
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13181-7
10.1016/j.heliyon.2024.e37150
e37150
Research Article
Application of nano-urea in conventional flood-irrigated Boro rice in Bangladesh and nitrogen losses investigation
Asif Md Abdullah Al asif.1702062@bau.edu.bd
a1⁎
Mahjabin Farah ad1
Singha Sourav Kumar b
Rahman Jahangir Mohammad Mofizur a
Hoque Sheikh Manjura manjura_hoque@yahoo.com
c⁎⁎
a Department of Soil Science, Bangladesh Agricultural University, Mymensingh, 2202, Bangladesh
b Bangladesh Food Safety Authority, Dhaka, 1000, Bangladesh
c Materials Science Division, Atomic Energy Centre Dhaka, Dhaka, 1000, Bangladesh
d School of Science, Western Sydney University, Richmond, NSW, Australia
⁎ Corresponding author. Bangladesh Agricultural University, Mymensingh – 2202, Atomic Energy Centre, Dhaka, 4 Kazi Nazrul Islam Avenue, Dhaka, 1000, Bangladesh. asif.1702062@bau.edu.bd
⁎⁎ Corresponding author. Bangladesh Agricultural University, Atomic Energy Centre, Mymensingh – 2202, Dhaka, Bangladesh. manjura_hoque@yahoo.com
1 These authors contributed equally.

30 8 2024
15 9 2024
30 8 2024
10 17 e371509 5 2024
1 8 2024
28 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Bangladesh stands third in global rice production while complete modernization of rice production is not fully enforced. The boon of nano agriculture might circumvent the challenge of increasing the yield with minimal ecological damage. Nanofertilizer might be one of the solutions to address the problem of modern agriculture confronting environmental hazards owing to the excessive use of synthetic fertilizers by farmers in Bangladesh. We synthesized nanourea by chemical co-precipitation (CP) and hydrothermal (HT) methods in an attempt to develop environmentally friendly nanofertilizers. We characterized the nanourea and confirmed the functionalization of nanohydroxyapatite (nHAP) with urea by scanning transmission electron microscopy (STEM)/EDS mapping. The CP method produced particle dimensions of 45.62 nm for length and 14.16 nm for width. In comparison, the readings obtained through the HT method were around 74.69 nm and 20.44 nm for length and width, respectively. The field application of nanourea demonstrated impressive results, indicating a significant relationship between the particle size of nanourea and its impact on several agricultural factors. The grain yield using traditional synthetic fertilizer (urea) ranged from 6.47 to 6.52 t ha−1 with a very low NUE of 35.8–36.34 %. Contrarily, the grain yield was found from 6.52 to 6.84 t ha−1 and the obtained NUE ranged from 57.58 to 71.0 % using nanourea of the same concentration calibrated with traditional urea by two methods. Additionally, nanourea treatments having 25 % less nitrogen (N) provided higher total N (TN) in grain suggesting possible nutritional enrichment while checking the yield penalty and substantial increase in N use efficiency (NUE). However, further upscaling of this research on a field scale is necessary to confirm the findings.

Highlights

• HAP nanoparticles synthesized via chemical co-precipitation and hydrothermal methods.

• Incorporation of synthetic N fertilizer (urea-H2N-CO-NH2) with HAP nanoparticles.

• First-ever field trial in Bangladesh on flood-irrigated Boro (winter) rice.

• Significant outcomes in terms of environmental safety, with N use efficiency (NUE) increased up to 70 %.

• Traditional urea with 35-40 % NUE requires three splits; nanourea is more efficient with two splits.

