Irrigation and reuse of rural domestic sewage after safe treatment is an effective way to alleviate the contradiction between supply and demand of water resources. In this paper, four kinds of irrigation water sources (primary and secondary treated water R1 and R2, purified water R3 and river water CK) and three kinds of water level regulation (low, medium, and high water level control of W1, W2 and W3) were set to study the impact on soil and crop safety, water and nitrogen utilization, biodiversity, and establish regulation mechanism of rural domestic sewage regenerative irrigation. The results showed that, the contents of Cd and Pb increased slightly, while the contents of Cr, Cu and Zn decreased in paddy field, heavy metals content showed that stem > leaf ≈ rice in rice plants, and did not increase significantly in rice grains. With the increase of field water level, the growth rate of PPCPs (Pharmaceutical and Personal Care Products) in 60–80 cm soil layer was accelerated, and the PPCPs content in rice husk was higher than that in grain, but they were at a very low level. Compared to CK, rural domestic sewage irrigation can effectively increase rice yield, RUE (rainwater use efficiency) and NUE (nitrogen use efficiency of reclaimed water) by 5.4% − 7.6%, 6.7% − 9.4% and 21.7% − 24.2%, respectively. Meanwhile, it can improve the species diversity, community diversity and richness of rice root layer, but the biodiversity decreased with the increase of field water level. Additionally, water level regulation of W3 with R2 water resource irrigation was conducive to the exertion of comprehensive benefits. The results of demonstration area showed that the amount of fresh water was reduced to 530 mm, the yield was increased by 9.6%, and the soil and crop was safety under the reclaimed sewage irrigation.
Research Article
Efficient and safe regulation mechanism of rural domestic sewage reclaimed irrigation of paddy field in southern China
https://doi.org/10.21203/rs.3.rs-1597908/v1
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Reclaimed water irrigation
Heavy metals
PPCPs
Water and nitrogen use efficiency
Microbial diversity
Weight TOPSIS
The contradiction of water shortage has become increasingly prominent in China. With the large consumption of water in industry and cities, the gap of agricultural irrigation water reaches 60 billion m3 every year, and it is increasing year by year. With the increasing discharge of rural domestic sewage, the problem of agricultural non-point source pollution is prominent. Compared with urban sewage, rural domestic sewage has the characteristics as follows: small difference in water quality, dispersed water volume, high content of N and P, good biodegradability and generally free of toxic substances such as heavy metals. It can be used as the fertilizer resource and easier to ensure the safety of irrigation reuse (Boecha et al., 2014; Dz et al., 2019). As a food crop with high water consumption, rice is an important carrier of regional ecological environment, it can maximize the consumption and utilization of domestic sewage (Zhang, et al., 2019; Xiao, et al., 2021). The core of irrigation regulation of rural domestic sewage is to improve the reclaimed water and nitrogen utilization efficiency, and promote rice yield by changing soil environmental quality and habitat characteristics (Arjun, et al., 2014).
Scholars in China and abroad have done a lot of researches on the effects of reclaimed water irrigation process on soil’s physical and chemical properties and crop yield. Liu et al. (2010) and khamisi et al. (2013) found that reclaimed water irrigation of winter wheat and maize had obvious yield increase effect. Alkhamisi et al. (2017) found that the cotton yield under reclaimed water irrigation was equivalent to that of clean water irrigation with equal nitrogen input, which was conducive to nitrogen transformation and crop absorption and utilization. A large number of studies have shown that reclaimed water irrigation can effectively improve nitrogen availability and increase the activities of enzymes-related and microorganisms-related transformation of N and P in soil. Chen et al. (2015) found that reclaimed water irrigation increased soil nutrient content and improved soil’s microbial activity and diversity. Cheng et al. (2012) showed that about 34% of the nitrogen entered the system with reclaimed water irrigation could be absorbed and utilized by crops, and 62% could be removed or adjusted the amount of nitrogen in soil nitrogen pool by denitrification.
Heavy metal pollution caused by reclaimed water irrigation is a hot issue in the research and application of reclaimed water reuse, and heavy metal pollution is a complex behavior process, in which all kinds of physical, chemical and biological reactions in soil, as well as the participation of crops are involved. The common heavy metals in rural domestic sewage include Cu, Cr, Pb, Cd and Zn, and their concentration is usually below 5 ug/L, among which Cr and Cd have the highest ecological risk (Chen et al., 2013; Ugulu et al., 2021). With the acceleration of urbanization and the improvement of living standards in urban and rural areas, the pollution of emerging pollutants of PPCPs increases rapidly. Agricultural and animal husbandry, household and hospital wastewater, being the main sources of PPCPs, directly or indirectly pollute the soil through the discharge of sewage disposal, and finally enter the surface or underground water body. It has been studied and found that rural sewage includes a trace of pollutants. Among them, the PPCPs contents, such as acetaminophen, ibuprofen and diclofenac, are higher, up to more than 50 ug/L, the concentration range of caffeine is 0.5–200 ug/L, and the concentration of pesticides and surfactants is 20–100 ug/L (Chen et al., 2013; Roberts et al., 2016). Different irrigation methods and water quality characteristics have different effects on the migration and accumulation of heavy metals and PPCPs in soil crop system. Therefore, analyzing the migration and transformation of heavy metals and PPCPs and evaluating the comprehensive ecological environment impact under farmland irrigation regulation are key to ensure the safe reuse of sewage.
