Effects of Different Wastewater Irrigation on Soil Properties and Vegetable Productivity in the North China Plain

: The interest in reusing wastewater for irrigation is being popularized in most countries. The objective of this study was to evaluate the effects of different wastewater and nitrogen fertilizer on soil fertility and plant quality, as well as to identify the optimal irrigation mode in the North China Plain. A total of nine treatments, including control (groundwater, no fertilizer), piggery wastewater, reclaimed water, and saline water, combined with nitrogen fertilizer (300 kg/ha and 200 kg/ha), were conducted in a greenhouse in 2019 (Xinxiang, Henan Province). Soil pH, electrical conductivity, organic matter, heavy metals contents, and cucumber yield and quality were analyzed. The results showed that: (1) compared with the underground water (control), soil pH value with a decrement of 0.21 units in piggery wastewater (PW), and 0.24 units in saline water treatments (SW). Soil electrical conductivity (EC) value signiﬁcantly increased by 5.8~20.9% in PW and SW treatments, while there was no signiﬁcant difference in EC in reclaimed water. The highest EC (770 µ S/cm) was recorded in SW treatment. (2) No dramatic difference on the concentrations of soil lead (Pb) and cadmium (Cd) in the PW, RW, and SW treatments, compared with the control, but soil organic matter, copper (Cu), and zinc (Zn) concentrations in wastewater treatments were increased by 2.1~43.4%, 24.4~27.0%, and 14.9~21.9%, respectively. (3) There were no signiﬁcant differences in cucumber yield and quality in RW treatment, while there was a slight decrease by 1.4% in yield in the SW treatment. The highest cucumber yield was observed in PWH treatment, with an increment of 17.5%. In addition, the contents of Vitamin C, soluble sugar, and protein were also improved by PW treatment. In this study, PW treatment showed the strongest ability to promote cucumber yield and quality, thus indicating that piggery wastewater irrigation with 300 kg/ha nitrogen would be the optimal practice in this region. Long-term study is necessary to monitor potential risk of heavy metals on the quality of soil and plant.


Introduction
The shortage of freshwater is a common problem in most regions of the world; it is estimated that the water shortage will reach 1.3 × 10 m 3 by 2030 in China [1,2].Extensive use of irrigation, environmental pollution, and decreasing water resources have threatened the sustainable development of agriculture; alternative sources of water should be considered to maintain the further development [3].Reusing wastewater is considered to an environmentally disposal practice that helps to prevent wasting the limited resources and minimize the environmental pollution (i.e., direct disposal of wastewater into surface or groundwater) [4].In addition, studies have shown that wastewater irrigation can enhance soil fertility and crop productivity [5].However, this practice may cause environmental problems if not appropriately treated and managed [6][7][8].
To a certain extent, wastewater includes aquaculture wastewater, urban sewage, and saline water, etc. [1].As an alternative resource, the safe utilization of wastewater has received significant attention, and much literature has been reported frequently [9][10][11][12][13][14].It has been reported that reclaimed water significantly increased tomato yields, soluble sugar, and titratable acidity content of the fruit, due to its high levels of nitrogen, organic matter, and other available nutrients [15].Xu et al. [16] showed that reclaimed water irrigation significantly improved the growth and yield of pakchoi, but had no significant effect on its quality.Within a certain range, saline water irrigation can improve water use efficiency and plant growth, without a dramatic decline in yield [17].
Cucumber (Cucumis sativus L.), due to its high controllability and economic benefits, is widely cultivated in several countries, especially China [18].How to make this cultivation has a greater economic return and higher yield than open-field cultivation does, which becomes the most critical issue for local farmers and drive farmers to overuse water and fertilizers to achieve higher vegetable yields [18].Clearly, scientific irrigation combined with fertilizer application for plant-soil is necessary to investigate.The interest in reusing wastewater of irrigation is an attractive option of disposal; because of its characteristics, it provides a direct or indirect influence, regarding the relationship between soil and plants.Although the wastewater irrigation is extensively popularized by many countries as an important measure to alleviate scarcity of water resource and reduce fertilizer application, it might aggravate soil salinity, nutritional disorder, poor soil structure, and toxic stress to crop growth [19][20][21][22][23].Therefore, the management of wastewater irrigation should consider the characteristics of the crop and soil, nature of water source, climate, and other factors.Most of the studies mainly focuses on the effects of single irrigation water quality, irrigation norms, and irrigation patterns [24][25][26][27], and there is a lack of systematic research on the effects of different wastewater irrigation on the water-soil-plant in North China Plain.Therefore, considering the regional importance and the uncertain influenced by wastewater irrigation in cultivated land fertility and crop productivity, the objectives of our experiment were to: (1) clarify the effects of different wastewater and N application on soil physicochemical properties, cucumber yield, and quality in North China Plain; (2) obtain the optimal irrigation mode of cultivated land; and (3) provide scientific support for optimizing the reuse of wastewater irrigation in this region.

