Currently, global crop production requires inputs of roughly 105 million tons of nitrogen (N), 20 million tons of phosphorus (P), and 23 million tons of potassium (K)  . The recovery efficiency of applied fertilizer is low: ≤50% for N, <10% for P, and ~40% for K   . However, in recent years, sustainable agriculture has received considerable attention from environmentalists, agriculturists, and consumers  .
Rice constitutes one of the most important staple foods for over half of the world’s population  . It is widely consumed by people throughout the world, irrespective of their race, religion, or political association  . In the central dry zone of Myanmar, rice is grown as a rain-fed crop, but is grown as an irrigated crop in areas where irrigation is available. The dry zones of the country are characterized by clay, sandy loam, and sandy soils that include gravel. According to Hadden  , soil series in dry zones are of low fertility with low organic matter content. Moreover, K levels in agricultural plots located in dry zones are low.
N is required for all non-legume crops on all soil types. In rice cultivation, it is usually applied in the form of inorganic fertilizer, whereas the straw, which is used for animal feed, is seldom returned as the fertilizer to rice fields. This has contributed to a progressive decline in important soil components in agricultural land, a situation worsened by soil-intensive cropping and the use of high doses of chemical fertilizers with little or no addition of organic manure. Consequently, previous fertile soils have become degraded, with limited organic matter and nutrient contents.
In Myanmar, as in other areas of the world, inappropriate fertilization and the excessive use of N fertilizer have resulted in considerable N losses through ammonia (NH3) volatilization and leaching   . Consequently, nitrogen use efficiencies (NUEs) are as low as ~35% (15% ± 20% lower than other major rice growing countries)  . One of the main challenges in field management is to reduce the amount of N fertilizers applied in the field while avoiding N deficiency of the soils. Among the measures recommended for lowering N losses and enhancing NUE are the application of N at a later growth stage  , adjusting the N rate based on chlorophyll readings  , applying controlled-release N fertilizer  , using urease inhibitors  , planting highly efficient rice varieties  , and combined organic and inorganic fertilizer applications  .
Glaser et al.  concluded that losses of soil organic matter can only be replenished in the short term by the application of organic matter such as manures. However, organic manures may be of relatively low nutrient content and thus unlikely to meet the requirement of high-yielding rice varieties when used alone. The combined application of organic and inorganic fertilizers increases nutrient synchrony and reduces losses by converting inorganic N to organic forms  , leading to sustainable productivity   . Narwal and Chaudhary  reported that the combined application of organic manure and chemical fertilizer accelerates microbial activity, increases NUE, and enhances the availability of native nutrients to plants, resulting in higher nutrient uptake rates. A later study confirmed that, to provide plant nutrients in readily available forms and maintain good soil health, resulting in optimum yields, agricultural fields should be treated with both organic manures and inorganic fertilizers  .
As demonstrated by Zadeh  , farmyard manure not only acts as a source of N and other nutrients, but also increases the efficiency of applied N. Cow manure contains sufficient amounts of N and K as well as fibrous materials that favorably regulate soil moisture and temperature in addition to preventing weed growth on soil surfaces  . For rice production, cow manure, but also poultry manure, is a good source of organic matter and can play a vital role in improving soil fertility, by supplying nutrients, especially N, sulfur (S), K, and zinc (Zn)   . Vermicompost has also been considered as a soil additive to reduce the use of mineral fertilizers, because it provides nutrients in the required amounts and increases both cation-exchange and water-retention capacities  . The application of fertilizers and manures of different composition will differentially influence the physical and chemical properties of the soil but ultimately enhance its biological activities  .
In rice, NUE is the main parameter used to determine nutrient uptake  . In Asia, N fertilizer use is very high but in rice cultivation it is inefficient, as only 30% - 40% of the applied N is recovered  . The low NUE of rice reflects N loss in soil-plant systems, which has been explained not only by the dependence solely on chemical fertilizer application, but also by volatilization of NH3, loss of nitrate (NO3) from leaching, inadequate soil conditions, surface run off, soil erosion, environmental factors, and the use of nitrification inhibitors  .