At present, the resource utilization of rural domestic sewage is still at a relatively blank stage. The research on the regulation mechanism of reclaimed water irrigation based on the safety of soil and crop system, efficient utilization of water and nitrogen, microbial diversity of paddy field and high yield of crops is still insufficient.The objectives of this study were included, (1) to analyzed heavy metals, PPCPs migration and accumulation, reclaimed water use efficiency and nitrogen utilization rate of reclaimed water under different grades of treated water and water level regulations, (2) to establish an efficient and safe reclaimed water irrigation regulation mechanism, (3) and to evaluate the comprehensive benefits in the demonstration area.
The results have important theoretical and practical significance for formulating a scientific and reasonable safe and efficient operation and management mode of reclaimed water irrigation and realizing the healthy and sustainable development of reclaimed water irrigation.
2.1. Experimental field
This study was carried out in Yongkang rural domestic sewage recycling base (28°48'N, 120°10'E) from May to October in 2019 and 2020. The area of study belongs to the landform of low mountains and hills, where is of the subtropical monsoon climate, with average annual precipitation of 1787 mm, maximum annual precipitation of 2385.8 mm and minimum annual precipitation of 1119.9 mm. in the local area, the annual average evaporation is 930.2 mm, the annual average temperature is 17.5 ℃, the annual maximum temperature is 39.9 ℃, the annual minimum air is - 14.5 ℃, the annual average sunshine is 1,909 h, and the frost free period is 245 d. The area of study includes a field experimental area and a 10,000 person scale demonstration area. It is composed of 36 standard experimental plots (size 20 m × 5 m per plot), the demonstration area is about 26.67 ha, mainly planted with rice and cash crops. For the location of area of study, see Fig. 1, for the environmental quality of different soil layers, see Table 1.
In the area of study, a domestic sewage disposal with design scale of 400 m3/d has been built, it was studied as the rural domestic sewage source in this study. The treatment process adopted the secondary biological treatment process (primary treatment was conventional process and secondary treatment adopted A2O process improved), and the effluent quality met the class I A standard in the discharge standard of pollutants for urban sewage treatment plants (GB18918-2002). An ecological pond with a sewage storage capacity of 3000 m3 was built to store and purify rural domestic sewage. The demonstration area transmitted reclaimed water to the paddy field by improving the sewage treatment terminal.
2.2. Experimental design
Four kinds of irrigation water sources were set in the field experimental area, namely primary treated water R1 and secondary treated water R2 from the sewage disposal, R3 of secondary treated water purified by ecological pond, and river water CK. The statistics of irrigation water quality indexes during the experimental period were shown in Table 2, and all indexes met the irrigation water quality standard (Chinese standard, 2021). Farmland water level refers to the water depth maintained by farmland after rainfall or irrigation and the buried depth of farmland groundwater level when there is no water layer. In this study, the farmland water level was used as the control index of irrigation and drainage in paddy field, and three kinds of water level regulation were set, with three repetitions for each treatment. The water level regulation in paddy field was strictly controlled in each growth period. When the water level dropped to the lower limit, the reclaimed water will be replenished. In case of rainstorm exceeding the upper limit of rain storage, the rainwater will be drained. The field water level control standards were shown in Table 3. There were 2 times of fertilization for all treatments during the growth, i.e., the basal fertilizer of 200 kg/ha compound fertilizer (N:P:K=18:8:15) and 100 kg/ha urea on Jun 25th, and the dressing fertilizer of 250 kg/ha compound fertilizer on July 12th. For the experimental layout, see Fig. 2. The irrigation water source and water level regulation in the demonstration area were controlled by the optimal control scheme in the field experimental plot, and the fertilization method was consistent with that in the field experimental plot.
2.3. Indicators and measurements
In the field experimental plot, water consumption (WCT) was measured by the change of water level in field surface according to the measuring needle when there is a water layer, while it was measured by the change of soil water content according to the soil moisture analyzer at 8:00 am every day when there is no water layer. Water leakage was measured by the leakage meter every day, and then the leakage amount was calculated according to the reading difference by the measuring needle before and after that. The evapotranspiration (ET) was the difference between WCT and leakage. In the demonstration area, irrigation water amount (IWA) was read by the electromagnetic flowmeter installed in each lift pump station, and the leakage rate of irrigation pipeline was taken as 5%.
In the paddy field, the content of Pb, Cr, Cu and Zn were determined by the flame atomic absorption spectrophotometry, Cd content was determined by graphite furnace atomic absorption spectrophotometry, PPCPs (Pharmaceutical and Personal Care Products) were determined by combining chromatography-mass and mass spectrometry method, and microbial diversity indexes were sequenced by MISEQ sequencing platform. In plant organs, the content of Cu and Zn were determined by flame atomic absorption spectrophotometry, and the content of Pb, Cr and Cd were determined by inductively coupled plasma mass spectrophotometry. The actual yield of different treatments was measured after rice harvest.
2.4. Statistical analysis
The potential ecological risk index (RI) was used to analyze the potential ecological risk of heavy metals in farmland soil irrigated by different water sources in the area of study. The calculation formula is as followed.
Where: RI is the comprehensive potential ecological risk index of various heavy metals in soil; Ei is the single factor ecological risk coefficient of each evaluation index in soil samples. Pi is the single factor pollution index in the soil evaluation index, which is the ratio of the measured value of the single pollution factor to the corresponding background value. Ti is the toxicity coefficient of heavy metals in soil.
WUE (water use efficiency) was defined as WUE = Y/W, where Y is the rice yield and W is the total IWA or WCT. RUE (rainwater use efficiency) was the ratio of effective rainfall to total rainfall. NUE (Nitrogen use efficiency of reclaimed water) was the ratio of nitrogen brought in by reclaimed water absorbed by crops and soil residues to the total nitrogen brought in by reclaimed water. Additionally, ANOVA analysis was carried out by SPSS Statistics 19. Multi objective decision model adopts Entropy weight TOPSIS carried out by SPSSAU.
