Mechanisms of impact on the soil environment of drip irrigation applying treated sewage effluent: a review in China Open Access

Rapid urbanization and population growth have intensified the contradiction between the supply and demand of global agricultural, industrial, and environmental water utilization. As the most populous and largest developing country, the shortage of water resources has become the main bottleneck restricting the sustainable development of societal economics in China. Furthermore, the amount of water resources per capita in China is approximately 2,237 m³ (Ministry of Water Resources 2021), which is only approximately one-quarter of the world's average. Deficient water resources and increasing water demands mean that the water scarcity in China has become more severe than ever before. Thus, the reuse of municipal treated sewage effluent (TSE) as an alternative water source for farmland and landscape irrigation can effectively relieve water shortage (Ofori et al. 2021). To encourage the utilization of TSE, guidelines on promoting the utilization of sewage as a resource were passed by the National Development and Reform Commission of China in 2021 (National Development and Reform Commission 2021). The guidelines explicitly indicated that the utilization rate of reclaimed water in water-deficient cities of China should be over 25% by 2025, while in the Beijing-Tianjin-Hebei region it should be over 35%. However, the improper utilization of TSE for irrigation can lead to pollution in soil and groundwater, thereby deteriorating the ecological environment, and posing risks to human beings.

Modernized drip irrigation was imported into China from Mexico in the 1970s. According to the International Commission on Irrigation and Drainage, the acreage of micro-irrigation in China increased at an annual rate of 27% during a decade and reached 5.27 × 10⁶ ha in 2015, ranking highest in the world (Li 2018). Compared to sprinkling and surface irrigation, drip irrigation (especially subsurface drip irrigation) has been recognized as a preferable method to use TSE. It can minimize human contact with effluent, reduce the effect of drift and overspray on the irrigated crop, and decrease soil surface bacteria survival, runoff into surface waters, and odor (Li 2020). Due to these advantages, drip irrigation is the most widely used method worldwide when applying TSE (Farhadkhani et al. 2018). However, drip irrigation does not necessarily reduce pollution in soil and groundwater while applying TSE. Indeed, drip irrigation can easily transport nutrients and water to the root zone and deeper into soil, imposing a threat to groundwater under conditions of high precipitation and irrigation. Therefore, appropriate technical parameters and management patterns of drip irrigation (such as emitter flow rate, lateral depth, irrigation amount, and fertilizer amount) need to be considered to minimize environmental risks when applying TSE. Furthermore, the effects of drip irrigation and reclaimed water on soil, microorganisms, and crops have been of great concern worldwide. Studies are expected to address the negative impact on health and the environment resulting from reclaimed water use in irrigation. Moreover, applying TSE can easily clog emitters, imposing a threat to drip irrigation systems.

Therefore, this paper reviews, outlines, analyzes and discusses the effects of reclaimed water on irrigation systems, soil, microorganisms, and crops for drip irrigation; it proposes suggestions and raises issues to provide a better understanding of the status of reclaimed water irrigation to promote the sustainable use of water resources.

Based on a large number of domestic and foreign studies about TSE drip irrigation, the study begins with a concise introduction highlighting the history of water reuse for irrigation in China. It then proceeds with an in-depth look at three broad issues: emitter clogging, migration and distribution of TSE, and environmental impacts. Using a schematic illustration (Figure 1), the effects of reclaimed water on irrigation systems, soil, microorganisms, and crops in the case of drip irrigation are outlined, analyzed, and discussed.

It is generally believed that the process of sewage reuse in irrigation of China began in the 1960s (Shi et al. 2014). Before 1957, industrial and domestic sewage was used for farmland irrigation in a small part of Beijing's outskirts, and had no obvious impact on the environment. Since 1957, China has made efforts to develop sewage irrigation and carry out experimental research on sewage irrigation of farmland in 12 cities. At that time, the Ministry of Construction and Industry allied with the Ministry of Agriculture and the Ministry of Health to put sewage irrigation into the national scientific research plan. As illustrated in Figure 2, the amount of land irrigated by sewage in China was 11,500 ha at the beginning of sewage irrigation. With the rapid growth of the national economy, the scale of cities and industries expanded, and the area of sewage irrigation in China increased from 0.333 million ha to 3.33 million ha from the 1970s to the 1990s. During this period, an increasingly serious environmental problem resulting from irrigation with raw or primary-treated sewage effluent appeared in some farmlands which slowed the increase in the area irrigated by sewage. By 2000, the sewage irrigation area in China had reached 4.3 million ha.

