1. Introduction
Groundwater poses a continuous threat to sustainable development of irrigated agriculture in the North China Plain (NCP). This plain generates about one-third of the country’s gross domestic product (GDP) in agriculture, and limited quantities of fresh water contribute a significant share in agricultural production [
1,
2], thus there is an urgent need to use this water resource more judiciously. Saline water irrigation is increasingly used, though such irrigation water is one of the major sources of soil salinity, which can result in crop yield reduction and loss of soil resources [
3,
4,
5,
6,
7]. Wang et al. [
8] reported that various irrigation modes of saline and fresh water could be altered to grow sensitive and salt tolerant crops. Therefore, not only the management and efficient use of limited fresh water resources is essential, but also discrete or alternate use of brackish water towards agricultural development is necessary for agricultural sustainability [
1,
8,
9,
10,
11].
Previous studies have reported that the saline/salty water can be effectively used to irrigate several crop species [
12,
13,
14,
15,
16,
17]. The continuous use of saline water for irrigation leads to long-term environmental problems, such as soil salinization. In a study, Fang and Chen [
18] noted that there is potential for high desalinization of upper soil layers if more than 300 mm rainfall occurs during monsoon months, but there is a significantly low chance of soil desalinization in the years with less rainfall occurring during monsoon months. According to Romy [
19], in the regions situated in littoral and semi-arid climatic areas, the annual rainfall ranges between 400–600 mm, and mostly occurs during summer season (which spans from mid-June to September), while, for the rest of period, a somewhat arid climate persists [
1,
20,
21]. Kiani and Mirlatifi [
22] have recommended the use of saline water for cultivation during the dry period. According to them, the salt build-up in soil profile during saline irrigation can be leached during the monsoon season under semi-arid climate. Kafi et al. [
23], Naresh et al. [
24], Malash et al. [
25] and Hassanli and Ebrahimian [
15], however, suggested that to counter-down the effect of saline water irrigation on crop yield and salt accumulation in soil profile, irrigation strategies should be adopted. The available strategies including the alternate use of saline to fresh waters are recommended to reduce salinity build-up in the soil. Although the indirect evidence favors alternate use of saline water [
26], the alternate saline and fresh water irrigation strategies at different growth stages still needs further investigation such as in the North China Plain (NCP).
Saline water can restrict physiological activities of crops and decrease biomass growth, fertile tillers and root development [
27,
28]. Under high ionic concentration, leaf area expansion is affected by water deficiency and nutritional imbalance, especially leading to K
+ deficiency in plant organs. It was reported that winter wheat is able to reduce Na
+ concentration while increase K
+ concentration and K
+:Na
+ ratio [
29,
30]. Oxidative stress and primary carbon metabolism of many crops are negatively affected due to osmotic effects [
31]. Similarly, due to high osmotic pressures, crop growth parameters are affected. These pressures restrict uptake of water by the roots that in turn are a major contributing factor to low crop water productivity and yields [
32,
33,
34]. Previous studies investigated continuous or alternate irrigation with varied saline water concentrations to maize and lower potentially toxic ion accumulation and improvement in K
+ and Na
+ balance in plant shoots were found [
35]. Grattan and Rhoades [
36] observed that the cyclic use of saline and non-saline water reduced soil salinity especially in the upper soil layers up to 30 cm depth. Therefore, the conjunctive use of irrigation water may not only increase crop production but also reduce risks of soil salinization in the NCP to achieve the maximum yield per drop of saline water.
Winter wheat (
Tritium aestivum L.) is a worldwide important grain crop. China is one of the most significant wheat-producing countries in the world and more than 75% of the winter wheat is produced in the NCP [
37,
38]. Winter wheat and summer maize are dominant sequences of the cropping system in the NCP. Liu et al. [
1] noted that the double cropping system requires significantly more water than that received from natural precipitation. Due to this fact, winter wheat requires additional irrigation water during peak dry months. Zheng et al. [
39] and Hu et al. [
40] reported that more than 50% of the area in the NCP is irrigated with groundwater. The continuous and over-pumping of groundwater for irrigation is not only the direct cause of a decline in water table and seawater intrusion in the aquifers [
41,
42,
43,
44], but also positively affecting hydrological water cycle and regional climate [
45]. However, as a general trend, lift irrigation operated by mechanical energy and excessive exploitation of groundwater will increase electricity consumption, thus increase carbon emissions, which will not only change global climate but also at the same time will have a massive impact on global economy [
46,
47].
