A Comparison of Nitrogen Transfer and Transformation in Traditional Farming and the Rice–Duck Farming System by 15N Tracer Method
A Comparison of Nitrogen Transfer and Transformation in Traditional Farming and the Rice–Duck...
EBISSA, Tchister Morrel;Yang, Bo;Guan, Yuanqing;Tan, Bingchang;Chen, Peizhen;Wang, Lili;Magwanga, Richard Odongo;Zheng, Xiangqun
2018-12-02 00:00:00
agronomy Article A Comparison of Nitrogen Transfer and Transformation in Traditional Farming and the Rice–Duck Farming System by N Tracer Method 1 , † 1 , † 1 , † 1 1 Tchister Morrel EBISSA , Bo Yang , Yuanqing Guan , Bingchang Tan , Peizhen Chen , 1 2 , 3 1 , Lili Wang , Richard Odongo Magwanga and Xiangqun Zheng * The Graduate School of Chinese Academy of Agricultural Sciences, Agro-Environmental Protection Institute, No.31 Fukang Road, Nankai District, Tianjin 300191, China; morreldalong2008@yahoo.com (T.M.E.); yangbopipi@126.com (B.Y.); lky6813567@163.com (Y.G.); tanbch@163.com (B.T.); chenpeizhencpz@sina.com (P.C.); lili0229ok@126.com (L.W.) Research Base in Anyang Institute of Technology, State Key Laboratory of Cotton Biology/Institute of Cotton Research, Chinese Academy of Agricultural Sciences (ICR, CAAS), Anyang 455000, China; magwangarichard@yahoo.com School of Physical and Biological Sciences (SPBS), Main Campus, Jaramogi Oginga Odinga University of Science and Technology, P.O. Box 210-40601, Bondo, Kenya * Correspondence: zhengxiangqun@126.com † These authors contributed equally to the work. Received: 20 September 2018; Accepted: 23 November 2018; Published: 2 December 2018 Abstract: A field experiment was conducted in Ninghe, Tianjin, China, using the N isotope method to determine the fate of N sources, application effect of organic fertilizer on the growth of rice plant organs, N uptake by rice, and N use efficiency. The experiment included eight treatments: CK-N (control + no-duck), CK-D (control + ducks), CF-N (chemical fertilizer + no-ducks), CF-D (chemical fertilizer + ducks), CM-N (chemical fertilizer + organic fertilizer + no-ducks), CM-D (chemical fertilizer + organic fertilizer + ducks), CD-N (chemical fertilizer 30% off + organic fertilizer + no-ducks), and CD-D (chemical fertilizer 30% off + organic fertilizer + ducks). The results showed that the application of organic fertilizer whether CM or CD in grain and leaf significantly increased N concentration; leaf and root P concentrations over control (CK) and chemical fertilizer (CF). In contrast, straw and root N concentrations, including grain and straw P concentrations did not show any difference between duck and no-duck treatment. Moreover, non-significant differences 15 1 were found in N fresh grain and husk concentration. Both organs ranged from 14.2–14.4 gkg and 6.2–6.3 gkg , respectively. Likewise, N uptake and N use efficiency in fresh grain and husk were not significantly differed within duck and without duck treatment. However, N uptake in fresh grain and husk ranged at the rates of 54.90–93.69 and 6.43–11.04 kg ha with duck and without duck treatment. N use efficiency in fresh grain and husk ranged from 21.55%–34.61% and 2.61%–4.24%, respectively. Overall organic fertilizer has a significant influence on rice growth and promotes crop productivity. Keywords: nitrogen; transfer; transformation; N uptake; nitrogen use efficiency 1. Introduction Rice is the most important staple food crop in the world [1] and contributes to more than 40% of the cereal yield [2]. In the last decades, rice yield in China rapidly increased due to the introduction of high yield varieties and increasing use of chemical N fertilizer [3]. Moreover, it has been mentioned that in order to meet the demand of the ever-increasing population, sustainable increases in rice yield by 1.21% annually is needed for food security in China [4,5]. As a result, the use of fertilizers, especially chemical Agronomy 2018, 8, 289; doi:10.3390/agronomy8120289 www.mdpi.com/journal/agronomy Agronomy 2018, 8, 289 2 of 13 fertilizer (CF) in China is more intensive and wide-spread than in any other country [6]. However, excessive use of N fertilizer has the consequence of severe environmental degradation with high potential for N loss in many pathways [7], decreased N use efficiency (NUE), decreased crop quality, and creation of environmental hazards in rice growing countries [8–11]. Therefore, an appropriate fertilizer input should be required and controlled to maintain rice yield. Adequate nitrogen (N) supply may enhance the rice growth and improve grain yield, and the application of appropriate levels of N fertilizer through improved management is key to increasing N use efficiency [12,13]. In