Effect of a reduced fertilizer rate on the water quality of paddy fields and rice yields under fishpond effluent irrigation

Agricultural Water Management 231 (2020) 105999

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Effect of a reduced fertilizer rate on the water quality of paddy fields and rice yields under fishpond effluent irrigation

T

Dongliang Qia,b, Jun Yana,b, Jianqiang Zhua,b,* a b

Engineering Research Center of Ecology and Agricultural Use of Wetland, Ministry of Education, Yangtze University, Jingzhou, Hubei, 434025, China College of Agriculture, Yangtze University, Jingzhou, Hubei, 434025, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Fishpond effluent Paddy w ater Nutrient harvest index Nutrient removal rates Grain yield

Aquaculture effluent irrigation has been widely adopted to replace freshwater irrigation to save water and providing additional fertilizer to the crop. There is limited information on the performance of fertilizer supply levels under fishpond effluent irrigation. The objectives of this study were to investigate the effect of reducing a rate of fertilizer on the purification of wastewater from fishponds by paddy fields and the yield of rice under fishpond effluent irrigation in central China in 2015. The treatments included 100 %, 80 %, and 60 % of the normal fertilizer rate (NFR, 150 kg N ha−1, 120 kg P2O5 ha−1 and 75 kg K2O ha−1) with fishpond (freshwaterpond aquaculture) effluent as an irrigation source, designated NFR-E, 0.8NFR-E, and 0.6NFR-E, respectively; with an additional NFR with freshwater as an irrigation source (NFR-F). The results showed that 5700 m3 ha-1 freshwater was saved by the use of the NFR-E, 0.8NFR-E and 0.6NFR-E treatments. The concentrations of total nitrogen (TN), total phosphorus (TP), dissolved phosphorus (DP), ammonia-nitrogen (NH4+-N), nitrate-nitrogen (NO3–N) and particulate phosphorus (PP) in the surface water and seepage water of the paddy field and the residual soil N and P in the 0−60 cm soil depth after the rice harvest decreased with the decreasing NFR. The removal rates of the TN in the surface water across the tillering, booting, heading and filling stages were 25.1 %, 38.9 % and 50.5 % on average for the NFR-E, 0.8NFR-E and 0.6NFR-E treatments, respectively. The corresponding removal rates of the TP were 56.4 %, 71.2 % and 76.2 %, respectively. These increased removal efficiencies were related to the lower N and P concentrations in the surface water of the paddy field and the efficient use of nutrients by rice under the reduced fertilization treatments. Compared to the NFR-F treatment, the 0.8NFR-E treatment resulted in a comparable accumulation of N and P and grain yield of rice, while decreasing the contents of N and P in the water of paddy fields and the residual soil N and P in the 0−60 cm soil depth after the rice harvest. Thus, reducing the normal fertilizer rate by 20 % could improve the water quality of the paddy field without deleterious effects on the rice yield and save 5700 m3 ha-1 of fresh water under fishpond effluent irrigation. These results can also provide a basis for in-depth understanding of the mechanism of aquaculture effluent purification through paddy field ecosystem in response to fertilizer supply levels.

1. Introduction The use of freshwater pond aquaculture has grown greatly worldwide. The productivity of freshwater pond aquaculture in China was 4.17 megatons in 1990, and it reached 20.08 megatons in 2006, an increase of almost five-fold (CAFM, 2006). However, pond aquaculture has also produced a lot of effluent, which resulted in serious environmental problems. Untreated effluent discharged from pond aquaculture operations often resulted in the eutrophication of the waters that received it (Naylor et al., 2002; Mcintosh and Fitzsimmons, 2003). To combat effluent pollution, the United States Environmental Protection

Agency (EPA) began the process of gathering data from the aquaculture industry on February 2000, and released a proposed ruling concerning aquaculture effluent on September 2002 (EPA, 2002; Mcintosh and Fitzsimmons, 2003). It has been proven that aquaculture effluent could be purified using approaches, such as pond-based recirculating systems (Feng et al., 2014), constructed wetlands (Sindilariu et al., 2009; Liu et al., 2014), water discharge reduction (Lin et al., 2001), particle removal via sedimentation and bivalves (Jones et al., 2001) and improve aquaculture feed and feeding practices (Froehlich et al., 2018). However, none of these solutions provide an additional use for the effluent water

⁎ Corresponding author at: Engineering Research Center of Ecology and Agricultural Use of Wetland, Ministry of Education, Yangtze University, Jingzhou, Hubei, 434025, China. E-mail address: [email protected] (J. Zhu).

https://doi.org/10.1016/j.agwat.2020.105999 Received 10 May 2019; Received in revised form 26 December 2019; Accepted 2 January 2020 0378-3774/ © 2020 Elsevier B.V. All rights reserved.