Even though the impact of saline water irrigation on plant vegetative growth, grain yield, biomass, ionic concentration as well as soil salinity profile under salinity stress have been reported, the fundamental understanding of enhanced crop yield and the alternate use of saline and fresh water at different growth stages are still poorly understood. Therefore, the effect of alternate saline water irrigation at the peak dry season in winter wheat is evaluated in the NCP. Wheat is classified as a moderately tolerant crop to salinity [
48]. The generally three growing phases are relevant for determination of grain yield of winter wheat. The jointing phase of winter wheat is more water sensitive compared to other phases [
49,
50,
51], as it is more salt tolerant than other phases. The main aim of this study was to examine the yield responses of winter wheat subjected to different irrigation modes of alternate use of saline and fresh water and to explore the tangible irrigation scheduling for winter wheat production using underground saline water during different growth stages of winter wheat in the county of NCP.
4. Discussion
A huge potential of water available in upper aquifers of NCP is characterized by brackish water at around 4.7 dS m
−1 [
54]. Utilization of this water for winter crops is important to cope up with the current tight water situation [
23,
55,
56,
57]. The NCP illustrates the challenges facing China as it deals with increased water demands and severe ground depletion [
41]. The mean water table is declining by 0.5 to 3 m year
−1 [
58]. In order to address water issues, Yuan, et al. [
42] suggested that the irrigation water requirement, groundwater pumping and depth, and crop yield in the region should be corrected.
Furthermore, the agricultural production per year is continually decreasing due to drought and fresh water shortage caused by other factors than the sum of the losses [
1]. In this context, the alternate use of saline and fresh water can be an alternative to increase crop productivity and simultaneously reduce the pressure on fresh water [
8,
15,
59]. In order to minimize the potentially hazardous effects of saline water on crop yield, farmers in the regions have not been able to adopt alternate saline and fresh water irrigation strategies. Thus, it is an urgent need to thoroughly evaluate the irrigation practices that can better perform for winter crops without affecting crop growth and yield. Therefore, in this study, the main aim was to investigate whether alternate saline and fresh water irrigation can sustain the yield without causing salinity accumulation in the soil. Use of saline and fresh water, particularly at stem elongation and the flowering stage of wheat, needs to be further understood.
The results of the study showed that effects of the alternate irrigation using saline water were predominated in this experiment. The soil pH and EC
1:5 were significantly different among treatments during entire periods (
Figure 3 and
Figure 4) when irrigation water was applied at different growth stages. During the peak dry months of winter season, soil electrical conductivity (EC
1:5) were significantly increased while decreased during the winter months. These results indicated that the average soil salinity in the root zone could be balanced annually. Winter wheat was able to utilize saline water during dry season (from March to mid-June). In the NCP, corresponding to about 70% of the annual rainfall occurs during summer season after the harvest of wheat, thus the salts accumulated in crop root zone leached. Similar results were reported by Sharma et al. [
60,
61], and they observed that in the monsoon climate areas characterized by a mean annual rainfall 500 mm or more, about 80% of the salts accumulated by irrigation application during winter wheat season is leached without any irrigation practices. However, the use of saline water for irrigation decreases water loss from the root zone (
Table 3). It slows down the deep percolation and evapotranspiration, thus soil water content under this condition minimally declines (
Figure 2). These results were in agreement with previous findings of Malash et al. [
12,
25], who observed that the field irrigated with fresh water had reached 70% soil moisture depletion and soil water content was depleted quickly. On the other hand, the fields that received saline water still had a soil water content greater than 70%, and so were irrigated a few days later. Total water consumptive values were lower when saline water was used (3390 m
3 ha
−1 for 2017–18), which could have been attributed to less evapotranspiration rate with saline water irrigation compared to threshold water quality for irrigation [
62]. These results are also supported in previous studies carried out by Hassanli and Ebrahimian [
15], Naresh, et al. [
24], Chauhan et al. [
63]. According to them, those options for the alternate use of saline underground and fresh water should hold greater assurance that produces higher grain yield of wheat for the similar salt load to soils.