addition, nitrogen is required to produce more food in agricultural systems. Therefore, the lack of N responds quickly to the addition of N fertilizers if applied in a timely manner and properly. Furthermore, nitrogen transformation in soil–plant systems involves the complex N cycling process, which increases the difficulty of N management. Basically, processes involving N in the soil–plant system are: mineralization, nitrification, immobilization, leaching, denitrification, and volatilization. In the present study, ducks were introduced to the field. Duck activities include walking, swimming, eating, grooming, paddling, and rubbing which can influence soil structure and fertility. Duck feces may be a good supply of organic fertilizer to the soil. Thus, it is estimated that the total excreted feces per duck can reach 10 kg, which contains 47 g N, 70 g P, and 31 g K [14]. Previous research has shown that duck stirring and intertillage can improve elements of the soil environment, such as soil air, texture, and structure [15]. Duck activities may enhance the decomposition of soil organic matter and nutrient transformation, which benefits the growth of rice plants. Moreover, we reduced the amount of inorganic fertilizer, maintained organic fertilizer application, and introduced ducks to the field in order to achieve the goal of clean rice production. Therefore, the amounts of fertilizers were strictly evaluated in order to avoid heavy N loss to the environment, environmental pollution, and to contribute to safe food production. Information about the combined application of organic and inorganic fertilizer and the use of N labeled sources in traditional farming and the rice–duck farming system is limited. The aims of this research were to determine the fate of labelled N sources, including the effect of the application of organic fertilizer on the growth rice plant organs, N uptake by rice, and N use efficiency in duck and no-duck fields. 2. Materials and Methods 2.1. Study Site The field experiment was conducted in 2017 during the rice-growing season at Ninghe district 0 0 2 of Tianjin City, China (39 18–39 50 N, 117 08–117 56 E). Ninghe covers an area of 1414 km , with a typical humid continental climate with large seasonal temperature differences. The annual average temperature is 54.0 F (12.2 C) and the warmest month is July, with an average temperature of 79.2 F (26.2 C). The coolest month is in January with an average of 24.4 F ( 4.2 C) and annual average precipitation of 591.8 mm. 2.2. Experimental Design and Operation The seeds were sown on 7 June 2017, and then the rice seedlings of Japonica rice were transplanted directly to the plots during the tilling stage on 3 July and harvested on 18 October 2017. The application of N-labelled fertilizer and ordinary fertilizers was performed during vegetative growth on 13 July 2017. Before applying isotope [ (NH ) SO ] fertilizer and ordinary ammonium sulphate fertilizers 4 2 4 to different treatments in two fields (duck and no-duck), the experiment field was drained and the soil was brought through the plots, irrigated, then the rice seedlings were transplanted. There were eight treatments replicated three times, arranged as a total of twenty-four boxes placed separately in each field (Figure 1). Eight treatments were used as follows: CK-N (control + no-duck), CK-D (control + ducks), CF-N (chemical fertilizer + no-ducks), CF-D (chemical fertilizer + ducks), CM-N (chemical fertilizer + organic fertilizer + no-ducks), CM-D (chemical fertilizer + organic fertilizer + ducks), CD-N (chemical fertilizer 30% off + organic fertilizer + no-ducks), and CD-D (chemical fertilizer Agronomy 2018, 8, x FOR PEER REVIEW 3 of 15 fertilizer + ducks), CM-N (chemical fertilizer + organic fertilizer + no-ducks), CM-D (chemical Agronomy 2018, 8, 289 3 of 13 fertilizer + organic fertilizer + ducks), CD-N (chemical fertilizer 30% off + organic fertilizer + no-ducks), and CD-D (chemical fertilizer + organic fertilizer + ducks). The plots covered an area of + organic fertilizer + ducks). The plots covered an area of 21 m (3 m 7 m) and were prepared as 21 m (3 m × 7 m) and were prepared as follows: first, a hole (1.2 m length, 1.1 width) was dug down follows: first, a hole (1.2 m length, 1.1 width) was dug down until the soil below the plough layer was until the soil below the plough layer was reached. A white nylon box (1 m length, 1 m width; open at reached. A white nylon box (1 m length, 1 m width; open at the top and bottom) was placed in the the top and bottom) was placed in the hole. Spacing of 50 cm × 50 cm was provided between rows. hole. Spacing of 50 cm 50 cm