Effects of water application intensity of micro-sprinkler irrigation and soil salinity on environment of coastal saline soils

Lin-lin Chu *, Yao-hu Kang , Shu-qin Wan

a College of Agricultural Science and Engineering, Hohai University, Nanjing 210098, China

b Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research,Chinese Academy of Sciences, Beijing 100101, China

Abstract To achieve the greatest leaching efficiency, water movement must occur under unsaturated flow conditions. Accordingly, the water application intensity of irrigation must be chosen carefully. The aim of this study was to evaluate the impact of the water application intensity of micro-sprinkler irrigation on coastal saline soil with different salt contents. To achieve this objective, a laboratory experiment was conducted with three soil salinity treatments (2.26, 10.13, and 22.29 dS/m) and three water application intensity treatments (3.05, 5.19, and 7.23 mm/h).The results showed that the effect of soil salinity on soil water content,electrical conductivity,and pH was significant,and the effect of the water application intensity was insignificant.High soil water content was present in the 40-60 cm profile in all soil salinity treatments,and the content was higher in the medium and high water application intensity treatments than in the low-intensity treatment.Significant salt leaching occurred in all treatments,and the effect was stronger in the high soil salinity treatment and medium water application intensity treatment.In the medium and high soil salinity treatments,pH exhibited a decreasing trend,with no trend change in the low soil salinity treatment,and the pH value was higher in the medium water application intensity treatment than in the other two treatments. These results indicated that the three intensities evaluated had no statistically different effect on the electrical conductivity of saturated soil-paste extracts (EC) in the upper 20 cm of the soil profile, and it would be better to maintain a lower value of the water application intensity.

© 2020 Hohai University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Soil water content; Salinity; Micro-sprinkler irrigation; Water application intensity; Saline soil environment

1. Introduction

Saline soil, either caused by natural factors or anthropogenic activities, is widely distributed globally, accounting for approximately 25% of the total land area and causing serious problems (Zhang et al., 2017; Wichelns and Qadir, 2014; Liu et al., 2017). Coastal saline soil is one of the main types of saline soil. Along the 6 000-km coastline that extends from Jiangsu Province to Liaoning Province in North China, there are 10 000 km2of coastal saline land, which is a potential resource for agricultural production and ecological landscape construction (Yu and Chen, 1999; Li et al., 2011). However,high soil salinity, a shortage of fresh water, a high sodium adsorption ratio,wind erosion,and cold damage have resulted in large areas of saline wasteland in this region (Sun et al.,2017; Guo and Liu, 2015; Feng et al., 2018). This is a significant obstacle to industrialization and urbanization in this region. Currently this constraint is becoming even more prevalent because there has been an urgent need to improve the landscape to meet the increasing demand of living environments for cities and districts (Li et al., 2015a, 2015b).

The key to salinity control and irrigation sustainability is leaching, and it interacts closely with crop growth, irrigation methods, and soil-physical properties (Wang et al., 2012a).Whereas most soils with high salinity in coastal saline land have low permeability, which is considered critical in reclamation, their infiltration capacity tends to decrease significantly.This is accompanied by the corruption of soil structure,as soils are saturated (Kang, 1998). Moreover, most lands in these regions are low-lying with a shallow groundwater table(Yu et al., 2016; Wang et al., 2012b).

Therefore, irrigation plays an important role in the reclamation of saline soils,and the method used is critical.Results from several experiments (Nielsen et al., 1966; Oster et al.,1972) have shown that the quantity of salts removed per unit quantity of water leached can be increased appreciably by assuring that leaching occurs at soil moisture contents less than saturation, i.e., under unsaturated conditions. Microirrigation, including drip irrigation and micro-sprinkler irrigation, can supply water at low discharge rates and high frequencies over an extended period and produce a better salt leaching effect, with saline soil becoming moderately or even slightly saline (Zhang et al., 2017; Sun et al., 2017; Li et al.,2015a, 2015b; Wan et al., 2012; Wang et al., 2013; Chu et al., 2015, 2016; Sun et al., 2016; Zhang et al., 2017).Sprinkler irrigation is an ideal method for applying small quantities of water at a time. Leaching of soluble salts is also accomplished more efficiently when the water application intensities are lower than the infiltration capacity of the soil,and such a condition cannot be achieved with flood irrigation methods. In a field experiment (Nielsen et al., 1966), flood irrigation required three times as much water as sprinkler irrigation to reduce soil salinity by the same increment.Sprinkler irrigation also has the advantage that small local differences in the field topography will not cause non-uniform water application and salt leaching; this is a superior result to that produced by drip irrigation, which supplies water continuously at a point source in the immediate vicinity of plant roots.