Experimental Site
The experiment was carried out in a greenhouse in April-July of 2019 at the Agricultural Water and Soil Environmental Field Science Research Station, Chinese Academy of Agricultural Sciences (Xinxiang City, Henan Province, 35 • 19 N, 113 • 53 E).The meteorological data of 60-year was obtained from Xinxiang Weather Station.The experimental site has a warm temperate continental monsoon climate, with annual average temperature was 14.1 • C, annual average precipitation was 588.8 mm, frost-free period was 210 d, and sunshine duration was 2398.8 h.The soil type is tidal soil, and the physical and chemical properties of 0-20 cm soil layer are as follows: soil bulk density: 1.35 g/cm 3 , pH value: 8.57, organic matter content: 7.3 g/kg, field capacity: 20.5%, available nitrogen: 0.78 mg/kg, available phosphorus: 18.5 mg/kg, and available potassium: 102.6 mg/kg.

Description of Experiment
Cucumber cultivar "CASS-106" (Cucumis sativus L.) seedlings was provided by the experimental station.A randomized block design (4 × 2) with three replications, and nine treatments in this experiment (total of twenty-nine plots), including control (groundwater, no fertilizer), unconventional wastewater (piggery, reclaim water and saline water), and two nitrogen applications (high: 300 kg/ha and low: 200 kg/ha); the details of experimental design are shown in Table 1.Each plot area was about 6 m 2 (1 m × 6 m), protective rows were set in each treatment block.Following the local conventional fertilizer application, chicken manure 30 t/ha (organic matter ≥ 45%) was applied before cucumber seedlings were transplanted (2 April).The amount of fertilizer was P 2 O 5 78 kg/ha, K 2 O 51 kg/ha.Throughout the cucumber growth period, irrigation occurred once every 3-5 days, and the total irrigation amount for the experiment was 2100 m 3 /ha.Groundwater was pumped from the nearby wells; piggery wastewater was taken from the biogas project in Xinxiang city, Henan Province.Reclaimed water was obtained from the Luotuowan domestic sewage treatment plant in Xinxiang City.Saline water was prepared with sea salt at a concentration of 4 g/L in the laboratory.The water quality of wastewater was judged by using the water quality standard for farmland irrigation (GB5084 2005), as shown in Table 2.After vegetable harvesting, soil samples were collected as the five-point mixing method.After air-dried naturally, the samples were ground and passed through a 2 mm sieve.
Soil pH was measured by using potentiometric method with soil and water extract at a ratio of 1:2.5 (w/v) (PHS-3E pH meter, Shanghai Electronic Scientific Instrument Co., Ltd., Shanghai, China).Soil electrical conductivity (EC) was measured in a 1:5 soil and water extract (w/v) by using portable conductivity meter measurement (DDB-303A, Shanghai Electronic Scientific Instrument Co., Ltd., Shanghai, China.)Soil bulk density was measured by cutting ring method described by Lu [28].Soil organic matter was determined by using potassium dichromate external heating method [28].Soil heavy metals were measured by using microwave digestion-atomic absorption spectrophotometry (AA7000F, Shimadzu, Kyoto, Japan).