Thus, not surprisingly, annual rice production is facing a sustainability problem due to production practices that often result in the indiscriminate use of chemical fertilizers and pesticides     . Alternative methods of rice production that are more economical and sustainable and can simultaneously improve NUE and minimize N losses are therefore needed. A possible approach to improving soil fertility and thereby crop productivity is the utilization of organic wastes, farm yard manure (FYM), compost, vermicompost, and poultry manures including their combined application with inorganic fertilizers. The use of organic matter to meet the nutrient requirement of crops would not only contribute to sustainable rice production but also ease the economic burden on poor farmers, who are unable to afford the increasingly costly chemical fertilizers. In fact, one of the few constraints on the use of fertilizer in crop production is its increasing cost  . In this study, we investigated the effects of organic fertilizer, alone or in combination with inorganic fertilizer, on N uptake (NU), NUE, and the N recovery efficiency (NRE) in the hybrid rice variety Palethwe-1.
2. Materials and Methods
2.1. Experimental Site
Two field experiments were conducted on the farm of the Department of Agro- nomy, Yezin Agricultural University, Nay Pyi Taw Division, Myanmar (19˚10'N, 96˚07'E) to study the combined effects of various organic manure and inorganic fertilizer levels on NUE and NRE of hybrid rice (Palethwe-1) (Figure 1). The first experiment (dry season) was conducted from January to April 2015 and the second (wet season) was conducted from August to November 2015.
Figure 1. Location of experimental site in Yezin Agricultural University, Nay Pyi Taw.
2.2. Experimental Design and Treatments
A split-plot design with three replications was used in the two field experiments. The size of each experimental plot was 3 × 5 m. The distances maintained between two replications and two plots were 2 and 0.5 m, respectively. Plots treated with inorganic fertilizers (NPK), applied in proportions of 0%, 50%, 75%, and 100% based on the recommended rates of 150 kg N ha−1, 70 kg P2O5 ha−1, and 120 kg K2O ha−1, were designated as the main plots. Plots treated with organic manures (cow manure, poultry manure, and vermicompost; 5 t・ha−1 each) and the no organic manure control, were designated as subplots. The position of each treatment was not changed between the first and second field experiment.
The land was irrigated so as to be easily plowed, subsequently harrowed, and then divided into four parts (as the main plot area) for the application of the different inorganic fertilizers levels. Each main plot was divided into four subplots. Double bunds were created to prevent seepage between adjacent plots. The full amount of organic manure was applied during land preparation. Inorganic fertilizer consisting of urea (as the N source) and muriate of potash (K2O source) was applied in three equal splits: one third before transplanting (baseline), one third at the active tillering stage, and one third at the panicle initiation stage. The full amount of triple superphosphate (P2O5 source) was applied as the basal dose.
2.3. Soil Sampling and Analysis
Baseline soil samples were collected from eight locations in the experimental field at a depth of 0 - 15 cm using a soil sampling tube (5 cm diameter). The samples were spread out, air dried at room temperature, crushed by hand, sifted through a 2-mm mesh sieve, and then analyzed for their physical and chemical properties.
The soil pHH2O (1:5, soil:water) in mass ratio was measured using a pH meter F-51 HORIBA and the 4A1-1:5 soil water suspension method  . Available N (mg kg−1) was extracted using the alkaline permanganate method  , and available P (mg kg−1) was analyzed using the 9C-Olsen P-Malachite green method  and a PD-303UV UV-VIS spectrophotometer. Available K (mg kg−1) was measured after its extraction using 1 N ammonium acetate  followed by atomic absorption flame emission spectrophotometry (AA-6200, Shimadzu). The amount of organic matter (%) was determined using Tyurin’s method  , and the cation exchange capacity (CEC) using the leaching method  . Soil samples were analyzed at the Department of Agricultural Research (DAR) Yezin, Nay Pyi Taw, Myanmar.