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in the Jianghan Plain (Liu et al., 2019), which 80–170 % higher the recommended fertilizer rate based on site-specific nutrient management (Dobermann et al., 2002; Liu et al., 2009). Large amounts of external N inputs and the unscientific use of fertilizers have contributed to low N and P use efficiency, resulting in serious soil degradation, eutrophication, groundwater pollution and the emission of ammonia and greenhouse gases (Ju et al., 2009; Spiertz, 2010; Ye et al., 2013; Wang et al., 2018). In addition, based on the “constitution of national scientific and technological innovation alliance for fertilizer reduction and its high efficient use” sponsored by the government of China, stakeholders are striving to achieve zero growth and gradually reduce fertilizer consumption for the primary crops by 2020. However, to the best of our knowledge, little information is available on the combined effects of reducing fertilizer inputs and aquaculture effluent irrigation on the removal of N and P by paddy fields, as well as their effects on the yield of rice. The objectives of this study were to investigate the surface and seepage water quality of paddy fields during the rice growing season and the yield of rice in response to reduced fertilizer rates under fishpond effluent irrigation. We hypothesized that with fewer fertilizer inputs, the paddy fields could more effectively remove the nutrients from fishpond effluent; consequently, the fertilizer inputs for rice production could be significantly reduced without deleterious effects on crop yields.

(Mcintosh and Fitzsimmons, 2003). Moreover, these approaches may not be adopted by most fish farms because of the expensive facility inputs or a failure to provide additional economic returns (Castro et al., 2006; Li et al., 2001). Thus, it is desirable to explore an appropriate way to manage the effluent that is highly efficient and inexpensive. Integrating aquaculture into agriculture appears to be a potential solution. The influence of the integrated farms on already scarce freshwater resources (Ingram et al., 2000) and their dependence on expensive chemical fertilizers can be decreased (Fernando and Halwart, 2000), resulting in improved economic return per unit of water (Mcintosh and Fitzsimmons, 2003). Following substantial research and practice, freshwater aquaculture effluent is utilized to irrigate several crops, such as cherry tomatoes (Castro et al., 2006), wheat (Lin and Yang, 2003) and cowpeas (Vigna unguiculata) (Azevedo et al., 2002). Moreover, the water quality of the effluent is obviously improved by the crops because a significant fraction of nutrients from the effluent are taken up by the crops (Wang et al., 2011). Therefore, using aquaculture effluent as an irrigation source is of substantial importance for both environmental and economic benefits, which has produced strong interest over the previous decades. Numerous studies have shown that the accumulation of soil nitrogen (N) and phosphorus (P) caused by the excessive application of fertilizers are the primary causes for the nutrient loss by surface runoff or seepage in many agricultural areas (Li et al., 2008; Ju et al., 2009; Chirinda et al., 2010). Moreover, earlier studies have shown that the efficiency of N fertilizer use of the cereal crops is only 20–40 % in China (Jiang and Cao, 2002), and at least 75 % more than the P applied is retained as different forms of residue in the soil (Shafqat and Pierzynski, 2013). Therefore, the management of soil fertility in modern agriculture is different from that of traditional agriculture. It is important to ensure that the soil fertility not only meets the nutrient demands of the crop but also does not ruin the environment through excessive nutrient accumulation in the soil (Ju et al., 2009). Thus, appropriately reducing fertilizer inputs should be an effective way to improve the water quality of paddy fields and their surroundings. A shortage of water resources is a serious problem in China, and its spatial and temporal distribution is extremely uneven (Dou, 2018). Moreover, the rice planting system accounts for 45–50 % of the total water consumption in China (Yao et al., 2015). In addition, intensive breeding technology models that utilize high density and a high input of bait are very popular in Chinese pond farming. An earlier report has shown that 70–80 % of the feed input discharged into water bodies in the form of dissolved matter and particulate matter and approximately 51 % of N and 64 % of P in the feed will eventually turn into the waste (Liu et al., 2002). Thus, the water shortage in rice planting and the water waste in fish breeding are simultaneous problems. In addition, Lin and Yang (2003) illustrated that the requirements of mineral nutrients by rice growth can be completely replaced by the effluent from hybrid catfish ponds. However, Wang et al. (2011) studied the efficiency of rice fields at treating pond aquaculture effluent and its responses to different fertilizer treatments (mineral fertilizer and potassium chloride) and found that paddy fields can purify pond aquaculture effluent efficiently without compromising the rice yield when supplied with a certain rate of mineral fertilizer (N:P2O5:K2O = 0.6:0.5:0.8). The reason behind this contradiction remains unclear. And, the amount of mineral fertilizer that can be saved without compromising crop yield if aquaculture effluent is used as an irrigation source remains largely unknown. The Jianghan Plain of China is rich with water and soil resources, and therefore is suitable for rice planting and aquaculture breeding. Whereas, rice planting and fish breeding have faced many contradictions, such as unexpected seasonal drought and increasing no-point pollution in this region in previous years (Wu et al., 2008; Yao et al., 2015). Moreover, the average rates of application of N and P have now increased to 200−360 kg N ha−1 and 140−200 kg P2O5 ha−1 owing to the fertilization practices of farmers in intensive rice cropping systems