Along with increasing attention given to the ecological environment, sewage treatment systems have been greatly developed in China. As illustrated in Figure 3, the percentage of sewage disposed in China increased at different rates in different periods. However, the treatment rate of domestic sewage was below 50% before 2006, which signifies that more than half of sewage was untreated and released directly into the environment imposing a threat to human beings. In 2006, the Chinese standards of reclaimed water quality (SL 368-2006) were first released, which significantly promoted the improvement of the sewage treatment rate. For instance, the percentage of sewage treated in China increased from 50.49% to 85.25% from 2006 to 2012. Despite the increase of treated sewage, there is rising concern over the relatively low utilization rate of TSE which increased from 4.79% to 22.52% during 2006–2020 (Figure 3). This may suggest a great potential for replacing fresh water with TSE in the future.

According to statistics, the urban sewage treatment capacity of China has reached 55.73 billion m³ with a treatment rate of 97.53% in 2020 (Ministry of Housing and Urban-Rural Development 2021). Nonetheless, reclaimed water consumption was only 10.89 billion m³, accounting for 3% of the total agricultural water consumption in 2020 (Figure 4) (Ministry of Water Resources 2021). There is great potential for the development of TSE reuse in China, especially in agricultural irrigation. A survey in 2008 illustrated that TSE was mainly used for agricultural irrigation (29% of the total reclaimed water), recreational/environmental enhancement (34%), miscellaneous urban use (12%), industry (23%), and groundwater recharge (2%). Furthermore, there exists a wide discrepancy in the development of reclaimed water reuse in the provinces of China. For instance, the utilization of reclaimed water resources in Beijing accounted for 65% of its total sewage treatment and ranked highest in the country. This was far ahead of the second-ranked Shandong province (44%) (Ministry of Housing and Urban-Rural Development 2021). Compared to Beijing where the total agricultural water consumption accounted for merely 0.09% of national total agricultural water consumption, the utilization of reclaimed water resources in Xinjiang, Gansu, Ningxia and other arid and semi-arid areas accounted for less than 36% of the total sewage treatment, while the agricultural water consumption in these regions accounted for 18% of the total agricultural water consumption in China (Ministry of Water Resources 2021). Compared with Israel and developed countries in Europe and America, a smaller amount of TSE is used for agricultural irrigation in China, which means that more reclaimed water can be used for agricultural irrigation in the future.

TSE contains a large amount of suspended solids, dissolved salts, chemical precipitation, dissolved organic matter, microorganisms and other substances, which have the potential to generate chemical and biological clogging of emitters. This would result in drip irrigation system performance degradation, which directly affects the operational life and application benefit of irrigation projects, and restricts the application and promotion of drip irrigation technology with TSE (Li 2020). In addition, the emitter clogging mechanism becomes more complex when using TSE for drip irrigation, due to a series of physical, chemical, and microbial dynamics factors present among the various components in the reclaimed water. Substantial research suggests that the blockage material in emitters irrigated with reclaimed water is a polymer (biofilm) composed of solid particles, microorganisms and their secreted viscous polymers (Hao et al. 2017; Song et al. 2017). More than 90% of microorganisms in the water environment will attach to the surface of solid substrates and exist in the form of biofilms (Taylor et al. 1995; Song et al. 2017), which means that microorganisms entering the drip irrigation system will form epiphytic biofilms in various parts of the system when using TSE. Furthermore, the chemical ions in the TSE impact the structural stability of the biofilm. For instance, in one study, Fe²⁺ accelerated the growth of microorganisms and promoted the precipitation of solid particles, increasing the risk of biochemical clogging (Hao et al. 2017). With increasing calcium concentration, the biofilm became thicker and denser. However, emitter clogging is a comprehensive result of water quality parameters and operational environments of drip irrigation systems as well as the internal structure of the emitters. When flowing TSE through emitters with flow channel depths of 0.75, 0.83, 1.01, and 1.08 mm, the minimum growth of epiphytic biofilm appeared in the emitters of 0.75 mm, while the maximum growth of epiphytic biofilm appeared in the emitters of 0.83 mm (Li et al. 2013). These findings may suggest that a comprehensive consideration of chemical, physical, and biological agents as well as the flow channel structure of emitters is important to address emitter blockage, especially in TSE irrigation.