3.1. Safety in soil and plant of sewage regeneration irrigation
3.1.1 Heavy metal content in paddy field
The content of heavy metals in each soil layer of paddy field was shown in Fig. 3. It was shown that, after irrigation in 2020 and 2021, the contents of Cd and Pb increased, but the contents of Cr, Cu and Zn decreased, which may be related to that the reclaimed water mainly came from domestic sewage, and the contents of Cr, Cu and Zn in this area were low. The Cd content in the surface layer of the soil was significantly higher than that in the deep layer, which was just due to the small mobility of Cd in the soil that it was easy to accumulate in the upper layer of the soil. With the increase of water level control and the extended of irrigation time, the Cd content in each soil layer showed an obvious increasing trend. The increase of Cd content in each soil layer was largest under R1 water source, moderate under R2 water source, and smallest under R3 water source, where the Cd content under R3 was close to that under CK. Such results indicated that the reclaimed water had a certain interception effect on Cd after purification by the ecological pond. In the year of 2020, Cr content in paddy field was decreased, except for the treatments of W1R2 and W3R2, but the Cr content was generally increased compared with the background value in 2021. With the extended irrigation time, the Cr content changed significantly. The results of ANOV were shown in Table 4. It showed that water level regulation and irrigation water source had a significant impact on Cd content in both 0-20 cm soil layer (P<0.01) and 20-40 cm soil layer (P<0.05), and irrigation water source had a significant impact on Cr content in 20-40 cm soil layer and Cu content in 40-60 cm soil layer (P<0.01). In addition, according to the calculation of the potential ecological risk index RI of heavy metals in the root layer (0-20cm), the RI of R1, R2, R3 and CK was 108.23, 128.33, 67.77 and 84.43 respectively. According to the heavy metal ecological risk rating standard, the ecological risk of each water source irrigation was mild, especially the ecological risk of heavy metals of R3 was lowest.
3.1.2 Heavy metal content in rice plant organs
Contents of heavy metals in rice plant organs were shown in Fig. 4, i.e.: stem>leaf≈grain. For rice stem, the content of total heavy metals in different irrigation water sources showed R1>R2>R3≈R4. Compared to R4, the content of total heavy metals under R1, R2 and R3 was increased by 1.52 times, 1.25 times and 1.15 times, respectively, and the total amount of heavy metals under R3 was close to that under R4. The content change of total heavy metals regulated by different water levels showed W1≈W2>W3. Compared with W3, the content of total heavy metals under W1 and W2 was increased by 1.07 times and 1.06 times, respectively. For rice leaf, the content of total heavy metals in different irrigation water sources showed R1>R4>R2≈R3. Compared with R3, the content of total heavy metals under R1, R2, and R4 was increased by 1.07 times, 1.04 times and 1.06 times, respectively. The content change of total heavy metals regulated by different water level regulations showed W3>W1>W2. Compared with W2, the content of total heavy metals under W1 and W3 was increased by 1.05 times and 1.17 times, respectively. For rice grain, the content change of total heavy metals in different irrigation water sources was consistent with that in stem. Compared to R4, the content of total heavy metals under R1, R2 and R3 was increased by 1.16 times, 1.09 times and 1.12 times, respectively, and the content change of total heavy metals regulated by different water levels was basically same. The results of ANOV were shown in Table 5. It showed that different irrigation water sources had significant effects on Zn content in both stem and leaf, and Pb content in grain, and had extremely significant effects on Pb content in stem, and Cu content in both leaf and grain. Different water level regulations had a significant effect on Cd content in stems and leaves (P<0.01), and Cr content in leaf (P<0.05).
3.1.3 PPCPs content in paddy field
In this paper, 23 kinds of PPCPs were detected, and 16 kinds of PPCPs were detected in paddy soil. The changes of PPCPs content in paddy field were shown in Fig. 5. It was shown that there were 6 kinds of PPCPs with high content in paddy field, namely ATE, MET, OLF, MAL, OXY, and MIN, with the change range of 0.10~0.40 ug/kg, 0.02~0.55 ug/kg, 0.003~0.68 ug/kg, 0.13~0.59 ug/kg, 0.001~0.23 ug/kg, and 0.001~0.23 ug/kg, respectively. With the increase of soil depth, the PPCPs content decreased, and the interannual change of PPCPs content in each soil layer showed an increasing trend. The growth rate of PPCPs in paddy field with different irrigation water sources was varied greatly, and showed R1>R2>R3≈R4. For water source of R1, compared to background value, the PPCPs content was increased by 1.58 times and 1.73 times in 0-20 cm soil layer, 1.27 times and 1.58 times in 20-40 cm soil layer, 1.50 times and 1.71 times in 40-60 cm soil layer, and 1.44 times and 1.70 times in 60-80 cm soil layer, respectively in 2020 and 2021. For water source of R2, compared to background value, the PPCPs content was increased by 1.48 times and 1.76 times in 0-20 cm soil layer, 1.30 times and 1.48 times in 20-40 cm soil layer, 1.20 times and 1.58 times in 40-60 cm soil layer, and 1.15 times and 1.21 times in 60-80 cm soil layer, respectively in 2020 and 2021. For water source of R3 and R4, in 2020 and 2021, the PPCPs content was increased by 1.24 and 1.19 times, 1.41 and 1.26 times in 0-20 cm soil layer, 1.25 and 1.12 times, 1.38 and 1.13 times in 20-40 cm soil layer, 1.14 and 1.13 times, 1.18 and 1.24 times in 40-60 cm soil layer, 1.11 and 1.09 times, 1.32 and 1.21 times in 60-80 cm soil layer, respectively. The growth rate of PPCPs regulated by different water levels was similar, namely W2>W3>W1 in 0-20 cm soil layer, W1≈W2≈W3 in 20-60 cm soil layer, and W3>W1>W2 in 60-80 cm soil layer, indicating that the growth rate of PPCPs in 60-80 cm soil layer was accelerated with the increase of water level control in paddy field.
