Abstract
To explore the effects of mixed irrigation on soil and crops, a pot experiment was conducted in two salinity levels of brackish water, four levels of mixed brackish-reclaimed water and freshwater irrigation as the control. The soil Na-Cl to Ca-SO4 contents changed, and activities of soil alkaline phosphatase and polyphenol oxidase changed, exhibiting a ‘V’-shaped curve with increasing the proportion of reclaimed water in the mixture. At the same brackish-reclaimed water level, there were no significant differences in alkaline phosphatase and polyphenol oxidase activities except for soil alkaline phosphatase activity decreasing significantly with the increase in salinity under brackish water irrigation. Mixed irrigation obviously improved superoxide dismutase activity but no significant influences on aboveground dry weight, underground biomass or crop physiological indexes (chlorophyll, soluble protein, malondialdehyde, peroxidase, catalase). Based on the integrated biological response index version 2 (IBRv2), the deviation of reclaimed water irrigation was the smallest, followed by 1:1 and 1:2 (3, 5 g/L brackish water salinities, respectively), with IBRv2 values of 7.94, 12.55 and 16.04. Therefore, considering the soil-crop characteristics, limited daily water amount and inadequate pipeline facilities for reclaimed water, the brackish-reclaimed water ratio should be 1:1 and 1:2 at 3, 5 g/L of brackish water, respectively.
HIGHLIGHTS
Mixed irrigation with brackish water-reclaimed water was introduced.
IBRv2 was used to evaluate the effects of mixed irrigation on soil-crop system.
Brackish-reclaimed water ratio should be 1:1 and 1:2 at 3 g/L and 5 g/L of irrigation water salinity, respectively.
Graphical Abstract
INTRODUCTION
China has water sources, but they are unevenly spread throughout the country. China's agricultural water consumption accounts for more than 60% of the total water consumption, but the mismatch between the temporal-spatial distributions of water resources and cultivated land resources makes it difficult to meet the demand for agricultural water in northern China, which seriously restricts the sustainable development of agriculture. China is rich in unconventional water resources such as reclaimed water and brackish water. In China, the irrigation amounts of reclaimed water and brackish water in 2015 reached 11.01 billion and 1.48 billion m3, respectively (Hu & Wu 2018). Therefore, it is important to use unconventional water resources for irrigation to relieve the pressure of insufficient freshwater resources.
In recent years, research on brackish water utilization has been carried out mainly from the aspects of brackish water salinity, soil texture, suitable crops and field management, and a relatively complete technical system has gradually formed. In most salinized agricultural areas lacking freshwater resources, brackish water irrigation has become an important way to improve saline-alkali land and increase crop yield (Huang et al. 2019). Soil salinity affects the water retention of silty clay mainly in the high-suction range and decreases the plant-available water capacity (Tang et al. 2021). The water electrical conductivities linked to irrigation levels have a direct effect on the morphophysiological characteristics of collards (Viana et al. 2021). Within a certain range, brackish water irrigation can stimulate crop growth without a significant reduction or increase in crop yield (United States Salinity Laboratory Staff 1954; Yuan et al. 2019) and can improve water use efficiency (Yuan et al. 2019). Short-term brackish water irrigation has no obvious effect on soil chemical properties and soil salinization, while long-term brackish water irrigation may cause soil salinization (Tahtouh et al. 2019) and affect crop growth (Feng et al. 2014). However, some researchers believe that brackish water irrigation can increase the grain protein content (Cucci et al. 2019). In addition, brackish water irrigation may induce the occurrence of soil water repellency (Liu et al. 2011). Freshwater irrigation (1.2 dS/m) in the vegetative growth stage can improve the potential yield, and saline water irrigation (7 dS/m) in the reproductive stage can improve fruit quality (Bustan et al. 2005). HYDRUS simulation results show that long-term saline water (3 g/L) used to irrigate wheat and corn is more suitable for homogeneous soil than heterogeneous soil in North China (Liu et al. 2019).
In addition, many studies have been conducted on the safe utilization of reclaimed water. Studies on the utilization of reclaimed water mainly involve the effects of reclaimed water irrigation on crop growth (Bhattacharyya et al. 2008; Han et al. 2018a; Perulli et al. 2019; Petousi et al. 2019) and quality (Sun et al. 2007; Hu et al. 2013; Pedrero et al. 2018), the soil environment (Durán-Alvarez et al. 2009; Gibson et al. 2010; Han et al. 2018a), soil microbial community structure (Guo et al. 2017), groundwater (Siemens et al. 2008; Lesser et al. 2018) and the appropriate irrigation technology of reclaimed water (Hassanli et al. 2008; Lu et al. 2016; Intriago et al. 2018).
However, at present, researchers are mainly focused on brackish water irrigation or reclaimed water irrigation. There are few reports on the related studies of combined irrigation. Therefore, this study set different mixing ratios of brackish water to reclaimed water in pot experiments to explore the effect of mixed irrigation on the soil environment and physiological characteristics of crops to provide a theoretical basis for the safe utilization of reclaimed water in areas irrigated with brackish water.
