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Deep placement of nitrogen fertilizer improves yield, nitrogen use efficiency and economic returns of transplanted fine rice

Rice (Oryza sativa L.) feeds to two-third of the global population by serving as staple food. It is the main export commodity of several countries; thus, contributes towards foreign exchange earnings. Unfortunately, average global rice yield is far below than its genetic potential. Low nitrogen (N) use efficiency (NUE) is among the major reasons for low average yield. Current study evaluated the impact of nitrogen fertilizer application methods (conventional and deep placement) on growth, yield-related traits, chlorophyll contents, photosynthesis rate, agronomic N-use efficiency (ANUE), partial factors productivity of applied N (PFP) and economic returns of two different transplanted rice varieties (Basmati-515 and Super-Basmati). Fertilizer application methods significantly affected allometry, yield-related traits, chlorophyll contents, photosynthesis rate, ANUE, PFP and economic returns. Deep placement of N-fertilizer (DPNF) observed better allometric traits, high chlorophyll contents, photosynthesis rate, ANUE, PFP, yield attributes and economic returns compared to conventional application of N-fertilizer (CANF). Similarly, Basmati-515 had better allometric and yield-related traits, chlorophyll contents, photosynthesis rate, ANUE, PFP and economic returns than Super-Basmati. Regarding interactions among N-fertilizer application methods and rice varieties, Basmati-515 with DPNF resulted in higher chlorophyll contents, photosynthesis rate, ANUE, PFP, allometric and yield related traits and economic returns than CANF. The lowest values of these traits were observed for Super-Basmati with no application of N-fertilizer. Both varieties had better yield and economic returns with DPNF compared to CANF. It is concluded that DPNF improved yield, ANUE and economic returns; therefore, should be opted to improve productivity of transplanted fine rice. Nonetheless, lower nitrogen doses need to be tested for DPNF to infer whether it could lower N use in rice crop.

Introduction Rice (Oryza sativa L.) feeds two-third of the global population by serving as staple food [1]. Archeological findings suggest that rice was first domesticated in river Yangtze, China some 10,000-14,000 years ago. Globally, rice is cultivated on 167.13 million hectares with an annual production of 782 million tons [2][3][4]. Nonetheless, it is main export commodity of several countries and contributes towards foreign exchange earnings. Rice grains contain thiamine, niacin, iron, riboflavin, vitamin D, calcium, carbohydrates and plant fibers [5][6][7]. The average global rice yield is far below than its genetic potential [8]. Rice cultivation faces numerous abiotic stresses, disease infestation, non-availability of quality seed and low nitrogen use efficiency (NUE). All of these are regarded as major reasons for low average yield [9][10][11][12][13].
Nitrogen (N) is a primary nutrient and often limits plant growth and productivity [14][15][16]. It plays a vital role in growth and development of rice [17]. The N requirement of crop plants, including rice, are high compared to the rest of the essential nutrients [18]. Therefore, excessive N is applied for better plant growth and productivity [19]. It is expected that N demands will be increased by 3-fold in the future [20]. Excessive N application pose several harmful impacts, including increase in nitrous oxide emissions [21], eutrophication [22] and low NUE [23]. Low NUE results in poor economic returns; therefore, must be improved. The NUE and rice yield could be improved with reasonable N management practices [24,25]. Several farm management practices, including modern fertilizer application methods [26] and selection of N-efficient genotypes could improve NUE [20]. Selecting genotypes with high N uptake, utilization and NUE is a challenging task [27]. Nonetheless, genotype selection for improved NUE would reduce expenses incurred on N fertilizers, which will result in low production cost and high economic returns [28].
Efficient use of applied fertilizers, particularly N, is necessary to meet the increasing global demand for rice. Nonetheless, better fertilizer use could minimize the negative environmental impacts and increase farmer profits by many folds. Deep placement (DP) of N-fertilizers could be used in rice crop for multiple benefits, i.e., good crop stand, higher number of tillers, balanced fertilizer use and improved benefit-cost ratio [25,29]. Numerous studies have reported that DP of N-fertilizers (DPNF) improved yield, NUE and economic returns in rice compared to conventional broadcasting method [30][31][32]. However, contrasting reports indicating no significant improvements in yield and economic returns with DPNF also exist [33]. The N placement depth also had a significant impact on NUE and yield of rice crop and 10-15 cm is considered optimum for better NUE and economic returns [31,34,35].
Globally, rice is categorized into two major groups, i.e., fine or non-sticky rice and course or sticky rice. Basmati varieties belong to fine rice category and have better cooking quality [5]. Different basmati verities exhibit significant differences for yield potential, and nutrient uptake and utilization [6,7]. Basmati varieties are mostly transplanted instead of direct seeding to acquire the desired aroma. Standing water in transplanted rice results in N leaching; thus, reduces NUE. Therefore, improving NUE of transplanted fine rice is mandatory to meet the global demands of fine rice. Although, earlier studies have reported that DPNF improved NUE and economic returns of transplanted rice, these studies mostly used coarse varieties. Nonetheless, widely cultivated and marketed rice verities, i.e., Basmati-515 and Super-Basmati have rarely been tested for their NUE and economic returns under various N-fertilizer application methods.
This two-year field study evaluated the impact of different N-fertilizer application methods on growth, allometric traits, yield and economic returns of two transplanted fine rice varieties. It was hypothesized that; i) N-fertilizer application methods will differ in growth, allometric traits, yield and economic returns, ii) rice varieties will differ growth, allometric traits, yield and economic returns and iii) DPNF will result in better growth, allometric traits, yield and economic returns compared to conventional N-application method. The results will help to improve NUE and economic returns of transplanted fine rice.