2.4. Organic Manure Analysis
The cow manure, poultry manure, and vermicompost were placed in a temperature controlled oven and their nutrient contents then extracted for gravimetric analysis  . Total N was analyzed using the Kjeldahl method  , followed by Kjeldahl digestion and distillation in a Vapodest 20 s distillation system (Gerhardt, Apparate GmbH & Co. KG, Germany). Total P was determined using the molybdovanadate-phosphoric acid method  and quantitated using a Jenway 6305 UV-VIS spectrophotometer. Total K was analyzed according to the wet digestion method  followed by atomic absorption spectrophotometry on an novAA 400 apparatus, and total S was determined using the turbidimetric method  and a Jenway 6305 UV-VIS spectrophotometer. Organic carbon was measured by the loss on ignition method  using a temperature-controlled oven (muffle furnace). The chemical composition of the organic manures (Table 1) was determined at the DAR.
2.5. Crop Management
The rice cultivated in this study was the hybrid rice (Oryza sativa L.) variety Palethwe-1. It is widely used in Myanmar because of its high potential yield. Rice seeds obtained from the farm of Yezin Agricultural University were soaked in water for 24 h and then incubated at 25˚C for 48 h. The sprouted seeds were sown on a well-prepared seed bed using the wet bed method of the International Rice Research Institute. The water level was gradually increased depending on the seedling height. On day 23, the seedlings were transplanted to hills with a spacing of 20 × 20 cm and two seedlings per hill. During the growing season, irrigation as well as insect, disease, and weed control were performed according to standard cultural practices. The plants were harvested at crop maturity, ~92 days after transplantation, in both seasons.
2.6. N Uptake Analysis of Straw and Grain
Samples obtained from two hills in each plot 2 - 3 cm above ground were oven-dried at 70˚C for 48 h and ground to a fine powder. The N contents of the straw and grain in the samples were analyzed separately using the Kjaldehl distillation method  and a Gerhardt Vapodest 20 S (Gerhardt, Apparate GmbH & Co. KG, Germany) at the DAR. Total NU was calculated as the sum of the products of the biomass and the straw and grain concentrations and expressed as kg・ha−1.
2.7. Computation of NUE and Related Parameters
The amount of applied N from the inorganic fertilizer and the organic manures was calculated. Based on the total NU of rice plants, NUE  was calculated as follows:
Table 1. Chemical compositions of different organic manures for field experiments.
Source: Soil and Plant Analysis Laboratory, Soil Science Section, Soil Science, Water Utilization and Agricultural Engineering Division, Department of Agricultural Research (DAR).
and the internal efficiency (IE)  as:
Agronomic N use efficiency (AUE)  was defined as:
The NRE  was calculated as follows:
2.8. Statistical Analysis
The data were evaluated using an analysis of variance. Means comparison among treatments were subjected to Tukey’s HSD test at the 5% probability level. The analyses were carried out using STATISTIX 8 software (Analytical Software, Tallahassee, FL, USA).
3.1. Soil Analysis
The physicochemical properties of the surface (0 - 15 cm) soil at the experimental site are shown in Table 2. The experimental soil was slightly acidic (pH 6.6) under yearly rice cultivation, with a low level of organic matter (1.8%) and a low CEC (8 cmol(+)/kg). There were moderate levels of available N (73 mg∙kg−1) and available P (20 mg∙kg−1), but a low level of available K (77 mg∙kg−1). An evalua-
Table 2. Physical and chemical properties of the surface (0 - 15 cm) profile of soil at the experimental site.
Source: Soil and Plant Analysis Laboratory, Soil Science Section, Soil Science, Water Utilization and Agricultural Engineering Division, Department of Agricultural Research (DAR).
tion of the soil data based on a previous report of the Federal Ministry of Agriculture and Natural Resources  showed that the soil (sandy loam) was of poor quality.