As emitter clogging is the primary cause impacting the irrigation uniformity and service life of drip irrigation systems, the prevention and treatment of emitter blockage is the critical problem for the safe operation of TSE drip irrigation systems. Chemical blockage caused by carbonate precipitation can be dissolved with an acid solution to ensure the normal operation of the drip system (Li 2020). Moreover, using irrigation water with a pH less than 6.8 can effectively alleviate the occurrence of chemical blockage of emitters (Hills et al. 1989). When compared to acidification, chlorination with lateral flushing to control drip irrigation emitter clogging is a more economical and practical method because it is effective for both biological and chemical blockage. Chlorine and hypochlorite (NaClO or CaClO₂) are commonly used as raw materials for chlorination. Chloric acid can dissolve chemical blockages due to inorganic salt precipitation. Moreover, the strong oxidation of chlorine can kill or inhibit the reproduction and growth of microorganisms (bacteria), preventing the formation of mucus and clumps and reducing the biological clogging problem of drip emitters (Hao et al. 2017; Li 2020). However, there is no standardized method for chlorination: concentration, frequency, and duration all vary. For instance, the chlorine concentration varies from 1 to 500 mg/L in the study results of different scholars (Feigin et al. 2012; Li et al. 2012). Field experiment results indicated that more frequent chlorination at a lower concentration was more effective in maintaining good system performance (Li et al. 2010). However, the opposite has also been concluded by an experiment conducted in the laboratory, which found that a higher concentration improved the emitter flowrate (Duarte Coelho & Souza Resende 2001). Furthermore, chlorination may lead to the accumulation of soil chloride ions, increasing the risk of soil salinization, impacting the soil biological environment, and imposing a threat to crop growth and yield (Hao et al. 2018). Therefore, much more effort should be made to study the emitter clogging mechanism and treatment as well as the impacts of chlorination on crops and the environment.

Drip irrigation is the application of water through point or line sources on or below the soil surface at a small operating pressure and at a low discharge rate, resulting in partial wetting of the soil surface that is substantially different from surface irrigation and sprinkling (Lamm & Ayars 2007). The technical parameters of drip irrigation (emitter flow rate, irrigation amount, lateral depth, etc.) are important factors affecting soil water distribution and nutrient migration and transformation under drip irrigation. The various technical parameters of drip irrigation differentially impact the soil, crops, and groundwater.

Among numerous technical parameters of drip irrigation, the emitter flow rate is one of the most important factors affecting the distribution of TSE. Studies have shown that a high emitter flow rate promotes the horizontal transport of water and salt in soil, while a low emitter flow rate facilitates the vertical transport of edaphic water and salt (Li et al. 2006; Liu et al. 2007). Statistical results have demonstrated that the emitter flow rate has a greater effect on horizontal soil water migration than on vertical transportation. Furthermore, drip irrigation using TSE has been carried out in sandy and saline-alkali soil to illustrate that a higher emitter flow rate resulted in a larger wetting front radius and more uneven water content distribution (Shang 2013). The distribution of TSE in soil results in the spatial variation of soil physicochemical properties. Li et al. (2003) compared the impacts of various emitter flow rates on nitrate migration in different soils. The results showed that nitrate accumulated near the wetting front and was uniformly distributed within 17.5 cm from the emitter in loam or sandy soil after fertigation. When the emitter flow rate increased from 2 L/h to 7.8 L/h, the average concentration of nitrate distributed within 17.5 cm from the emitter increased from 56.6 mg/L to 66.1 mg/L. A field irrigation experiment using residential effluent confirmed that a greater emitter flow rate (4 L/h) resulted in a noticeably higher concentration of phosphorous in topsoil along the dripline and perpendicular to the lateral direction, while the concentration of phosphorous at a lower emitter flow rate (3.4 L/h) was higher in deeper soils (Jnad et al. 2001). Similar results were found in the transportation of Escherichia coli (E. coli) in sandy loam when applying TSE using drip irrigation (Wen et al. 2016). An increasing rate of emitter flow accelerated the E. coli transport rate horizontally, resulting in a larger distributed volume of E. coli. Haynes (1990) investigated the effect of nitrogen fertigation on soil pH with different emitter flow rate. The results indicated that in treatment with a 4 L/h emitter flow rate, soil acidification mainly occurred in the topsoil within a 30 cm radius of the emitter, although significant acidification appeared at 40 cm depth in the treatment with a 2 L/h emitter flow rate. These results may imply that to utilize TSE to raise the pH of deep soil (Hassanli et al. 2008; Bastida et al. 2018), it is suitable to use a lower emitter flow rate, while a greater emitter flow rate is beneficial for the acidification of soils since simple liming is traditionally used to correct topsoil acidity. However, there are not sufficient studies on the effects of drip irrigation of TSE on soil acidification.

