2.1 Analysis of soil physical and chemical properties
You can see from Table 2, the amount of salt in the soil of the 4 plants (OP, PT, PB, GB) irrigated by reclaimed water was significantly increased, compared with the contrast treatment. According to the local standard of China, when the total salt mass fraction of the soil is higher than 1.2 g kg− 1, some plant growth is inhibited or damaged, and the soil is in danger of salinization. The total salt mass fraction in the soil of this study is 0.199–0.501 g kg− 1 when irrigated by reclaimed water in 5 years, which is below the standard of 1.2 g kg− 1, but averagely 160.3% (ranging at 71.1%-210.2%) higher than the contract treatments (0.098–0.446 g kg− 1). Thus, concern should be paid attention to the salt accumulation caused by reclaimed water irrigation in green space to avoid the harm of salinization. Chlorides ion content ranged at 71.25–103.60 mg kg− 1 after 5-year reclaimed water irrigation. Under two kinds of irrigation water quality, chlorides ion content had significant differences with the contrast treatments in soil of SJ, PB, MD and HF, averagely 83.3% (ranging at 44.6%-156.6%) higher than that after tap water irrigation. And, water soluble sodium ranged at 52.33–80.21 mg kg− 1 after 5-year reclaimed water irrigation, which showed significant differences in six kinds of plants (SJ, PT, PB, GB, MD, HF), compared with contrast treatments, averagely 67.5% (ranging at 20.1%-123.2%) higher than that after tap water irrigation. In other plants, water soluble sodium and chloride content in soil showed increased trend after reclaimed water irrigation, although there were no significant differences. It indicated that the years of reclaimed water irrigation have resulted in the accumulation of chloride ions and sodium ions in the soil. These results are consistent with previous findings, which indicate that salt accumulation in soils and plants may often occur under reclaimed water irrigation and reclaimed water irrigation is one of significant processes enhanced soil salinization (Zalacáin et al. 2019;Chen et al. 2013;Wang et al.2017). For example, Zalacáinet al. (2019) indicated the increase of Cl− and Na+ after 5-years of irrigation by reclaimed water. Chen et al. (2013) also indicated that the soil salinity increased about 20% in the top 0.20 m after irrigation by reclaimed water than those by drinking water in Beijing, where is approximately 150 km away from the experimental area in this study. Compared with tap water resources, the reclaimed water is much easier to increase the risk of salinization due to its high saline content, as reported from 600 to 1700 µS cm− 1 (2017).
Table 2
Soil data analysis under different water quality irrigation conditions
Index
|
Irrigation water
|
OP
|
SJ
|
PT
|
PB
|
GB
|
MD
|
HF
|
EJ
|
pH
|
RW
|
7.87
|
8.12
|
7.76
|
7.94
|
7.85
|
8.65
|
8.08
|
7.97
|
TW
|
8.29
|
8.31
|
7.96
|
8.30
|
8.02
|
8.73
|
8.14
|
8.26
|
Total salinity (g kg− 1)
|
RW
|
0.361*
|
0.279
|
0.449**
|
0.381**
|
0.304**
|
0.199
|
0.501
|
0.229
|
TW
|
0.211
|
0.251
|
0.103
|
0.307
|
0.098
|
0.149
|
0.446
|
0.213
|
Chloride ions (mg kg− 1)
|
RW
|
86.25
|
103.60*
|
91.25
|
90.02*
|
71.25
|
80.37*
|
99.8*
|
90.69
|
TW
|
83.41
|
55.89
|
70.33
|
61.31
|
63.46
|
55.57
|
38.90
|
84.24
|
Water soluble sodium (mg kg− 1)
|
RW
|
65.34