2. Materials and methods 2.1. Experimental site The experimental plot was located in the farm of Taihu Lake, Jingzhou City in Hubei Province, central China (29°41′25.2″N, 112°39′40.8″E), which is situated near the Yangtze River and is part of the Jianghan Plain of China. The site is a typical mixed farming plain lake area surrounded by paddy fields and fishponds. It is also in a typical subtropical monsoon climate zone with a mean annual precipitation of 1096 mm. The mean annual duration of sunshine is more than 1,742 h, and the mean annual temperature is 16.5 °C. The maximum and minimum temperatures during the rice growing season were 36.9 °C and 14.2 °C in 2015, respectively. The average air, temperature precipitation, and hours of sunshine during the rice growing season in 2015 that were measured at a weather station within the experimental site are shown in Table 1. The soil type is an Acrisol (classified by the FAO), and the soil pH was 7.28. The amount of organic matter was 24.05 g kg−1; the total N was 1.91 g kg−1; the total P was 0.34 g kg−1; the total potassium (K) was 3.34 g kg−1; the available N was 69.4 mg kg-1; the available P was 24.1 mg kg−1, and the available K was 118.7 mg kg−1. The soil chemical properties were measured from a soil depth of 0−20 cm in the paddy field. 2.2. Experimental design The experiment was comprised of four treatments with three replicates. The treatments included 100 %, 80 %, and 60 % of the normal Table 1 Precipitation, hours of sunshine and mean temperature during the growing season of rice in 2015 at the experimental site. June

July

Precipitation (mm per month) 208 164 Sunshine (h per month) 98 197 Mean temperature (℃) 25.2 27.1

August

September

October

145

89

180

260

146

142

27.5

24,1

19.1

Temperatures are the monthly averages. 2

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Table 2 The amount of nutrient input (kg ha−1) for different water and fertilizer managements. Nutrient input from irrigation

NFR-F NFR-E 0.8NFR-E 0.6NFR-E

Basal fertilization

Fertilization at the tillering

N

P2O5

K2O

N

P2O5

K2O

N

P2O5

K2O

0-3 17-21 17-21 17-21

0-0.4 4-6 4-6 4-6

0 0 0 0

105 105 84 63

84 84 67.2 50.4

75 75 75 75

45 45 36 27

36 36 28.8 21.6

0 0 0 0

fertilizer rate (NFR) with effluent from a fishpond (freshwater-pond aquaculture) as an irrigation source (designated NFR-E, 0.8NFR-E, and 0.6NFR-E, respectively); with an additional NFR using freshwater as an irrigation source (NFR-F). The NFR with 150 kg N ha−1, 120 kg P2O5 ha−1 and 75 kg K2O ha-1 was determined based on the local farming practices. Fertilizer applications were split over two developmental stages of the rice crop (Ye et al., 2013): basal on before transplant on June 26, 2015, and the tillering on July 15, 2015. The size of the rice field used in this experiment was 0.67 ha (133 m × 50 m). The experimental field was divided into 12 plots on average. Each plot had an area of 320 m2 (40 m × 8 m). The exact amount of nutrient input for each treatment is shown in Table 2.

were determined immediately after sampling. A 500 ml plastic bottle was used to collect water samples along the long side of the plot at 0 m (at the water inlet) and at 10, 20, 30 and 40 m (at the water outlet). The soil seepage solution was collected at the clay head, which was pressed into 30 cm in the soil depth to collect seepage water samples at 10, 20 and 30 m along the long side of the plot. Soil samples were collected after the rice harvest on October 23, 2015, using a stainless auger. The S-shaped 5-point collection method was used for the sampling. The samples were collected from 0 to 20, 20–40 and 40−60 cm soil depths in each plot. The root debris and soil samples were sieved and stored as described by Si et al. (2018). Briefly, the plant root remnants and rocks were removed, and the samples were passed through a 0.85 mm sieve and stored at 4 °C to determine the relevant indices. The water quality parameters determined included the following: TP, TN, dissolved phosphorus (DP), ammonia-nitrogen (NH4+-N), nitrate-nitrogen (NO3−-N) and particulate phosphorus (PP). The TN concentration was determined with a Unico UV-2800 spectrophotometer after digestion using potassium peroxodisulfate. The concentrations of NH4+-N, NO3−-N, and the TP in the water samples were analyzed by the indophenol blue, disulfonic acid phenol, and ammonium molybdate spectrophotometric methods using a UV-2800 spectrophotometer (SEPA, 2002). The DP was determined using colorimetry as described by Murphy and Riley (1962). The PP was calculated as the difference between the TP and DP. Soil nutrient parameters that were monitored included the following: TN, TP and available N and P. The TN was determined by the semi-micro Kjeldahl method, and the TP was determined colorimetrically after wet digestion with H2SO4 + HClO4 (Parkinson and Allen, 1975). The available N (NH4+-N + NO3−-N) was determined using a micro-diffusion technique after alkaline hydrolysis (Conway, 1978). The available P was determined using the Olsen method (Emteryd, 1989).