MATERIALS AND METHODS
Tested soil description
The tested soil was collected from 0 to 20 cm topsoil of a field at Qiliying Experimental Base, Xinxiang city, Henan Province. The soil was air-dried, crushed and sieved (2 mm). The bulk density of the soil was 1.40 g/cm3, the field water-holding capacity of the soil was 23.02%, total nitrogen and total phosphorus contents in soil were 0.99 and 1.11 g/kg, and the alkali hydrolyzed nitrogen, available phosphorus and available potassium were 90.98, 26.82 and 208.60 mg/kg, respectively (Li et al. 2019). The electrical conductivity of a 1:5 soil-water extract was 372 μS/cm, and the mass fraction of organic matter was 2.66%. The particle size distribution of the soil samples was analysed by a BT-9300HT laser particle analyser. The proportions of clay (<0.002 mm), silt (0.002–0.02 mm) and sand (0.02–2 mm) were 13.05%, 62.46% and 24.49%, respectively, and the soil had a silty loam texture (International System).
Experimental device and scheme
The pot experiment was performed in the greenhouses of the Agriculture Water and Soil Environmental Field Science Research Station at the Chinese Academy of Agricultural Sciences in Xinxiang city, Henan Province, from May to June 2020. The station is located at 35°19′ N, 113°53′ E, 73.2 m above sea level, with an average annual temperature of 14.1 °C and multiyear average annual precipitation and evaporation of 588 mm and 2,000 mm, respectively. The frost-free period lasts for 210 days, and the average annual sunshine duration is 2,398 hr.
The pots had an upper diameter of 25 cm, lower diameter of 14.5 cm and a height of 19 cm. Each pot was loaded with 7 kg of soil, and all treatments received compound fertilizer (the ratio of N-P2O5-K2O was 15-15-15). All of the treatments received fertilizer as a basal application, following the local conventional fertilizer application. The tested crop was Shanghai green, and all treatments were irrigated with freshwater before sowing to maintain moisture. Seeds were sown on 27 May 2020 and were spread evenly in each pot. Five seedlings were left in each pot at the two-leaf stage (11 June), and then the irrigation treatment was started. In the early stage, irrigation was carried out approximately once every 2 days with 400 mL of water (the lower limit of irrigation was 75% of the field capacity); in the later stage, irrigation was performed approximately once per day (400 mL) as the crop's water demand increased. In the experiment, the two salinity levels of the brackish water were set for 3 and 5 g/L, and four mixed ratio levels were set for the mixing ratio of brackish water and reclaimed water, namely, reclaimed water, brackish water-reclaimed water 1:2, brackish water-reclaimed water 1:1, and brackish water. The control group consisted of cultivated crops under freshwater irrigation. The pot was randomly laid, and there were three replicates for each treatment. The specific experimental design is shown in Table 1. The water quality of the brackish water and reclaimed water is shown in Table 2. The reclaimed water was obtained from the Luotuowan Domestic Sewage Treatment Plant in Xinxiang city, Henan Province, and the sewage treatment plant used the A/O process. The water quality after sewage treatment was in line with the Farmland Irrigation Water Quality Standard (GB5084-2005). The freshwater source was local groundwater, and brackish water was prepared by adding sea salt to the freshwater.
Treatment . | CK . | T1 . | T2 . | T3 . | T4 . | T5 . | T6 . | T7 . |
---|---|---|---|---|---|---|---|---|
Solution | FW | 3 g/L of BW | 5 g/L of BW | 1:1 of BW(3 g/L) to RW | 1:1 of BW(5 g/L) to RW | 1:2 of BW(3 g/L) to RW | 1:2 of BW(5 g/L) to RW | RW |
Treatment . | CK . | T1 . | T2 . | T3 . | T4 . | T5 . | T6 . | T7 . |
---|---|---|---|---|---|---|---|---|
Solution | FW | 3 g/L of BW | 5 g/L of BW | 1:1 of BW(3 g/L) to RW | 1:1 of BW(5 g/L) to RW | 1:2 of BW(3 g/L) to RW | 1:2 of BW(5 g/L) to RW | RW |
FW, freshwater; BW, brackish water; RW, reclaimed water.
Water source . | EC1:5/(μS·cm−1) . | SAR . | Ion content/(mg·L−1) . | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Na+ . | K+ . | Ca2+ . | Mg2+ . | Cl− . | HCO3− . | SO42− . | CO32− . | |||
Fresh water | 321 | 0.34 | 10 | 1.6 | 39 | 15 | 30 | 120 | 104 | – |
Reclaimed water | 2,120 | 5.82 | 310 | 13.9 | 91 | 74 | 314 | 278 | 507 | – |
Brackish water (3 g/L) | 6,100 | 43.30 | 1,330 | 2.1 | 43 | 17 | 1,921 | 142 | 92 | – |
Brackish water (5 g/L) | 9,432 | 67.19 | 2,000 | 2.6 | 37 | 18 | 3,222 | 139 | 109 | – |
Water source . | EC1:5/(μS·cm−1) . | SAR . | Ion content/(mg·L−1) . | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Na+ . | K+ . | Ca2+ . | Mg2+ . | Cl− . | HCO3− . | SO42− . | CO32− . | |||
Fresh water | 321 | 0.34 | 10 | 1.6 | 39 | 15 | 30 | 120 | 104 | – |
Reclaimed water | 2,120 | 5.82 | 310 | 13.9 | 91 | 74 | 314 | 278 | 507 | – |
Brackish water (3 g/L) | 6,100 | 43.30 | 1,330 | 2.1 | 43 | 17 | 1,921 | 142 | 92 | – |
Brackish water (5 g/L) | 9,432 | 67.19 | 2,000 | 2.6 | 37 | 18 | 3,222 | 139 | 109 | – |
Measured indexes and methods
Measured indexes and methods were as follows:
Soil physical and chemical properties. After crop harvesting, soil samples were collected from the pots and were air-dried, ground and passed through a 2-mm sieve. The soil water content was determined by the oven drying method. The conductivity of a 1:5 soil-to-water extract (EC1:5) was determined by a conductivity meter. The low-temperature external-heat potassium dichromate oxidation-photocolourimetric method was used to determine the soil organic matter (SOM) content. The 1:5 soil-to-water extracts were prepared, the Na+ and K+ contents were determined by flame photometry, the Ca2+ and Mg2+ contents were determined by EDTA titration, the Cl− content was determined by AgNO3 titration, the CO32− and HCO3− contents were determined by double indicator-neutralization titration, and SO42− was determined by EDTA indirect complexometric titration. The water drop penetration time (WDPT) method was used to determine the soil WDPT.