was provided between rows. The boxes were distributed in eight The boxes were distributed in eight plots across the open fields with and without ducks. The chosen plots across the open fields with and without ducks. The chosen boxes were given N ammonium boxes were given N ammonium sulfate (20.20% atom enrichment, produced by the Shanghai sulfate (20.20% atom enrichment, produced by the Shanghai Research Institute of Chemical Industry) Research Institute of Chemical Industry) instead of normal ammonium sulfate. Steel fencing was instead used to of separa normal te the duck a ammoniumnsulfate. d no-duc Steel k field fencing s. Duc was ks wer usede re to separate leased to the fa the duck rm a andt the vegeta no-duck fields. tive Ducks were released to the farm at the vegetative stage. The fertilizer application rates of different stage. The fertilizer application rates of different treatments are shown in Table 1. The rates of tr fert eatments ilizers applied are shown were in t Table he sa 1me . The inrates bothof fie fertilizers lds. To av applied oid any wer diesea the ses same and in yiboth eld los fields. s, peT st os wer avoide any diseases and yield loss, pests were controlled using recommended pesticides. controlled using recommended pesticides. Figure 1. Schematic of the experimental set up. The N-15 ammonium sulphate fertilizer was applied Figure 1. Schematic of the experimental set up. The N-15 ammonium sulphate fertilizer was applied only in the first box of each treatment from plot 2 to plot 4. Both duck and no-duck fields had a total only in the first box of each treatment from plot 2 to plot 4. Both duck and no-duck fields had a total of twenty-four boxes. Plot 1 refers to CK treatment; plot 2 = CF; plot 3 = CM; plot 4 = CD treatment. Duck and no-duck fields underwent identical treatment. Agronomy 2018, 8, 289 4 of 13 Table 1. Application rates of common fertilizers and N isotope fertilizer in different treatments at the experimental farm. 1 2 15 2 Year Site Treatment Fertilizers Applied per ha (kgha ) Fertilizers Applied (gm ) N Applied (gm ) CK Control with no fertilization Control with no fertilization Control with no fertilization Calcium superphosphate N: 200 Ca(H PO4) : 124.8 ( NH ) SO 2 2 4 2 4 CF P: 90 Potassium sulfate (K SO ): 186.584 35.73 2 4 K: 120 Ammonium sulfate (NH ) SO : 754.2844 4 2 4 Ca(H PO4) : 92.74 Ninghe 2 2 CF: N: 163, P: 67, K: 105 K SO : 163.104 2 4 CM OF: N: 37, P: 23, K: 15 29.03 (NH ) SO : 616.5111 4 2 4 Organic fertilizer: 1194 Organic fertilizer: 2507.4 CF: N: 114, P: 47, K: 74 Ca(H PO ) : 65.06 2 4 2 OF: N: 37, P: 23, K: 15 K SO : 115.064 2 4 CD 20.54 Organic fertilizer: 1194 (NH ) SO4: 429.9333 4 2 (Rapeseed) Organic fertilizer: 2507.4 CK: Control; CF: Chemical fertilizer; CM: CF (Chemical fertilizer) + OF (Organic fertilizer); CD: CF (Chemical fertilizer 30% off) + OF (Organic fertilizer amount unchanged); Organic fertilizer it is estimated at TN: 15.96 g/kg, P O : 24.92 g/kg, K O: 18.1 g/kg. 2 5 2 Agronomy 2018, 8, 289 5 of 13 2.3. Sampling and Measurement Rice and soil samples were collected on 18 October 2017 at the end of the growing season. The soil samples were randomly collected with the auger from three points within each box at 0–20 cm and 20–40 cm depth. Two rice plant samples were randomly chosen inside each box at physiological maturity. During the harvest time, no noticeable crop damage was observed due to weeds, insects, or other diseases. After being harvested, plants were divided into grain, straw, leaf, and root. The last harvest was done on 4 November 2017 in the whole plots to determine rice yield (Tables A1 and A2). A small part of fresh Japonica rice was taken from the large bags and then separated into grain and husk to determine the content of nitrogen-15 isotope from both fresh organs. Soil and plant samples were brought to the laboratory for the analysis. The soil samples were air-dried and ground to pass through 100-mm mesh sieve for the determination of total N, P, NH -N, NO -N, soil organic matter 4 3 (SOM), and N analysis. Soil pH was measured through a 0.9-mm sieve with air-dried soil and 0.01 M of calcium chloride (CaCl ), using a balance (METTLER TOLEDO). Soil total N, total P, NH -N, NO -N, 2 4 3 and rice plant organs were measured by flow injection analysis (Automatic Analyzer AA3 type), and soil moisture content was measured by the oven-drying method (Table 2). To analyze NH -N and NO -N, a 5-g sample of fresh soil was extracted with 50 mL of 1 M KCl by shaking for half an hour, followed by centrifugation and filtering. Soil organic matter (SOM) was measured by the potassium dichromate oxidation method. Soil texture was determined by hydrometer. Rice plant organs, such as grains, leaves, straw, and roots, were oven-dried for three days at 75 C and powdered in order to 15 15 determine the content of nitrogen, phosphorus, and N. The nitrogen content, both unlabeled and N labeled, of rice plant organs, including grain, straw, leaf, and root, were measured. Isotope analysis was carried out with an elemental mass spectrometer. 