Effects of Reclaimed Water Irrigation On The Soil Characteristics And Microbial Populations of Plant Rhizosphere

In this paper, the effects of irrigation with different water quality on the soil characteristics of 8 kinds of garden plants were analyzed. The results showed that soil pH (ranging at 7.76–8.73) had no signicant difference in different soils compared with the contrast treatment. Under the reclaimed water irrigation, the content of soil total salinity, chloride ions and water soluble sodium in soil of most plants was averagely 160.3%, 83.3% and 67.5% higher than that of tap water, respectively. The inuences of reclaimed water irrigation on soil nutrients were changed with the types of plants. The content of soil organic matter and the available potassium showed no signicant differences in most plants. Compared with the tap water irrigation, the content of alkaline nitrogen in in 5 plants increased (averagely 25.8%) after 5-year irrigation with reclaimed water. In terms of soil microorganism, the increase of soil microbial population, including bacteria, fungus and Actinomycetes, has been promoted by different levels of reclaimed water irrigation, showing increasing trend with the increasing content of reclaimed water from 0%, 50–100%. Moreover, the number of bacteria and fugus is closely related with the content of soil organic matter, available potassium and effective phosphorus. However, the number of Actinomycetes is determined by the content of available potassium. 5-year reclaimed water irrigation and did not show signicant difference compared with the contrast treatment. However, the content of soil total salinity (0.199-0.501 g kg -1 ), water soluble sodium (52.33-80.21 mg kg -1 )and chloride ions(71.25-103.60mg kg -1 ) were higher after reclaimed water irrigation than that of tap water irrigation. In terms of soil nutrient, the content of alkaline nitrogen, effective phosphorus and available potassium in soil after reclaimed water irrigation was highly dependent on the types of potted plants. Because the water quality is the most direct factors affecting soil microbial (no signicant interactions between plants and water quality), the content of different indices in irrigation water quality is different and the total number of soil bacteria also has a signicant impact. The number of microorganism in soil, including bacteria, fungus and Actinomycetes were increased with the content of reclaimed water when irrigated, which is also closely related with the changing of soil nutrient. Also, the number of bacteria and Actinomycetes may increase with the irrigation years by reclaimed water. The content of soil organic matter kept unchanged in different irrigation water qualities, which may be attributed to the increasing number of Actinomycetes in soil. All these results suggest that reclaimed water is one of the valid alternative sources for irrigation in green space in Hebei Province, where is suffering water scarce. When reclaimed water irrigation is implemented in Hebei province, total salinity of soil needs to be monitored to prevent the risk of soil salinization. The change of soil characteristics and microbial population in different seasons is of great signicance to the application of reclaimed water irrigation. We will analyze soil characteristics and seasonal changes in microbial populations. Moreover, the soil microbial community structure and diversity should be studied in depth.


Introduction
The source of reclaimed water is generally urban sewage, including sewage, partial industrial e uent and trapped rainwater. In addition to containing conventional pollutants, heavy metals and dissolved salts, urban sewage contains a variety of refractory organics, pathogens, viruses and certain parasitic eggs (Zhou 2002;Al-Nakshabandi et al. 1997). Modern sewage treatment technology to treat wastewater as reclaimed water is very effective and reliable. According to the long-time experience of reclaimed water reuse and a large number of scienti c studies, reclaimed water recycling is normally accepted for both public health and environmental safety(Al-Lahham et al. 2003; Oron et al.1999). It was reported that the reuse of reclaimed water for farmland had reached 20 million, and the amount is supposed to be increasing greatly due to the water crisis (Zhang et al. 2018).
However, the hazards from substandard industrial wastewater in the urban sewage system increased the risk of reclaimed water recycling. Soil properties may change with different pollutions in the reclaimed water. For example, pH would be in uenced by the acidity pollutions, soil elements would change with organic chemicals, and soil capacity of holding nutrient elements would alter Electrical conductivity (EC) (Smith et al.1996; Wang et al.2003). Toxic elements and organic pollutions may be transferred to soils by reclaimed water irrigation (Gu et (Cameron and McLaren 1997). For example, the reclaimed lawn irrigation could increase the population of root microbe, while have no signi cant effects on the microbial community in the root layer . The Shannon index (H'), Pielou evenness index (J si ) and the Margalef richness index (R1, R2) changed with the reclaimed water irrigation as well ).
Water quality should be identi ed according to the needs of different types of crops and irrigation methods when applying the reclaimed water irrigation (Manta et Gwenzi and Munondo 2008). Therefore, standards for reclaimed water developed in many countries. The potential harm of the reclaimed water irrigation to green space plants, soil pollution and the decline of groundwater quality were discussed in the previous studies (Wang et al. 2011;Smith and Cook 1996;Bouwer and Idelovitch 1987;Rattan et al. 2005). Currently, no harmful effects were demonstrated in short term in domestic studies of tertiary reclaimed water irrigation on green space, lawns and golf course (Lu, S., Wang, J., & Pei, L. 2016;Lu, S., Zhang, X., & Pei, L. 2016)). However, the effects of reclaimed water irrigation on soil environment of garden plants are not well explained. For this situation, this study investigated the effects of reclaimed water with different water quality on soil physical and chemical properties of several common garden plants, with comparison with contract treatment irrigated by Tap water. Potted experiments in 8 kinds of plants were carried out in Baoding city, Hebei province. The annual average water resource per capita in Baoding city is 282 m 3 , which suggest that Baoding City is belonging to the extreme water shortage area. Hence, alternative water resources are signi cant for the regional sustainability. Meanwhile, surface water suffers serious pollution. The river with water quality worse than V standard was approximately 147 km long in 2010s, according to China environmental quality standard for surface water (GB3838-2002), which means the water is too polluted to be used in these rivers. The main pollution source of surface water is COD, ranging at 34-89 mg L − 1 in serious polluted river, and the average COD in the polluted river is approximately 50 mg L − 1 , about 25% higher than the V standard (Huang et al.2019). The in uence of reclaimed water irrigation on the conventional soil nutrients and microbial population structure were also investigated to provide experimental basis for the standards setting when applying reclaimed water in garden irrigation.