2.3. Experimental management A fishpond was located northwest of the paddy field with a total area of 2000 m2. The fish that were raised were primarily composed of approximately 1000 kg black, grass, silver and bighead carp in total. The fish were fed at approximately 8:00 AM each day. The feed was primarily composed of chicken and pig manure and wheat, weeds and crop straw that had been fermented. A total of 20 kg of feed was used at each feeding. The circumference of each plot was isolated by double layer plastic sheeting placed in backfilled trenches to a depth of 0.5 m to form a barrier. The experimental rice used was late-rice (Oryza sativa L.) variety “Jingchuyou148.″ The rice was sown on June 1, 2015. The 4week-old seedings were artificially transplanted on June 30, 2015 with a planting space of 25 cm × 30 cm. The panicles were initiated on August 24, 2015, and harvested on October 22, 2015. The whole growth period was 142 d in all the treatments. Disease, weeds, and pests were well controlled in each treatment. The fishpond was connected to the rice field by water pipes and pumps, and the effluent from the fishpond was pumped to irrigate the wastewater. The rice was irrigated 15, 31, 56, 71, 82 and 93 days after transplanting (DAT) (95 mm each time). The amount of irrigation water was measured with a water meter installed at the discharging end of the pipe. In the freshwater irrigation, the irrigated water from a reservoir in the local area (Yaoxin Reservoir) with an electrical conductivity of 251 μS cm−1, pH of 7.1, total suspended solids (TSS) of 3.4 mg L−1, and contents of total N (TN) and total P (TP) of 0.18 mg L−1 and 0.02 mg L−1, respectively. In the fishpond effluent irrigation, the contents of TN and TP in the fishpond effluent were 2.80–3.40 mg L-1 and 0.2-0.3 mg L-1, respectively. The effluent had an electrical conductivity of 401–446 μS cm−1, a pH of 7.1–8.2, a chemical oxygen demand (COD) of 7.5–10.1 mg L−1, dissolved oxygen (DO) of 3.3–5.0 mg L−1 and TSS of 78.2–82.5 mg L−1. The contents of TN and TP in the aquaculture effluent in the local area were 2.50–4.00 mg L-1 and 0.180.35 mg L-1, respectively (Zhao and Zeng, 2012). Thus, the concentrations of N and P in the effluent used in this study are representative.

2.4.2. Nutrient uptake, yield and yield components Five representative plants in each plot were cut at the maturity stage (harvest) on October 22, 2015, for the determination of the accumulation of nutrients. The plants were divided into stems, leaves and panicles. All the plant samples were oven-dried at 70 °C to a constant biomass and weighed. The samples were passed through a 0.15 mm sieve, and then the subsamples were collected for the total determination of N and total P as described by Ye et al. (2013). The concentration of N in the tissues was analyzed using the semi-micro Kjeldahl method, and the concentration of P in the tissues was analyzed using the vanadium molybdate yellow colorimetric method (Bao, 2000). The total N uptake was calculated from the sum of the products of dry matter and N concentration of the different plant parts. The total P uptake was calculated from the sum of the products of dry matter and the P concentration of the different plant parts. The grain yield was measured from a 10 m2 area in the center of each plot at the maturity stage on October 22, 2015. The rice was harvested and the yield component determined as described by Ye et al. (2013). In short, the rice was harvested manually, and the grains were air dried. The yield components included the effective panicle number per plant, spikelet number per panicle, grain filling percentage, and 1000-grain weight.

2.4. Data collection 2.4.1. Water quality and soil nutrients The surface water and leakage water samples were sampled at the tillering, booting, heading and filling stages of rice at 34, 58, 73 and 95 DAT, respectively. The concentrations of N and P in the different forms 3

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2.5. Data analysis

Table 4 Removal rates of NH4+-N, NO3−-N, total nitrogen (TN), particulate phosphorus (DP), particulate phosphorus (PP) and total phosphorus (TP) in the surface water by the paddy field at the tillering, booting, heading and filling stages as affected by different water and fertilizer management strategies.

The removal rates (R) of the N and P in different forms from the fishpond effluent by paddy field were calculated as follows:

R=

C0 − Cd × 100% C0

Growth stage

(1)

where C0 is an initial concentration of the N and P in the wastewater from the fishpond effluent before fishpond irrigation (mg L−1), and Cd is the concentration of the N and P in the surface water of paddy field after fishpond irrigation (mg L−1), which was the means of the concentrations along the long side of the plot at 0 m (at the water inlet) and 10, 20, 30 and 40 m (at the water outlet) in the paddy field. An analysis of variance (ANOVA) was performed by a one-way ANOVA using SPSS 17.0 software. The mean values among the different treatments were compared for significant differences (P = 0.05) using least significant difference tests.

Tillering

Booting

3. Results

Heading

3.1. Total irrigation amount There were six irrigation events (950 m3 ha−1 per time) during the rice growing season for each treatment, resulting in a total irrigation amount of 5700 m3 for all the treatments. Thus, 5700 m3 of freshwater could be saved from the NFR-E, 0.8NFR-E and 0.6NFR-E treatments.

Filling

3.2. N and P content in the surface water

Index

NH4+-N NO3−-N TN DP PP TP NH4+-N NO3−-N TN DP PP TP NH4+-N NO3−-N TN DP PP TP NH4+-N NO3−-N TN DP PP TP

C0(mg L−1)

1.303 0.595 3.398 0.180 0.405 0.585 1.734 0.477 3.47 0.243 0.368 0.611 1.574 0.418 2.809 0.081 0.283 0.364 1.593 0.600 3.003 0.131 0.355 0.486

Removal rate(%) NFR-E

0.8NFR-E

0.6NFR-E

54.5a 82.7a 32.4a 62.2a 52.1a 55.2a 23.1a 55.8a 15.5a 51.4a 52.7a 52.2a 31.1a 86.6a 37.1a 30.9a 64.7a 57.1a 28.7a 72.2a 15.4a 35.1a 70.7a 61.1a

61.6ab 88.6b 45.0ab 73.3a 69.8ab 70.9a 33.2ab 58.3ab 42.2b 82.3b 57.9a 67.6a 34.8a 90.0a 38.4a 45.7ab 76.0a 69.2a 40.6a 79.0b 29.9a 45.0ab 88.7b 77.0b

63.2b 92.1c 52.6b 82.2b 73.8b 76.4b 47.1b 66.9b 49.8c 86.4b 60.1a 70.5b 51.3b 90.2a 52.6b 61.7b 83.0b 78.3b 54.6b 82.8b 46.9b 58.8b 87.0b 79.4c

Note: C0 represents the initial concentration of N and P in different forms in the effluent from fishponds as an irrigation source. Values within the same row and growth stage followed by different letters are significantly different at P < 0.05.