|
56.78*
|
80.21**
|
61.21**
|
52.33**
|
61.12**
|
70.44**
|
70.48
|
TW
|
67.21
|
47.29
|
50.23
|
38.67
|
29.48
|
36.77
|
31.56
|
69.22
|
Organic matter (g kg− 1)
|
RW
|
20.12
|
13.78
|
9.21
|
16.43*
|
13.22
|
18.90
|
21.22
|
20.54
|
TW
|
22.88
|
16.89
|
10.33
|
10.59
|
14.37
|
17.47
|
16.48
|
21.20
|
Alkaline nitrogen (mg kg− 1)
|
RW
|
121.22*
|
106.78*
|
79.21
|
89.79*
|
80.23
|
89.78
|
128.66*
|
109.48*
|
TW
|
110.32
|
89.66
|
75.44
|
70.23
|
77.41
|
80.34
|
108.76
|
71.22
|
Effective phosphorus (mg kg− 1)
|
RW
|
35.78
|
58.21
|
34.32*
|
33.45**
|
41.44
|
35.33
|
26.78
|
43.22
|
TW
|
50.22
|
60.20
|
20.22
|
17.13
|
37.21
|
34.28
|
28.31
|
50.98
|
Available potassium (mg kg− 1)
|
RW
|
137.22
|
108.89
|
98.46
|
132.45*
|
123.33*
|
120.21
|
158.49**
|
100.25
|
TW
|
141.54
|
107.54
|
89.55
|
101.21
|
99.58
|
130.41
|
109.87
|
121.38
|
Note: * indicates significant difference (p < 0.05); ** indicates extremely significant difference (p < 0.01); RW indicates reclaimed water irrigation; TW indicates Tap water irrigation; OP indicates Oriental plane; SJ indicates Sophora japonica; PT indicates Pinus tabulaeformis; PB indicates Pinus bungeana; GB indicates Ginkgo biloba; MD indicates Magnolia denudate; HF indicates Hemerocallis fulva; and EJ indicates Euonymus japonicas. |
According to the test data (Table 2), the soil pH in different plants is in the range of 7.76 ~ 8.65 after 5-year reclaimed water irrigation. Compared with contrast treatments, soil pH did not show significant changes after 5-year reclaimed water irrigation, but showed a bit decrease trend in reclaimed water. This is similar with the previous results, which also showed no significant differences in soil pH after years of reclaimed water irrigation, comparing with the tap water irrigation (Liu et al.2011).
Nitrogen, phosphorus, potassium and organic matter in soil are essential nutrients for plant growth and development, and thus be assigned as the main indices to determine soil fertility. It is reported that these soil nutrients are significantly increased by long-term reclaimed water irrigation (Chen et al. 2015;Urbano et al. 2017). For example, Chen et al. (2015) indicated that soil total nitrogen, available phosphorus and organic matter content increased by 6–17% in 7 urban areas of Beijing with different reclaimed water irrigation histories. In this study, the influences of reclaimed water irrigation on soil nutrients were changed with the types of plants. As shown in Table 2, the content of soil organic matter only showed significant increasing trend in the plant of PB with the increase rate at approximately 55%. However, no significant differences were found in other kinds of plants.
Compared with the tap water irrigation, the content of alkaline nitrogen in soil significantly increased (averagely 25.8%) in 5 plants (OP, SJ, PB, HF, EJ) after 5-year irrigation with reclaimed water. The content of the effective phosphorus only showed significant increases in 2 kinds of plants (PT, PB) after 5-year irrigation with reclaimed water is approximately 82.5% higher than the contrast treatments.
Compared with the tap water irrigation, the content of available potassium in soil showed significant increases in 3 kinds of plants (PB, GB, HF), with the average increasing rate at 34.1% after 5-yearreclaimed water irrigation.