Keywords

Nanourea
Scanning transmission electron microscopy (STEM)
EDS mapping
Hydroxyapatite (HAP)
N use efficiency (NUE)
==== Body
pmc1 Introduction

In the quest to solve the expanding global crisis of hunger, food insecurity, and malnutrition, new UN studies highlight a disturbing trend: the world is becoming progressively away from its aim of eradicating these challenges by 2030. The number of people impacted by hunger increased to an estimated 828 million in 2021, up around 46 million from the previous year and 150 million since the commencement of the COVID-19 epidemic [1]. With rice providing a basic food for millions of people, the importance of rice growing in Bangladesh becomes evident against this backdrop. Winter rice, also known as boro rice, is a crucial component of Bangladesh's agricultural environment. Accounting for approximately 53.8 % of the nation's annual rice production [2], Boro rice stands as a cornerstone of the country's agricultural output and food security endeavors. However, against the backdrop of burgeoning global population projections—expected to exceed 9.7 billion by 2050—the imperative to enhance agricultural practices for sustainable rice productivity becomes increasingly urgent.

In order to fulfill rising food demands, the intensification of current agricultural methods has long relied upon the extensive use of chemical pesticides, fertilizers, and crop types that have undergone genetic modification [3]. The use of nitrogen (N) fertilizers to improve soil inadequacies and support crop development is central to this paradigm. Nevertheless, the careless use of traditional nitrogen fertilizers has led to a host of environmental issues, including reduced nitrogen use efficiency (NUE), acidification of the soil, emissions of nitrous oxide, and eutrophication of aquatic environments [[4], [5], [6]]. In the context of paddy fields, nitrogen retention within the soil fluctuates between 10 % and 35 %, with substantial nitrogen losses occurring via gas (ammonia volatilization and denitrification) and water (runoff, leaching, and lateral seepage) pathways [[7], [8], [9], [10]]. Moreover, the proximity of urea application to seeds poses a risk of seedling damage due to ammonia release, further complicating nitrogen management practices [11]. The use of nitrogen (N) fertilizers to improve soil inadequacies and support crop development is central to this paradigm. Nevertheless, the careless use of traditional nitrogen fertilizers has led to a host of environmental issues, including reduced nitrogen use efficiency (NUE), acidification of the soil, emissions of nitrous oxide, and eutrophication of aquatic environments [[12], [13], [14]].

Because of their biocompatibility and biodegradability, hydroxyapatite (HAP) nanoparticles in particular have attracted interest as a promising option for sustainable nutrient delivery in agricultural contexts [[15], [16], [17]]. HAP nanoparticles have several uses, but one that deserves further investigation is as possible nitrogen transporters for plant nutrition Kottegoda et al., 2017. Additionally, it has also been found that nanourea synthesized using HAP and urea has a slower N release rate in the field condition than the conventional urea due to the presence of a moderately strong H-bond [18]. But even with the bright future of nano fertilizers, there is still little study being done in Bangladesh on the manufacture and use of nanourea.

In this vein, the current study endeavors to address this gap by developing two distinct types of nanourea—utilizing nanohydroxyapatite (nHAP) and synthetic N fertilizer (urea)—via chemical co-precipitation (CP) and hydrothermal (HT) methods. Designated as SW and SH, respectively, these nanourea variants are anticipated to exhibit differential characteristics in particle size, shape, and crystallinity, thereby influencing crop yield and nitrogen release dynamics. Moreover, this study undertakes a comprehensive investigation into the interactions between these nanohybrids and the soil-plant environment within a rice field, encompassing diverse agronomic properties. It is hypothesized that the incorporation of urea into the matrix of nHAP will confer slow-release properties upon the resulting nanourea, thereby reducing solubility in field conditions and necessitating fewer application frequencies compared to the recommended rate of the conventional urea in Bangladesh—thus offering potential economic savings for farmers and the nation at large.

2 Materials and methods

2.1 Materials

Calcium hydroxide [Ca(OH)2] (Merck, Germany), phosphoric acid [H3PO4] (RCI Labscan, 85 %) and Ethanol [C2H5OH] (RCI Labscan) were used during the synthesis of nanoparticles. Urea was purchased from the local market to ensure farmer's grade.