The correct intensity is important for the successful use of sprinkler irrigation on coastal saline soil. Some researchers(Sun et al., 2008; Sor and Bertand, 1962) have found that,compared to lower intensities, a higher sprinkler intensity produces greater changes in properties such as soil structure and influences deeper soil layers more significantly.Zhao et al.(2003) reported that light rainfall had no effect on desalination, but moderate rainfall and heavy rainfall had a detectable and strong effect on desalination, respectively. Chu et al.(2015, 2016) claimed that the water application intensity of sprinkler irrigation influenced the soil water content,electrical conductivity of saturated soil-paste extracts (EC), and pH of saline soils. For coastal saline silt soil, low water application intensity was more desirable than high intensity.

Considering the effects on salt leaching in coastal saline sandy-loam soils, high water application intensity is more advantageous than low intensity under unsaturated flow conditions, as high intensities cause greater water movement in the soil. In recent years, the authors’ group reclaimed coastal saline soils(16.7 dS/m ＜EC＜ 66.7 dS/m)in the Bohai Gulf of China using sprinkler irrigation. After one year, the soil salts were well leached and the soil gradually became moderately saline (Li et al., 2019; Li and Kang, 2020). The most important step for successful use of sprinkler irrigation on highly saline soil is to select the appropriate sprinkler intensity.

The water application intensity must maintain the soil in an unsaturated state to ensure the highest leaching efficiency.Thus,it is critically important to keep sprinkler intensity lower than the soil's saturated infiltration rate. However, soils in a coastal region have different salt contents, and during the soil reclamation process the soil salinity changes, which in turn affects the infiltration rate. The questions of which water application intensity is appropriate and whether the water application intensity selected at the beginning of the reclamation process is suitable throughout the process are important. In order to make the physical and chemical properties of the coastal soil as consistent as possible,except for the varying salt content, the investigated soil consisted of the original high-salinity soil and the lower-salinity soil,which came from the original soil after leaching. The primary objectives of this study were (1) to test the possibility of reclaiming coastal saline soil using sprinkler irrigation, and (2) to investigate the effects of different sprinkler application intensities on soil reclamation under various ranges of soil salinity.

2. Materials and methods

2.1. Soil

The soil in this study was collected from the Industrial Zone of the Caofeidian District (39°03′N, 118°48′E) south of Tangshan City,in East China (Fig.1).This area is adjacent to the Bohai Sea, which borders the Pacific Ocean. It has a typical oceanic monsoon climate with an annual precipitation of approximately 580 mm,most of which occurs between June and September. The average annual evaporation and temperature are 1 677 mm and 11.4°C, respectively.

Fig. 1. Location of experimental site in Bohai Gulf region, China.

Disturbed soil samples were taken from the upper 20 cm layer at more than six locations in an area that was reclaimed from the sea in 2005. The soil in the 0-20 cm soil layer was classified as sandy loam.In the composition,0.45%of the soil particles were smaller than 0.002 mm in diameter, 43.15% of the particles were between 0.002 and 0.05 mm,and 56.39%of the particles were greater than 0.05 mm.The soil bulk density of the 0-20 cm soil layer ranged from 1.43 to 1.55 g/cm3.The saturated soil water content,EC, sodium absorption ratio (SAR), and pH value were 40%, 22.29 dS/m,50.76 (mmol/L)0.5, and 7.65, respectively (Table 1).

The collected soil was leached with water to remove salt and produce relatively low-salinity soil havingECand pH values of 2.26 dS/m and 8.41, respectively. The leached soil was air-dried, crushed, sieved through a 2-mm sieve, and thoroughly mixed. Finally, this prepared low-salinity soil was mixed with the original soil to obtain new mixed soil havingECand pH values of 10.13 dS/m and 7.76, respectively (Table 1).The three soil salinity treatments thus consisted of the prepared low-salinity soil(S1),the mixture of prepared soil and original soil (S2), and the original high-salinity soil (S3).

2.2. Experimental setup

Laboratory experiments were conducted using disturbed soil columns encased in acrylic glass.The volume of each soil column was 0.005 7 m3(110 mm in diameter and 600 mm in depth), with 1-mm-diameter holes drilled in the bottom to allow for drainage. A 0.1-mm-diameter polyester mesh was placed on the bottom of each column to prevent the soil from leaking out of the drainage holes. The mixed soil was placed in the soil column in 5-cm layers to achieve a soil bulk density of 1.5 g/cm3.

