soil between 0 and 50 cm, and sandy loam characterized between 51 and 100 cm. Air drying was used to measured saturated and initial water content. Soil bulk density was

measured using a steel ring (volume, 100 cm3, height, 5.0 cm). The basic

physical properties of the experimental soil are shown in Table 1.

2.2. Experimental design and crop management

The research objects were 7-years-old (as of 2018) Hui–jujube (Yu et al., 2020) trees approximately 3.5 m in height. Row and tree spacing

was 3.0 m. Most of the jujube roots were concentrated in the upper layer (approximately 0–50 cm) (Dai et al., 2019). Hui-Jujube physiological and growth characteristics were observed and classified in five stages

during the growing season: (I) Leaf emergence period (Ear- ly-April–Mid-May); (II) Blossom and young fruit period (Late-- May–Mid-June); (III) Fruit swelling period (Mid-June–Late-July); (IV) Fruit maturation period (Early-August–Late-September); (V) Deciduous period (Early-October–Late-October).

Irrigation water comes from the Tarim River, which is the water salinity is 2.01–3.40 g L—1, and PH is 7.4–8.0 (Between June-July 2018 and 2019). Between March and April of both experimental years, spring

irrigation was performed to wash away some of the salt in the soil in accordance with local agronomic practices. After salt washing, the

average soil salinity in the 0–100 cm soil layer was 2.65 g kg—1 (2018) and 3.15 g kg—1 (2019), respectively. Standard agronomic measures

such as fertilization, trimming, insecticide spraying, and weed control were uniform for all trees. In March of both years, the chemical fertilizer

(CO(NH2)2) was applied with each tree (300 g tree—1). Top-dressing

with K2HPO4⋅3H2O and NH4H2PO4 was applied twice over each growing season (150–200 g tree—1).

To explore the influence of irrigation technologies on the yield and quality of jujube trees, and vertical tube irrigation and surface drip

irrigation (a commonly used method for planting jujube trees in Xin- jiang) were selected for the experiment to irrigate Hui–jujube trees. Fig. 3 presents details of the vertical tube irrigation methods used. The emitters in the vertical tube irrigation systems were composed of poly- vinyl chloride, and the inner diameter of the outlets was 1.6 cm (Fig. 3).

Each tree was irrigated using four emitters placed 30 cm away. Their outlets were buried 40 cm below the soil surface. The water supply heads, which were between the water outlet of the vertical tube and the air inlet of the water tank, were 0.8, 1.0, 1.2 m, respectively, and designated VI0.8, VI1.0 and VI1.2 accordingly. Under surface drip irriga- tion (designated DI), the lateral pipe was 30 cm away from the tree rows.

The drip emitters were spaced at 20 cm intervals and had a discharge

rate of 4.0 L h—1. In this experiment, each jujube tree was applied as an independent research plot. Nine and three trees were selected to

Table 1

The basic physical properties of the experimental soil.

undergo vertical tube irrigation (VI0.8, VI1.0, VI1.2) and surface drip irrigation (DI), respectively. Each of the four treatments was repeated three times.

Vertical tube irrigation was continuous implemented during Ear- ly–June (the late in blossom and young fruit period) to Mid–July (the middle to late in fruit swelling period). In 2018, this period was between June 13 and July 20, a total of 36 days (water was shut off on June 17, 18 and 23 and July 18). In 2019, this period spanned 34 days, from June

13 to July 16. Surface drip irrigation was implemented over 16 h, three times a year. In 2018, the dates were June 13, 27, and July15, respec- tively. In 2019, they were June 5, July 5, and 18, respectively.

2.3. Measurement and calculation data

Water conservation is the key to the sustainable development of agriculture in arid regions, and irrigation technology directly affects the

effect of irrigation. For this reason, the irrigation amount, SWC, Hui–jujube yield and quality were measured to compare the effects of different irrigation technologies. The specific measurement methods are as follows.

2.3.1. Irrigation amount

A cuboid tank (inner dimensions 29.6 cm 24.6 cm 35.0 cm) was used to supply water for vertical tube irrigation. The daily irrigation amount (mm) was calculated according to the difference in water level. Under surface drip irrigation, water meters were installed on the lateral to record the irrigation amount. IVI and IDI denote the total irrigation amount over the growing season under vertical tube irrigation and surface drip irrigation. They were summed from the irrigation amount over each day or at each irrigation, respectively.