3.2. Effect of the Combined Application of Organic and Inorganic Fertilizers on NU
The differences in NU as a function of inorganic fertilizer level and organic manure type were significant (p < 0.01). The interaction effect of the combined application of organic and inorganic fertilizers was significantly higher (p < 0.01) during the dry than the wet season (Table 3).
During the dry season, NU was highest (162.21 kg・ha−1) in the combined 100% inorganic fertilizer + poultry manure plot (I100-Op) (Figure 2(a)). A similar NU level (143.83 kg・ha−1) was determined in the 100% inorganic fertilizer + vermicompost plot (I100-Ov) and in the 75% inorganic fertilizer + poultry manure plot (I75-Op, 146.65 kg・ha−1). NU was lower (140.22 kg・ha−1) in the 100% inorganic fertilizer + cow manure plot (I100-Oc) but higher than achieved with inorganic fertilizer alone (I100-O0, 116.85 kg・ha−1). With 50% NPK, NU of 136.11 kg・ha−1 was achieved in combination with poultry manure (I50-Op) was similar to that obtained with I75-Oc, I100-Oc, and I100-Ov. By contrast, a decrease in the amount of chemical fertilizer could not be compensated by vermicompost (I75- Ov, 116.28 kg・ha−1; I50-Ov, 116.83 kg・ha−1). However, NU in the I75-Oc and I50-Oc subplots (140.15 and 126.36 kg・ha−1, respectively) was higher than in the combined vermicompost subplots for both I75 and I50. In the complete absence of inorganic fertilizer, optimal NU was not achieved with cow manure, poultry manure, or vermicompost (85.00, 98.72, and 90.00 kg・ha−1, respectively).
The trends in the NU during the wet season were similar (Figure 2(b)), with the highest value achieved with I100-Op (137.23 kg・ha−1), followed by I75-Op (130.97 kg・ha−1) and I50-Op (130.09 kg・ha−1). When half the amount of inorganic fertilizer was used (I50), poultry manure provided a similar NU to that determined with I100-Op (137.23 kg・ha−1). Lower values were achieved with I100-Oc (124.71 kg・ha−1), I75-Oc (122.44 kg・ha−1), and I50-Oc (121.68 kg・ha−1) but the NU of the cow manure-treated subplots was consistently higher than that of the
Table 3. Probability values using ANOVA of nitrogen use efficiency and related parameters of hybrid rice (Palethwe-1) in both season (dry and wet season), 2015.
NU = Nitrogen uptake, NUE = nitrogen use efficiency, IUE = internal use efficiency, AUE = agronomic nitrogen use efficiency, NRE = nitrogen recovery efficiency, ns = non-significant difference.
Figure 2. Nitrogen uptake of hybrid rice (Palethwe-1) as affected by combined application of organic manures and inorganic fertilizers in (a) dry season and (b) wet season, 2015. The histograms with the same letter are not significantly different by the Tukey HSD test (p < 0.05). The bar on each histogram indicates standard deviation. The numbers followed by I show the percentage of NPK applied based on 150 kg N ha−1, 70 kg P2O5 ha−1 and 120 kg K2O ha−1. I = Inorganic fertilizer; Oc = cow manure, Op = poultry manure and Ov = vermicompost.
Ov-treated subplots, even at the highest inorganic fertilizer level (I100-Ov, 120.13 kg・ha−1).