2.2 Preparation of nanourea

2.2.1 Preparation of HAP by CP method

Two different precursor substances calcium hydroxide (Ca(OH)2) and phosphoric acid (H3PO4) were selected for the synthesis of HAP [Ca10(PO4)6(OH)2] nanoparticles. At first, 0.6M of H3PO4 solution (1.73 ml, 43.82 ml deionized water) was added dropwise in the suspension of 1M Ca(OH)2 under stirring. The magnetic stirring was carried out at 500 rpm at room temperature for 30 min. The reaction that was used to produce Ca10(PO4)6(OH)2 nanoparticles. The precipitated sample was then washed three times by centrifugation at 13000 rpm for 20 min using deionized water and dried at 75–80 °C for approximately three days.

2.2.2 Preparation of HAP by HT method

A pressure reactor vessel (Parr Instrument Model- 4576, 5000 psi and 500 °C) was used for the synthesis of HAP [Ca10(PO4)6(OH)2] by HT method. Initially, 180 ml of Ca(OH)2 and H3PO4 with the correct stoichiometric ratio was transferred into the reactor vessel. Then the reactor was assembled and placed into its position and the stirring shaft was put into its place. The water line related to the vessel was turned on. The gas pressure was controlled with the Ar gas followed by turning on the heater. The temperature was set at 300 °C, with the accompanying pressure of 1250–1300 psi inside the reactor. The pressure, temperature, and reaction time were monitored in the controller. The reaction was carried out for 4 h. When the reactor temperature came down to room temperature, the reactor vessel was ready to be dismantled. Before dismantling, it was ensured that the pressure inside the reactor chamber was released. The synthesized HAP was collected from the reactor, centrifuged at 12000 rpm for 20 min, and dried for three to 4 h in the hot plate at 75–80 °C.

2.2.3 Synthesis of nanourea

The synthesized HAP nanoparticles by CP and HT were then separately mixed with 30–40 ml of water (enough to wet the whole powder roughly) and sonicated for 45 min to disperse the aggregated and powdered HAP nanoparticles into individual nanoparticles. A saturated urea solution was prepared to mix with the sonicated HAP nanoparticles maintaining a ratio of urea: nHAP to 5:1. The nHAP and urea mixtures were stirred overnight to ensure thorough mixing. Then, the excess liquid was decanted followed by removing excess urea by centrifugation at 11000 rpm for 15 min using deionized water with subsequent drying.

2.3 Characterization of structural properties of nanourea

2.3.1 X-ray diffraction

The XRD patterns of all synthesized samples were recorded using an X-ray diffractometer (XRD), Model: SmartLab SE, Rigaku, Japan. Thermofisher USA's Transmission Electron Microscopy (TEM), model TALOS 200X was used to examine the sample's size and morphology operated at 200 kV.

2.3.2 Fourier transform infrared spectroscopy (FTIR)

The Fourier Transform Infrared Spectroscopy analysis was made using the Jupiter, UK-based STA, 449 F3 FTIR spectroscopy, to determine the chemical structure and the bonding nature of the composites.

2.3.3 Evaluation of thermal properties (thermogravimetric analysis)

A thermogravimetric analyzer (model: TG/DTA6300, Seiko Instruments Inc. EXSTAR6000, Japan) was employed to conduct thermal analysis on various materials within a temperature span of 25–1000 °C under both air and nitrogen environments. This analysis encompassed thermogravimetric, differential thermogravimetric, and differential thermal measurements.