Cucumber Sample
Mature cucumbers were harvested from plants in each plot and weighed; cucumber count was converted to number per hectare, and cucumber weight was converted to kilogram per hectare to standardize yield measurements.
To estimate the quality of cucumber, four fruits per plot were sampled and washed in deionized water to determine vegetable quality.
Fruit soluble sugar content was measured by anthrone sulfuric acid colorimetry, according to the method of Lu [28].Vitamin C content was analyzed by using 2,6-dichlorophenol titration, following the method of Wu et al. [29].Titratable acidity was determined using sodium hydroxide titration [30].The protein content was determined by the Kjeldahl nitrogen determination method [30].The content of nitrate was determined by the phenol sulfonic acid method, according to the procedures described by Li et al. [30].

Data Analysis
Data was calculated by Microsoft excel 2016.Statistical analyses were conducted via the two-way analyses of variance (ANOVA), and the least significant difference (LSD) test for significance using the SPSS 26.0 software (IBM Crop.).Origin 2018 software was used for graphical presentation of results.

Soil pH after Irrigating
Effect of irrigation with different wastewater on soil pH value at 0-40 cm layer, as shown in Table 3. Within 0-20 cm soil layer, soil pH decreased by 0.08 units in PWH and 0.21 units in PWL treatment, compared with the control, but with no significant difference between them.A similar decreasing trend was observed in SW, with 0.24 units in SWH and 0.22 units in SWL treatment.The soil pH values in RW treatment were higher than that in the control, with increment of 0.09 in RWH and 0.11 units in RWL treatment.Except for RW treatment, other treatments decreased by 0.08-0.13units, compared with the control, in the 20-40 cm layer.The highest pH value of 8.58 was recorded in RWL treatment, followed by 8.56 in RWH treatment (Table 3).The ANOVA showed that the interaction between nitrogen application and water quality had no significant effect on soil pH (Table 3).

Soil Electrical Conductivity after Irrigating
Results on the effects of different water quality irrigation on soil electrical conductivity (EC) were presented in Figure 1.Compared with the control, the EC values significantly increased by 5.8~20.9% in PW and SW treatments (p < 0.05), and there was a dramatic difference in EC variability between the two nitrogen levels of the PW treatments (p < 0.05).However, the EC of RW treatment had no significant change, compared with the control (Figure 1).The highest increment was observed in the SWL treatment (770 µS/cm), followed by the SWH treatment (760 µS/cm), which increased by 20.9% and 19.3%, respectively.Moreover, a marked difference was observed in water quality among the unconventional irrigation, as shown in Table 4.

Soil Organic Matter after Irrigating
Compared to that in the control, a highly significant difference in soil organic matter change was observed in different wastewater irrigation, as shown in Figure 2. Except for the RWH treatment, the soil organic matter content of other treatments was significantly higher than that of the control, but there was no significant difference between nitrogen levels.In the 0-20 cm soil layer, organic matter content of PW treatment was significantly higher than that of the control, with increment of 2.1~43.4% (p < 0.05).The highest organic matter content was recorded in PWH treatment, with the value of 19.36 g/kg (Figure 2).In this study, there was significant difference in soil organic matter between water quality and nitrogen application, and the same behavior was found in their interaction (Table 5).The treatment effect for soil organic matter was not pronounced or significant, except for the PWH treatment in the 20-40 cm soil layer.Meanwhile, our results demonstrated that the interaction between nitrogen application and water quality had no dramatic difference in organic matter variability, but a highly significant difference in organic matter change was observed with different water quality for irrigation (p < 0.01, Table 5).