When compared to the emitter flow rate, the effect of irrigation volume on TSE distribution largely overlaps with the influence of gravity. At equivalent emitter flow rates and as irrigation volume increases, the rate of transportation of water and salt in the vertical direction is larger than that in the horizontal direction. Using a 15° wedge-shaped plexiglass container to conduct experiments, the effect of irrigation levels on the distribution of water and nitrate was evaluated by Li et al. (2003). Their results indicated that the quantity of water and nitrate near the wetting front increased as the irrigation level increased. Bielorai et al. (1984) found that the residual soil moisture remaining at depths of 60–150 cm increased as the irrigation amount increased at the end of the irrigation season in TSE drip irrigation plots. This suggested that much more salt stemming from TSE accumulated in deep soil at larger irrigation levels, resulting in a higher osmotic potential and a reduced level of water extraction by plants. A field experiment with drip irrigation further confirmed that TSE irrigation could promote soil nutrient downward movement. Compared to sufficient irrigation applying saline water (Electrical conductivity (EC) = 1 dS m⁻¹), the salt content at a 60 cm depth irrigated with TSE (EC = 3 dS m⁻¹) was higher, while the salt content at a 0–30 cm depth was much less than that of deficit irrigation using TSE (Mounzer et al. 2013). The variation in soil conductivity consequently gives rise to a change in soil pH. A few studies have found that soil pH decreased with salinity accumulation, and there was a significant negative correlation between them (Yin et al. 2011; Qiu et al. 2017a; Li 2020). Nevertheless, the effects of irrigation level on the distribution of E. coli and soil enzyme activities in soil were unnoticeable and unstable during the experimental periods when applying TSE with drip irrigation (Li & Wen 2016; Qiu et al. 2017a). It may reasonably be expected that more efforts should be made to find the mechanism behind these phenomena.

Different from the emitter flow rate and irrigation amount, the lateral (pipes or tubes) depth directly dominates the transportation and distribution of TSE in soil. Lamm & Ayars (2007) indicated that the depth of soil water and nutrient migration increased as the lateral depth increased, while the transport of water and nutrients above laterals was sluggish. Utilizing reclaimed wastewater for irrigation in the arid region of Israel, the difference in chemical characteristics of the soil with various dripline depths was evaluated (Sacks & Bernstein 2011). The results showed that a greater lateral depth resulted in noticeably higher chemical characteristics in deeper soils, whereas chemical characteristics at a shallow lateral depth were higher in topsoil. For example, the content of sodium at 40–60 cm depth with surface drip irrigation was 233 mg kg⁻¹, but that with subsurface drip irrigation at 40 cm depth was 306 mg kg⁻¹. Li (2020) reported similar findings on the distribution of soil nitrate, EC, pH, E. coli, and enzyme activities when using TSE with subsurface drip irrigation. This study specifically pointed out that the distribution of E. coli in the soil was influenced by lateral depth but the difference was not statistically significant. Coincidentally, Asgari & Cornelis (2015) demonstrated that the emitter depth in drip irrigation does not play a significant role in the accumulation of heavy metals from TSE in sandy loam soil. However, an increasing lateral depth increased the seasonal NO₃–N leaching, especially in drip irrigation using TSE (Qiu et al. 2017b). This may be due to the nature and quality of the TSE, as well as the irrigation frequency and watering period varying within and between the studies. The abovementioned facts clearly show that the choice of an appropriate lateral depth is affected not only by crop, soil, and climate characteristics, but also by the components of the reclaimed water.

Irrigation water displaces air in the soil pore spaces depriving roots of oxygen during irrigation, especially in poorly drained soils in which rainfall or irrigation water cannot easily enter (infiltrate) or move downward through the soil (percolate). When the balance of components between air and water in the soil is broken, low oxygen content in the rhizosphere hinders respiration and negatively impacts plant uptake of water and nutrients, resulting in constrained yield performance (Bhattarai et al. 2011). This may be the principal reason resulting in drip irrigation lacking a significant yield benefit when compared to traditional practice, since there is a sustained wetting front around emitters. Reclaimed water has high biochemical oxygen demand (BOD), a lot of organic matter, and often records poor dissolved oxygen concentrations. Once it enters the soil, the decomposition of microorganisms and organic matter in TSE will accelerate the consumption of soil oxygen content and impose a hypoxia on the soil causing an increase in soil nutrient (especially nitrate) residual content and a rising risk of soil environmental pollution. Therefore, the application of aerated drip irrigation of reclaimed water can effectively meet the oxygen demand of microorganisms and crop roots to improve biological activity of the soil, reduce the chance of fertilizer residue polluting the soil environment, and facilitate the safe and efficient utilization of TSE. Soil aeration and structure improved and the biochemical environment was altered from aerated drip irrigation with reclaimed water. Furthermore, the irrigation fluid phase changed from liquid flow to gas-liquid two-phase flows, and the migration and transformation of nutrients in the soil subsequently changed. The migration and transformation processes of soil nutrients differ from those of non-aerated irrigation and reclaimed water irrigation. Further information concerning the effect of aerated drip irrigation on soil structures, biochemical environment, and nutrient migration and transformation associated with TSE is still needed.