2.4 Field trial

2.4.1 Experimental site and soil description

A field trial of nanourea in Bangladesh was undertaken at the Soil Science field laboratory (Latitude: 24°43′04.8″ N; Longitude: 90°25′32.7″), Bangladesh Agricultural University, Mymensingh. A pot experiment at the field site was carried out where all the biotic and abiotic environmental factors were alike in a real rice field. The soil was collected from a field where typically R-R-R (Boro rice- T. Aus rice- T. Aman rice) cropping pattern is followed and the soil is characterized as a Non-calcareous Dark Gray Floodplain soil (Aeric Haplaquept in US Soil Taxonomy) and belongs to agro-ecological zone-9, Old Brahmaputra floodplain soil [19]. The soil had a silt loam texture, an almost neutral pH (6.4), and a total nitrogen (TN) content was 0.126 %. The area experienced a sub-tropical monsoon climate with average annual temperatures of 26 °C, 1800 mm of rainfall, and 85 % relative humidity.

2.4.2 Experimental setup and cultural management

In a Completely Randomized Design (CRD), the nanourea SW, received four (4) replications and five (5) treatments, whereas, SH received four (4) replications and four (4) treatments. The following treatments were assigned in the pots for this study: T0: Control (no nutrient or fertilizer), T1: Urea (Conventional N fertilizer) at 100 % recommended dose (RD), T2: 100 % SW and SH at 100 % RD of urea, T3: 75 % SW and SH at 75 % RD of Urea, T4: 50 % SW at 50 % RD of Urea. A field layout is provided for better understanding (Fig. 1). Around 600–700 kg of soil was collected from the field and hard clods were broken into numerous small fractions so that the seedlings would not have to face difficulties to be established. Unexpected plants, residues, weeds, and stubbles were removed from the soil before being placed in the pots. The number of pots was 36 of which 20 received SW and 16 received SH nanourea. Pots were filled with around 10 kg of soil, which were air and sun-dried. The area of the pot was 0.064 m2. A composite sample of the dried soil was appropriated for chemical analysis, specifically for pH and TN. Rice was grown during the winter season, popularly known as the Boro season and the variety used was BRRI dhan28 as a trial crop. Seedlings were transplanted on 25 January 2023 in our experimental pots. From that day, seedlings were kept flooded with water for about 15 days for seedling establishment.Fig. 1 Layout of the pots in the field.

Fig. 1

Urea 144 kg N ha−1, TSP (Triple Superphosphate) 21 kg P ha−1, MoP (Muriate of Potash) 60 kg K ha−1, Gypsum 8 kg S ha−1 and Zinc sulphate (ZnSO4) 1.5 kg Zn ha−1. Fertilizers were applied based on the amount of soil in each pot (weight basis). Except for urea, all other chemical fertilizers were applied just before transplanting. Urea was applied in three splits (3) whereas nanourea was applied in two. For each pot, maintaining the BARC fertilizer recommendation, 0.7g TSP, 0.8 g MP, 0.3g Gypsum, and 0.05g ZnSO4 were provided. For T1, a total of 2.09 g of urea was provided maintaining 0.7g for each of the three splits. Nanourea (SW and SH) were applied in T2 in total amounts of 3.03g and 1.51g for each of the two spits due to the 45 % surface coating of urea. For T3 and T4, a total of 2.27g and 1.51g of nanourea were supplied where 1.13g and 0.76g were used respectively for each of the two splits. Split application of urea was maintained conventionally, generally 15, 30, and 45 days after transplanting (DAT) whereas the first and the last split of nanourea were provided at 15 DAT and 40 DAT.

In nursery beds, boro rice seedlings were raised for 35–40 days before being carefully taken out and planted in pots. Each pot contained one seedling. Intercultural operations such as irrigation, weeding, and pest control were carried out as necessary to provide and maintain a conducive environment for regular agricultural growth and development. The crops were harvested when they had reached physiological maturity. The whole seedling from each pot was collected and threshed on a threshing floor.

2.5 Assessment of yield after harvesting

Initial weights of the rice grains and straw were recorded after harvesting. The final weights of the grain samples were appraised after oven drying (65 °C, 72 h) until a constant weight was found. Straw yields were calculated using a sun-dry basis.