Soil enzyme activity. Detection kits (Solarbio, Beijing) were used to measure the activities of soil sucrase (S-SC), soil alkaline phosphatase (S-AKP/ALP), soil urease (S-UE) and soil polyphenol oxidase (S-PPO).
Crop growth indexes. After harvesting, crop samples were collected and divided into two parts, aboveground and underground, which were rinsed with distilled water and air-dried. The aboveground fresh weight (AFW) and underground fresh weight (UFW) were weighed using a balance. Subsequently, the samples were placed in a 105 °C oven for 15 min and were then dried to a constant weight at 75 °C, after which the samples were weighed to calculate the aboveground dry weight (ADW) and underground dry weight (UDW).
Physiological characteristics of crops. The chlorophyll a, chlorophyll b and total chlorophyll contents were measured using a chlorophyll content detection kit (Solarbio, Beijing, China). The content of soluble protein was determined by the Coomassie brilliant blue G-250 method, and the catalase (CAT) activity, superoxide dismutase (SOD) activity, peroxidase (POD) activity and malondialdehyde (MDA) content in the plants was determined by UV absorption, nitrogen blue tetrazolium (NBT) photoreduction, guaiacol colourimetry, and thiobarbituric acid methods.
- Integrated biological response version 2 (IBRv2), proposed by Sanchez et al. (2013), was applied to explain the effect of irrigation management on arid soil enzyme activities (Diaz et al. 2021). The IBRv2 index was calculated using the main indexes, i.e., soil and crop, following Equation (1). Higher IBRv2 scores indicated larger deviations for all selected factors with regard to the chosen reference level.where Ai, calculated using Equation (2), is a deviation value for each factor with respect to that measured in the reference plots.
For each factor, single data points (xi) were divided by the corresponding reference value (x0) and were log-transformed to reduce the variance. The log-transformed data were standardized considering the general mean (μ) and the standard deviation (σ). Then, Ai, obtained by subtracting the mean of the reference standardized data (Z0) from the mean of the standardized value, was plotted in a radar graph to visually check for the induction (positive Ai or area in the radar plot above the reference level) or inhibition (vice versa) of each factor.
Data analysis
Excel 2010 software was used to calculate the experimental data, SPSS 25.0 software was used to conduct univariate analysis of variance, followed by the least significant difference (LSD) test (P< 0.05).
RESULTS
Effects of mixed irrigation on soil
Effects on soil water and salt
Soil moisture is an important factor in soil fertility and is the main source of crop water absorption. The soil salt content is the main parameter of soil salinity and the main index of soil salinization. Soil salinity is highly positively correlated with EC1:5, which is easy to determine. Generally, the EC1:5 value represents the soil salt content. The variations in the soil water content and EC1:5 under mixed irrigation with brackish water and reclaimed water after crop harvesting are shown in Figure 1.
As shown in Figure 1, compared with CK, the soil moisture content increased slightly by 10.30% in T7, but this was not significant. At the same salinity of brackish water, the soil moisture content decreased gradually with the increase in the proportion of reclaimed water in the mixture. For example, when the salinity of brackish water was 3 g/L, there was no significant difference between T1 and T3 or T3 and T5 (P > 0.05), but there was a significant difference between T1 and T5 (P < 0.05); when the salinity of brackish water was 5 g/L, T2 was significantly different from T4 and T6 (P < 0.05), but there was no significant difference between T4 and T6 (P > 0.05). At the same mixing ratio of brackish water and reclaimed water, the higher the salinity, the greater the soil moisture content, and there was no significant difference among the other mixed treatments (P > 0.05) except for the significant difference between the two brackish water irrigation treatments. Therefore, there were no obvious impacts of reclaimed water and freshwater irrigation on the soil water content, and the soil moisture content decreased gradually with the increase in the proportion of reclaimed water in the mixture.
When the salinity of brackish water was constant, the EC1:5 in different mixed irrigation treatments was significantly higher than that of CK (P < 0.05), and soil salinity increased obviously with the increase in the proportion of reclaimed water in the mixture (P < 0.05). At the same mixing ratio of brackish water and reclaimed water, there was a positive correlation between EC1:5 and the salinity of brackish water, and the difference between treatments was significant (P < 0.05). Therefore, soil salinity was mainly determined by the salt content in irrigation water. This was consistent with the changing trend of the soil moisture content because the higher the salt content, the stronger the limiting effect on crop water absorption, resulting in more water remaining in the soil.