Compared to the NFR-F treatment, NH4+-N, NO3−-N, TN, dissolved phosphorus (DP), particulate phosphorus (PP) and TP contents in the surface water at the tillering, booting, heading and filling stages were comparable in the NFR-E treatment with the exception of NO3−-N at the tillering stage and DP at the booting stage (Table 3). The 0.8NFR-E treatment decreased the NO3−-N content at the tillering stage by 21.8 %, the TN content at the booting stage by 9.4 %, and the NO3−-N and TP contents at the filling stage by 14.9 % and 37.1 %, respectively. The slight increase in the N and P measured were also observed for the 0.8NFR-E treatment. However, the 0.6NFR-E treatment decreased the NH4+-N, NO3−-N, TN, DP, PP and TP contents at the four growth stages with the exception of the content of NO3−-N at the heading stage (Table 3). Across the four growth stages, the 0.8NFR-E and 0.6NFR-E treatments decreased the TN content by 14.6 % and 23.4 % on average, respectively, and decreased the TP content on average by 22.3 % and

35.5 %, respectively, in comparison with the NFR-F treatment. 3.3. N and P removal by the paddy fields The removal rate of NH4+-N, NO3−-N, TN, DP, PP and TP in the effluent from fishpond by the paddy field at the tillering, booting, heading and filling stages increased as the NFR decreased. The removal rate was the greatest for NO3−-N, followed by NH4+-N, TN or PP, DP and TP at the four growth stages. The removal rates of the NH4+-N, NO3−-N, TN, DP, PP and TP were the greatest at the tillering stage, followed by the booting, filling and heading stages (Table 4). Across the

Table 3 NH4+-N, NO3−-N, total nitrogen (TN), particulate phosphorus (DP), particulate phosphorus (PP) and total phosphorus (TP) contents (mg L-1) in the surface water of paddy field at the tillering, booting, heading and filling stages as affected by different water and fertilizer management strategies. Growth stage

Treatment

NH4+-N

NO3−-N

TN

DP

PP

TP

Tillering

NFR-F NFR-E 0.8NFR-E 0.6NFR-E NFR-F NFR-E 0.8NFR-E 0.6NFR-E NFR-F NFR-E 0.8NFR-E 0.6NFR-E NFR-F NFR-E 0.8NFR-E 0.6NFR-E

0.587a 0.593a 0.501ab 0.479b 1.153a 1.334a 1.158a 0.918b 1.092a 1.085a 1.026a 0.767b 0.940a 1.136a 0.946a 0.723b

0.087b 0.103a 0.068b 0.047c 0.196a 0.211a 0.199a 0.158b 0.036a 0.056a 0.042a 0.041a 0.148a 0.167a 0.126b 0.103b

2.064a 2.298a 1.868ab 1.612b 2.215ab 2.932a 2.007b 1.742c 1.693a 1.766a 1.729a 1.332b 2.249a 2.541a 2.105a 1.594b

0.040ab 0.068a 0.048a 0.032b 0.044b 0.118a 0.043b 0.033b 0.044ab 0.056a 0.044ab 0.031b 0.062ab 0.085a 0.072ab 0.054b

0.210a 0.194a 0.122ab 0.106b 0.216a 0.174a 0.155ab 0.147b 0.058ab 0.100a 0.068ab 0.048b 0.116a 0.104a 0.040b 0.046b

0.250a 0.262a 0.170a 0.138b 0.260a 0.292a 0.198b 0.180b 0.102a 0.156a 0.112a 0.079b 0.178a 0.189a 0.112b 0.100b

Booting

Heading

Filling

Note: Values were the means of the content along the long side of the plot at 0 m (at the water inlet), and 10, 20, 30 and 40 m (at the water outlet). Values within the same column and grown stage followed by different letters are significantly different at P < 0.05. 4

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Table 5 NH4+-N, NO3−-N, total nitrogen (TN), particulate phosphorus (DP), particulate phosphorus (PP) and total phosphorus (TP) contents (mg L-1) in the seepage water of paddy field at the tillering, booting, heading and filling stages as affected by different water and fertilizer management strategies. Growth stage

Treatment

NH4+-N

NO3−-N

TN

DP

PP

TP

Tillering

NFR-F NFR-E 0.8NFR-E 0.6NFR-E NFR-F NFR-E 0.8NFR-E 0.6NFR-E NFR-F NFR-E 0.8NFR-E 0.6NFR-E NFR-F NFR-E 0.8NFR-E 0.6NFR-E

1.138ab 1.243a 0.981bc 0.875c 0.767a 0.862a 0.663ab 0.547b 0.613a 0.614a 0.543a 0.491a 0.728a 0.788a 0.666ab 0.517b

0.211a 0.241a 0.178ab 0.134b 0.157a 0.192a 0.126a 0.107a 0.143a 0.147a 0.089a 0.061a 0.156a 0.165a 0.126b 0.127b