Compared with tap water, the higher content of nitrogen in reclaimed water only resulted in the significant increases in the content of soil nutrients in parts of plants. It is noticeable that all kinds of nutrient (alkaline nitrogen, effective phosphorus, available potassium and organic matter) in soil significantly increased in PB plant after reclaimed water irrigation. In other plants, the differences of soil nutrients content in soil varied with the types of plants, which are mainly related with the taken-up capacity of different plants. Previous studies have found that different plants have different absorption efficiency to different substances under different tree ages. Some plants are more efficient at absorbing certain substances when the fruit is growing, while others are more efficient at absorbing certain substances when the root is growing. We will continue to explore this area in the future (Chen et al. 2013).
2.2 Analysis of soil microbial quantity
2.2.1 Soil bacteria content in different water quality irrigation
Contrary results were found in the previous researches. For example, the total number of soil bacteria and Actinomycetes, fungi in grassland soil of Kentucky bluegrass were lower after years of irrigation by reclaimed water than that of tap water irrigation (Han et al.2006). However, Guo et al. (2006) indicated that the reclaimed water irrigation was not only beneficial to the increasing of the number of lawn grass rhizosphere microbes, but also helpful to the microbial community diversity of lawn grass, comparing with the conventional irrigation. In this study, the number of soil bacteria after irrigation with different water qualities followed the order as reclaimed water > 50% Tap water 50% reclaimed water > Tap water in the majority of the testing plants (Fig. 2(a),(b),(c)). Figure 2(d) showed that the number of soil bacteria showed an increasing trend in the most selected plants from 2013 to 2015. The number of soil bacteria in 8 kinds of plants in 2015 was higher than soil bacteria number in 2014 and 2013 except for the OP, which may be related to the contents of refractory organic compounds, pathogenic bacteria, viruses and parasitic ovum in the reclaimed water. In addition, the reclaimed water usually needs to be disinfected with chlorides. When chlorides gas enters into the reclaimed water, the organic matter or suspended substance in the water can be broken down and decomposed, resulting in some volatile phenols and odors. When the reclaimed water with high salt content enters the soil, it will affect the soil's EC, which will affect the change of the total number of soil bacteria (Klay et al. 2010).
2.2.2 Soil fungus content in different irrigation water quality
Soil fungus is a common soil microorganism, which is suitable for acidity, and the number of fungi is generally low due to the high pH in the experimental soils. The number of fungus showed increasing trend with the increasing content of reclaimed water from 0%, 50–100% in the most plants in 2013 (Fig. 3 (a)), although this increasing trend was weaken with the irrigation years. However, the number of fungus demonstrated different trends with the irrigation time with reclaimed water depending on the types of the testing plants. As shown in Fig. 3 (d), soil fungus content showed increasing trend with the irrigation years (2013–2015) in 4 kinds of plant (OP, GB, MD,PB), decreasing trend in 2 kinds of plants (SJ and EJ), and fluctuations in other plants. Specifically, the content of soil fungus increased in 2014 and then decreased in 2015 in 2 kinds of plants (PT and HF). During 2015, the differences of soil fungus content reduced in three kinds of irrigation water qualities in SJ, EJ, PT and HF, due to the decreasing trend of fungus in reclaimed water irrigation in 2015.
In addition, the root secretions of trees have a great influence on soil microorganisms. The root secretion causes a great change in the variety and quantity of the rhizosphere microorganism, and the root secretion causes the number of rhizosphere microorganisms to exceed the number of non-rhizosphere soil microorganisms. The number of rhizosphere microorganisms depends largely on the amount of sugar, organic acids and amino acids in the root secretions. The more secretions, the more microbes grow. The amount of soil microorganism is also affected by the plant itself, soil type and soil management measures. The results of the study suggest that the role of plants varies from plant community structure diversity, plant species, genotypes of the same plant, and even the different root regions of the same plant (Wang et al. 2006).