TSE reuse for drip irrigation presents numerous environmental benefits and challenges. The scale of impacts is dependent on the quality of the treated effluent. When compared to the reclaimed water quality guidelines recommended by the US Environmental Protection Agency (USEPA), the upper limits of pollutant indexes in the Chinese reclaimed water quality standard for farmland irrigation (GB20922-2007) are relatively high (Table 1). For instance, the upper limit of suspended solids for vegetables, paddy grain, dryland grain and fiber crops are 60, 80, 90, and 100 mg/L respectively, which are substantially higher than the 30 mg/L in the USEPA guidelines. Additionally, the components of reclaimed water fluctuate with time due to the interaction between components. The great difference in water quality components is an important reason for the differential experimental results when irrigating TSE.

Drip irrigation and irrigation water impact the structure and properties of soil, especially with long-term irrigation regimes and high strength irrigation practices. A cavity appearing in soil near an emitter, soil penetration resistance increasing, and soil infiltration rate decreasing were reported under drip irrigation (Sinobas & Rodríguez 2012). Changes in soil physicochemical properties resulting in waterlogging near an emitter can restrict crop roots obtaining water and nutrients from a humid area (Yang et al. 2019). Furthermore, salts (cations and anions), organic matter, microorganisms, and suspended solids in irrigation water inevitably alter the chemical composition of the soil solution and aggregate existing conditions, which leads to changes of the soil structure and hydraulic properties. In a study conducted by Sou/Dakouré et al. (2013), a drastic decreasing in soil structural porosity with was reported with wastewater irrigation. Their results revealed that organic matter contained in wastewater was largely dissolved due to a sharp soil pH increase, leading to black alkali formation at the soil surface which interferes with water infiltration.

A long-term treated wastewater drip irrigation exercise adversely impacted soil hydraulic properties in Israel (Assouline & Narkis 2011). The results demonstrated that hydraulic conductivity, sorptivity, and infiltration rates were consistently reduced. These changes of soil hydraulic properties would result in a smaller root zone with lower oxygen availability. Levy et al. (2014) highlighted that exchangeable sodium would accumulate in subsurface layers to degrade the soil structure through swelling of clay minerals when using TSE drip irrigation. The degradation may impose an adverse effect on water flow in soil layers and possibly create an anaerobic environment. Using column infiltration experiments, the effects of TSE drip irrigation on soil water repellency was evaluated by Shang et al. (2012) in China. Their results indicated that soil water repellency increased with an increase in irrigation quantity and duration, and that sandy soil is more suitable for irrigating with TSE compared to other types of soil. However, several studies hold contrary opinions. In one study conducted by Hu et al. (2020), using reclaimed water improved the structural properties and hydraulic conductivity of tidal soil.

Point source infiltration and inferior water are bound to impact soil structure. However, the effect felt depends on water quality, irrigation parameter, and soil types. The changes in the soil-water relationship arising from drip irrigation using TSE can significantly affect crop growth and production since soil-water is the fundamental media supporting plant growth (Ofori et al. 2021).

Compared to conventional water sources, reclaimed water is more saline and excessive salt and sodium could impose the risk of soil salinization. Moreover, drip irrigation alters the rules of water and salt movement in soil that improved the risk of salinity accumulation in surface soil and secondary salinization in farmland, especially in arid and semi-arid regions with high rates of evaporation and a long dry season. The effects of TSE drip irrigation on soil salinity accumulation have been evaluated by various studies in China. Han et al. (2020) found that the reclaimed water irrigation significantly increased the salinity at 0–60 cm soil depth compared to tap water. They also suggested strictly controlling the salt content in the reuse water to avoid soil salinization with a long-term irrigation exercise. Pan et al. (2012) collected surface soil samples from urban green lands and suburban farmlands of Beijing to evaluate the potential risks posed by long-term reclaimed water irrigation. Their results indicated that the electrical conductivity and sodium adsorption ratio in the soil are both significantly higher than those of farmland irrigated with conventional water sources. Similar results were also supported by other studies conducted around the world. An example from an arid region of American showed that irrigation with reclaimed water exhibited 187 and 481% higher EC and sodium adsorption ratio (SAR) compared with fresh water irrigation, respectively (Elgallal et al. 2016). However, a study on the effects of drip irrigation using municipal effluent in Iran presented opposite results. The results demonstrated that soil salinity was reduced from 8.2, 6.8 and 7.0 dS m⁻¹ to 1.07, 1.12 and 3.5 dS m⁻¹ at a soil depth of 0–30, 30–60 and 60–90 cm, respectively (Hassanli et al. 2008). These consequences may be dependent on the higher EC of the experimental field (EC = 7.3) than that of the irrigation water (EC = 1.5) and superior amount of irrigation (933 mm). Furthermore, a field experiment was carried out to investigate the effect of drip irrigation on reclamation and sustainability of salt-affected land in China. Results showed that both soil salinity and pH value at 0–40 cm soil depth decreased dramatically after 3 years of reclamation (Tan & Kang 2009). It may reasonably be expected to prevent soil salinization with appropriate application of drip irrigation parameters as well as water and fertilizer management when using reclaimed water.