Experimental site description
The current study was conducted at farmer's field, Kala Shah Kaku during kharif seasons of 2018 and 2019. There were no specific field permit required to conduct the study, as it did not involve any endangered species. The experimental site lies in Kallar tract, which is famous for cultivation of transplanted fine rice and development of desired aroma in the harvested grains. Soil of the experimental site was collected and analyzed before initiating the experiments during both years of the study. The soil properties of the experimental site are summarized in Table 1.

Experimental details
The experiment consisted of two factors, i.e., rice varieties and N-fertilizer application methods. Two fine rice verities, i.e., Basmati-515 and Super-Basmati were included in the experiment due to their widespread cultivation in Kallar tract and high demand due to superior cooking quality. The N-fertilizer application methods, i.e., conventional application (CANF) via broadcasting and deep placement (DPNF) along with no application (control) of N were the second factor of the experiment.

Nursery sowing
Seed of both varieties were procured from Rice Research Institute, Kala Shah Kaku. Nursery was sown on 1 st and 3 rd June during 2018 and 2019, respectively. The plant production and protection measures recommended by Rice Research Institute, Kala Shah Kaku were opted for nursery raising.

Crop husbandry
The experimental site was irrigated to 10 cm depth before seedbed preparation. Seedbed was prepared by ploughing followed by planking after soil reached workable moisture regime. Puddling was done with a manual planker to meet requirements of transplanted rice. One-monthold seedlings were transplanted keeping plant population of 200,000 per hectare. Two seedlings were transplanted per hill. The row-to-row and plant-to-plant distance was kept 22.5 cm.
Fertilizers were applied at the rate of 150 kg ha -1 N, phosphorus (P) and potassium (K). Urea, di-ammonium phosphate and potassium chloride were used as source of N, P and K, respectively. All of the P and K fertilizers were applied at the time of seedbed preparation. The N-fertilizer was applied as broadcasting in three equal splits in CANF method. The 1 st , 2 nd and 3 rd splits were applied at the time of sowing, 20 and 40 days after sowing, respectively. Whole amount of N-fertilizer was manually deep-placed at 15 cm depth one time as a basal fertilizer in DPNF. None of the fertilizers were applied in the control treatment. The recommended plant protection measures were opted to keep the crop free form diseases, insect pests and weeds. The crop was harvested at physical maturity.