2.3.2. Jujube yield and quality

After the maturation stage (October 2 on both years), five Hui–jujube fruits were picked from the eastern, southern, western, and northern

aspects of each tree. Soluble reducing content was measured using IR240 hold refraction instrument (IR240, InsMark, Shanghai, China) and soluble reducing sugar content was measured using the anthrone method (2008). The acid-sugar ratio (Solomakhin and Blanke, 2010), an indicator of taste and quality, was calculated from the ratio of organic acid content (determined through the titration method), to soluble reducing content. Vitamin C (VC) content was measured using the 2,6 dichloroindophenol sodium salt method (Cui et al., 2008).

On the last 2–3 days of October of each year, the fruit was harvested and the yield from each tree was calculated through weighing, mark as Y (t ha—1). From each tree, ten jujubes were randomly selected and

weighed, with the averaged weight denoted as y (g). The total yield (i.e., number of fruits) was mark as N. The cracked jujube weighed, mark as Y1.

The fruit cracking rate of jujube (CR) was calculated using the

following equations Eq. (1):

Depth (cm) Particle composition (%) Bulk density (g cm—3) Soil Texture Saturated water content (cm3 cm—3)

< 0.002 mm 0.002–0. 2 mm 2–0.2 mm

0–10 5.23 46.88 47.89 1.43 ± 0.01 Silty loam 0.41 ± 0.02

10–20 5.01 49.34 45.65 1.51 ± 0.02

20–30 6.64 46.71 46.65 1.46 ± 0.02

30–40 5.64 48.72 45.64 1.44 ± 0.01

40–50 5.32 49.21 45.47 1.45 ± 0.03

50–60 3.60 37.78 58.62 1.43 ± 0.02 Sandy loam 0.45 ± 0.05

60–70 3.95 28.50 67.55 1.46 ± 0.02

70–80 4.51 37.55 57.94 1.37 ± 0.04

80–90 4.87 33.60 61.53 1.35 ± 0.02

90–100 5.57 32.85 61.58 1.37 ± 0.03

Soil physical properties sample date: 2018/3/10. The bulk density data are the means ± SD of three replicate samples.

Fig. 3. The emitters and TDR arrangement of vertical tube irrigation. a. Longitudinal section. b. Schematic diagram of experiment layout.

CR = Y1 × 100 (1)

where CR is the fruit cracking rate (%), Y is the jujube yield (t ha—1), and

Y1 is the cracked jujube weighed(t ha—1).

2.3.3. SWC

Volumetric SWC in each field plot was measured using a TRIME-T3 tubular time-domain reflectometer (TRIME-PICO-IPH, IMKO GmbH, Ettlingen, Germany). The vertical SWC profile was measured at 20 cm intervals over the 100 cm soil layer. The TDR tube of the reflectometer was installed 30 cm from each tree, at a depth of 1.5 m (Fig. 3).

2.3.4. Climate data

Climate data in the experimental site, specifically solar radiation, air temperature, humidity and wind speed, were recorded every 15 min by using a portable weather station (Watchdog 2000, Spectrum Technolo- gies, Inc, Illinois, America).

2.3.5. The irrigation water use efficiency

Irrigation water use efficiency (IWUE) (Du et al., 2017) was calcu- lated using the following equations Eq. (2):

3.
Results

3.1. Irrigation water amount

3.1.1. Comparison of the total irrigation water amount between the two irrigation techniques

Annual irrigation water amounts over study period and under different irrigation techniques are shown in Table 2. In 2018, the average irrigation water amount for vertical tube irrigation over the three treatments was 183.42 ± 3.11 mm, 197.56 ± 86.06 mm, and

309.14 ± 55.19 mm, respectively. The irrigation amount per tree under

surface drip irrigation of 496.32 27.69 mm, was 2.30–2.71 times that

under vertical tube irrigation. Overall, irrigation amounts across all treatments were significantly lower in 2019 than in 2018. In 2019, the irrigation water amount for vertical tube irrigation over the three treatments was 150.28 ± 34.49 mm, 198.31 ± 25.20 mm,