Soil Heavy Metal after Irrigating
As shown in Table 6, the concentrations of soil heavy metals under different wastewater irrigation were significant different.Soil total copper (Cu) concentration of eight treatments was significantly higher than that of the control, with the highest increment of 27% in PWH treatment, followed by PWL treatment (26.8%).A similar increasing trend was also detected, and the treatment effect was dramatic in total zinc (Zn) concentration modification.The concentration of Zn increased by 21.9%, 21.3%, 17.7%, and 17.2% for PWH, PWL, SWL, and SWH treatments, respectively, compared with the control (p < 0.05, Table 6).The highest Zn content was obtained in PWH treatment, with the value of 25.07 mg/kg.The ANOVA indicated that only the quality of wastewater had significant effects on soil Cu and Zn concentrations.The results regarding the concentrations of lead (Pb) and cadmium (Cd) indicated that there was no significant difference in the three unconventional water treatments, compared with the control.Similarly, the results showed that the interaction between nitrogen application and water irrigation had no significant difference on soil Pb and Cd concentration (Table 6).

Cucumber Yield
Results on the effect of irrigation treatments on single fruit weight and yield are presented in Table 7. Data on single fruit weight indicated that no significant differences in SW treatments compared with the control.However, irrigating plant with wastewater, resulted in a percentage increase in single fruit weight in PWH, PWL, RWH, and RWL treatments (23.5%, 17.9%, 8.7%, and 9.2%, respectively), compared with the control.The highest increment occurred under PWH treatment, followed by PWL treatment (Table 7).Based on our results, it was concluded that the interaction between water quality and nitrogen application had no significant difference on single fruit weight.Moreover, a marked difference in cucumber yield was recorded in PW treatment, which increased by 17.5% and 14.4%, respectively, compared to that of the control.Interestingly, the yield was significantly different between the two-level nitrogen application, indicating that the cucumber yield increased with increasing nitrogen application.However, data on the yield of cucumber indicated a slight decrease of 1.4% and 1.3%, respectively, in the SWL and SWH treatments, compared with the control.Additionally, there was no significant difference in cucumber yield under RW treatment.Treatments with wastewater irrigation significantly enhanced cucumber yield, compared with the control, and PWH treatment showed the most pronounced promoting effect.The ANOVA suggested that the interaction between water irrigation and nitrogen application had significant difference in the yield of cucumber (p < 0.05, Table 7).

Cucumber Quality
The effects of different wastewater treatments on the quality indexes of cucumber are demonstrated in Table 8.Data regarding the content of vitamin C, soluble sugar, and nitrate in cucumber showed no significant changes in the RW and SW treatments, compared with the control.The highest content of titratable acidity (1.49 g/kg) was observed in the SW treatment, with a percentage increase of 18.2% (p < 0.05).The contents of vitamin C, soluble sugar, and protein in PWH treatment were significantly higher than that of the control, which increased by 43.2%, 13.4%, and 42.5%, respectively (p < 0.05).Results showed that quality of cucumber was significantly influenced by different wastewater irrigation, except for nitrate content.However, the interaction between water quality and nitrogen application only had a significant difference in protein content (p < 0.05, Table 8).

Correlation Analysis between Soil Properties and Yield and Quality of Cucumber
Cucumber yields were positively correlated with OM, Vc, SS, protein, and soil total Zn concentration, but negatively correlated with soil pH value.Soil EC had a significant positive correlation with TA and soil total Cu concentration.There were dramatically positive correlations between the OM and Vc, SS, Cu, Zn, Pn, and Cd concentrations.The Vc content of cucumber had significantly positive correlations with protein, Cu, Zn, and Cd concentrations, while the nitrate content of vegetable had no significant correlation with the tested indexes in our experiment (Table 9).