2.150ab 2.215a 1.885bc 1.647c 1.793a 1.968a 1.538ab 1.320b 1.871a 1.896a 1.155b 1.084b 1.437a 1.743a 1.318ab 1.177b

0.112b 0.153a 0.060bc 0.050c 0.056a 0.079a 0.049a 0.023a 0.062a 0.071a 0.038a 0.024a 0.030b 0.053a 0.023c 0.017c

0.167a 0.173a 0.149a 0.097b 0.168a 0.174a 0.112b 0.097c 0.056a 0.058a 0.049a 0.037b 0.055a 0.060a 0.051a 0.033b

0.279ab 0.326a 0.209bc 0.147c 0.224a 0.253a 0.161b 0.120b 0.118a 0.129a 0.087ab 0.061b 0.085b 0.113a 0.074c 0.050c

Booting

Heading

Filling

Note: Values within the same column and growth stage followed by different letters are significantly different at P < 0.05.

3.6. N and P accumulation in rice, N harvest index (NHI) and P harvest index (PHI)

four growth stages, the NFR-E, 0.8NFR-E and 0.6NFR-E treatments resulted in the following average removal rates: NH4+-N 34.3 %, 42.5 % and 54.0 %, respectively; NO3−-N 74.3 %, 79.0 % and 83.0 %, respectively; TN 25.1 %, 38.9 % and 50.5 %, respectively; DP 44.9 %, 61.6 % and 72.3 %, respectively; PP 60.1 %, 73.1 % and 76.0 %, respectively, and TP 56.4 %, 71.2 % and 76.2 %, respectively. These results suggest that the efficiency of the removal of TN and TP in the effluent by the paddy field increased with the decrease in the normal fertilizer rate.

The accumulation of N and P in the leaves and the accumulation of N in the stems at the maturity stage were comparable among the different treatments. However, the accumulation of P in the stems and panicles was significantly smaller in the 0.6NFR-E treatment than in the other treatments (Table 7). The NFR-E treatment produced the greatest accumulation of N in the panicles among the four treatments (Table 7). Compared with the NFR-F treatment, the accumulation of N and P in the shoot, the N harvest index (NHI, ratio of panicles N to total N accumulation in rice) and the P harvest index (PHI, ratio of panicles P to total P accumulation in rice) were comparable in the NFR-E and 0.8NFR-E treatments, while these values were significantly smaller in the 0.6NFR-E treatment.

3.4. N and P contents in the seepage water Compared to the NFR-F treatment, the NFR-E treatment increased the NH4+-N, NO3−-N, TN, DP, PP and TP contents in the seepage water at the tillering, booting, heading and filling stages, although most of the differences did not reach a significant level. However, the NH4+-N, NO3−-N, TN, DP, PP and TP content were decreased by the 0.8NFR-E and 0.6NFR-E treatments (Table 5). The TN and TP contents at the four growth stages were significantly smaller in the 0.6NFR-E treatment than in the NFR-F treatment (Table 5). Across the four growth stages, the 0.8NFR-E and 0.6 NFR-E treatments decreased the TN content by 18.3 % and 27.5 %, respectively, and decreased the TP content by 23.1 % and 45.8 % on average, respectively, in comparison with the NFR-F treatment.

3.7. Residual soil N and P in the 0−60 cm soil depth The soil TN, available N, TP and available P contents after the rice harvest in all the treatments decreased with the deepening of soil depth. The NFR-E treatment generated the greatest TN, available N, TP and available P contents in the each soil depth (Fig. 1). The soil available N in the 0−20 cm soil depth, soil TN content in the 40−60 cm soil depth and soil available P content in the 20−40 cm soil depth were significantly greater in the NFR-F and NFR-E treatments than those in the 0.8NFR-E and 0.6NFR-E treatments (Fig. 1a, b and d). Compared with the NFR-F treatment, the soil TN and available N contents in the 20−40 cm soil depth, soil TP and available P contents in the 0−20 cm soil depth, and the soil available P content in the 40−60 cm soil depth were significantly greater in the NFR-E treatment, while these values were smaller in the 0.8NFR-E and 0.6NFR-E treatments (Fig. 1a-d). The soil TP content in the 20−60 cm soil depths was significantly smaller in the 0.6NFR-E treatment compared with the other treatments (Fig. 1c).

3.5. Grain yield and yield components The effective panicle numbers per plant and 100-grain weight were comparable among the different treatments. Compared with the NFR-F treatment, spikelet number, grain filling percentage and grain yield were comparable in the NFR-E and 0.8NFR-E treatments, while these values were significantly smaller in the 0.6NFR-E treatment (Table 6). This suggests that reducing the normal fertilizer rate by 20 % did not compromise the yield of rice under fishpond effluent irrigation.