3.1.4 PPCPs content in rice plant
The analysis of 3.1.3 showed that different water level regulations had little impact on the PPCPs content in paddy field. Therefore, in this paper, the changes of PPCPs content in rice plant were analyzed under different irrigation water sources (Fig. 6). For rice grain, the high content PPCPs mainly included ATE, OFL, MAL, OXY, MET and MIN, the change of PPCPs content in rice grain showed that R2 (6.33 ug/kg)>R1 (6.82 ug/kg)> R3 (5.00 ug/kg)≈R4 (4.79 ug/kg). Compared to R4, the PPCPs content of R1, R2 and R3 was increased by 32.3%, 42.4% and 4.4%, respectively. For rice husk, the high content PPCPs mainly included ATE, OFL, ACE (not detected in grain), MAL, OXY, MET, MIN and TET (not detected in grain), the change of PPCPs content in rice husk showed that R1 (11.48 ug/kg)>R2 (11.30 ug/kg)>R4 (9.92 ug/kg)≈R3 (9.31 ug/kg). Compared to R3, the PPCPs content of R1, R2 and R4 was increased by 23.3%, 21.4% and 6.4%, respectively. The average PPCP content in rice grain was 5.73 ug/kg, and that of rice husk was 10.50 ug/kg, namely the latter was 1.83 times of the former. In general, the PPCPs content in both rice grain and husk was at a very low level, and had a cumulative effect under irrigation of R1 and R2 water source.
3.2 High efficiency of sewage regeneration irrigation
3.2.1 Utilization of water and nitrogen
Utilization of water and nitrogen in paddy field under different irrigation water sources and water level regulations was shown in Table 6. It showed that the yield can be effectively improved by reclaimed water irrigation regulation. Compared to CK, the yield under R1, R2 and R3 has increased by 6.3%, 7.6% and 5.4% respectively. IWA showed R3≈R4>R1≈R2. The reason may be the one that the content of COD, nitrogen and organic matter in reclaimed water (R1 and R2) was relatively high, which affected the ventilation and leakage of paddy soil, thus resulting in large water consumption and slightly high irrigation water amount irrigated by R3 and R4. WCT, WUEI and WUEET under irrigation of R1 and R2 were slightly higher than those under irrigation of R3 and R4. Meanwhile, RUE and NUE can be significantly improved by reclaimed water irrigation. Compared to CK, RUE and NUE of R1, R2 and R3 was increased by 6.7% and 24.2%, 9.3% and 22.6%, 9.4% and 21.7%, respectively. Under different water level regulations, yield, IWA, WCT, RUE and NUE were increased, but WUEI and WUEET was decreased with as the field water level control increased. Compared to W1, yield, IWA, WCT, RUE and NUE was increased by - 0.1%, 18.0%, 6.8% and 3.4%, 2.1%, 30.6% under W2, 7.3% and 14.0% under W3, respectively, WUEI and WUEET was decreased by 15.4% and 3.3% under W2, 21.9% and 5.2% under W3, respectively. In addition, water level regulation had a significant impact on all indicators of utilization of water and nitrogen except WUEET, irrigation water source had a significant impact on yield, IRA, RUE and NUE, and interactions between water level regulation and irrigation water source had a significant impact on yield, RUE and NUE.
3.2.2 Microbial diversity in root layer of paddy field
In this paper, the effective tags of all samples were clustered, and the sequence was clustered into OTUs (operational taxonomic units) with 97% identity. At the same time, the diversity of microbial community was analyzed by alpha diversity (Table 7). OTUs reflects species diversity, Shannon index reflects community diversity, and Chao1 index reflects community richness. Under different irrigation water sources, it showed that the peak values of OTUs, Shannon and Chao1 were appeared in R3 water source irrigation. Except for R1, the indexes of OTUs, Shannon and Chao1 of R2 and R3 were higher than those of CK, indicating that the species diversity, community diversity and richness of root layer soil under reclaimed water irrigation were higher than those of CK. Under different water level regulations, all indexes showed W1>W3≈W2, indicating that species diversity, community diversity and richness were decreased with the increase of field water level control. In addition, the effects of irrigation water source, water level regulation and their interactions on microbial diversity in paddy field were extremely significant.
3.3 Safe and efficient regulation mechanism of sewage regeneration irrigation
The entropy weight TOPSIS multi-objective decision-making model was used to optimize the regulation schemes of different irrigation water sources and water level regulations. Nine indexes were selected as the evaluation factors to establish the initial matrix [Y] (Fig. 7), related to soil and crop safety (potential ecological risk index RI and PPCPs content in soil, content of heavy metals and PPCPs in grain), efficient utilization of water and nitrogen (WUEI, RUE, and NUE), microbial diversity (Shannon index), and economic benefit (yield). According to the principle that the larger the RUE, NUE, Shannon and yield are, the smaller the WUEI, RI and PPCPs content in paddy field and heavy metals and PPCPs content in rice grain are, the better the regulation schemes are, thereby the standardized decision matrix [R] can be constructed (Fig. 7). The entry weight W (w1, w2, w3, w4, w5, w6, w7, w8, w9) was calculated to be equal to 0.157, 0.099, 0.087, 0.175, 0.107, 0.086, 0.086, 0.084, and 0.119. The ideal solution x+ and negative ideal solution x- was determined, as x+ = (0.0000, 0.0000, 0.0000, 0.0864, 0.0862, 0.0836, 0.1190), x- = (0.1574, 0.0989, 0.0870, 0.1750, 0.1065, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000). The euclidean distance of ideal solution and negative ideal solution (di+ and di-), and the relative closeness of ideal solutions (Si) under different regulation schemes were respectively calculated, and the results therefrom were ranked from large to small (Table 8). As what it was shown, the order of comprehensive benefits showed R2>R1>R3>R4 under different water sources, W3>W1> W2 under different water level regulations, indicating that the high water level regulation under R2 water source was conducive to the exertion of the comprehensive benefits of rural domestic sewage regeneration irrigation regulation. Therefore, in the process of rural domestic sewage irrigation and reuse regulation, R2 was given as the priority water source, following R3 and CK as the supplementary water sources.