Focusing on the application of alternate saline water at different growth stages, the results of the present study indicated positive influence of irrigation with different water qualities on plant height and dry biomass of winter wheat was more pronounced in stem elongation and flowering stages as compared to SS treatment. While at the flowering stage, growth parameters of winter wheat were slightly affected [
1,
24,
63,
64]. Nonetheless, when irrigated with fresh water at the stem elongation stage, higher plant height considerably produced more tillers (
Table 4).
Figure 5 revealed that the dry biomass was significantly increased by additional irrigation [
8,
22,
65]. Saline water irrigation applied during the flowering stage led to significant influences on spike length, spikelet number, grain weight and RGR, whereas, grain number was not affected in the applied saline water irrigation (
Table 4 and
Table 5).
The alternate irrigation method using saline and fresh water significantly influenced crop yield (
Figure 6). Irrigation with alternate saline and fresh water only showed mild decline of grain yield as followed by fresh water treatment (FF). Similarly, the grain yield slightly declined between FS treatment with respect to SF treatment. The successful amalgamation of alternate irrigation water in this region offers great potential of saline water resources. Irrigation with SS treatment produced the lowest crop yield compared to other treatments [
3,
55]. If fresh water were available during initial crop stages for the better tillering of winter wheat, the saline water irrigation can be more effectively applied at other crop development stages. The threshold salinity of irrigation water for wheat was 7~8 dS m
−1 that could be used after germination [
1,
63]. Several researchers have also suggested the positive effects of saline irrigation on wheat production. Salinity of irrigation water between 6 to 9 dS m
−1 has been suggested by Mass and Grattan [
66], while water salinity ranging from 3 to 8 dS m
−1 has been rated within the permissible limit and water with 4.7 dS m
−1 in irrigation for winter wheat was not so high [
1,
65]. Moreover, previous studies reported that saline irrigation affected Na
+ and K
+ concentrations in plant biomass [
67,
68,
69,
70,
71]. Although saline water was applied at either or during the two growth stages, and the Na
+ and K
+ concentrations were increased significantly compared with NI and FF treatments in the leaves, the saline irrigation did not affect Na
+ and K
+ concentrations as well as K
+:Na
+ ratios in the roots and grains.
The results revealed that the water quality affected crop harvest index when water was applied at the flowering stages (
Table 6). Previous studies reported that saline water irrigation reduced water uptake efficiency, transpiration rate and net CO
2 assimilation due to these reductions, and in turn crop growth and nutrients transport into plant is affected [
72,
73,
74,
75]. Similarly, Poustini [
76] reported that there was no effect of water quality having an EC
1:5 ranging between 3.5 and 6.9 dS m
−1 on net assimilation rate. However, winter wheat could be salt tolerance to water and soil salinity up to 6 dS m
−1 [
77]. According to Alarcon et al. [
78], the reduction in leaf area index led to a reduced light interception and thus reduced dry biomass production. Moreover, Pang et al. [
79] have reported the comparisons between alternate irrigation and blended/ mixing ratio. According to the results, the alternate application of fresh and saline water increased irrigation water productivity if compared to the application of saline water during the whole season. In other comparisons, Hassanli and Ebrahimian [
15], Naresh et al. [
24] and Zhao et al. [
26] concluded that the grain yield of winter wheat were higher with the alternate use of saline and fresh water than blended as well as saline water. Consequently, the amount of irrigation water devoted to field that received the saline water at flowering stage was less than those at fresh water and this led to the increase in crop water productivity [
32].
In the NCP, the limited availability of fresh water resources and to release the pressure on fresh resources, the alternate irrigation using saline water can leave the groundwater salinity level (4.7 dS m−1) unaffected and is a promising option for the sustainable agriculture development. Proper saline water management is required besides selection of a salt tolerant variety and water application timing for saline irrigation alternated with fresh water.