2.2.3 Soil Actinomycetes content in different irrigation water quality
Actinomycetes play important roles in soil organic matter decomposition process, because they are characterized by having high G + C contents and can recycle the dead organic matter by breaking down the compounds that is not well utilized by other microorganisms (Hozzein et al. 2019). In addition the Actinomycetes can fix nitrogen by the genus Frankia group and their symbiotic relations with plant roots (Hozzein et al. 2019;Zhang et al. 2012). The number of Actinomycetes in soil increased with the higher content of reclaimed water in the irrigation water resources, which is similar as the changing of the soil bacteria content in different irrigation water qualities during years 2013–2015 (Fig. 4(a), (b), (c)). Also, the number of Actinomycetes is increasing with the reclaimed water irrigation years, showing the similar increasing trend as the number of bacteria (Fig. 4(d)).
2.3 The relationship between soil microorganism and soil nutrient factors
Although the types of plants have influences on soil fertility, soil nutrient content, especially the content of alkaline nitrogen, effective phosphorus and the available potassium in soil may increase in the long-term reclaimed water irrigation, due to the higher content of nitrogen and phosphorus in the reclaimed water than that in tap water. The growth of microorganisms is enhanced by the soil fertility, to increase the number of different kinds of groups, like bacteria, fungus and Actinomycetes after irrigation by reclaimed water.
Soil bacteria are the main components of soil microorganisms, which can decompose various organic substances. Because the reclaimed water contains rich nutrition such as N, P and K, it provides abundant carbon source and nitrogen source for the growth of bacteria. These nutritions can increase the permeability of the soil and stimulate the growth and development of bacteria, thus greatly increasing the number of bacteria. It is also related to the various refractory organisms, pathogens, viruses and certain parasitic eggs in reclaimed water.
Fungus is one of the most common soil microbial communities. In terms of quantity, they are significantly lower than other kinds of microorganisms, but they are extremely important in terms of biomass.
Actinomycetes are a class of bacteria which are second only to bacteria in quantity, playing an important role in the decomposition of organic compounds in soil and the synthesis of soil humus.
Meanwhile, soil microorganisms not only controlled the soil organic matter and important nutrient elements (such as N, P, S) bioconversion, but also deeply influenced soil physical and chemical properties, such as the formation of soil aggregate structure, pH changes, etc.. For example, the increasing number of Actinomycetes help to decompose the organic matter decomposition and decrease the content of soil organic matter, which result in the insignificant changes of soil organic matter after irrigation by different water qualities. The unchanged pH of soil after irrigation by different water qualities is mainly due to the high soil buffer capacity (Guo et al. 2017).
It is noticeable that the BOD substance in the reclaimed water (oxygen-consuming organic matter) is too much, and when it enters the soil, it will rapidly deplete the oxygen in the soil. In this way, the anaerobic microorganism is developed, the denitrification of soil bacteria is strengthened, and the N in soil is oxidized to the gas N2 to volatilize.
The relations between soil microorganism and soil nutrients are regressed as equations (1), (2) and (3), where soil organic matter (x1), available potassium (x2), effective phosphorus (x3) and alkali solution nitrogen (x4) as the independent variable, total number of bacteria in the soil with reclaimed water irrigation (y1), the total number of fungus (y2) and the total number of Actinomycetes (y3) as the dependent variable,
y1 = 19092.743 x1-20401.654 x2 + 60315.622 x3 + 2978554.112 (1)
y2 = -1496.243 x1 + 281.403 x2 + 36.511x3 + 5960.143 (2)
y3 = -2015.214 x2 + 504811.654 (3)
The regression equation of the total number of soil bacteria (Eq. (1)) showed that the soil organic matter, effective phosphorus and alkali solution nitrogen were beneficial to the improvement of the total number of bacteria, but the content of the soil available potassium was negatively correlated with the total number of bacteria.
The regression equation of soil fungus content (Eq. (2)) showed the total number of fungus was improved by the content of the available potassium and the effective phosphorus in the soil, but decreased by the content of soil organic matter.
The regression equation of soil Actinomycetes content (Eq. (2)) showed that the content of available potassium had great influence on the total number of Actinomycetes.