Effects on soil water-soluble ions
Soil water-soluble salt is an important attribute of saline-alkali soil and a restraint on crop growth. In addition to the determination of the pH value and total salt content, the analysis of soil water-soluble salt also includes the determination of anions (CO32−, HCO3−, Cl−, SO42−) and cations (K+, Na+, Ca2+, Mg2+). The changes in the water-soluble ions in the soil after mixed irrigation with brackish water and reclaimed water are shown in Figure 2.
Figure 2 shows (1) the same change trends for the soil K+, Ca2+ and SO42− contents, and the contents of soil K+, Ca2+ and SO42− in T7 were significantly higher (P < 0.05) than those in CK (except for the Ca2+ content). At the same salinity of brackish water, the contents of soil K+, Ca2+ and SO42− increased gradually with the increase in the proportion of reclaimed water in the mixture. For example, when the salinity of brackish water was 3 g/L, the contents of soil K+, Ca2+ and SO42− in T7 were the highest. The soil K+ content in T7 was significantly different from those of T1 and T5 (P < 0.05) but not significantly different from that of T3, while the contents of Ca2+ and SO42− were significantly higher in T7 than in the other treatments (P < 0.05). When the mixing ratio of brackish water and reclaimed water was constant, the contents of K+, Ca2+ and SO42− in soil increased with the increase in the salinity of brackish water, but the difference was not significant (P > 0.05).
(2) The soil Na+ and Cl− contents exhibited the same trend, and the contents in T7 were significantly higher than those in CK (P < 0.05). At the same salinity of brackish water, the contents of soil Na+ and Cl− decreased with the increase in the proportion of reclaimed water in the mixture, and the difference between different treatments was significant (P < 0.05) (but there was no significant difference between the T3 and T5 treatments). At the same ratio of brackish water to reclaimed water, the contents of soil Na+ and Cl− increased significantly with increasing salinity of brackish water (P < 0.05).
(3) With respect to the contents of Mg2+ and HCO3− in soil, CK had higher values than T7, and the difference in the soil Mg2+ content reached a significant level (P < 0.05). Under the same salinity of brackish water, the soil Mg2+ content decreased gradually with the increase in the proportion of reclaimed water in the mixture, but the difference was not significant (P > 0.05) (except in T7). The soil HCO3− content also decreased gradually, and the soil HCO3− content in brackish water was higher than that in reclaimed water, but there was no significant difference between the 1:1 and 1:2 mixtures of brackish water and reclaimed water (P > 0.05).
Effects on soil enzyme activity
Soil phosphatase is an enzyme that catalyses the mineralization of soil organophosphorus compounds. Its activity level directly affects the decomposition and transformation of organophosphorus in soil and its bioavailability. In northern China, soil is alkaline, so soil phosphatase activity in the test refers to alkaline phosphatase (S-AKP/ALP). S-PPO is mainly derived from the decomposition and release of soil microorganisms, plant root exudates and animal-plant residues. This enzyme catalyses the oxidation of aromatic compounds in soil into quinone, which reacts with proteins, amino acids, carbohydrates, minerals and other substances in soil to generate organic matter and pigments and completes the cycling of soil aromatic compounds for soil environmental remediation. The enzymatic products of S-SC are closely related to the contents of organic matter, nitrogen and phosphorus in the soil, the abundance of microorganisms and the soil respiration intensity. S-UE activity is positively correlated with the soil microbial abundance, organic matter content, total nitrogen and available nitrogen, reflecting the nitrogen status of soil. The changes in the activities of S-AKP/ALP, S-PPO, S-SC and S-UE after mixed irrigation with brackish water and reclaimed water are shown in Figure 3.
As shown in Figure 3, there was no significant difference between T7 and CK in terms of S-AKP/ALP and S-PPO activity (P > 0.05). At the same salinity of brackish water, the activities of S-AKP/ALP and S-PPO changed, exhibiting a ‘V’-shaped curve with the increase in the proportion of reclaimed water in the mixture, and the higher the salinity of brackish water, the earlier the inflection point. At the same ratio of brackish water to reclaimed water, there was no significant difference in S-AKP/ALP and S-PPO activities (P > 0.05) except that the activity of S-AKP/ALP decreased significantly (P < 0.05) with the increase in salinity under brackish water irrigation.
There was no significant difference in S-SC activity between T7 and CK (P > 0.05). At a given salinity of brackish water, the S-SC activity showed an initial increase followed by a decrease with the increase in the proportion of reclaimed water in the mixture, and the activity was the highest at a brackish water/reclaimed water ratio of 1:2, which was significantly higher (P < 0.05) than that of reclaimed water irrigation and brackish water irrigation (but brackish water with a salinity of 3 g/L was not significantly different from reclaimed water irrigation and brackish water irrigation). At the same mixing ratio of brackish water and reclaimed water, the S-SC activity increased with the increase in the proportion of reclaimed water in the mixture, but the difference was not significant (P > 0.05). In addition, there was no significant difference in S-UE activity among the different treatments (P > 0.05) and there was no obvious change trend.
Effects on SOM
SOM is one of the important indexes of soil fertility, in addition to basic conditions to ensure the normal growth of plants, and an important measure to increase production. The activities of microorganisms, the synthesis and decomposition of organic matter, the ability of soil to retain nutrients and the availability of elements in soil are all related to soil pH. The variations in the SOM content and pH after mixed irrigation with brackish water and reclaimed water are shown in Figure 4.