2.4. Calculation N use efficiency was calculated according to Zhu et al. [16]. 15N uptake NUE(Isotopic method) = 100 (1) 15N input 2.5. Statistical Analysis Statistical analyses were executed by SPSS version 20 statistical software (IBM, Chicago, IL, USA). A one-way analysis of variance (ANOVA) was undertaken to assess differences between duck and no-duck treatments. The means of different treatments were compared based on the least significant difference test (LSD) with multiple comparisons. Significant differences at p < 0.05 between treatments are indicated by different letters. Graphing was performed with Origin 8.5 (Origin Lab) software and MS word was used to generate tables. Agronomy 2018, 8, 289 6 of 13 Table 2. Physicochemical proprieties of soil at the experimental farm. NH -N NO -N Distribution of Soil 4 3 Total N (gkg ) SMC (%) 1 1 (mgkg ) (mgkg ) Particles (%) Total P SOM Year Duck pH 1 1 (Location) (gkg ) (gkg ) Levels 0–20 20–40 0–20 20–40 0–20 20–40 0–20 20–40 Clay Silt Sand cm cm cm cm cm cm cm cm 2017 Duck 7.42 1.03 0.89 0.89 3.92 1.56 25.98 32.94 36.6 34.69 22.75 31.5 62.75 5.75 (Ninghe) No-duck 7.47 0.92 0.91 0.90 2.23 2.01 35.65 26.15 36.9 34.96 19.59 35.25 55.75 9 SMC: soil moisture content; SOM: soil organic matter. Agronomy 2018, 8, x FOR PEER REVIEW 8 of 15 Agronomy 2018, 8, 289 7 of 13 3. Results and Discussion 3. Results and Discussion 3.1. Effect of Duck Presence on Rice Plant Growth 3.1. Effect of Duck Presence on Rice Plant Growth The results indicated that grain N concentration significantly differed among CK, CF, and CM The results indicated that grain N concentration significantly differed among CK, CF, and CM treatment when comparing the presence and absence of ducks in the field (Figure 2A), whereas CD treatment when comparing the presence and absence of ducks in the field (Figure 2A), whereas CD did not differ significantly between conditions. Likewise, straw N concentration did not respond did not differ significantly between conditions. Likewise, straw N concentration did not respond significantly between treatments whether ducks were present in rice field or not (Figure 2B). significantly between treatments whether ducks were present in rice field or not (Figure 2B). Moreover, Moreover, significant differences between treatments were observed in leaf N concentration (Figure significant differences between treatments were observed in leaf N concentration (Figure 2C). 2C). In contrast, root N concentration was not affected by the presence or absence of ducks in the In contrast, root N concentration was not affected by the presence or absence of ducks in the field field (Figure 2D), (Table 3). Grain and straw P concentrations were not significantly affected by the (Figure 2D), (Table 3). Grain and straw P concentrations were not significantly affected by the presence presence and absence of ducks in rice field (Figure 3A,B). In contrast, leaf and root were significantly and absence of ducks in rice field (Figure 3A,B). In contrast, leaf and root were significantly affected affected by the presence of ducks (Figure 3C,D). In most cases, the presence of ducks alone did not by the presence of ducks (Figure 3C,D). In most cases, the presence of ducks alone did not show show differences between treatments. There were, however, some significant differences when differences between treatments. There were, however, some significant differences when comparing comparing duck to no-duck conditions. duck to no-duck conditions. Aa ND (A) (B) Aa Aa Aa Aa Aa Aa Bb Bbc Cc Aa Aa Aa Aa Aa Aa 0 0 Aa (C) (D) Aa Aa Aa Aa Aa 16 Aa Aa Aab Aa Aa Aa Aa Bbc Bc Aa 0 0 CK CF CM CD CK CF CM CD Treatment Treatment Figure 2. Effects of organic fertilizer with duck presence (D) and without duck presence (ND) on grain Figure 2. Effects of organic fertilizer with duck presence (D) and without duck presence (ND) on N content (A), straw N Content (B), leaf N content (C), and root N content (D). Values are mean SE grain N content (A), straw N Content (B), leaf N content (C), and root N content (D). Values are mean (n = 3). Different upper-case letters indicate significant differences at the 5% level among treatments. ±SE (n = 3). Different upper-case letters indicate significant