Data collection
Allometric traits. Leaf area was recorded at biweekly intervals from 35 days after sowing (DAS) using a leaf area meter (DT Area Meter, model MK2; Delta-T Devices, Cambridge, UK). A 0.5 m 2 area was harvested, weighed and leaf area of pre-weighed leaves was measured. Leaf area index (LAI) was calculated as the ratio of leaf area to ground area. Crop growth rate (CGR) and net assimilation rate (NAR) were computed according to the procedures devised by Hunt [36].
Biochemical attributes. Nitrogen concentration (mg g −1 leaf DW) was recorded by using Dualex 4 Scientific+, Force-A, Orasy, France. Leaf chlorophyll content (mg g −1 leaf) was recorded by SPAD 502 Plus meter. Photosynthetic rate (μmol m −2 s −1 ) was measured by using Infrared Gas Analyzer. Agronomic N use efficiency ANUE (kg grain kg −1 N) and partial factors productivity of applied N (PFP) were recorded by following Sakai [37].
Yield and related traits. Ten spikes were randomly selected, threshed separately and number of gains were counted per spike. Five samples of 1000-grains were randomly taken from the seed lot of each experimental unit, weighed and averaged. All experimental plot were harvested separately and threshed manually for recording grain yield. The grain yield of the experimental units was converted to t ha -1 using unitary method.

Statistical analysis
The collected data were tested for normality and homogeneity of variance, which indicated a normal distribution. Paired t test was used to infer the differences among experimental years, which indicated significant differences. Therefore, data of each year were analyzed and interpreted separately. Two-way analysis of variance (ANOVA) was used to infer significance in the data [38]. Least significant difference test at 5% probability level was used as a post-hoc test to separate the means. Allometric traits were presented in the form of line graphs using means ± standard errors of means. All statistical analyses were performed on SPSS version 20.0 [39] and graphs were produced in Microsoft Excel version 2016.

Economic analysis
An economics analysis was performed to compute economic returns of different fertilizer application methods following Byerlee et al. [40]. The production cost included seed costs, land rent, irrigation, fertilizer cost, land preparation and labor charges. Gross income was computed by multiplying grain yield with existing market price of rice paddy. Total expenses were subtracted from gross income to calculate net income, while the benefit-cost ratio (BCR) was calculated by dividing gross income by the total cost of production.

Allometric traits
Leaf area index (LAI) increased up to 55 days after sowing (DAS) and then declined during both years (Fig 1). Overall, Basmati-515 had higher LAI than Super-Basmati. Deep placement of N-fertilizer (DPNF) resulted in the highest LAI at all sampling dates during both years, which was followed by convention application of N-fertilizer (CANF). Nonetheless, control treatment observed the lowest LAI during both years at all sampling dates (Fig 1).
Different N-fertilizer application methods had significant impact on crop growth rate (CGR) of transplanted fine rice varieties. Higher CGR was recorded for Basmati-515 compared to Super-Basmati during both years of study. The highest CGR was noted for DPNF at all sampling dates during both years, which was followed by CANF. Nonetheless, control treatment observed the lowest CGR during both study years at all sampling dates (Fig 2).

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Net assimilation rate (NAR) was significantly affected by different N-fertilizer application methods. Basmati-515 had higher NAR compared to Super-Basmati during both years. The highest NAR was noted for DPNF at all sampling dates followed by CANF. Nonetheless, control treatment observed the lowest NAR during both years at all sampling dates (Fig 3).