192.72 ± 29.15 mm, respectively. The corresponding amounts per tree

under surface drip irrigation of 502.26 6.46 mm was 2.53–3.34 times

that under vertical tube irrigation. The irrigation water amount differed significantly (P < 0.05). The irrigation water amount in vertical tube irrigation was higher under higher water supply pressure. Over the experiment period, it exhibited a gradual reduction. The irrigation water

IWUE = 100 Y

(2)

amount under surface drip irrigation increased slightly over the same duration but was essentially maintained at approximately 500 mm.

Where IWUE is the irrigation water use efficiency (kg m—3), Y is the jujube yield (t ha—1), and I is the irrigation amount (mm).

2.4. Data analysis

The data were analyzed by variance, performed with the SPSS sta- tistical software (version 24.0, SPSS). The significance of the treatment

effect was set at P < 0.05 and was determined using the F-test. Data for

2018 and 2019 were analyzed separately.

3.1.2.
Irrigation processes

Fig. 4 shows the processes of the two irrigation methods over the study period. The single irrigation water amount differed by method. In general, the surface drip irrigation was performed when the SWC was low and stopped once the SWC had increased to a sufficient level or even reached the upper limit. Soil evaporation and tree transpiration lead to gradual reductions in SWC before drip irrigation begins. In other words, drip irrigation is intermittent, with a higher irrigation amount (over 100 mm), and a longer interval between irrigation sessions (Battam et al., 2003).

As mentioned, the emitters used for vertical tube irrigation can automatically adjust the discharge according to the SWC. Specifically, discharge was lower when the SWC was high (Dan et al., 2016a). Thus, the trees were continuously irrigated under this method. In the initial

Table 2

Irrigation amount per jujube tree under different irrigation methods from 2018 to 2019.

502.26 ± 6.46a

The data are the means ± SD of three replicate samples. Different letters indicate values significantly different comparisons within same year (P < 0.05).

Fig. 4. The irrigation process of different irrigation techniques in 2018 and 2019.

stage of vertical tube irrigation, the SWC was low. The growth re- quirements of jujube trees, and soil evaporation water consumption, meant that the daily irrigation water amount under vertical tube irri- gation was relatively high in the initial stage in 2018. It decreased gradually and varied little in the later stages. In the initial stage in 2019 of approximately 10 days, this amount was higher than that in 2018. And it decreased gradually over the irrigation season. At the end, the daily irrigation amount was less than that in 2018. These changes stem from the effects of temperature on the tree water consumption (Montoro et al., 2020). The average temperatures over the two irrigation seasons

were 24.57 ◦ C and 24.35 ◦ C, and the number of days when the average

temperature exceeded 25 ◦ C was 15 and 11 days, respectively (Fig. 2). A rise in temperature was accompanied with an increase in the water absorption rate of the jujube root system, leading to a change in the matric potential of the soil around it and a subsequent increase in the vertical tube emitter discharge.

3.2. Variations in SWC

The effects of irrigation on vertical direction soil water distribution in the jujube tree’s root systems were analyzed. Figs. 5 and 6 present the results of the analysis of vertical direction water content in different soil layers under vertical tube irrigation from 2018 to 2019. The SWC changes were similar over the two experimental years. The SWC at

0–20 cm increased with the increases in water supply pressure but stayed at a low level (approximately 20–25 cm3 cm—3) after approxi-

mately 10 days of continuous irrigation. The SWC in 2019 (Fig. 6e) was relatively stable compared to 2018 (Fig. 5a), which was mainly due to rainfall during the irrigation period in 2018, which caused slight

fluctuations in surface soil water content. In the initial stage, the SWC at

61–100 cm fluctuated slightly, but in the later stages, it was less than 30 cm3 cm—3. Overall, The SWC was the highest in the soil layer between 21 and 60 cm (approximately 30–40 cm3 cm—3, less than the saturated water content 45 cm3 cm—3) because the outlets were buried at a depth

of 40 cm and water diffuses from the outlets to the surroundings. Although the irrigation amount of vertical tube irrigation is relatively small, the SWC of this soil layer is relatively stable. Higher irrigation amounts under higher water supply pressure the upward migration of water and increased the surface SWC (Figs. 5b, c, and 6e, f). Conversely, when the irrigation water amount was lower, the surface SWC was also low (Figs. 5a and 6d).