differences at the 5% level among Different lower-case letters indicate significant differences at the 5% level between D and ND at each treatments. Different lower-case letters indicate significant differences at the 5% level between D and field. CK: control, CF: chemical fertilizer, CM: chemical fertilizer + organic fertilizer, CD: chemical ND at each field. CK: control, CF: chemical fertilizer, CM: chemical fertilizer + organic fertilizer, CD: fertilizer 30 off + organic fertilizer. chemical fertilizer 30 off + organic fertilizer. Overall, most of the results indicated that N and P concentrations were higher when ducks were Overall, most of the results indicated that N and P concentrations were higher when ducks present in the field. Our results strongly supported the findings of Zhang et al. [17] who showed that were present in the field. Our results strongly supported the findings of Zhang et al. [17] who the presence of ducks might stimulate rice growth and cause changes of shape, height, stalk thickness, showed that the presence of ducks might stimulate rice growth and cause changes of shape, height, and effective tilling. Duck activities not only stimulate rice growth but can also increase its lodging stalk thickness, and effective tilling. Duck activities not only stimulate rice growth but can also resistance [18]. Comparable results were found by other researchers, although they did not evaluate increase its lodging resistance [18]. Comparable results were found by other researchers, although the effect of ducks on the concentration of rice plant organs such as grain, straw, leaf, and root, but they they did not evaluate the effect of ducks on the concentration of rice plant organs such as grain, found that the presence of ducks on rice land caused increases in rice height, grain number per panicle, straw, leaf, and root, but they found that the presence of ducks on rice land caused increases in rice and grain yield [19]. Moreover, ducks’ movement and feeding activity in rice plots cause variations -1 -1 N content (g.kg ) N content (g.kg ) -1 -1 N content (g.kg ) N content (g.kg ) Agronomy 2018, 8, x FOR PEER REVIEW 9 of 15 Agronomy 2018, 8, 289 8 of 13 height, grain number per panicle, and grain yield [19]. Moreover, ducks’ movement and feeding activity in rice plots cause variations in soil distribution, thus resulting in improved soil physical properties which subsequently improve the root systems of rice plants [20]. Mutual rice–duck in soil distribution, thus resulting in improved soil physical properties which subsequently improve organic farming takes advantage of controlling plant diseases, insect pests, and increases in rice the root systems of rice plants [20]. Mutual rice–duck organic farming takes advantage of controlling production [21]. plant diseases, insect pests, and increases in rice production [21]. ND Aa 1.4 (A) Aa Aa Aa (B) 3.0 Aa Aa Aa 1.2 Aa Aa Aa Aa 2.5 Aa 1.0 Aa Aa Aa Aa 2.0 0.8 1.5 0.6 1.0 0.4 0.5 0.2 0.0 0.0 CK CF CM CD CK CF CM CD (C) Aa Aa Aa (D) 1.4 Aa Aa Aa Aab 1.2 4 Aa Bbc Aa Aa Ccd 1.0 Aa 0.8 Aab Bb 0.6 Bb 0.4 0.2 0.0 0 CK CF CM CD CK CF CM CD Treatment Treatment Figure 3. Effects of the application of organic fertilizer with duck presence (D) and without duck Figure 3. Effects of the application of organic fertilizer with duck presence (D) and without duck presence (ND) on grain P content (A), straw P Content (B), leaf P content (C), and root P content (D). presence (ND) on grain P content (A), straw P Content (B), leaf P content (C), and root P content (D). Values are mean SE (n = 3). Different upper-case letters indicate significant differences at 5% level Values are mean ±SE (n = 3). Different upper-case letters indicate significant differences at 5% level among treatments. Different lower-case letters indicate significant differences at 5% level between D among treatments. Different lower-case letters indicate significant differences at 5% level between D and ND at each field. and ND at each field. 