Three experiments were conducted in 2012 and the total area of each experiment was 12.56 m2(all of the soil columns were placed into a concentric circle with a radius of 2 m). A micro-sprinkler irrigation system was installed for each treatment and the number of small swivel micro-sprinklers(Beijing Lu¨yuan Co.) that was used in each system depended on the water intensity treatment required. The system is presented in Fig.2.The micro-sprinklers were deployed at the center of the circular arrangement of soil columns and were thus 4 m from the soil columns. The micro-sprinklers were mounted on a 125-cm-high riser such that the sprinklers were 65 cm higher than the upper surface of the soil columns. The nozzle operating pressure was maintained at 0.2 MPa using a hydraulic pressure control valve. Three rainfall gauges were placed near each sampling location at the height of the soil surface and at the same distance from each sprinkler as the soil columns.

The experiment was performed in a laboratory to reduce the influences of air flow and evaporation. When the leaching water amount reached 200 mm,water application was stopped and then the soil column was covered with plastic film and placed for 24 h so that consistent soil water moisture levels could be maintained.

Fig. 2. Schematic maps of laboratory experiments for three water application intensity treatments.

2.3. Measurements

Water application intensity is defined as the water application depth distributed over an experimental area in a given unit of time (Kang, 1998). Different water application intensity treatments were formulated by adjusting the number of micro-sprinklers in different combinations. The lowest and highest water application intensities were achieved using two and four sprinklers, respectively. When each irrigation event had finished, the water application intensity for each soil column was recorded as the water application intensity of the nearest measuring glass. Three water application intensity treatments were investigated in the laboratory experiment: a low-intensity treatment of 3.05 mm/h (W1), a mediumintensity treatment of 5.19 mm/h (W2), and a high-intensity treatment of 7.23 mm/h (W3).

Soil water content and salinity were measured from soil cores, which were retrieved from each column using an auger(3.0 cm in diameter and 100 cm in length) at the end of irrigation. All samples were obtained at 5-cm-depth increments:0-5, 5-10, 10-15, 15-20, 20-25, 25-30, 30-35, 35-40,40-45,45-50,50-55,and 55-60 cm.The soil water content was measured gravimetrically using a subsample of each sample. The remaining soil subsamples were air-dried and sieved through a 1-mm sieve. A saturated soil paste was prepared for chemical analysis using centrifugation (4 000 rpm for 20 min). Values forECand pH, based on extracts of saturated soil, were determined using a conductivity meter(DDS-11, REX, Shanghai, China) and a pH meter (PHS-3C,REX),respectively.In the experiments,the average soil water content,EC, and pH values within the whole soil profile were combined to account for spatial variations.

2.4. Calculations and statistical analyses

Analysis of variance (ANOVA) was conducted to evaluate the effects of soil salinity (the treatment number was 3) andwater application intensity treatment (the treatment number was 3) on soil content,EC, and pH. Duncan's significant difference was used to compare and rank the treatments at the 0.05 probability level forEC. Analyses were performed with SPSS 13.0 (SPSS Inc., Chicago, Illionois, USA).

Table 1 Soil EC, pH, and SAR for each treatment.

3. Results

3.1. Soil water content

The distribution of soil water content with depth for each salinity-intensity treatment combination when irrigation ended is shown in Fig.3.In the S1 treatment(2.26 dS/m)(Fig.3(a)),the soil water content increased with depth in the soil profile in the W1 (low-intensity) treatment, but the water content deceased with depth in the W2 (medium-intensity) and W3(high-intensity) treatments. The water content at a soil depth of 0-20 cm of W1 treatment was lower than those of the other two treatments.The water content at a soil depth of 55-60 cm was higher in the W1 treatment than in the W2 and W3 treatments. In the S2 treatment (Fig. 3(b)), the pattern of changes in soil water content with depth in the three water application intensity treatments was the same as that in the S1 treatment, but with a slight change at the soil depth of 0-20 cm, where the water content varied from 33.50% (W1)to 39.64% (W3). In the S3 treatment(Fig.3(c))the soil water content increased as the depth increased (W3 ＞ W2 ＞ W1),varying from 29.51% (W1) at a soil depth of 0-20 cm to 38.93% (W3) at a soil depth of 50-60 cm.

ANOVA results are presented in Table 2 and indicate that the effect of water application intensity on the soil water content was statistically non-significant at soil depths of 0-20 cm, 20-40 cm, and 0-60 cm. This may be because differences among the irrigation water intensities were not large enough,or because the irrigation volumes were too high to provide differences due to leaching. The effect of soil salinity on soil water content was significant at soil depths of 40-60 cm (W1 treatment), 0-20 cm (W2 treatment), and 0-40 cm (W3 treatment), but insignificant in the other cases,for example, at soil depths of 0-40 cm (W1 treatment),20-60 cm (W2 treatment), and 40-60 cm (W3 treatment).