3.3. Effect of the Combined Application of Organic and Inorganic Fertilizers on NUE
In both seasons, the differences in NUE among inorganic fertilizer levels and among organic manures were significant (p < 0.01 and p < 0.05, respectively) (Table 3). As the amount of inorganic fertilizer increased, the NUE of the subplots treated with combined organic and inorganic fertilizer decreased (Figure 3(a) and Figure 3(b)). At 50% NPK, the highest NUE value of 104.13 kg grain ha−1 (kg N applied)−1 was obtained with I50-Op, with similar efficiencies of 102.93 and 101.65 kg grain ha−1 (kg N applied)−1 measured in the I50-Oc and I50-Ov subplots, respectively (Figure 3(a)). NUEs were lower in the I75-Oc, I75-Op, and
Figure 3. Nitrogen use efficiency of hybrid rice (Palethwe-1) as affected by combined application of organic manures and inorganic fertilizers in (a) dry season and (b) wet season, 2015. The histograms with the same letter are not significantly different by the Tukey HSD test (p < 0.05). The bar on each histogram indicates standard deviation. The numbers followed by I show the percentage of NPK applied based on 150 kg N ha−1, 70 kg P2O5 ha−1 and 120 kg K2O ha−1. I = Inorganic fertilizer; Oc = cow manure, Op = poultry manure and Ov = vermicompost.
I75-Ov subplots [75.45, 77.34, and 81.02 kg grain ha−1 (kg N applied)−1, respectively], but also lower in the I75-O0 subplot [80.30 kg grain ha−1 (kg N applied)−1]. The application of any of the manures in combination with 100% NPK during the dry season resulted in NUEs that were lower than those of any of the other tested conditions.
During the wet season, all of the NUEs obtained in the combined applications of organic and inorganic fertilizer were very similar to those of the dry season. The highest NUE was that of the I50-Op subplot [99.00 kg grain ha−1 (kg N applied)−1] followed by the I50-Ov [96.51 kg grain ha−1 (kg N applied)−1] and I50-Oc [91.00 kg grain ha−1 (kg N applied)−1] subplots. The NUE achieved with the various combinations of organic and inorganic fertilizer correlated negatively with the amount of applied inorganic fertilizer (Figure 3(b)).
3.4. Effect of the Combined Application of Organic and Inorganic Fertilizers on the IE
The IE, defined as the grain yield per total N uptake, was significantly (p < 0.01) influenced by both organic and inorganic fertilizer application during both seasons. The interaction between organic and inorganic fertilizers was also significant (Table 3). The highest IE value was 73.37 kg grain (kg NU)−1 in the I50-Ov subplot, followed by 71.88, 68.23, 69.40, 71.90, 70.12, and 67.65 kg grain (kg NU)−1 in the I50-Op, I50-Oc, I75-Op, I100-O0, I100-Oc, and I100-Ov subplots, respectively, with higher, albeit similar grain yields per increasing NU. The IE values in fields treated only with inorganic fertilizer decreased gradually with decreasing amounts of NPK, from 71.90 kg grain (kg NU)−1 in the I100-O0 subplot, to 60.00 and 48.88 kg grain (kg NU)−1 in the I75-O0 and I50-O0 subplots, respectively. In the presence of 50% NPK, the grain yields (kg) per NU (kg) obtained with I50-Op, I50-Oc, and I50-Ov were similar to those of I75 and I100 plus any of the organic manures (Figure 4(a)).
During the wet season, the IE of the I50-Op subplots was also the highest [85.00 kg grain (kg NU)−1], with a slightly lower value in I75-Op subplots [76.00 kg grain (kg NU)−1]. An increase in the amount of inorganic fertilizer to I75 and I100 resulted in similar yields in the fields treated with the organic manures (Figure 4(b)). The IEs of Oc and Ov were the same in both seasons regardless of the inorganic fertilizer level.