3.2. Effects of Organic Fertilizer in the Field 3.2. Effects of Organic Fertilizer in the Field The highest grain N concentration was observed in CM when ducks were present in the field The highest grain N concentration was observed in CM when ducks were present in the field (Figure 2A). The order of grain N concentration was CK < CD < CF < CM in the presence of ducks and (Figure 2A). The order of grain N concentration was CK < CD < CF < CM in the presence of ducks gradually increased from lower to higher grain N concentration (CK < CF < CM < CD) in the absence and gradually increased from lower to higher grain N concentration (CK < CF < CM < CD) in the 1 1 of ducks. Grain N concentrations ranged from 11.47 gkg to 12.54 gkg with duck presence and −1 −1 absence of ducks. Grain N concentrations ranged from 11.47 g·kg to 12.54 g·kg with duck presence 9.99–12.32 gkg in the absence of ducks. A similar trend was observed in straw and leaf, with the −1 and 9.99–12.32 g·kg in the absence of ducks. A similar trend was observed in straw and leaf, with highest N concentration occurring in CM when ducks were present in the field (Figure 2B,C). Root N the highest N concentration occurring in CM when ducks were present in the field (Figure 2B,C). concentration was higher in CF when ducks were present in rice field (Figure 2D). Root N concentration was higher in CF when ducks were present in rice field (Figure 2D). Moreover, grain P concentration was higher in CM when ducks were present in comparison Moreover, grain P concentration was higher in CM when ducks were present in comparison to to other treatments (Figure 3A,C). By contrast, straw P concentration was higher in CD (Figure 3B) other treatments (Figure 3A,C). By contrast, straw P concentration was higher in CD (Figure 3 B) with duck presence and higher in CM without ducks. P concentration with duck presence ranged with duck presence and higher in CM without ducks. P concentration with duck presence ranged 1 1 1 1 from 2.73–2.91 gkg , 0.79–1.08 gkg , 1.17–1.35 gkg , and 1.61–2.48 gkg in grain, straw, leaf, −1 −1 −1 −1 from 2.73–2.91 g·kg , 0.79–1.08 g·kg , 1.17–1.35 g·kg , and 1.61–2.48 g·kg in grain, straw, leaf, and and root, respectively. In most of our findings, CK showed lower N and P concentrations. Leaf P root, respectively. In most of our findings, CK showed lower N and P concentrations. Leaf P concentration was higher in CM with duck presence and in CD without duck presence, while root P concentration was higher in CM with duck presence and in CD without duck presence, while root P concentration was higher in CF than that in other treatments when ducks were present in the field. concentration was higher in CF than that in other treatments when ducks were present in the field. Root P concentration showed the highest concentration in CK when ducks were absent (Figure 3D). Root P concentration showed the highest concentration in CK when ducks were absent (Figure 3D). 1 1 1 P concentration without ducks ranged from 2.19–2.62 gkg , 0.69–0.92 gkg , 0.99–1.18 gkg , and 3.19–4.41 gkg in grain, straw, leaf, and root, respectively (Table 3). -1 -1 P content (g.kg ) P content (g.kg ) -1 P content (g.kg ) -1 P content (g.kg ) Agronomy 2018, 8, 289 9 of 13 Moreover, the results of this study demonstrated that the combined application of chemical and organic fertilizer (CM) may be beneficial for the growth of rice plants and for maintaining grain N content with higher quantity compared to CF (chemical fertilizer) when applied alone. In addition, a large amount of N content was observed when chemical fertilizer was reduced (CD) when compared to no fertilization treatment. The results were similar to those reported by other researchers, showing that the combined application of organic and inorganic fertilizers is advantageous, making full use of the on-farm organic fertilizers, which is beneficial for increasing crop yield and the maintenance of soil fertility [22]. Therefore, an important strategy to sustain and enhance soil fertility and improve fertilizer utilization efficiency is to combine the application of chemical and organic fertilizers [23]. However, to avoid heavy nitrogen loss and environmental pollution, the nitrogen application should be strictly controlled. Our trial field experiment may help farmers use the appropriate amount and combination of fertilizers. Over-application could be considered a result of N loss and a source of environmental risk. It has been demonstrated in previous research that inappropriate fertilization patterns and excessive use of N fertilizer result in considerable N losses through ammonia (NH ) volatilization and leaching [24,25]. It has also been demonstrated that overuse of chemical N fertilizer may promote soil acidification in the long term [26]. Therefore, decreasing inorganic fertilizer use may solve environmental issues. Other studies showed that decreasing N rates from 0.74 g pot 1 1 (equivalent to the recommended field rate of 150 kg ha ) to 0.44 g pot (equivalent to 60% of the recommended rate) resulted in lower fertilizer N loss rates [27]. Furthermore, the findings of Siavoshi et al. [28] proved that organic fertilizer has a significant influence on growth and productivity in rice. Based on the results, the present study indicated that organic fertilizer strongly influenced rice plant growth compared to chemical fertilizer applied alone. In most cases, organic fertilizer is more suitable for green production of healthy food and may be of lower cost to the environment than chemical fertilizer. 