Figs. 7 and 8 present the changes in SWC of different layers in the first meter of soil under surface drip irrigation from 2018 to 2019. Drip irrigation SWC were not changes with the experimental years. The SWC changes periodically with irrigation: the SWC was low before irrigation; increased after the process, and then decreased gradually. The most

substantial changes were observed in the 0–20 cm soil layer. Specif-

ically, approximately 5 days after irrigation, the SWC decreases from 35 to approximately 20 cm3 cm—3. In the soil layer between 21 and 60 cm,

the SWC increases after irrigation and then dropped by degrees and stayed at approximately 25 cm3 cm—3. Between 61 and 100 cm, it increased after irrigation and then stayed at 30 cm3 cm—3. In general,

SWC gradually increases with increase in soil depth. Under the action of gravity, water was easily transported to deeper soil layers and was therefore mostly inaccessible to crops (Song et al., 2018).

After the surface drip irrigation, the water consumption increased of jujube trees over time, and the mean SWC gradually decreases with time until the next irrigation. It gradually decreased from 39 to

Fig. 5. The soil water content (0–100 cm soil profile depth) variations with the different vertical tube irrigation treatments in 2018 irrigation season. a, b, and c are the distribution of soil water content under 0.8, 1.0, and 1.2 m water supply pressure, respectively.

Fig. 6. The soil water content (0–100 cm soil profile depth) variations with the different vertical tube irrigation treatments in 2019 irrigation season. d, e, and f are the distribution of soil water content under 0.8, 1.0, and 1.2 m water supply pressure, respectively.

approximately 20 cm3 cm—3 over 10 days. Under surface drip irrigation, water enters the soil from a point source on the surface, forming a wet body centered on the soil below the dripper. At the end of irrigation

session, both the maximum horizontal wetting distance and maximum SWC could be determined from the surface.

The distribution of SWC in the soil layers differed by method.

Fig. 7. The soil water content (0–100 cm soil profile depth) variations with surface drip irrigation in 2018.

Fig. 8. The soil water content (0–100 cm soil profile depth) variations with surface drip irrigation in 2019.

Although the irrigation amount under vertical tube irrigation was low, the SWC in the main root layer of jujube trees can be maintained within an appropriate range tends to the growth of crops (Reyes-Cabrera et al., 2016); Although the irrigation amount under surface drip irrigation was high, moisture on the soil surface evaporated easily (Feng et al., 2017), causing the SWC to fluctuate considerably before and after irrigation. Notably, the diffusion of water into the deeper soil layer will reduce the water content of the main root layer, which easily causes the main root to lack water and negatively affecting jujube trees growth (Patel and Rajput, 2007).

3.3. Jujube yield, quality and IWUE

Jujube tree yields obtained from the diverse treatments in the experimental years are presented in Fig. 9. In 2018, the jujube yield was the highest and lowest under the VI1.0 and VI1.2 treatments, at

23.90 ± 4.80 and 17.47 ± 2.01 t ha—1, respectively. The jujube yield

was higher in 2019, but again the highest and lowest under the VI1.0 and VI1.2 treatments, at 22.33 ± 1.34 and 17.61 ± 4.74 t ha—1, respectively.

Fig. 9. The jujube tree’s yield statistical analysis under among treatments in difference years. Error bars represent standard deviations.

The yield under the VI0.8 treatment essentially remained at 22 t ha —1

over 2 years. The yield from surface drip irrigation (10.07 1.36 and

12.85 0.42 t ha—1 in 2018 and 2019, respectively) was positively correlated with the year. The yield under vertical tube irrigation was

significantly higher than that under surface drip irrigation over both years (P < 0.05), and the yield did not differ considerably by treatment. The association between irrigation water amount and yield was not linear. Notably, although vertical tube irrigation consumed less water than surface drip irrigation, the yield was not lower under this method.