3.5. Effect of the Combined Application of Organic and Inorganic Fertilizers on AUE
The AUE was significantly influenced by inorganic fertilizer application (p < 0.01) during both seasons, but was influenced by organic manure only during the dry season (Table 3). The maximum AUE [62.62 kg additional grain yield (kg N applied per∙ha)−1] was achieved in the I50-Op subplot, but with similar values of 56.25 and 55.37 kg additional grain yield (kg N applied per∙ha)−1 determined in the I50-Oc and I50-Ov subplots, respectively. A 50% NPK application alone resulted in a lower AUE [I50-O0, 22.22 kg additional grain yield (kg N applied per∙ha)−1]. As the amount of applied N from inorganic fertilizer and organic manures increased, the AUE decreased gradually, due to the similar yields among I50, I75, and I100 subplots treated with the different manures. The lowest AUE was that of the I100 plus manures subplot, because of the high inputs of N from both organic and inorganic fertilizers vs. the grain yield (Figure 5(a)). The overall trends in the AUE during the wet season were the same, with the highest value in the I50-Op subplot [45.00 kg additional grain yield (kg N applied per∙ha)−1] followed by 40.00 and 39.95 kg additional grain yield (kg N applied per∙ha)−1 for the I50-Oc and I50-Ov subplots, respectively (Figure 5(b)).
3.6. Effect of the Combined Application of Organic and Inorganic Fertilizers on the NRE
The application of inorganic fertilizer significantly influenced the NRE, both during the dry (p < 0.01) and the wet (p < 05) seasons. The differences in the NRE achieved with the different manures were also significant (p < 0.01). The NRE was highest in the I50-Op subplot [51.25 kg additional NU (kg N applied)−1] but the differences compared to the other subplots were not significant with values of 45.63, 43.66, 44.61, and 43.95 kg additional NU (kg N applied)−1 for I50-Oc, I75-Oc, I75-Op, and I100-Op, respectively. Generally, the NRE decreased steadily with increasing amounts of N from organic and inorganic fertilizers. In the sandy loam soil, the high losses of N in I100-Oc, I100-Op, and I100-Ov resulted in lower NREs of 32.96, 43.95, and 35.08 kg additional NU (kg N applied)−1, re-
Figure 4. Internal use efficiency of hybrid rice (Palethwe-1) as affected by combined application of organic manures and inorganic fertilizers in (a) dry season and (b) wet season, 2015. The histograms with the same letter are not significantly different by the Tukey HSD test (p < 0.05). The bar on each histogram indicates standard deviation. The numbers followed by I show the percentage of NPK applied based on 150 kg N ha−1, 70 kg P2O5 ha−1 and 120 kg K2O ha−1. I = Inorganic fertilizer; Oc = cow manure, Op = poultry manure and Ov = vermicompost.
Figure 5. Agronomic N use efficiency of hybrid rice (Palethwe-1) as affected by combined application of organic manures and inorganic fertilizers in (a) dry season and (b) wet season, 2015. The histograms with the same letter are not significantly different by the Tukey HSD test (p < 0.05). The bar on each histogram indicates standard deviation. The numbers followed by I show the percentage of NPK applied based on 150 kg N ha−1, 70 kg P2O5 ha−1 and 120 kg K2O ha−1. I = Inorganic fertilizer; Oc = cow manure, Op = poultry manure and Ov = vermicompost.
spectively) (Figure 6(a)).
During the wet season, the highest NUE was also in the I50-Op subplot [48.06 kg additional NU (kg N applied)−1], in accordance with its higher NU. Nonetheless, similar NREs of 43.98 and 37.96 kg additional NU (kg N applied)−1 were obtained in the I50-Oc and I50-Ov subplots, respectively. Because of the high N losses, the NREs in the I75 plus manures and I100 plus manures subplots were lower. In the subplots treated with 100% inorganic fertilizer alone, the NRE was lower [24.11 kg additional NU (kg N applied)−1] than in the I50-Op [48.06 kg additional NU (kg N applied)−1] subplot (Figure 6(b)).
Figure 6. Nitrogen recovery efficiency of hybrid rice (Palethwe-1) as affected by combined application of organic manures and inorganic fertilizers in (a) dry season and (b) wet season, 2015. The histograms with the same letter are not significantly different by the Tukey HSD test (p < 0.05). The bar on each histogram indicates standard deviation. The numbers followed by I show the percentage of NPK applied based on 150 kg N ha−1, 70 kg P2O5 ha−1 and 120 kg K2O ha−1. I = Inorganic fertilizer; Oc = cow manure, Op = poultry manure and Ov = vermicompost.