3.4 Effectiveness evaluation in demonstration area
The rice planting area in the demonstration area is about 10 ha. According to the conclusion of 3.3, the field irrigation was regulated by W3, R2 was given as priority water source, R3 and CK were used as supplementary water sources during the rice growth stages. Effectiveness indicators of crop and soil system in rice planting demonstration area were shown in Table 9. It showed that the irrigation water amount was 635 mm, including 215mm of R2 water source, 300 mm of R3 water source, and 120 mm of river water CK. Compared to the background value, the amount of fresh water can be reduced to 530mm, for paddy field, the Cd content was remained unchanged, the content of Pb, Cr and PPCPs was increased slightly, with an increase of 0.3%, 0.9% and 2.5%, respectively, for rice grain, the Pb content was decreased, the Cd content was remained unchanged, and the content of Cr and PPCPs was increased slightly, with an increase of 1.3% and 1.5% respectively, and rice yield was increased by 9.6%. It indicated that, on the basis of ensuring the safety of soil and crops, the amount of fresh water can be significantly reduced, and the increase benefit of yield was significant.
4.1 Effects mechanism on changes of heavy metals and PPCPs
The degree of heavy metals pollution is related to the source of reclaimed water, soil type, irrigation regulation mode and so on. The accumulation of heavy metals in soil will directly affect crop growth and quality. Reasonable irrigation methods can reduce the accumulation of heavy metals in soil. Li et al. (2015) proposed that drip irrigation or alternate root irrigation technology can significantly reduce the content of heavy metals in soil compared with conventional irrigation. In this paper, field water level regulations were carried out for rice, and it was found that water level regulation had a significant impact on Cd content in both 0-20 cm and 20-40 cm soil layers in paddy field, it further confirmed the effective effect of irrigation regulation on the accumulation and distribution of heavy metals. Li et al. (2021) found that there were great differences in the accumulation of heavy metals in various parts of crops, and the lowest one in grains, whereas the rhizome leaf system played a barrier role in the migration of heavy metals to fruits, which was basically consistent with the research conclusions of this paper. Although PPCPs content is at a continuously low level in the reclaimed water system, it will still lead to significant ecotoxicity. Gottschall (2012) monitored more than 80 kinds of PPCPs in groundwater and soil after one year of sludge application in the drainage network and activated sludge in Ottawa, Canada. Seven kinds of PPCPs in the drainage network, four kinds of PPCPs in groundwater and five kinds of PPCPs in soil were detected, respectively. The results of this study showed that the PPCPs content in paddy field and rice plant irrigated by R1 water source was enriched, followed by R2, and the irrigation effect of R3 water source was basically consistent with CK. Therefore, in order to reduce the pollution risk of PPCPs, direct irrigation by R1 water source should be avoided, and R3 water source was relatively safe as a supplementary water source.
4.2 Effects mechanism on water-nitrogen use and paddy field microbial diversity
Reclaimed water irrigation changes the process of water and fertilizer absorption and utilization of crops. On the one hand, it changes the water-soluble salts in soil, and physically hinders water and fertilizer absorption. On the other hand, it affects soil environmental quality, causes soil permeability to change (Christou et al., 2014), affects indirectly the process of crop metabolism and water and fertilizer absorption (Bastida et al., 2019), moreover changes the activity and biodiversity of soil microorganisms, affects the nutrient transformation process (Chopyk et al., 2020). Reclaimed water is rich in nitrogen, phosphorus and organic matter. Proper regulation of water and nitrogen can improve soil buffering performance and soil fertility and promote crop growth (Li et al., 2012; Li et al., 2018). The research in this paper showed that the rural domestic sewage regenerative irrigation can increase production by 6.7%-9.4%, and improve biodiversity, which effectively confirmed the above conclusions. The NH4+-N concentration of rural domestic sewage was from 3.52 mg/L to 11.9 mg/L, the nitrogen accumulation brought in by reclaimed water can reach 10.43-27.90 kg/ha, and NUE can be increased by 21.7%-24.2%, which provided a basis for fertilizer nitrogen application strategy of reclaimed water irrigation. These research conclusions fully showed that reasonable irrigation regulation of reclaimed water can improve nitrogen availability and utilization, and promote crop yield (Maestre-Valero et al., 2018). In this paper, three kinds of field water level regulations were set up, in addition to the drainage affected by typhoon, there was basically zero direct drainage, which effectively improved the utilization efficiency of reclaimed water, and reduced the amount of fresh water up to 2000-6000 m3/ha.