Table 6 Grain yield and yield components as affected by different water and fertilizer management strategies. Treatment

Effective panicle number(plant−1)

Spikelet number (panicle−1)

Grain filling percentage (%)

1000-grain weight (g)

Grain yield (kg ha−1)

NFR-F NFR-E 0.8NFR-E 0.6NFR-E

13 14 13 12

188 198 185 178

82.2 85.9 82.0 78.4

26.3 27.0 26.0 25.9

8030 8211 7981 7261

± ± ± ±

0.58a 0.58a 0.58a 0.58a

± ± ± ±

4.15ab 2.42a 6.57b 2.39c

± ± ± ±

1.6ab 1.2a 0.3b 1.8c

Note: Values within the same column followed by different letters are significantly different at P < 0.05. 5

± ± ± ±

0.3a 0.7a 0.1a 0.1a

± ± ± ±

71ab 120 a 160ab 81c

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Table 7 Distribution of nitrogen (N) and phosphorus (P) in the leaves, stems and panicles and total nitrogen (TN) and total phosphorus (TP) accumulation in shoot (leaves + stems + panicles) at the maturity stage (kg ha−1), nitrogen harvest index (NHI) and phosphorus harvest index (NHI) of rice as affected by different water and fertilizer management strategies. Treatment

NFR-F NFR-E 0.8NFR-E 0.6NFR-E

Leaves

Stems

Panicles

Shoot

N

P

N

P

N

P

TN

TP

27.9a 27.2a 29.0a 29.1a

4.8a 4.5a 4.9a 4.8a

23.8a 23.2a 24.7a 24.8a

7.9a 8.3a 7.7a 6.6b

67.6b 75.3a 64.1b 56.7c

20.1a 21.6a 19.3a 16.3b

119.3ab 125.7a 117.8ab 110.6b

32.8ab 34.4a 31.9ab 27.7b

NHI (%)

PHI (%)

56.7ab 59.9a 54.4ab 51.3b

61.3ab 62.9a 60.5ab 58.7b

Note: Values within the same column followed by different letters are significantly different at P < 0.05.

4. Discussion

These indicate that the 0.8NRF-E and 0.6NRF-E treatments performed better on the removal of TN and TP compared to the NFR-E treatment. In addition, an earlier study demonstrated that the rice crop removed 32 % of TN and 24 % of TP from hybrid catfish pond effluent (Lin and Yang, 2003), in which the removal efficiencies of N and P were lower compared to those in the 0.8NFR-E and 0.6NFR-E treatments. The enhanced nutrients removal rates may be explained as follows: first, reducing the fertilizer inputs resulted in a reduced concentration of N and P from the source (Malhi et al., 2002; Wang et al., 2014). The relatively lower NH4+-N, NO3−-N, TN, DP, PP and TP contents in the surface water of paddy field under the reduced fertilizer treatments (Table 3) confirmed this. Second, this was attributed to nutrients that could be better utilized by the crop under reduced fertilization treatments (Luo et al., 2018), as indicated by the 0.8NRF-E treatment, which had a comparable N and P accumulation in the shoots of rice compared to the

4.1. Water quality of the paddy field The paddy field ecosystem is an important artificial wetland, which not only mediates important grain production functions but also plays a unique role in purifying water quality (Wang et al., 2011; Liu et al., 2019). The previous work showed that after aquaculture pond effluent water was restored by the paddy field, the TN and TP contents in the effluent decreased by 66.0–69.3 % and 41.4–42.5 % (Chen, 2009). Consistently, in this study, the removal rates of TN in the effluent from fishponds across the tillering, booting, heading and filling stages by the paddy field were 25.1 %, 38.9 % and 50.5 % on average for the NFR-E, 0.8NFR-E and 0.6NFR-E treatments, respectively. The corresponding removal rates of TP were 56.4 %, 71.2 % and 76.2 %, respectively.

Fig. 1. Soil total nitrogen, available nitrogen, total phosphorus and available phosphorus contents in the 0−60 cm soil depths as affected by different water and fertilizer managements. Note: Values (means ± standard error, n = 3) within the same soil depth followed by different letters are significantly different at P < 0.05. 6

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4.3. Residual soil N and P

NFR-F treatment (Table 7). Our earlier work has shown that reduced fertilization treatments improve the leaf area index at the post-stages of rice growth in reduced fertilization treatments under fishpond effluent irrigation (Zhou et al., 2011). Moreover, appropriate N fertilizer reduction can enhance the chlorophyll content and net photosynthetic rate of cotton, and thereby delay leaf senescence and increase nitrogen use efficiency (Luo et al., 2018). However, the mechanism behind the respective contributions of input mineral nutrients and nutrients from the fishponds to nutrient requirements by the crop under reduced fertilization treatments is not clear and merits further study. In this study, the NH4+-N and NO3−-N contents accounted for approximately 70 % of the TN content, and the DP content only accounted for approximately 30 % of the TP content in the initial effluent from fishpond (Table 4). This is consistent with the findings of Bergheim and Brinker (2003) that more than 80 % of the TN was NH4+-N and NO3−N in aquaculture effluent and that 30–84 % of the TP was bound in particles. Moreover, the removal efficiencies of the NH4+-N, NO3−-N and PP were all the highest at the tillering, followed by the booting, filling and heading stages (Table 4), since nutrient demand and the uptake rate by rice vary among the different growth stages. Generally, the most important uptake happens at the tillering stage (Shimono et al., 2012). In addition, the original concentrations of N and P in different forms in the effluent as an irrigation source were comparable among the six irrigation events. Thus, it is still not clear whether the removal efficiencies of N and P by the paddy field are related to the load of nutrients in the fishpond effluent and environmental factors, such as light, temperature and microorganisms, which merit additional study. Before this study, little information was available on the N and P dynamics in response to reducing the fertilizer rate under aquaculture effluent irrigation. In this study, we found that compared with the NFRF treatment, both the 0.8NFR-E and 0.6NFR-E treatments decreased the contents of TN and TP in the surface and seepage water (Tables 3 and 5). This suggests that reducing fertilizer inputs could effectively lower the concentrations of N and P of the discharge water from paddy fields if fishpond effluent is used as an irrigation source.