4.1. Beneficial Effects of Combined Inorganic and Organic Fertilizer Application
This study compared the effect of inorganic fertilizer application, as recommended by Myanmar’s DAR for the hybrid rice variety Palethwe-1 (150 kg N ha−1, 70 kg P2O5 ha−1, 120 kg K2O ha−1), with that of the combined application of organic and inorganic fertilizers in terms of NU, NUE, and related parameters. Generally, hybrid rice with a high yield potential has high nutrients requirements. However, the use of large amounts of inorganic fertilizers, including N fertilizers  , alone causes high losses and a low recovery of N, especially in sandy loam soil such as that of the study site. These high N losses lead not only to a low NUE but also to environmental pollution and high costs to farmers.
Our study showed that the combined application of organic and inorganic fertilizer effectively enhances the NUE of hybrid rice, as the maximum NUE was achieved in the I50-Op subplot in both seasons. The higher NU achieved with I50-Op resulted in a larger yield, as a high NUE was generated with less N loss, presumably due to the higher nutrient availability and major nutrient content of Op. In fact, this manure had the highest nutrient content of all tested manures. Its use as a soil amendment for agricultural crops was previously shown to provide appreciable quantities of all important plant nutrients  . The application of organic manure not only increases NU through mineralization but also reduces N losses from the soil  . However, in this study, the NUEs for I50-Oc and I50-Ov were generally lower than the NUE of Op, due to their lower NPK content. In this study, as the amount of inorganic fertilizer was increased, the NUE of each combined application steadily decreased, with a very low NUE recorded in the I100-O0 subplot. This result was consistent with the previously reported low NUE of chemical N fertilizer due to heavy losses  . By contrast, Xu et al.  reported a higher NUE following the combined application of organic and inorganic fertilizers than with chemical fertilizers alone. In fact, the yields obtained in the I50, I75, and I100 subplots to which manures were added did not differ substantially. A similar trend was seen in terms of NU, with the maximum value occurring with I100-Op, which suggested that the use of Op with chemical fertilizer can minimize N losses. However, with 100% inorganic fertilizer alone, NU was low and N losses were high, resulting in yields lower than those achieved with I100-Op, I100-Oc, and I100-Ov. Myint et al.  concluded that the high level of plant available nutrients in organic manure was a product of slow nutrient release throughout the growing season. Liu et al.  reported that the combined application of organic and inorganic fertilizers improved the NUE of fertilizer N and reduced N losses; however, in the subplots without inorganic fertilizer (I0-Oc, I0-Op, and I0-Ov), NU in both seasons was low, which demonstrated that organic manure alone cannot provide optimum NU and high grain yields. This finding is in agreement with that of Bair  , who found that for sustainable crop production higher crop yields were not possible using organic manure alone.
4.2. Correlations between Applied N and N Accumulation in the Treated Subplots
The correlation coefficient (R2) between applied N and N accumulation among the subplots treated with organic and inorganic fertilizer was significant (p < 0.05) (Table 4). In both seasons, applied N correlated positively with N accumulation for each combined application, especially in the Op subplot, in which the strongest the correlation was measured for both the dry and the wet season. Generally, the NU of applied manures depends on their mineralizable N content, which together with the nutrient content is higher in poultry manure than in
Table 4. Regression equations and coefficients of determination between applied N (kg・ha−1) and N uptake (kg・ha−1) of hybrid rice (Palethwe-1) as affected by combined application of organic and inorganic fertilizers in both season (dry and wet season), 2015.
y = N accumulation (kg・ha−1), x = Applied N (kg・ha−1), ** indicates significance at p = 0.01, * indicates significance at p = 0.05.
cow manure or vermicompost  .
However, in the subplots receiving inorganic fertilizer alone, the correlation between applied N and N accumulation was relatively weak, reflecting N losses from the sandy loam soil. Together, these results recommend the use of combined applications of organic and inorganic fertilizer, especially Op, to maximize NU and therefore the yield of hybrid rice (Palethwe-1).