Irrigation with Different Wastewater on Soil Properties
It is necessary to investigate the reuse of inferior water-soil-plant relationship because the maintenance of soil health and plant fertility is conducive to the rational construction of sustainable development [31].Soil pH is an important attribute of saline-alkali, and directly reflect soil nutrient availability and crop growth [17,32].Liu et al. [33] showed that livestock wastewater irrigation slightly decreased the soil pH, compared with the freshwater.Yang et al. [34] reported that the soil pH in saline water was slightly lower than that of the control.These results were consistent with the findings of this study, i.e., that piggery wastewater and saline water irrigation reduced the soil pH value, as compared with the control.The decrease may be related to the high organic or inorganic nitrogen in piggery wastewater, which promotes an intense nitrification reaction and releases a certain number of protons in the process [17,33].This slight changes in soil pH could be attributed to the release of exchangeable cations during soil organic matter mineralization process [32,35].
Soil EC is an important indicator for judging soil acidification and secondary salinization [36,37].The results of this study showed that soil EC was not significantly influenced by reclaimed water irrigation, which was consistent with Wang et al. [5].However, Liu et al. [38] reported that EC value was greater than 16% in reclaimed water irrigation.A general increase in EC was noted in piggery wastewater irrigation in this study, this result was supported by the findings of Kiziloglu et al. [32], who found that cauliflower and red cabbage having high EC of the wastewater with the slaughterhouse effluent.The discrepancy in EC may be related to the nature of water sources, irrigation frequency, and irrigation amount.The previous study [5,17] showed that EC increased dramatically under saline water irrigation, which was also supported by our result, showed that the highest EC value was obtained in saline water treatment, increased by 20.9% than that of the control.The results in this experiment showed that organic matter significantly increased by 2.1~43.4% in unconventional water irrigation in the 0-20 cm soil layer, and the highest value was observed in piggery water treatment.The same results were obtained in a study by Lu et al. [39], who indicated that biogas slurry irrigation significantly increased the organic matter of rhizosphere soils.The results of this study suggest that piggery wastewater irrigation could be favorable to improve soil structure and aeration [40].
Although many studies have been conducted to examine the impacts of wastewater irrigation on soil heavy metals (such as Cd, Cu, Ni, Zn, Cr, Pb, etc.), there is no consensus on them [32,40,41].In this study, there were no significant differences regarding the concentrations of the soil's total Pb and Cd under wastewater irrigation, while there were dramatically increased Cu and Zn concentrations.Similar results were supported by the findings of Liu et al. [33], who reported that reclaimed water and piggery wastewater signif-icantly increased Cu and Zn content, with no marked changes in Pb and Cd concentrations in the North China Plain.Huang et al. [41,42] also found that piggery wastewater irrigation caused a notable increase in Cu and Zn contents.The possible reason for this phenomenon may be related to the high Cu and Zn concentrations in the treatments.Though higher concentrations of Cu and Zn were observed in wastewater irrigation, the concentrations of them still within the permissible limits of the Soil Pollution Risk Control Standard for Soil Environmental Quality and Agricultural Land [43].