When using reclaimed water, pathogens can reside in the soil and the concentration of pathogens in the irrigation water usually determines the survival period in the soil (Vergine et al. 2015; Li & Wen 2016). To reduce the risk of disease, many countries have developed reclaimed water irrigation quality standards to strictly control the concentration of pathogens in TSE. Simultaneously, drip irrigation using reclaimed water could substantially help to minimize the risk of pathogens exposure to humans and animals (Lamm & Ayars 2007). Field experiments were conducted to compare subsurface drip irrigation with furrow irrigation on crop contamination when using TSE (Song et al. 2006). The study illustrated that furrow irrigation resulted in generally greater microbial contamination of lettuce, bell pepper and soil surface than subsurface drip irrigation. These results have been confirmed by Fonseca et al. (2011), who, when comparing spray, furrow, and subsurface drip irrigation of lettuce, found that almost all lettuce samples from subsurface drip irrigation were free of E. coli while spray and furrow irrigation resulted in substantially higher concentrations of E. coli contamination. A study was conducted to evaluate the effects of TSE drip irrigation on tomato production, and its results indicated that drip irrigation neither resulted in the transfer of fecal bacteria nor microbial pathogens to the irrigated soil and crop (Orlofsky et al. 2016). Nevertheless, drip irrigation (especially subsurface drip irrigation) weakens the exposure of pathogens to solar and ultraviolet radiation and thus may lead to a prolonged survival of pathogens in the soil. In fact, Oron (1996) reported that a high accumulation of pathogens in the soil was produced by subsurface drip irrigation using sewage effluent. Similar results were provided by Li & Wen (2016) and Qiu et al. (2017b). Their results showed that lateral depth affected the distribution of E. coli in soil: a substantial accumulation of E. coli was found in surface soil with surface drip irrigation while a high accumulation of E. coli at 30–50 cm depth was found with subsurface drip irrigation. Halalsheh et al. (2008) found pathogens at 0–60 cm soil depth 10 days after the last irrigation using mulched drip irrigation of sewage effluent in a greenhouse. These studies demonstrated that there was some risk of pathogens contamination even using surface or subsurface drip irrigation applying TSE. Thus, the transportation and distribution of pathogens in soil resulting from drip irrigation parameters as well as water and fertilizer management require further investigation. Additionally, it is still unclear whether pathogens could penetrate into deep soil with irrigation and rainfall under field conditions.

Although efficient treatment technologies can eliminate most heavy metals from sewage effluent, it has been reported that heavy metals can remain in TSE and accumulate in the soil (Wang et al. 2017). Since irrigation frequency, intensity, and irrigation level are substantially different from that of conventional irrigation, this has led to different spatial and temporal distribution of heavy metals in the soil when using drip irrigation applying TSE. The effects of drip irrigation using reclaimed water and fresh water on the migration and transformation of pollutants As and Cd in soil were evaluated by Pei & Liao (2018) and Pei et al. (2018). Their results indicate that the pollutants' concentrations in topsoil were higher one day after drip irrigation, while the pollutants' concentrations at depth of 100 cm were higher on the third day after drip irrigation. Qi et al. (2008) compared the effects of irrigation method (furrow irrigation and subsurface drip irrigation) and management (full irrigation and partial rootzone drying irrigation) on heavy metal residue in the soil when using TSE. Their results illustrate that both irrigation method and management method substantially affected the accumulation of heavy metals in the soil. For example, Cd content in the soil with full irrigation was higher than that with partial rootzone drying irrigation. It can be inferred that the potential accumulation of heavy metals in the soil greatly depends on the heavy metal concentrations in the irrigation water and the amount of irrigation water used. Therefore, attention should be given to heavy metal migration and transformation in the soil, plants, and underground water systems for the long-term application of TSE.