Materials And Methods
Test site was in Hebei agricultural university in Baoding City, Hebei province. Reclaimed water (RW) was from the drainage group of Baoding sewage treatment plant. The water resource for contrast treatment was tap water. The experimental irrigation period was from 2012 to 2016. The initial water quality of tap water and reclaimed water was compared in Table 1. The organic matter and nitrogen content was much higher in reclaimed water than that in tap water. The suspended sediment concentration was also much higher in reclaimed water. pH did not show signi cant differences between reclaimed water and tap water. We put the owchart (Fig. 1) here to assist the comprehension of the research process in this study.   (1) Soil physicochemical properties were determined using conventional methods (Bao 2000). Soil acidity (pH value) was determined by potentiometric method (PHK-613acidity meter). Soil pH in distilled water at a soil-to-solution mass ratio of 1:5. The total salinity in soil was determined by residue drying -quality method. The chloride ions were measured using silver nitrate titration method and water soluble sodium was measured by ame photometry (FP6410 ame photometer). The organic matter was determined using Potassium dichromate -capacity method. The alkaline nitrogen was measured using alkali solution -diffusion method. The effective phosphorus was determined by NaHCO 3 leaching molybdenum blue colorimetry (752 spectrophotometers). The available potassium was measured using NH 4 OAc leaching -ame photometry (FP6410 ame photometer).
(2) Soil microbial group number is calculated by conventional method according to document (Rivero-Huguet and Marshall 2011). The bacteria were cultured by beef extract-peptone medium, the fungus was cultured by Martin's medium and the Actinomycetes were cultured by Gao's medium (No. 1). The volume fractions of soil suspensions for bacteria were 10 − 5 , 10 − 6 , and 10 − 7 . The volume fraction of soil suspensions for fungus and Actinomycetes were 10 − 3 , 10 − 4 and 10 − 5 . Each concentration of medium was cultured in the incubator and repeated 3 times for the culture. All materials, such as glass rod, straws, triangle bottle and glass beads, were treated by uperization and the inoculation was taken place on ultra-clean bench to ensure the operation process was strictly controlled under aseptic conditions.

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 signi cantly 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 signi cant 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

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,  indicated that the reclaimed water irrigation was not only bene cial 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)). 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).

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) In addition, the root secretions of trees have a great in uence 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).

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  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)).

The relationship between soil microorganism and soil nutrient factors
Although the types of plants have in uences 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 signi cantly 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 in uenced 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 insigni cant 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 denitri cation of soil bacteria is strengthened, and the N in soil is oxidized to the gas N 2 to volatilize.
The relations between soil microorganism and soil nutrients are regressed as equations (1), (2) and (3) (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 bene cial 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 in uence on the total number of Actinomycetes.

Conclusions
The in uences of irrigation water qualities on the soil physiochemical properties and microorganism were examined in 8 kinds of potted plants in this study. The pH of soil ranged at 7.76-8.65 after 5-year reclaimed water irrigation and did not show signi cant difference compared with the contrast treatment. However, the content of soil total salinity (0.199-0.501 g kg -1 ), water soluble sodium (52.33-80.21 mg kg -1 )and chloride ions(71.25-103.60mg kg -1 ) were higher after reclaimed water irrigation than that of tap water irrigation. In terms of soil nutrient, the content of alkaline nitrogen, effective phosphorus and available potassium in soil after reclaimed water irrigation was highly dependent on the types of potted plants. Because the water quality is the most direct factors affecting soil microbial (no signi cant interactions between plants and water quality), the content of different indices in irrigation water quality is different and the total number of soil bacteria also has a signi cant impact. The number of microorganism in soil, including bacteria, fungus and Actinomycetes were increased with the content of reclaimed water when irrigated, which is also closely related with the changing of soil nutrient. Also, the number of bacteria and Actinomycetes may increase with the irrigation years by reclaimed water. The content of soil organic matter kept unchanged in different irrigation water qualities, which may be attributed to the increasing number of Actinomycetes in soil. All these results suggest that reclaimed water is one of the valid alternative sources for irrigation in green space in Hebei Province, where is suffering water scarce. When reclaimed water irrigation is implemented in Hebei province, total salinity of soil needs to be monitored to prevent the risk of soil salinization. The change of soil characteristics and microbial population in different seasons is of great signi cance to the application of reclaimed water irrigation. We will analyze soil characteristics and seasonal changes in microbial populations. Moreover, the soil microbial community structure and diversity should be studied in depth.

Declarations
Ethics approval and consent to participate Not applicable.
Consent for publication Not applicable.
Availability of data and materials The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
Competing interests The authors declare that they have no competing interests" in this section.