Table 2 Average soil water contents at different water application intensities and soil salinity levels at end of irrigation.

Soil water content is an indicator of soil aeration.When the water content reaches saturation, little air can enter the soil pore space to form a favorable aeration environment, especially on coastal saline soils that contain a large proportion of sodium (Wan et al., 2012). In the S1 treatment, the soil water content reached 35.31% at the 0-10 cm depth of the soil profile in the W1 (3.05 mm/h) water application intensity treatment, and 33.79% at the 0-5 cm depth in the other two water application intensity treatments,respectively,which was close to the saturated water content. In the S2 treatment, the soil water content at the 0-10 cm depth in the W3 treatment was greater than the saturated water content. In the S3 treatment, the average soil water content for the entire soil profile was lower than the saturated water content. It is believed that the surface soil, with low salt content and low saturated hydraulic conductivity, was more easily saturated during the irrigation process.Consequently,we found that the use of low water application intensity can be beneficial in reclaiming saline soils, a finding similar to that of Chen et al. (2015).

Fig.3.Spatial distribution of soil water content in soil profile in response to three water application intensities(3.05,5.19,and 7.23 mm/h)and different soil salinities.

3.2. Variation of EC

During an irrigation period,salts were moved downward in the soil columns as water moved.Thus,after irrigation ceased,ECdecreased in each treatment compared with the original values (Fig. 4). Soil salinity increased with depth along the soil profile, and the smallestECvalues occurred at the soil depth of 0-40 cm.

In the S1 treatment,the salts in all layers were leached,but there were no significant differences for the three water application intensity treatments (Fig. 4(a)). The averageECvalues in the soil profile for the three water application intensity treatments were 0.47, 0.71, and 1.01 dS/m in the 0-20 cm,20-40 cm,and 0-60 cm depth ranges,respectively.TheECvalues slightly changed after irrigation and leaching because the initial content of soil salt was relatively low.

In the S2 and S3 treatments, theECvalues at the 0-40 cm depth were not significantly affected by any of the water application intensity treatments (similarly to the final soil water content).However,theECvalues at the 40-60 cm depth in the W1 treatment were 6.70 and 14.67 dS/m in the S2 and S3 treatments,respectively.These values were higher than the corresponding values in the W2 treatment(3.05 and 2.38 dS/m in the S2 and S3 treatments, respectively) and the W3 treatment (3.68 and 3.34 dS/m in the S2 and S3 treatments,respectively).

ANOVA results are presented in Table 3,indicating that the effect of soil salinity treatment onECwas statistically significant, and the highestECvalues occurred in the S3 treatment. This was because the initial soil salinity in the S3 treatment was very high (resulting in the soil being classified as very strongly saline).

The variation of soil salinity in the soil profile was similar to that found by Oster et al. (1972) and Moreno et al. (1995),who observed that soil salinity decreased with increasing soil depth as was the pattern in the present study.Although the soil profiles were saturated in some treatments,the soil salt content was still low. For example, at the high water application intensity(W3)in the S1 and S2 treatments,ECreached 0.76 and 0.78 dS/m in the 0-5 cm soil layer, respectively. This was possibly due to the high sand content in the soil, which had a high infiltration capacity (Hu et al., 2012).

Table 3 Average EC at different water application intensities and soil salinity levels at end of irrigation.

3.3. Desalination effectiveness

The desalination effect at the 0-20,20-40,and 40-60 cm depths from all three water application intensity treatments showed identical response behaviors to soil salinity (Fig. 5).Desalination as a function of soil salinity was best represented by a logarithmic equation as follows:

whereYis the desalination change in a given depth range(0-20, 20-40, or 40-60 cm depth) (%);xis the soil salinity(dS/m); andaandbare empirical coefficients.

Regression parameters and statistical analysis for each salinity level shown in Fig. 5 are presented in Table 4. Using Eq. (4) with the statistically determined coefficients demonstrated the increasing effect of soil salinity on desalination as salinity increased. For example, at the soil salinity level(2.26 dS/m) at the 0-20 cm depth, increasing the water application intensity from 3.05 to 5.19 mm/h and from 5.19 to 7.23 mm/h decreased the desalination by 12.13% and 4.67%,respectively (Fig. 5(a)). However, as salinity levels increased,the differences in desalination effects due to different water application intensity treatments gradually decreased. As soil salinity increased from 2.26 to 22.29 dS/m in the 0-20 cm soil layer, desalination went from 46.75% to 93.73% (W1),62.77% to 93.96% (W2), and 39.79% to 94.96% (W3),respectively. In other words, when the initial soil salinity was high, approximately the same desalination effectiveness was achieved in the 0-20 cm soil layer in all three water application intensity treatments (Fig. 5(a)).