Soil microbial communities are an essential part of the soil ecosystem, while soil enzymes play key biochemical functions in the overall process of organic matter decomposition and nutrient cycling in a soil system (Shukla & Varma 2011; Ofori et al. 2021). As the indicators of soil biological activity and soil health, soil microbial biomass and enzyme activities are used to assess the effects of irrigation practices on soil properties. Since soil microbe and enzyme activity are susceptible to the soil moisture content and ion environment, the distribution of microbe and enzyme activity in the soil for drip irrigation may differ from either furrow or spray irrigation.

A soil column experiment was carried out to reveal the effects of different irrigation levels of TSE on bacteria community structure (Han et al. 2020). The results show that TSE drip irrigation decreased the bacteria diversity and operational taxonomic units (OTUs) in the 0–60 cm soil compared with tap water irrigation, while full irrigation of TSE reduced the bacteria diversity and species number in the deep soil compared to deficit irrigation. This may be due to the decomposition of microorganisms and organic matter in TSE accelerating the consumption of soil oxygen content and anoxic environment inhibiting microbial growth (Bhattarai et al. 2011). During a one-year drip irrigation study, it was observed that high concentration of TSE stimulated the propogation of fungus, ammonifying bacteria, and denitrobacteria, while the quantities of bacteria, actinomycetes, cellulose-decomposing bacteria, nitrobacteria, nitrosobacteria, and azotobacteria decreased with decreasing TSE concentration (Pei et al. 2015). Their results suggested that reclaimed water for crop irrigation should be mixed with conventional water in half and half quantities. Similar situations have recurred in soil enzyme activity. Soil enzyme activities were compared between municipal reclaimed water and rain fed irrigation (Brzezinska et al. 2006). The results reported a prominent increasing of enzyme activities in the soil irrigated with municipal reclaimed water. These results have been confirmed by Chen et al. (2008), who highlighted that the enhanced activities of enzymes involved in cycling of elements in the soil signified the biogeochemical processes governing elements cycling in the soil and the integrity of the ecosystem functions were not breached and the use of TSE could be environmentally sustainable. However, use of TSE with high salinity or heavy metal ion concentration could inhibit soil enzyme activity. Field experiments indicated that the accumulation of soluble salt in soil imposed a potential negative effect on enzyme activities, and decreased with the increasing of soil electrical conductivity (Frankenberger & Bingham 1982). Additionally, the enzyme activities in soil are susceptible to irrigation practices. The activities of alkaline phosphatase, urease, and invertase all decreased with increasing horizontal and vertical distance from the emitter in a saline sodic soil while using drip irrigation (Kang et al. 2013). The effects of lateral depth, irrigation level, and water quality on soil enzyme activities were evaluated by Qiu et al. (2017a). Their results indicated that a greater lateral depth resulted in noticeably higher enzyme activities in deeper soils, while enzyme activities at a shallow lateral depth were higher in topsoil. Compared to lateral depth, the effects of irrigation level on soil enzyme activities were unstable during the growing seasons of maize. It can be inferred that there is a complex effect of TSE drip irrigation on microbial biomass and enzyme activities in the soil due to various materials contained in TSE and the distribution pattern of water and nutrient under drip irrigation. Since the increase in microbial biomass and enzyme activity may therefore promote crop growth and ensure nutrient cycling, additional information concerning the drip irrigation practice impact on soil biological activities when using TSE is still required in China.

The use of TSE for drip irrigation provides the important macro- and micro-nutrients to the soil and can therefore improve soil quality, promote crop growth, and reduce fertilizer use. In contrast to fresh water, TSE is a valuable source of nitrogen, phosphorus, and microelements. Studies have reported that total N and P in the TSE used for irrigation were in the range of 5–60 and 0.2–1.0 mg/L, respectively (Levy 2011). After a six-year TSE irrigation, Ganjegunte et al. (2017) indicated that an increase was found in soil nitrate and potassium. For example, the average nitrate concentration in the soil increased from 295 to 1,590 mg/L. In addition, the minerals in TSE are more liable to be absorbed and utilized by crops. An 8-year field experiment compared the impact of water quality on nutrient availability (Minhas et al. 2015). Their results reported that about 40, 33, 75, and 20% of NP fertilizer with TSE was sufficient for similar production as with the recommended NP with groundwater for food grain, agroforestry, fodder, and vegetable production, respectively. Moreover, pot experiments using the ¹⁵N isotope tracer method were conducted to evaluate the N availability of treated effluent (Guo et al. 2017). The results illustrated that TSE drip irrigation could promote the assimilation of effluent N for maize, while higher N fertilizer application rates hampered the N uptake derived from effluent. Qiu (2017) reported that a greater irrigation level and lateral depth could increase the nitrogen content in the subsurface soil and the migration depth of nitrate as well as the risk of nitrate leaching when using TSE. The regulation of reclaimed water drip irrigation on dynamic variation of nitrogen in the soil was assessed by Wang et al. (2016). Their results indicated that the nitrate content in 0–50, 50–100, 100–150, and 150–200 cm respectively increased by 38.2, 44.7, 34.9, and 30.9% at the end of the cucumber growing period compared to that prior to planting. Excessive soil residual nitrogen could permeate into groundwater and impose a threat to environmental quality and human health. Therefore, suitable water and fertilizer management is essential for soil nutrient migration and transformation while using TSE drip irrigation.