An earlier study had shown that the long-term or excessive use of sewage from livestock and poultry breeding as an irrigation source leads to a significant increase in the accumulation of N and P in the 0−40 cm soil depths of paddy fields (Zhang et al., 2014). However, in this study, the 0.8NFR-E and 0.6NFR-E treatments effectively reduced the contents of TN in the 20−60 cm soil depths and TP in the 0−20 cm soil depth in the paddy soil compared with the NFR-F treatment (Fig. 1), indicating that reducing the normal fertilizer rate by 20–40 % would not result in the accumulation of N and P in the soil profile under fishpond effluent irrigation, thereby reducing the possibility of N and P leaching. This contradiction may be explained by the differences in crop varieties, soil types, soil microorganisms, nutrient load in water bodies and environmental factors, such as light and temperature. In addition, our study primarily focused on the monitoring and evaluation of water quality and rice yield, and the test was only conducted for one year. Therefore, further research is needed to study the soil quality status of the return of long-term aquaculture effluent to farmland. 5. Conclusions A paddy field system could effectively remove NH4+-N, NO3−-N, TN, DP, PP and TP from fishpond effluent, and the removal efficiencies of TN and TP all increased with the decreasing normal fertilizer rate (NFR) at the tillering, booting, heading and filling stages of rice. The improved removal efficiencies were attributed to the lower N and P contents in the surface water of paddy field and the efficient N and P uptake by rice under reduced fertilization treatments. Using fishpond effluent as an irrigation source, the concentrations of NH4+-N, NO3−-N, TN, DP, PP and TP in the surface water and seepage water of the paddy field and the residual soil N and P in the 0−60 cm soil depths after the rice harvest decreased with the decrease in the NFR. The N and P concentration in the fishpond effluent could meet the N and P requirement of rice and obtain a comparable grain yield with a 20 % reduction in the NFR input under fishpond effluent irrigation. Therefore, considering the yield effect of rice and the environmental risk, 20 % of the fertilizer input and 5700 m3 ha-1 of fresh water could be saved under aquaculture effluent irrigation during the rice growing season. These results can also provide a basis for in-depth understanding of the mechanism of aquaculture effluent purification through paddy field ecosystem in response to fertilizer supply levels.

4.2. Grain yield of rice N and P are the two essential fertilizer resources for regulating plant growth and development. It has been proven that the management of water and nutrients has a substantial influence on rice yield formation (Ye et al., 2013). In this study, the accumulation of N and P in the shoots of rice at the maturity stage (harvest) were comparable between the NFR-F and 0.8NFR-E treatments (Table 7), suggesting that reducing the normal fertilizer rate by 20 % could meet the nutrient requirements of rice under fishpond effluent irrigation. Moreover, herein we observed that the NHI and PHI were comparable between the NFR-F and 0.8NFRE treatments (Table 7). Based on an analysis from the perspective of physiology, when a certain nutrient is deficient, the rice will prioritize the transfer of the nutrient lacking to the grain to promote its differentiation and maturation (Whitbread et al., 2003). This indicates that reducing the normal fertilizer rate by 20 % did not induce a deficiency of N and P in rice under aquaculture effluent irrigation. We also observed that the NFR-E and 0.8NFR-E treatments resulted in a comparable grain yield of rice, while the 0.6NFR-E treatment significantly decreased the grain yield compared with that of the NFR-F treatment (Table 6), suggesting that the normal fertilizer rate of rice could be reduced by 20 % without deleterious effects on yield under fishpond effluent irrigation. In addition, the fact that the NFR-E treatment did not lead to a greater rice yield could be related to a sufficient amount of fertilizer that supplied all the nutrients needed by the plant, so that irrigation with the aquaculture effluent did not influence the crop yield (Castro et al., 2006).

Declaration of Competing Interest The authors declare that they have no competing interests. Acknowledgments We are grateful to the National Key Research and Development Project, China (2016YFD0800500), the Innovation System Found Project of Ecological and Circular Agriculture in Hubei Province, China (2018skjcx01) and the Engineering Research Center of Ecology and Agricultural Use of Wetland, Ministry of Education, China (KFT201904) for providing funding in the form of research grants. References Azevedo, C.B., Gurreto, A.K., Filho, J.L., Maia, C.E., 2002. Stimulating nodulation and growth in cowpea with fish effluent. J. World Aquac. Soc. 33, 49–64. Bao, S., 2000. Soil and Agricultural Chemistry Analysis, ed. iii. Chinese Agriculture Press, Beijing, pp. 42–49 264–268 (in Chinese). Bergheim, A., Brinker, A., 2003. Effluent treatment for flow-through systems and European environmental regulations. Aqua. Eng. 27, 61–77. CAFM (China Agriculture Fisheries Ministry), 2006. China Fisheries Yearbook. China Agriculture Press, China in Chinese. Castro, R.S., Azevedo, C.B., Bezerra, F., 2006. Increasing cherry tomato yield using fish effluent as irrigation water in Northeast Brazil. Sci. Hortic. 110, 44–50.

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