4.3 Safe and efficient regeneration irrigation and drainage regulation of rural domestic sewage
Rural domestic sewage after treatment can meet the requirements of irrigation water and be used as a relatively ideal irrigation water source. At present, the research on the irrigation regulation of reclaimed water has a single regulation goal, and the quantitative degree of describing the impact of technical parameters is not enough, which limits the exertion of the comprehensive benefits of reclaimed water irrigation. This paper considered the interaction of soil and crop safety, utilization efficiency of water and nitrogen, yield and other factors, and an efficient and safe regulation mechanism of reclaimed water irrigation in paddy field was established. The results showed that short-term rural domestic sewage irrigation will not cause emerging pollutants and heavy metals pollution to soil and crops, not affect crop yield and benefits, and it can meet the requirements of efficient and safe reuse of reclaimed water irrigation and drainage. However, the impact of long-term reclaimed water irrigation is not clear, so it is still necessary to track and monitor the effect on the system of soil, crop, and underground water under the condition of long-term rural domestic sewage recycling and reuse.
Field experiments were conducted to study the effects on soil and crop safety, water and nitrogen utilization efficiency, and microbial diversity, established efficient and safe regulation mechanism of rural domestic sewage regenerative irrigation, and evaluated the demonstration effects. The main conclusions were described as follows.
1) The content of Cd and Pb was increased slightly, while the content of Cr, Cu and Zn was decreased in paddy field under rural domestic sewage regeneration irrigation. The ecological risk of four kinds of water source irrigation was mild, and the RI under R3 water source irrigation was lowest. The heavy metals content in rice plants showed stem>rice≈leaf. Water level regulation had little effect on the accumulation of heavy metals in all parts of rice plants. The heavy metals content in rice grain irrigated by reclaimed water did not increase significantly, which met the requirements of pollutant limit in rice.
2) The growth rate of PPCPs in paddy field with different irrigation water sources was quite different, and showed R1>R2>R3≈R4, but was similar regulated by different water level regulations. With the increase of field water level, the growth rate of PPCPs in 60-80 cm soil layer was accelerated. The PPCPs content in rice husk was much higher than that of rice grain, but they were at a very low level. The PPCPs content of rice grain and husk was similar irrigated by R3 and CK water sources.
3) Rice yield, RUE and NUE was effectively improved by rural domestic sewage irrigation, compared to CK, the yield, RUE and NUE was increased by 5.4%-7.6%, 6.7%-9.4%, and 21.7%-24.2%, respectively. Meanwhile, with the increase of field water level control, yield, IWA, WCT, RUE and NUE was shown a significant upward trend, WUEI and WUEET was shown a downward trend. The species diversity, community diversity and richness of root layer under reclaimed water irrigation in paddy field was higher than that under CK, but showed a downward trend with the increase of field water level control.
4) The regulation and evaluation system of rural domestic sewage regenerative irrigation mainly considered four aspects including soil and crop safety, efficient utilization of water and nitrogen, microbial diversity and economic benefits. The entropy weight TOPSIS multi-objective decision-making model was established. The regulation of high water level control with R2 water source irrigation was conducive to the exertion of comprehensive benefits. Therefore, in the process of rural domestic sewage irrigation, R2 can serve as the priority water source, R3 and CK as the supplementary water sources.
5) Based on the field mechanism experiment, the monitoring of the demonstration area was carried out. Compared to the background value, rural domestic sewage regenerative irrigation can reduce the amount of fresh water to 530 mm, increase the yield of rice by 9.6%, slightly increase the contents of Pb, Cr and PPCPs in paddy field by 0.3%, 0.9% and 2.5%, respectively, and slightly increase the contents of Cr and PPCPs in rice grain by 1.3% and 1.5%, respectively. The soil and crop safety had been basically guaranteed in the demonstration area.
Acknowledgements
This study was financially supported by National Key Research and Development Program (2019YFC0408803), Basic Public Welfare Research Project of Zhejiang Province (LGN20E090001), and Water conservancy science and technology in Zhejiang Province (RC1918, RC2029).
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Table 1
PH, electrical conductivity (EC), soluble salt, total nitrogen (TN), total phosphorus (TP), Organic matter, ammonium-nitrogen (NH4-N) and nitrate-nitrogen (NO3--N) in area of study.
|
Soil depth |
pH |
EC (mS/m) |
Soluble salt (g/kg) |
TN (%) |
TP (%) |
Organic matter (g/kg) |
NH4+-N (mg/kg) |
NO3--N (mg/kg) |
|
0-20cm |
5.56 |
2.6 |
0.44 |
0.12 |
0.069 |
17.7 |
8.24 |
2.84 |
|
20-40cm |
5.88 |
2.9 |
0.27 |
0.09 |
0.032 |
14.8 |
5.75 |
2.69 |
|
40-60cm |
6.15 |
2.8 |
0.26 |
0.07 |
0.027 |
12.9 |
4.71 |
2.5 |
|
60-80cm |
6.61 |
2.8 |
0.26 |
0.04 |
0.03 |
8 |
3.17 |
2.16 |
Table 2
Description and statistics of chemical oxygen demand (COD), linear alkylbenzene sulfonates (LAS), ammonium-nitrogen (NH4-N) and nitrate-nitrogen (NO3--N) in different irrigation water sources (mg/L).