As seen from Figure 4, no significant difference in soil pH was found between T7 and CK, and soil pH decreased slightly with the increase in the proportion of reclaimed water in the mixture at the same salinity of brackish water. The soil pH in T4 was significantly higher than that in T6 and T7 (P < 0.05) when the salinity of brackish water was 5 g/L, while the soil pH decreased (but not significantly) with the increase in the salinity of brackish water at the same mixing ratio of brackish water and reclaimed water.
The SOM in T7 was slightly higher than that of CK, but the difference was not significant (P > 0.05). At the same mixing ratio of brackish water and reclaimed water, SOM decreased with increasing salinity of brackish water, and the difference reached a significant level (P < 0.05) when the mixing ratio of brackish water and reclaimed water was 1:2. With the increase in the proportion of reclaimed water in the mixture, SOM increased first and then decreased in response to brackish water with a salinity of 3 g/L, while SOM decreased gradually in response to brackish water with a salinity of 5 g/L. Therefore, regardless of the salinity of brackish water, the SOM in T7 was significantly lower than that in the other treatments (P < 0.05).
Effects on soil WDPT
Soil water repellency refers to the physical phenomenon in which water cannot or has difficulty moistening the surface of soil particles (Yang et al. 1994). Soil water repellency will lead to an uneven distribution of soil water, soil water loss and dry soil surfaces, and enhanced surface runoff and soil erosion after rainfall or irrigation and will thus hinder the growth of crops (Dekker & Jungerius 1990). The strength of soil water repellency is generally characterized by WDPT. A WDPT that exceeds 5 seconds indicates soil water repellency. The changes in soil WDPT after mixed irrigation with brackish water and reclaimed water are shown in Figure 4. The WDPT in T7 was slightly lower than that in CK, but the difference was not significant (P > 0.05). At the same salinity of brackish water, WDPT decreased with the increase in the proportion of reclaimed water in the mixture, and WDPT in T1 was significantly higher (P < 0.05) than that in T5 and T7 in response to brackish water with a salinity of 3 g/L. The higher the salinity of brackish water, the higher the WDPT, but there was no significant difference at the same ratio of brackish water to reclaimed water (P > 0.05).
Effects of mixed irrigation on crops
Crop growth
Biomass plays a very important role in the formation of ecosystem structure and function. The changes in aboveground biomass and underground biomass (fresh weight and dry weight) of crops after mixed irrigation with brackish water and reclaimed water are shown in Figure 5.
As shown in Figure 5, there were no significant differences in AFW and ADW between T7 and CK (P > 0.05). At the same salinity of brackish water, AFW and ADW under brackish water irrigation were significantly lower than those under reclaimed water irrigation (P < 0.05), while AFW and ADW under mixed irrigation were generally higher than those under brackish water irrigation, but the difference was not significant (P > 0.05). At a given mixing ratio of brackish water to reclaimed water, the higher the salinity of brackish water, the lower the AFW and ADW; significant differences in AFW were observed (P < 0.05) between brackish water irrigation and 1:1 mixed irrigation with brackish water-reclaimed water, but there were no other significant treatment differences (P > 0.05). Regardless of the different salinities of brackish water or the ratio of brackish water to reclaimed water, no significant treatment differences were observed for UFW and UDW (P > 0.05).
Physiological and biochemical characteristics of crops
(1) Chlorophyll content. The chlorophyll content, which is closely related to photosynthesis and nutritional status, is an important indicator of plant growth. The chlorophyll in the chloroplasts of higher plants mainly includes chlorophyll a and chlorophyll b. The contents of chlorophyll a, chlorophyll b and total chlorophyll of leaves after mixed irrigation with brackish water and reclaimed water are shown in Figure 6.
As observed in Figure 6, there were no significant differences in chlorophyll a, chlorophyll b or total chlorophyll among all treatments (P > 0.05), and their change trends were consistent. T7 had chlorophyll a, chlorophyll b and total chlorophyll contents that were 3.97%, 5.61% and 4.47% higher, respectively, than those of CK. Compared with T7, under 1:2 and 1:1 mixed irrigation and brackish water irrigation, the chlorophyll a content decreased by 4.98%–7.63%, 3.82%–7.85% and 5.15%–9.26%, the content of chlorophyll b decreased by 10.88%–13.99%, 8.2%–12.73% and 9.28%–13.46%, and the total chlorophyll content decreased by 9.61%–9.76%, 6.12%–9.37% and 6.43%–10.15%, respectively. Therefore, mixed irrigation with brackish water and reclaimed water had no significant effect on the chlorophyll content of the crops.
(2) Enzyme activity in the leaf. Antioxidant enzymes, such as SOD, POD and CAT, can convert peroxides into less toxic or harmless substances. SOD, which scavenges superoxide anion free radicals by disproportionation reactions, plays an important role in biological antioxidant systems. POD can eliminate the toxicity of hydrogen peroxide, phenols and amines. CAT plays an important role in the active oxygen scavenging system. The change in the antioxidant enzyme activity in the crop under mixed irrigation of brackish water and reclaimed water is shown in Figure 7.