Growth attributes
Different rice varieties, N-fertilizer application methods and their interaction significantly altered growth attributes, including plant height, number of tillers hill -1 , panicle length, number of spikelets panicle -1 , percentage of filled spikelets and 1000-grain weight ( Table 2). Basmati-515 recorded the highest plant height, number of tillers hill -1 , panicle length, number of spikelets panicle -1 , percentage of filled spikelets and 1000-grain weight, whereas the lowest values of these traits were noted for Super-Basmati (Table 2). Similarly, DPNF recorded the

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Deep placement of nitrogen in rice highest values for plant height, number of tillers hill -1 , panicle length, number of spikelets panicle -1 , percentage of filled spikelets and 1000-grain weight, whereas control treatment had the lowest values of these parameters ( Table 2).
Regarding varieties × N-fertilizer application methods' interaction, Basmati-515 with DPNF recorded the highest values of plant height, number of tillers hill -1 , panicle length, number of spikelets panicle -1 , percentage of filled spikelets and 1000-grain weight, whereas Super-Basmati with control treatment had the lowest values of these traits during each year ( Table 2).
Nitrogen uptake, chlorophyll contents, photosynthesis rate, agronomic N use efficiency (ANUE), and partial factors productivity of applied N (PFP) of transplanted fine rice were significantly affected by varieties, N-fertilizer application methods and their interaction. Basmati-515 recorded the highest N uptake, chlorophyll contents, photosynthesis rate, ANUE and PFP

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during both years (Table 3). Similarly, DPNF observed the highest values of N uptake, chlorophyll contents, photosynthesis rate, ANUE and PFP, whereas the lowest values were recorded for control treatment except ANUE and PFP, which were lower in CANF than DPNF during both years (Table 3).
Regarding interactions, the highest N uptake, chlorophyll contents and photosynthesis rate were recorded for Basmati-515 during both years, whereas the lowest values were record for Super-basmati with control treatments. Similarly, Basmati-515 with DPNF observed the

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Deep placement of nitrogen in rice highest values of ANUE and PFP, whereas Super-basmati with CANF recorded the lowest values of ANUE and PFP (Table 3). Different rice varieties, N-fertilizer application methods and their interaction significantly altered grain yield, gross income, net income and benefit-cost ratio during both years of study (Table 4). Basmati-515 had the highest grain yield, gross income, net income and benefit-cost Table 3. The impact of nitrogen fertilizer application methods on nitrogen uptake, chlorophyll contents, photosynthesis rate, agronomic N use efficiency (ANUE), and partial factors productivity of applied N (PFP) of transplanted fine rice. ratio (Table 4). Similarly, the highest values of grain yield, gross income, net income and benefit-cost ratio were recorded for DPNF during both years (Table 4). Regarding interactions, the highest grain yield, gross income, net income and benefit-cost ratio were recorded for Basmati-515 with DPNF during both years, whereas the lowest values were record for Super-basmati with control (Table 4).