|
Water sources |
Indicator |
Maximum value |
Minimum value |
Standard deviation |
Mean value |
Kurtosis |
Skewness |
|
R1 |
COD |
84 |
15 |
26.794 |
29.5 |
5.855 |
2.410 |
|
LAS |
0.88 |
0.06 |
0.315 |
0.25 |
5.199 |
2.247 |
|
|
NH4+-N |
11.9 |
8.25 |
1.645 |
9.647 |
-1.782 |
0.916 |
|
|
NO3--N |
0.061 |
0.016 |
0.019 |
0.034 |
-1.452 |
0.642 |
|
|
R2 |
COD |
59 |
10 |
16.783 |
24.1 |
0.719 |
1.291 |
|
LAS |
0.16 |
0 |
0.058 |
0.048 |
-0.425 |
0.827 |
|
|
NH4+-N |
11.9 |
3.52 |
2.837 |
7.712 |
-0.946 |
-0.174 |
|
|
NO3--N |
6.25 |
0.01 |
2.455 |
1.364 |
1.238 |
1.687 |
|
|
R3 |
COD |
62 |
11 |
12.735 |
24.15 |
0.710 |
1.553 |
|
LAS |
0.32 |
0 |
0.132 |
0.042 |
-1.215 |
0.826 |
|
|
NH4+-N |
5.45 |
2.34 |
0.634 |
4.415 |
0.478 |
0.473 |
|
|
NO3--N |
3.16 |
0.345 |
0.928 |
0.823 |
1.382 |
1.275 |
|
|
CK |
COD |
56 |
7 |
15.712 |
23.45 |
0.710 |
1.251 |
|
LAS |
0.1 |
0 |
0.041 |
0.035 |
-1.875 |
0.418 |
|
|
NH4+-N |
1.49 |
0.116 |
0.394 |
0.711 |
0.143 |
0.393 |
|
|
NO3--N |
2.56 |
0.624 |
0.578 |
1.048 |
4.680 |
2.078 |
Table 3
Standard of water level control in paddy field (mm).The values of the upper and lower limit are water depth maintained by farmland, when it is lower than the lower limit of sewage, irrigated to the upper limit of sewage, when it is higer than upper limit of storage after rainfall, drained to upper limit of storage.
|
Water control |
Upper and lower limit |
TG |
ET |
LT |
JB |
HF |
MK |
|
W1 |
Upper limit of sewage |
0 |
3-5 days exposing field |
1-2 days exposing field |
1-2 days exposing field |
1-2 days exposing field |
3-5 days exposing field |
|
Lower limit of sewage |
30 |
30 |
exposing field |
40 |
40 |
30 |
|
|
Upper limit of storage |
50 |
70 |
80 |
80 |
60 |
||
|
W2 |
Upper limit of sewage |
0 |
10 |
10 |
10 |
10 |
10 |
|
Lower limit of sewage |
30 |
50 |
exposing field |
50 |
50 |
50 |
|
|
Upper limit of storage |
50 |
70 |
100 |
100 |
100 |
||
|
W3 |
Upper limit of sewage |
0 |
40 |
40 |
40 |
40 |
10 |
|
Lower limit of sewage |
30 |
60 |
exposing field |
60 |
60 |
60 |
|
|
Upper limit of storage |
50 |
100 |
150 |
150 |
100 |
||
TG, ET, LT, JB, HF and MK represent turning green, early tillering, later tillering, jointing and booting, heading and flowering, milky of rice growth stages, respectively.
1-2 days and 3-5 days exposing fieldindicate 1mm and 2mm cracks in the paddy field after exposing field.
Table 4
Two factor analysis of variance (ANOVA) of heavy metals in paddy field.
|
Soil layer |
Cd |
Pb |
Cr |
Cu |
Zn |
|||||
|
W |
R |
W |
R |
W |
R |
W |
R |
W |
R |
|
|
0-20 cm |
** |
** |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
|
20-40 cm |
* |
* |
NS |
NS |
NS |
** |
NS |
NS |
NS |
NS |
|
40-60 cm |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
** |
NS |
NS |
|
60-80 cm |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
NS |
W and R represent water level control and water resources of irrgation, respectively.
NS = not significant at the 0.05 level.
* = significant at the 0.05 level.
** = significant at the 0.01 level.
Table 5
Two factor analysis of variance (ANOVA) of heavy metals in plant organs.
|
Organ |
Zn |
Pb |
Cd |
Cr |
Cu |
|||||
|
W |
R |
W |
R |
W |
R |
W |
R |
W |
R |
|
|
Stem |
NS |
* |
NS |
** |
** |
NS |
NS |
NS |
NS |
NS |
|
Leaf |
NS |
* |
NS |
NS |
** |
NS |
* |
NS |
NS |
** |
|
Grain |
NS |
NS |
NS |
* |
NS |
NS |
NS |
NS |
NS |
** |
W and R represent water level control and water resources of irrgation, respectively.
NS = not significant at the 0.05 level.
* = significant at the 0.05 level.
** = significant at the 0.01 level.
Table 6
Utilization of water and nitrogen in paddy field under different irrigation water sources and water level regulations.