As shown in Figure 7, no significant difference occurred in SOD and CAT activities between T7 and CK (P > 0.05). At the same salinity of brackish water, the activities of SOD and CAT first increased and then decreased with the increase in the proportion of reclaimed water in the mixture, reaching a maximum at a 1:2 ratio of brackish water to reclaimed water. At 3 g/L salinity of brackish water, SOD activity in T7 was significantly lower than that in T3 and T5 (P < 0.05), but there was no significant difference between T1, T3 and T5 (P > 0.05); there was no significant difference in CAT activity between treatments (P > 0.05). When the salinity of brackish water was 5 g/L, SOD activity in T6 was significantly higher than that in other treatments (P < 0.05), CAT activity in T6 was significantly higher than T2 (P < 0.05), and there were no significant differences in SOD and CAT activity between T2 and T4 (P > 0.05), but SOD activity in T6 was significantly higher than that in T7 (P < 0.05). At a given mixing ratio of brackish water to reclaimed water, the activities of SOD and CAT increased with increasing salinity of brackish water, but the difference did not reach a significant level (P > 0.05).
There was no significant difference in POD activity among the different treatments (P > 0.05), but the change trend was different under different salinities of brackish water. For example, mixed irrigation improved POD activity to a certain extent compared to brackish water irrigation at 3 g/L salinity, while the opposite trend occurred at 5 g/L salinity.
(3) MDA. The production of MDA aggravates membrane damage. Understanding the degree of membrane lipid peroxidation by MDA can indirectly determine the degree of damage caused to the membrane system and the stress resistance of plants. The change in the MDA content of crops after mixed irrigation with brackish water and reclaimed water is shown in Figure 8. There was no significant difference in the MDA content among the different treatments (P > 0.05). At the same salinity of brackish water, the MDA content increased with the increase in the proportion of reclaimed water in the mixture but decreased slightly under reclaimed water irrigation; at the same ratio of brackish water to reclaimed water, the higher the salinity of brackish water, the higher the MDA content.
(4) Soluble protein. Soluble proteins are important osmotic regulators and nutrients. Their increase and accumulation can improve the water retention capacity of cells and protect the life substances and biofilms of cells. The change in the soluble protein content of crops under mixed irrigation of brackish water and reclaimed water is shown in Figure 8. At the same salinity of brackish water, the soluble protein content decreased with the increase in the proportion of reclaimed water in the mixture, and the content increased slightly under reclaimed water irrigation. At the same mixing ratio of brackish water to reclaimed water, the content of soluble protein was positively correlated with the salinity of brackish water. However, there was no significant difference in the soluble protein content among the different treatments (P > 0.05).
Evaluation of mixed irrigation in a soil-crop system based on IBRv2
IBRv2 was first used to evaluate the toxicity of environmental pollution to organisms and then gradually applied to explain the effect of irrigation on enzyme activity. The effect of mixed irrigation with brackish water and reclaimed water on the soil-crop system was evaluated by IBRv2. Based on the above results, ten crop and soil indexes (θ, EC1:5, AFW, SOM, WDPT, S-AKP/ALP, S-PPO, S-SC, CAT, SOD), with a relatively obvious influence and no repeated affecting factors, were selected to calculate the IBRv2 value with the measured value of CK as the control reference. The changes in IBRv2 and the radar map are shown in Figure 8.
As seen in Figure 8, all treatments had activation effects on θ, EC1:5, SOM, S-PPO and S-SC and certain inhibitory effects on CAT, SOD, S-AKP/ALP and AFW as a whole except for certain inhibitory effects on S-SC under T7, S-PPO under T4 and T5 and activation effects on CAT under T7. WDPT was activated under brackish water irrigation and 1:1 mixed irrigation with brackish water and reclaimed water but was inhibited under reclaimed water irrigation and 1:2 mixed irrigation of brackish water and reclaimed water.
The IBRv2 value in T7 was the smallest and significantly lower than other treatments (P < 0.05), indicating that the deviation between reclaimed water irrigation and freshwater irrigation was the smallest. However, the combined utilization of brackish water and reclaimed water could be considered when using brackish water for irrigation in areas where freshwater resources are insufficient due to the constraints of reclaimed water resources such as limited daily discharge and imperfect reclaimed water networks. At brackish water salinities of 3 g/L and 5 g/L, the most appropriate IBRv2 occurred in T3 and T6, with scores of 12.55 and 16.04, respectively. Therefore, the 1:1 ratio of mixed irrigation of brackish water and reclaimed water was suitable at 3 g/L of the salinity of brackish water but the 1:2 ratio at 5 g/L of the salinity of brackish water.
DISCUSSION
Effects of mixed irrigation on soil water, salt and water-soluble ions
According to previous results, the soil moisture content and electrical conductivity increased with increasing salinity of brackish water at the same soil depth (Yang et al.,2020b). In addition, the soil salt content increased with increasing salinity of brackish water (Zhang et al. 2020a). Soil moisture increased with increasing salinity of brackish water (1–6 g/L) but decreased above a certain salinity (9–12 g/L) (Yang et al. 2020a). The results in the current paper show that the soil moisture content and soil salt content decreased gradually with the increase in the proportion of reclaimed water in the mixture, which is consistent with previous research results. The reason is that the salinity of reclaimed water was higher than that of freshwater, the salinity in the mixture decreased with the increase in the proportion of reclaimed water in the mixture, and the soil salt content was lower after irrigation. Soil salinity will inhibit crop water absorption to a certain extent, so at the same irrigation amount, the soil moisture content will be higher due to the decrease in crop water absorption with the increase in salinity.