3.3. Total N Content Total N significantly differed between treatment conditions in the 0–20 cm soil layer when ducks were present in the field (Table 4). However, the same soil layer did not respond significantly in the absence of ducks. At the 20–40 cm soil depth, total N was not affected by duck presence or absence. In addition, grain N concentration was not significantly affected by the presence or absence 15 1 of ducks. However, the highest grain N content was observed in CF (14.30 gkg ), followed by CM 1 1 (13.75 gkg ) and CD (13.50 gkg ) when ducks were present in the field. In contrast, when ducks 15 1 were absent, the highest grain N concentration was observed in CM (13.90 gkg ), followed by CF 1 1 15 (12.85 gkg ) and CD (12.25 gkg ), respectively. The grain N concentration order was CF > CM > CD with duck presence and CM > CF > CD without ducks. On the other hand, straw N content significantly differed with and without ducks. The highest straw N content was observed in CM 1 1 (12.70 gkg ) with duck presence. A similar trend was observed in CM with no ducks (9.85 gkg ) when compared to CF and CD. The order for straw N concentration was CM > CF > CD in duck presence and CM > CD > CF without ducks. Moreover, leaf N content significantly differed with and without ducks, while leaf N content did not respond significantly to duck presence. The highest leaf N content was observed in CM with and without duck presence. Our results suggested that the addition of organic fertilizer is a good supply for enhancing and maintaining rice productivity. Moreover, root N content significantly differed among the no-duck conditions and the highest root 15 15 N content was observed in CF without duck presence. In contrast, root N content did not show any difference with duck presence (Table 4). From the above discussion, it is clear that replacing chemical fertilizer with organic fertilizer significantly influenced the growth of rice plant organs. Furthermore, our results are in agreement with the findings of Chen et al. [29], showing that N content in grain was higher when compared with straw N content. Agronomy 2018, 8, 289 10 of 13 Table 3. Results from an ANOVA analysis evaluating the effects of organic fertilizer with and without duck presence on rice plant organs. 1 1 N Content (gkg ) P Content (gkg ) Year Duck Treatment (Location) Presence Grain Straw Leaf Root Grain Straw Leaf Root Aa Aa Aa Aa Aa Aa Aa Bb CK 11.47 0.31 6.13 0.45 14.60 1.04 7.73 0.66 2.73 0.06 0.79 0.05 1.28 0.08 1.85 0.19 Aa Aa Aa Aa Aa Aa Aa Aa CF 12.10 0.26 7.71 1.00 16.68 0.45 9.62 1.16 2.88 0.08 1.04 0.11 1.32 0.01 2.48 0.25 Duck Aa Aa Aa Aa Aa Aa Aa Aab CM 12.54 0.53 7.86 1.04 17.05 1.11 8.56 0.51 2.91 0.17 0.82 0.06 1.35 0.03 1.87 0.23 Aa Aa Aa Aa Aa Aa Aa Bb CD 11.88 0.42 6.67 0.75 14.08 0.95 7.93 0.74 2.88 0.23 1.08 0.22 1.17 0.07 1.61 0.15 (Ninghe) Cc Aa Bbc Aa Aa Aa Bbc Aa CK 9.99 0.37 5.21 0.44 12.05 0.77 7.45 0.34 2.19 0.11 0.83 0.06 1.02 0.03 4.41 0.29 Bbc Aa Bc Aa Aa Aa Ccd Aa CF 5.39 0.48 11.64 0.89 7.78 0.78 2.46 0.13 0.81 0.08 3.19 0.44 10.72 0.33 0.99 0.02 No-Duck Bb Aa Aa Aa Aa Aa Aab Aa CM 11.11 0.38 6.41 1.25 14.81 1.13 9.19 0.62 2.33 0.11 0.92 0.28 1.16 0.06 3.21 0.24 Aa Aa Aab Aa Aa Aa Aa Aa CD 12.32 0.38 5.47 1.52 14.39 0.64 8.94 0.96 2.62 0.08 0.69 0.39 1.18 0.06 3.44 0.38 CK: control, CF: chemical fertilizer, CM: chemical fertilizer + organic fertilizer, CD: chemical fertilizer 30 off + organic fertilizer. Organic fertilizer unchanged amount. Mean SE (n = 3), different small letters within column and capital letters with the same row for treatment indicate significant differences at p < 0.05, according to LSD tests. Table 4. Mean comparison of total N with and without duck presence in the field. 15 1 Total N (gkg ) Year Treatment Duck Presence (Location) 0–20 cm 20–40 cm Grain Straw Leaf Root Aa Aa Aa Aab Bb Aa CF 1.97 0.006 1.34 0.03 14.30 0.00 12.60 0.10 18.00 0.05 14.05 0.20 Bbc Aa Aa Aa Aa Aa Duck CM 0.98 0.0005 0.78 0.006 13.75 0.17 12.70 0.03 20.15 0.02 12.10 0.14 Bb Aa Aa Cc Cc Aa CD 1.06 0.015 0.77 0.001 13.50 0.00 7.52 0.04 14.40 0.03 8.90 0.04 (Ninghe) Aa Aa Aa Aa Aa Aa CF 0.94 0.08 0.86 0.002 12.85 0.01 6.85 0.03 13.25 0.10 9.59 0.02 Aa Aa Aa Aa Aa Aab CM 1.09 0.006 1.47 0.054 13.90 1.11 9.85 0.13 17.55 0.11 8.97 0.004 No-duck Aa Aa Aa Aa Aa Cc CD 1.4 0.024 1.19 0.038 12.25 0.005 7.03 0.02 13.85 0.03 6.04 0.031 CF: chemical fertilizer, CM: chemical fertilizer + organic fertilizer, CD: chemical fertilizer 30 off + organic fertilizer. Organic fertilizer unchanged amount. Mean SE, different small letters within column and capital letters with the same row for treatment indicate significant differences at p < 0.05, according to LSD tests. CK was not considered in the analysis for the N isotope determination. It is known as natural abundance (0.3663 at.