Crop growth greatly depends on soil nutrients, and an excess of as well as deficiency in nutrients can both limit growth and productivity. Extensive research has shown that crop growth and yield increased significantly with reclaimed water irrigation due to the nutrients present in reclaimed water providing extra effective nutrients for plant (Levy et al. 2011; Li 2020). A 3-year drip irrigation experiment was conducted in Israel to explore the effect of TSE on the growth and production of cotton (Bielorai et al. 1984). Their results indicated that cotton plants irrigated with TSE grew taller, with more vegetative growth than did the plants irrigated with conventional water. In a greenhouse experiment, the lettuce production cultivated on TSE drip irrigation with partial conventional fertilization was greater than that cultivated on fresh water with conventional fertilization (Urbano et al. 2017). However, TSE drip irrigation did not always impose a positive effect on the growth and production of crops. Several studies have reported that the concentrations of chloride and salinity in TSE exceeding grass allowable values would inhibit the growth of grass (Lamm & Ayars 2007; Levy et al. 2011). In a drip irrigation study conducted by Li & Li (2010) in a greenhouse in Beijing, the influence of chlorine concentration and injection frequency on tomato production, quality and nitrogen uptake were evaluated. Their results showed that a slightly greater yield of tomato was observed for TSE drip irrigation than that irrigated with groundwater, while chlorination resulted in a decreased nitrogen uptake and yield. A pot experiment conducted by Miao et al. (2008) reported that reclaimed water irrigation restrained the maize absorption of nutrients from soil, resulting in a decrease of maize biomass. The abovementioned facts suggested that there are more complicated relationships between nutrient supply, conversion, uptake, and crop production under TSE drip irrigation. Furthermore, the nutrients in TSE can be excessive for crops, leading to overfertilization, reduction in crop size, and nutrient leaching (Wang et al. 2017). Further studies of crop response to nutrients contained in TSE will be essential to improve the application of TSE for irrigation.

Since irrigation equipment, technical parameters, water quality, and fertilizer management are different from conventional irrigation, the distribution and transformation of TSE as well as the materials contained in TSE in the soil-plant-groundwater system are different. The application of TSE drip irrigation in agriculture presents environmental, health, and economic challenges as well as a positive effect on crop growth and yield. At present, there are some issues of great concern that should be further studied for the sustainable use of TSE drip irrigation:

• (1)

  Perfecting of wastewater treatment equipment as well as the standards of TSE for irrigation. Since the effects of using TSE on drip irrigation systems, soil, microorganisms, and crops are highly dependent on the materials contained in TSE, improving TSE quality for irrigation will be conductive to reducing the risk of pollution.

• (2)

  Development of the mechanisms and anti-clogging equipment needed for drip irrigation. Decreasing the clogging of emitters and ensuring the uniformity of drip irrigation can help to promote TSE drip irrigation.

• (3)

  Configuration of drip irrigation parameters based on risk control. Effectively reducing the exposure to TSE of both human beings and crops is an important way to control irrigation risk. The technical parameters of drip irrigation should be reasonably determined to enhance the safety and efficiency of TSE irrigation.

• (4)

  Continuous monitoring and evaluation of TSE drip irrigation. The impact of TSE drip irrigation on the environment is a continuous and cumulative process, strengthening the need for long-term persistent monitoring and research to ensure the safety of crops, the quality of soils and groundwater, and finally the health of human beings.

This study was financially supported by the Natural Science Foundation of Hunan Province (grant no. 2020JJ5022) and General project of Education Department of Hunan Province (grant no. 19C0373).

The authors declare that there is no conflict of interests. We do not have any possible conflicts of interest.

All relevant data are included in the paper or its Supplementary Information.