Irrigation with Different Water Quality on Cucumber Yield and Quality
Health soil physicochemical properties and nutritional status can produce crop yield and quality [32,[44][45][46].In the present study, the single fruit weight of cucumber under piggery wastewater and reclaimed water irrigation was significantly increased by 8.7~23.5%.Wu et al. [47] found that cucumber yield in reclaimed water treatment was 24% higher than that of freshwater, while there was no significant change in our study.The study also showed that a higher cucumber yield was observed in piggery wastewater treatment than in the control, which increased by 17.5% and 14.4%, respectively, indicating that yield increased with the increasing nitrogen application in the tested area.In addition, piggery wastewater treatment had the highest value of cucumber yield in this experiment, as a result of the complex factors in the soil.Our results were consistent with the findings of Kiziloglu et al. [32], who found that wastewater irrigation positively affected cauliflower and cabbage yield.Piggery wastewater has an abundance of mineral elements and organic matter, and these characteristics of biogas slurry may enhance crop yield [40,48].Furthermore, the correlation analysis showed that cucumber yield was significantly correlated with soil organic matter (Table 9), thus indicating that organic matter plays a positive role in increasing vegetable yield.This study found piggery wastewater led to a slight increase in the contents of Vc, soluble sugar, and protein, compared with the control, but there was only significant difference in the protein content in reclaimed water treatment.
Saline water irrigation has been reported to play a negative role in plant growth and crop yield with high salinity [49,50].In this study, no significant change was recorded in single fruit weight in saline water treatment, but cucumber yield was 1.4% lower than that of the control.The result was consistent with Huang et al. [23], who found that saline water irrigation reduced melon yield in northwest China.Mugwanya et al. [21] also reported that cucumber growth and yield was significantly decreased by saline water irrigation.The reason for a slight decrease in yield may be due to the highest value of soil EC (770 µS/cm), which was observed in saline water treatment, and high salt concentration may cause the disturbance of several physicochemical and physiological attributes, including soil bulk density, water uptake, nutritional disorder, and photosynthetic capacity, etc. [51][52][53][54].This study only found that the content of titratable acidity was significantly influenced by saline water treatment.Unconventional water irrigation was still limited by various objective or subjective factors, such as the nature of water, irrigation amount and frequency, soil texture, crop, and other uncertainties [55][56][57].Therefore, it is necessary to conduct in-depth research on promoting the advantages of unconventional water.The above-mentioned results indicated that piggery wastewater had the most positive effect on vegetable yield and quality in this study.

Conclusions
Three unconventional water irrigation types affect the soil properties, cucumber yield, and quality, apparently differently, in this study.Therefore, the characteristics of wastewater, soil, and crops should be considered in managing wastewater irrigation.Compared with groundwater irrigation, piggery wastewater and saline water irrigation decreased the soil pH but significantly increased the soil EC; the highest soil EC was observed in saline water.The results of wastewater irrigation indicated no significant changes in soil Pb and Cd concentrations, but remarkably increased in soil organic matter, Cu, and Zn concentrations, but the concentrations of Cu and Zn were far lower than the standard limit, indicating that short-term irrigation would not cause the accumulation of heavy metals pollution in soil.
Saline water irrigation resulted in a percentage decrease in cucumber yield.Piggery wastewater irrigation not only significantly improved the yield of cucumber, but also increased the content of Vc, soluble sugar, and protein.In this study, piggery wastewater was superior to all the other irrigation and had the highest cucumber yield.Considering the economic and environmental benefits, piggery wastewater irrigation combined with 300 kg/ha nitrogen was the optimal mode for this region.Further experimental research is needed.

Figure 1 .
Figure 1.Effects of different irrigation treatments on soil electrical conductivity.Note: control (CK): groundwater; PW: piggery wastewater; RW: reclaimed water; SW: saline water; H: 300 kg/ha nitrogen application; L: 200 kg/ha nitrogen application.Different letters indicate a significant level of 5% between the different treatments.

Table 1 .
Experimental design in this study.

Table 3 .
Changes in soil pH under different irrigation treatments.
Note: control (CK): groundwater; PW: piggery wastewater; RW: reclaimed water; SW: saline water; H: 300 kg/ha nitrogen application; L: 200 kg/ha nitrogen application.Different letters indicate a significant level of 5% between the different treatments.NS: no significant difference.

Table 4 .
ANOVA analysis for effects of different irrigation treatments on soil EC.

Table 6 .
Effects of different irrigation treatments on the concentration of soil heavy metals.

Table 7 .
Effects of different irrigation treatments on weight per fruit and cucumber yield.
L: 200 kg/ha nitrogen application.Different letters indicate a significant level of 5% between the different treatments.** means p < 0.01; NS: no significant difference.

Table 8 .
Effects of different irrigation treatments on the quality of cucumber.

Table 9 .
Correlation analysis between soil properties, cucumber yield, and quality.