Discussion
Different nitrogen (N) fertilizer application methods and transplanted rice varieties, as hypothesized, significantly differed for growth, allometric traits, yield and economic returns. Similarly, the results also confirmed our hypothesis that DPNF would have better allometric traits, yield and economic returns than CANF. The results of both years differed from each other; however, observed the same trend. The differences among years are attributed to the variations in the climatic conditions of the experimental site. Allometric traits of both rice varieties were significantly altered by different N application methods. Overall, DPNF had better allometric traits during both years than CANF. The LAI is an important growth attribute contributing towards photosynthesis. Higher LAI is linked to greater photosynthesis, assimilate partitioning, yield and economic returns. Similarly, CGR and NAR represent the total dry matter accumulated during the growth period of the crop. Higher LAI, CGR and NAR result in better yield and economic returns. Different rice varieties included in the study differed for these traits. The differences among genotypes are explained by their inherent genetic makeup, which enabled them to behave differently. Several earlier studies on different crops have indicated that different varieties exhibit significant differences for growth and allometric traits [41][42][43]. The increase in these traits up to a certain time and then decline is directly linked to crop growth stage. Leaf growth and dry matter accumulation increase with time and then start declining due to senescence. Significant differences were noted in N application methods for allometric traits. These can be explained by better NUE in DPNF compared to CANF. Numerous earlier reports have indicated that DPNF results in better allometric traits than CANF [25,28,34,44]. Better allometric traits under DPNF could be attributed to better N uptake and vegetative growth, which enabled the plants to partition more assimilates than CANF and control treatments of the study.
The DPNF exhibited positive relation with plant height [45][46][47]. Higher number of tillers per hill were noted for Basmati-515 with DPNF, while control treatment resulted in lower number of tillers. Several earlier reports have indicated that DPNF improves tillering capacity of rice genotypes [48,49]. The possible reason of improved tillering capacity under DPNF could be better N uptake and lower leaching and evaporative losses than CANF. Panicle development was directly affect by N-fertilizer application methods. Basmati-515 and Super-basmati with DPNF had longer panicles, whereas both varieties with control treatment observed the lowest panicle length during both years. The positive impact of DPNF on panicle length has been described earlier [50,51]. Nitrogen is regarded among the key nutrients, which could limit the yield potential of cereal crops [15]. Studies have indicated that N application during early panicle differentiation stage increases number of spikletes per panicle and panicle length [52,53]. The N uptake at late panicle differentiation stage and number of spikelets per panicle are linearly associated to each other [54,55]. However, significant variations are observed for amount, timing and method of N application for number of spikelets per spike and panicle length [15,16]. Results demonstrated that DPNF enabled the plants of both varieties to develop the highest numbers of spikelets per panicle, while control treatment resulted in the lowest numbers of spikelets per panicle. The differences among N application methods for spikletes per panicle have been described in earlier studies [56,57].
Empty grains are regarded as a severe problem of rice crop due to kernel abortion [58,59]. If the nutrient are not available at the required time, empty grains are developed in rice. Mostly micronutrients, and especially boron is used to overcome empty grain problem in rice crop [59]. Fertilizer application method is an important factor to reduce the number of empty grain. The current study indicated that DPNF resulted in higher filled spikelets percentage, which contributed towards higher yield of both varieties. On the contrary, control treatment resulted in higher number of empty grains and low numbers of filled spikelets. The empty grains significantly influence rice yield and fertilizer application method overcoming this issue results in better yield [56,60]. The highest 1000-grain weight was noted for DPNF, which was significantly different from control treatment ( Table 2). The differences in 1000-grain weight for various N application methods have been reported in earlier studies [61,62].
Rice crop responds to different N application methods with differential growth and yieldrelated traits [63]. The results of this two-year study indicated that N application in the root zone positively affected growth and other yield-related parameters. The DPNF resulted in improved yield than CANF and control treatments and the same have been reported in earlier studies [64,65]. Nitrogen uptake was higher in Basmati-515 with DPNF, while Super-basmati resulted in lower N uptake. Varietal differences for N uptake have been reported earlier and owed to inherent genetic makeup [66,67]. Chlorophyll contents and photosynthesis rate were higher in both varieties under DPNF. Chlorophyll contents are directly linked to improved N uptake and utilization in DPNF compared to other methods [68,69]. Photosynthesis rate is also linked with improved N uptake and higher chlorophyll contents under DPNF [70,71].
Economic feasibility of any method is mandatory in agricultural crops [72]. The economic analysis indicated that DPNF resulted in higher income and benefit:cost ratio compared to conventional N application method. Higher economic returns of DPNF render it as a feasible technique for transplanted rice. However, lower N doses need to be tested for DPNF to infer whether it could lower N use in rice crop.

Conclusions
Different nitrogen application methods significantly impacted growth, allometry, yield and economic returns of both rice varieties used in the study. Overall, Basmati-515 and deep placement of nitrogen had higher yield and economic returns along with better growth and allometric traits. Nonetheless, deep placement of nitrogen fertilizer improved nitrogen use efficiency compared to conventional nitrogen fertilizer application method. Basmati-515 with deep placement of nitrogen resulted in the highest yield and economic returns during both study years. It is recommended that Basmati-515 with deep placement of nitrogen should be used for higher yield and economic returns of transplanted fine rice. Nonetheless, lower nitrogen doses need to be tested for deep placement of nitrogen to infer whether it could lower N use in rice crop.