|
Treatment |
Yield |
IWA |
WCT |
WUEI |
WUEET |
RUE |
NUE |
|
(kg/hm2) |
(mm) |
(mm) |
(kg/m3) |
(kg/m3) |
(%) |
(%) |
|
|
W1R1 |
10021bcd |
367.2ef |
804.8e |
2.73a |
1.25ab |
72.9ab |
83.2de |
|
W1R2 |
9813cd |
356.1f |
815.5e |
2.76a |
1.20abc |
72.6ab |
85.4cd |
|
W1R3 |
10174b |
405.5cd |
823.5de |
2.51ab |
1.24ab |
59.1d |
81.2ef |
|
W1CK |
9152f |
387.1de |
815.5e |
2.36ab |
1.12abc |
53.8e |
72.3g |
|
W2R1 |
10054bc |
425.0c |
841.7bcde |
2.37ab |
1.19abc |
58.6 d |
91.2b |
|
W2R2 |
9564e |
429.9c |
824.5de |
2.22ab |
1.16abc |
71.2ab |
86.3cd |
|
W2R3 |
10224b |
462.4b |
860.5abcd |
2.21ab |
1.19abc |
75.2a |
88.2bc |
|
W2CK |
9271f |
471.9b |
836.3cde |
1.96b |
1.11bc |
71.0b |
67.3h |
|
W3R1 |
9920cd |
492.5ab |
871.8abc |
2.01b |
1.14abc |
72.1b |
96.5a |
|
W3R2 |
10968a |
478.0b |
868.2abc |
2.29ab |
1.26a |
64.8c |
95.8a |
|
W3R3 |
9340f |
488.9b |
877.5ab |
1.91b |
1.06c |
74.4ab |
96.2a |
|
W3CK |
9790d |
519.6a |
886.5a |
1.88b |
1.10abc |
66.0c |
78.6f |
|
W |
** (P=0.00) |
** (P=0.00) |
** (P=0.00) |
** (P=0.00) |
NS (P=0.18) |
** (P=0.00) |
** (P=0.00) |
|
R |
** (P=0.00) |
** (P=0.00) |
NS (P=0.32) |
NS (P=0.12) |
NS (P=0.08) |
** (P=0.00) |
** (P=0.00) |
|
W×R |
** (P=0.00) |
NS (P=0.15) |
NS (P=0.77) |
NS (P=0.94) |
NS (P=0.20) |
** (P=0.00) |
** (P=0.00) |
IWA, WCT, WUEI, WUEET, RUE andNUE represent irrigation water amount, water consumption, water use efficiency of irrigation, water use efficiency of evapotranspiration, rainwater use efficiency and nitrogen use efficiency of reclaimed water, respectively.
W and R represent water level control and water resources of irrgation, respectively.
The values followed by the same letter in the column are not significantly different at a probability level of 0.05.
NS = not significant at the 0.05 level.
* = significant at the 0.05 level.
** = significant at the 0.01 level.
Table 7
Operational taxonomic units (OTUs) number and alpha indices statistics.
|
Sample |
Total tags |
OTUs |
Shannon |
Chao1 |
|
W1R1 |
42,774de |
1,775e |
8.609de |
2218.349gh |
|
W2R1 |
50,321a |
1,784e |
8.245e |
2009.174h |
|
W3R1 |
45,546b |
2,652b |
9.569a |
3249.588bc |
|
W1R2 |
44,407bcd |
3,076a |
9.611a |
3707.506a |
|
W2R2 |
40,950e |
2,302c |
7.586f |
2880.793de |
|
W3R2 |
42,654de |
2,075d |
9.065bc |
2320.385fg |
|
W1R3 |
45,324bc |
3,102a |
9.632a |
3125.846c |
|
W2R3 |
44,528bcd |
3,215a |
9.375ab |
3372.359b |
|
W3R3 |
45,680b |
3,084a |
8.937cd |
3098.724cd |
|
W1CK |
43,052cde |
2,429c |
8.504e |
2756.360e |
|
W2CK |
44,253bcd |
2,407c |
8.464e |
2883.915de |
|
W3CK |
43,885bcd |
1,978de |
8.343e |
2483.521f |
|
W |
NS(P=0.13) |
**(P=0.00) |
**(P=0.00) |
**(P=0.00) |
|
R |
**(P=0.00) |
**(P=0.00) |
**(P=0.00) |
**(P=0.00) |
|
W×R |
**(P=0.00) |
**(P=0.00) |
**(P=0.00) |
**(P=0.00) |
Shanon and Chao1 represent the indicators of alpha diversity.
W and R represent water level control and water resources of irrgation, respectively.
The values followed by the same letter in the column are not significantly different at a probability level of 0.05.
NS = not significant at the 0.05 level.
* = significant at the 0.05 level.
** = significant at the 0.01 level.
Table 8
Distance of ideal solution (di+), negative ideal solution (di-) and the relative closeness of ideal solutions (Si) of different irrigation water sources and water level regulations.
|
Scheme |
di+ |
di- |
Si |
Sequencing |
|
W1R1 |
0.2504 |
0.1274 |
0.6628 |
3 |
|
W1R2 |
0.2955 |
0.1182 |
0.7143 |
2 |
|
W1R3 |
0.1447 |
0.2618 |
0.3560 |
9 |
|
W1CK |
0.1129 |
0.2798 |
0.2876 |
12 |
|
W2R1 |
0.2235 |
0.1451 |
0.6064 |
5 |
|
W2R2 |
0.2596 |
0.1737 |
0.5992 |
6 |
|
W2R3 |
0.1540 |
0.2707 |
0.3627 |
7 |
|
W2CK |
0.1265 |
0.2846 |
0.3077 |
9 |
|
W3R1 |
0.2540 |
0.1337 |
0.6551 |
4 |
|
W3R2 |
0.2953 |
0.1124 |
0.7244 |
1 |
|
W3R3 |
0.1335 |
0.3033 |
0.3056 |
11 |
|
W3CK |
0.0961 |
0.2770 |
0.2575 |
13 |
Table 9
Effectiveness indicators of crop and soil system in rice planting demonstration area.
|
Before and after irrigation |
Irrigation water amount (mm) |
Heavy metals in soil (mg/kg) |
PPCPs in soil (ug/kg) |
Heavy metals in grain (mg/kg) |
PPCPs in grain (ng/kg) |
Yield (kg/ha) |
||||||
|
R2 |
R3 |
CK |
Cd |
Pb |
Cr |
Cd |
Pb |
Cr |
||||
|
Back ground |
/ |
/ |
650 |
0.05 |
33.23 |
24.36 |
0.922 |
0.017 |
0.115 |
0.715 |
4.78 |
9875 |
|
Reclaimed irrigation |
215 |
300 |
120 |
0.05 |
33.31 |
24.78 |
0.945 |
0.017 |
0.114 |
0.724 |
4.85 |
10824 |
No competing interests reported.