The higher the soil electrical conductivity, the higher the concentration of soluble ions in the soil and the higher the total salt content. Soil electrical conductivity can represent soil salinity, and different soil salinities will affect the content of salt ions (Zamanian et al. 2016). In this paper, the results showed that after crop harvest, the soil K+, Ca2+ and SO42− contents generally exhibited the same trend, and their contents under reclaimed water irrigation were all significantly higher than those under freshwater irrigation (except for the Ca2+ content). The ion content tended to increase gradually with the increase in the proportion of reclaimed water in the mixture at the same salinity of brackish water, while the contents of K+, Ca2+ and SO42− in soil exhibited an increasing trend with the increase in the salinity of brackish water at the same mixing ratio of brackish water to reclaimed water. The reason is the relatively high contents of K+, Ca2+ and SO42− in reclaimed water. The results also showed that the contents of soil Na+ and Cl− increased significantly under mixed irrigation, and the contents of soil Na+ and Cl− decreased significantly with the increase in the proportion of reclaimed water in the mixture, which may be because the changes in soil Na+ and Cl− were mainly related to the contents of Na+ and Cl− in irrigation water and water absorption by roots. Na+ and Cl− move with water to the root, and water is absorbed by the root. However, less Na+ and Cl− were absorbed by the roots and then accumulated in the soil around the roots. In addition, the contents of soil Ca2+ and Mg2+ under mixed irrigation were lower than those under freshwater irrigation and reclaimed water irrigation, indicating that mixed irrigation may promote the absorption of Ca2+ and Mg2+ by crops, which may be related to the competitive adsorption of ions, but the reasons need to be further discussed.
Effects of mixed irrigation on SOM and WDPT
Soil organic matter is an important index of soil fertility. The content of SOM was reported to increase, but not significantly, under reclaimed water irrigation (Chen et al. 2014), which is consistent with the results in this paper. However, it was found that the SOM content under reclaimed water irrigation was significantly higher than that under freshwater irrigation (Han et al. 2018b), which is slightly different from the results of this experiment. In this paper, the SOM content increased slightly with no significant difference under reclaimed water irrigation. The reason may be due to the differences in the quality of reclaimed water or that crops were planted in our experiment, while bare land irrigation was used in the previous experiment (Han et al. 2018b). Our results also showed that the SOM content under mixed irrigation was higher than that under reclaimed water irrigation, which may be due to the high salt content. A high salt content would reduce organic matter and the absorption of nutrients by crops; thus, the SOM was relatively higher after crop harvest.
Soil water repellency is ubiquitous. Extracellular polymers are formed by soil microorganisms under reclaimed water, and soil water repellency may be caused by the cover of extracellular polymers on the surface of soil mineral particles or soil aggregates (Morales et al. 2010). It was reported that the WDPT in the profile of four different soil types increased under reclaimed water irrigation (Shang et al. 2012), which slightly differed from the experimental results in this paper; that is, soil WDPT under reclaimed water irrigation was slightly lower than that under freshwater irrigation, and the reason may be due to the different components in the reclaimed water or soil texture types. Shang et al. (2012) compared the WDPT of soil after infiltration with that of air-dried soil before experiments based on indoor soil column tests. In the current experiment, however, changes in soil WDPT under reclaimed water and freshwater irrigation were compared, and different soil moisture contents and irrigation processes may also have effects on soil WDPT. Liu et al. (2011) found that weak water repellency occurred in many loamy soil profiles after infiltration with 3.5 g/L brackish water in Yangling in Shaanxi and in saline-alkali soil in Manas in Xinjiang, especially the latter. This is consistent with the experimental results that the soil WDPT increased with the increase in the proportion of brackish water in the mixture. A positive correlation was found between WDPT and the SOM content (Ren et al. 2017), but there was no perfect positive correlation in our results, which was closely related to the complex water quality. Our results also showed that compared with brackish water irrigation, WDPT decreased with the increase in the proportion of reclaimed water in the mixture. Related studies on mixed irrigation with brackish water and reclaimed water have not been reported, and this paper only preliminarily discussed mixed irrigation. Therefore, the related theoretical mechanism of mixed irrigation with brackish water and reclaimed water needs to be discussed further.
Effects of mixed irrigation on soil enzyme activity
There were some differences in the effects of reclaimed water irrigation on soil enzyme activities. Reclaimed water irrigation had no significant effect on soil enzyme activity (Ndour et al. 2008). Soil urease, alkaline phosphatase and sucrase activities increased to some extent under reclaimed water irrigation, but the difference was not significant compared with the control (Pan et al. 2012). Soil enzyme activity was affected by soil nutrients and heavy metal pollution (Zhang et al. 2006), and the timing of reclaimed water irrigation had different impacts on soil enzyme activities (Guo et al. 2012). In this paper, the effects of reclaimed water irrigation on soil enzyme activities (alkaline phosphatase, sucrase, urease and polyphenol oxidase) were not significant, which is basically consistent with previous research results. In addition, mixed irrigation with different ratios of brackish water to reclaimed water had a certain effect on soil enzyme activity, i.e., mixed irrigation with a 1:2 ratio of brackish water and reclaimed water enhanced soil sucrase activity, and mixed irrigation with a 1:1 ratio of 5 g/L brackish water and reclaimed water enhanced urease activity to some extent. Therefore, the best appropriate mixing ratio of brackish water to reclaimed water can improve soil enzyme activity, but the specific ratio and the regulatory mechanism of mixed irrigation with brackish water and reclaimed water on soil enzyme activity need to be further discussed.