The IE of Palethwe-1 hybrid rice was highest in the I50-Op subplot, because of the higher grain yield and higher NU. With the application of increasing amounts of inorganic fertilizer, the IE decreased, as the yields of the manure-treated I50, I75, and I100 subplots were similar. The lower NU of the Oc and Ov subplots accounted for their lower IEs, which were still higher than the IE in the subplots treated with inorganic fertilizer alone (I50-O0, I75-O0, and I100-O0). Liu et al.  also reported that the integrated use of organic waste and chemical fertilizer was more effective than either one alone.
A lower AUE was obtained only for 100% inorganic fertilizer alone whereas the values with I100-Oc, I100-Op, and I100-Ov were similar. Although the amount of applied N from organic and inorganic fertilizer was maximized, the yields of the different subplots were similar. Thus, half the amount of inorganic fertilizer resulted in the maximum AUE in both seasons when poultry manure was also applied. This result demonstrates that the maximum yield of hybrid rice depends on the application of the optimal amount of applied N from organic as well as inorganic fertilizer. In addition to N, organic manure provides important micronutrients and increases the CEC of soil, which in turn improves nutrient availability and, in combination with inorganic fertilizers, enhances plant growth and grain yield  . The higher grain yield associated with the higher NU correlated with the higher recovery efficiency of the applied N. The high NRE of the I50-Op subplot may have been due to the delayed the conversion of organic N to mineralizable N, leading to slow N release and a better match between the N supply and demand of hybrid rice. Poultry manure acts as a slow-release fertilizer, supplying nutrients, especially N, on a continuous basis throughout the growing season  . However, with the combined application of I75 and I100 and Op, Oc, or Ov, high N losses were incurred. A decrease in the nutrient recovery percentage with increasing levels of fertilizer application was also reported by Culley et al.  , Motavalli et al.  , and Cusick et al.  , and low NRE in rice plants treated with cow manure was reported by Uenosono et al.  and Nishida et al.  , among others.
In our experiments, 5 t∙ha−1 of each manure (cow manure, poultry manure, and vermicompost) was combined with different amounts of inorganic fertilizer. Both cow and poultry manures are readily available in Myanmar and are thus economical for the country’s farmers. The use of vermicompost is less wide spread because of inadequate production and the high cost. Efficient and inexpensive methods of organic and inorganic fertilizer application are important for Myanmar farmers. Our results demonstrate that 50% inorganic fertilizer applied in combination with manures, especially poultry manure, is sufficient to provide high but economical hybrid rice yields. Specifically, in the central dry zone of Myanmar, the use of I50 (75 kg N ha−1) + Op (5 t poultry manure ha−1) will greatly enhance the NU, NUE, and NRE of hybrid rice. If poultry manure is not easily available, cow manure (5 t∙ha−1) plus I75 (112.5 kg N ha−1) is the next best alternative for reducing chemical fertilizer use. Antil and Singh  also reported that the application of organic manures, such as cow or poultry manure, in conjunction with mineral fertilizer was very effective in sustaining crop productivity. Moreover, the integrated use of organic and inorganic fertilizers also has environmental benefits, as it reduces both chemical fertilizer usage and N losses, and allows for sustainable crop production  . Further evaluation of the different available manures and their interactions with inorganic fertilizers, together with the development of new rice varieties, will lead to continued improvements in rice production throughout the world.
This study was supported by Japanese Government (MEXT) Scholarship Program 2016-2019, Japan. We thank to Dr. Kyaw Kyaw Win (Professor) and Kumudra Win Mg (M. Agr. Sc), Department of Agronomy, Yezin Agricultural University, Myanmar for their contribution to this research.
Conflict of Interest: Disclosure Statements
I have disclosed that there are no conflicts of interest regarding publication of this article.
*Equally contributed to this research.
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