Fig. 4. Spatial distribution of EC in soil profile in response to three water application intensities (3.05, 5.19, and 7.23 mm/h) and different soil salinities.

Fig.5.Extent of desalination at depths of 0-20,20-40,and 40-60 cm as a function of EC for three water application intensity treatments(3.05,5.19, and 7.23 mm/h).

Table 4 Regression parameters for Eq. (1) (Y = a ln x+ b) and statistical parameters for fitted lines presented in Fig. 4.

The quantitative effects of increasing soil salinity level and increasing water application intensity on desalination at the 20-40 cm depth both increased. When soil salinity in this layer increased from 2.26 to 22.29 dS/m, the desalination effectiveness ranged from 46.95% (W1) to 88.50% (W3),respectively (Fig. 5(b)). However, the change in desalination effectiveness at the 40-60 cm depth as soil salinity increased from 2.26 to 22.29 dS/m was from-34.27%to 34.18%(W1),50.20% to 89.34% (W2), and 19.40% to 85.03% (W3),respectively (Fig. 5(c)).

3.4. pH

Soil pH in the profile changed concomitantly with changes in theECdistribution(Fig.6).However,unlike the significant changes inEC, the changes in pH were slight. Before the experiments began, the average initial values of soil pH in the 0-60 cm soil profile were 8.41 (S1 treatment), 7.71 (S2 treatment), and 7.65 (S3 treatment), respectively.

In the S1 treatment, the average pH for the whole soil profile changed little and there were only small differences between soil layers at the end of the experiments. In the S2 treatment, the pH value gradually increased in the 0-20 cm soil layer, but then gradually decreased in the 20-60 cm soil layer. In the S3 treatment, the pH value decreased in the 0-20 cm soil layer, and then increased gradually with depth.

Fig. 6.Spatial distribution of pH in soil profile in response to three water application intensities (3.05, 5.19, and 7.23 mm/h) and different soil salinities.

There were no obvious differences in the spatial distribution of pH due to water application intensity treatments in the S1 and S2 treatments.Compared to the initial pH,the average pH of the whole soil profile changed by-0.13%to 0.70%and 3.57%to 5.71%in the S1 and S3 treatments,respectively,and by 5.79% to 11.08% in the S2 treatment. These results indicated that although soil salinity and water application intensity played important roles in salt leaching, their effects on pH were less obvious. The apparent lack of effect on pH value may be due to the soil buffering capacity arising from the presence of calcite (as confirmed by a strong formation of bubbles upon addition of HCl to dry samples of the soil obtained from all depths).

We found that salt content affected the physical and chemical characteristics of soil, and these responses also affected the selection of sprinkler water application intensity.The soil with high salinity had a high saturated hydraulic conductivity and was resistant to alkaline pH; on such soils,the water application intensity of sprinkler irrigation should be slightly lower than the saturated hydraulic conductivity.As the irrigation (and salt leaching) progressed, the soil salinity and saturated hydraulic conductivity decreased continuously, and lower water application intensity for sprinkler irrigation could be beneficial.

4. Conclusions

Micro-sprinkler irrigation was used in reclamation of saline soils varying in water application intensity and initial soil salinity. An unsaturated soil moisture environment, which is beneficial to soil leaching, was created in soils with higher salinity and lower water application intensity,in which a better leaching effect was found in the upper soil layers, and the pH value was higher compared with soils that had lower salinity and higher water application intensity.

The salinity of soil has a more significant impact on the soil environment than the water application intensity of a microsprinkler. During reclamation of salinity soil, it is necessary to pay attention to the decrease of soil permeability and to note that the soil is easily saturated with the decrease of soil salinity. Moreover, soil alkalization should receive more attention during the salt leaching process.

Although the optimal water application intensity depended on the initial soil salinity,the three intensities evaluated had no statistically different effects onECin the upper 20 cm of the soil profile.In other words,all three water application intensity treatments were equally effective in desalinating the root zone.Therefore, any intensity greater than the lowest intensity was excessive because the higher intensity would increase water(and energy) usage but produce no measureable effect on salinity.

Declaration of competing interest

The authors declare no conflicts of interest.