% N). Agronomy 2018, 8, 289 11 of 13 15 15 3.4. Fresh Grain N, Husk N Content, N Uptake, and N Use Efficiency There was no significant effect on fresh grain and husk N content resulting from duck presence 15 1 1 (Table 5). However, fresh grain and husk N ranged from 14.2 to 14.4 gkg and 6.2–6.3 gkg , 15 15 respectively. Likewise, fresh grain and husk N uptake and N use efficiency were not significantly 15 1 affected by the presence of ducks. N uptake ranged from 54.90–93.69 kg ha in grain and 1 15 6.43–11.04 kg ha in husk, respectively. N use efficiency ranged from 21.55%–34.61% in fresh grain and 2.61%–4.24% in fresh husk, respectively. We examined the effects of duck presence on fresh grain and husk by using the N tracer technique. Our results demonstrated that fresh grain N concentration was higher than fresh husk N concentration. A trend of stalks > leaves > grains > husks was reported elsewhere [30]. Table 5. Mean comparison of fresh grain and husk with duck presence or absence. NUE 15 1 15 1 N Content (gkg ) N Uptake (kgha ) (Isotopic Method) Year Site Treatment Grain Husk Grain Husk Grain Husk a a a a a a Duck 14.2 0.02 6.3 0.04 54.90 13.41 6.43 1.96 21.55 2.61 Ninghe a a a a a a No-Duck 14.4 0.02 6.2 0.05 93.69 5.97 11.04 1.36 34.61 4.24 Means in each column followed by the same letter are not significantly different at the 5% probability level. Data are means SE (n = 3). NUE: N use efficiency. According to the yield data CK, CF, CM, and CD were simplified to duck and no-duck treatments, respectively. 4. Conclusions N is an essential nutrient for improving crop productivity and is also the most widely applied fertilizer because it is usually considered the main limiting factor in most agricultural systems. However, excessive application of fertilizers may be harmful to the environment and cause N loss to the environment, environmental risks, environmental pollution, and non-point pollution. To ensure high-quality rice, practical and safe production methods and measures should be adopted. Therefore, organic fertilizer is preferred for clean rice production. The results showed that the application of organic fertilizer is the key to maintaining productivity in the soil–rice plant system instead of inorganic fertilizer applied alone. Thus, it is important to consider organic fertilizer when estimating N transfer and transformation in traditional farming and rice–duck farming systems. Moreover, there was no difference between fresh grain and husk N uptake and N use efficiency. However, the ducks’ feces were not examined in our study. Therefore, further studies may require an appropriate technique for examining duck feces. Author Contributions: X.Z. and T.M.E. contributed to the conceptualization of this project. B.Y. and T.M.E. conceived the study design, methodology, and also revised the manuscript. Y.G., B.T., P.C., R.O.M., and L.W. participated in formal analysis, drafting the manuscript, and proof reading the final version. All authors reviewed and approved the final manuscript. Funding: This research received no external funding Acknowledgments: We would like to thank Zheng Xiangqun at Agro-Environmental Protection Institute, Ministry of Agriculture, Tianjin, China, for supporting this project, and also Bo Yang for her valuable suggestions and comments to the manuscript. We are grateful to farmers at Ninghe district for the help of conducting and managing the field experiment. We would like also to thank all members of the Agro-Environmental Protection Institute, Tianjin, China, for their valuable services. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations N Nitrogen-15 P Phosphorus NH -N ammonium nitrogen NO -N nitrate nitrogen 3 Agronomy 2018, 8, 289 12 of 13 Appendix A Table A1. The weight measurement of whole rice plants harvested in all plots at the Ninghe experimental farm in 2017. Duck Presence Treatment Plot No Grain (Small Bag) (g) Husk (Small Bag) (g) Rice (Large Bag) (kg) CK 1 78.62 19.32 7.75 CF 2 63.60 15.10 5.00 Duck CM 3 85.45 21.07 12.80 CD 4 86.74 24.63 12.95 CK 1 77.15 19.72 14.45 CF 2 71.68 17.96 15.50 No-duck CM 3 75.91 20.52 20.25 CD 4 73.41 21.66 16.55 Table A2. Yield and yield components at Ninghe in 2017. 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