Effects of mixed irrigation on the biomass and physiological characteristics of crops
Biomass is one of the important indicators of crop yield, especially that of vegetables. The results in this paper showed that the mixed irrigation of brackish water-reclaimed water had no significant effect on the aboveground dry weight, but the aboveground fresh weight was significantly lower compared with reclaimed water irrigation, which was consistent with the change in the soil moisture. The soil moisture content under mixed irrigation was higher than that under freshwater irrigation, indicating that the lower the amount of water absorbed by crops under the same irrigation amount and meteorological conditions, the lower the plant water content. Therefore, the fresh weight decreased significantly, but there was no significant difference in the dry weight. In previous research, compared with freshwater irrigation, reclaimed water irrigation significantly improved the yield of fruits and vegetables (Wu et al. 2010). The photosynthesis and yield indexes of cucumber irrigated with reclaimed water were higher than those irrigated with tap water (Wang et al. 2020). Our results also showed that there was no significant difference in biomass between reclaimed water irrigation and freshwater irrigation, which may be related to the quality and composition of the reclaimed water in different areas. The fresh fruit yield of processing tomato generally exhibited low yield under high salt levels (Zhang et al.,2020b), which is basically consistent with the results in this paper. It was found that the aboveground biomass (fresh weight and dry weight) of sweet and crisp pea decreased significantly with the increase in the salinity of irrigation water (P < 0.05) (Feng et al. 2020), which slightly differed from the result in this paper due to the difference in crop type or the difference between substrate cultivation and soil cultivation.
The chlorophyll content of leaves is an important index to represent plant growth. At the same irrigation quota, the SPAD value under reclaimed water irrigation was 5.54% higher than that under tap water irrigation (Wang et al. 2020). Reclaimed water irrigation could significantly increase the chlorophyll content of turfgrass (Lei & Li 2020), which is similar to the experimental results of this paper, that is, the content of chlorophyll under reclaimed water irrigation was higher than that under freshwater irrigation. Reclaimed water irrigation increased the MDA content and CAT activity of turfgrass (Lei & Li 2020), but our results showed no significant differences in the MDA content and CAT activity between reclaimed water and freshwater irrigation, which may be mainly due to the difference in water quality. The reclaimed water used in the former was a mixture of industrial and domestic effluent, while the effluent from domestic sewage treatment was used in this experiment. It was found that the POD and CAT activities and the MDA content of leaves increased significantly under recycled mine water irrigation (Yan et al. 2014), among which the change in the POD activity was similar to our experimental result, but the changes in the CAT activity and MDA content were slightly different. This may have been caused by different sources of reclaimed water or different water qualities or may be due to different crop types. No significant differences were found in the chlorophyll and MDA contents between brackish water irrigation and freshwater irrigation (Li et al. 2020), and Li et al. (2016) also reported no significant differences in the chlorophyll and MDA contents between salt water irrigation (2–10 g/L) and freshwater irrigation, which is consistent with the experimental results in this paper.
CONCLUSIONS
The following conclusions were drawn:
At the same salinity of brackish water, the soil water content, salt content and WDPT decreased with the increase in the proportion of reclaimed water in the mixture of brackish water and reclaimed water. At the same ratio of brackish water to reclaimed water, the higher the salinity of brackish water, the higher the soil water content, salt content and WDPT.
Overall, soil cations changed from Na+ to Ca2+, and anions changed from Cl− to SO42− with the increase in the proportion of reclaimed water in the mixture.
Different mixing ratios of brackish water to reclaimed water had different effects on soil enzyme activities. At the same salinity of brackish water, the activities of S-AKP/ALP and S-PPO changed, with an initial decrease followed by an increase with the increase in the proportion of reclaimed water in the mixture, and the higher the salinity of brackish water, the earlier the inflection point.
Mixed irrigation with brackish water and reclaimed water had a certain effect on the AFW of the crops, and the higher the salinity of brackish water, the more obvious the difference. Mixed irrigation had no significant effects on the ADW, underground biomass or crop physiological indexes (chlorophyll content, soluble protein content, MDA content, POD activity, CAT activity), but significantly improved SOD activity.
Based on the relative calculated results of IBRv2, the deviation of reclaimed water irrigation was the smallest, with a value of 7.94, followed by 1:1 (salinity of brackish water: 3 g/L) and 1:2 (salinity of brackish water: 5 g/L) mixed irrigation with brackish water and reclaimed water, with values of 12.55 and 16.04. Considering the soil environmental quality index, the limited daily reclaimed water amount and imperfect pipeline facilities, it is recommended that reclaimed water be considered an alternative source of freshwater to utilize with brackish water together in areas where freshwater resources are limited, and the suitable mixing ratio of brackish water to reclaimed water is 1:1 (at 3 g/L) or 1:2 (at 5 g/L).
ACKNOWLEDGEMENTS
This research was funded by the Scientific and Technological Project of Henan Province, grant number 202102110264, the Natural Science Foundation of Henan Province of China, grant number 202300410552 and Central Public-interest Scientific Institution Basal Research Fund, grant number FIRI202001-02, FIRI20210302, FIRI2019-04-02.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.