2.6 Determination of total nitrogen, nitrogen uptake, and nitrogen use efficiency

A few days after harvesting, we collected soil samples from each pot to determine the final nitrogen concentration in the soil. TN concentration (%) was determined using the semi-micro Kjeldahl method [20].

Total N (TN), N uptake and N use efficiency (NUE) were calculated using the following formulae:TotalN(%)=(Sample−Blank)ml×0.01×0.014×Dilutionfactor×100WeightofSample(g)

NutrientUptake(kg/ha)=NutrientConc.(%)×Drymassproduction(tha−1)100

NUE(%)=NuptakeFertilizedplot(kgha−1)−NuptakeControlplot(kgha−1)TotalNinput(kgha−1)×100

2.7 Statistical analysis

Descriptive statistics such as mean, median, standard deviation, standard error, etc. were calculated to summarize the key features of the data set which provided a comprehensive knowledge about the data's central tendencies, dispersion, and distribution. Analysis of variance (ANOVA) and Tukey–Kramer multiple comparisons test were carried out using Statistix 10 and considered significant at p < 0.05.

3 Results and discussion

3.1 Structural properties of nanourea

3.1.1 XRD

Fig. 2 shows X-ray diffraction patterns of urea, SW, SH, and HAP in the bare state. The lattice parameters, a and c of SW were 6.847 and 9.334 Å, and of SH were 6.877 and 9.394 Å, respectively. The nanourea showed the lattice parameters for SW as 6.831 and 9.329 Å and SH as 6.887 and 9.386 Å, respectively. The grain size and lattice strain of SW in the as-synthesized condition is 9 nm and 0.01349 and for SH 20 nm and 0.0065. The lattice parameters of SH closely match with the bulk hydroxyapatite. The average grain size of SW is much smaller than the SH, while the lattice strain of SW is higher than the SH. The XRD patterns of both SW and SH bear the planes of urea and hydroxyapatite, for instance, 110 and 111 planes of urea and 002, 211, 300, and 202 planes of HAP. Peak shifts were prominent in the XRD patterns of HAP and nanourea revealing functionalization of HAP after the formation of nanourea. There is a large change in the lattice parameters between HAP and nanourea.Fig. 2 XRD patterns show peak overlapping among urea, nanourea, and HAP.

Fig. 2

3.2 Fourier-transform infrared spectroscopy (FTIR)

Fig. 3 shows the FTIR spectrum of several characteristic bands of the HAP, urea, and nanourea synthesized by both chemical co-precipitation and hydrothermal methods. It revealed unique functional groups of urea molecules (N-C-N, H-N-H, and C=O) and HAP nanoparticles (PO43−, HPO42−) [21,22]. In the case of the chemical co-precipitation method, this is evident from the figure that the spectra of urea and nanourea coincided with each other from 1456 cm−1 to 1458 cm−1 for N-C-N bond, 1590 cm−1 to 1622 cm−1 for the functional group of C=O and 3218 cm−1 to 3430 cm−1 for N-H-N. In the case of HAP and nanourea, spectra coincided between 560 cm−1 to 564 cm−1 and 1024 cm−1 to 1026 cm−1 due to the presence of PO43−. Additionally, peaks between 594 cm−1 and 600 cm−1 were aligned with HPO42−. Contrarily for the hydrothermal method, the spectra of urea and nanourea coincided with each other between 1458 cm−1 to 1460 cm−1 due to the presence of N-C-N group, 1590 cm−1 to 1624 cm−1 for C=O bond and 3330 cm−1 to 3430 cm−1 for N-H-N. In the case of HAP and nanourea, spectra coincided between 560 cm−1 to 564 cm−1 and 1022 cm−1 to 1024 cm−1 for PO43−. Additionally, peaks between 594 cm−1 and 600 cm−1 were aligned with HPO42−. Therefore, it can be said that the peak positions of urea, nanourea, and HAP for both methods coincided nearly in a similar manner. However, the spectral shifts in both cases were visible. The nanourea showed spectrum shifts in the bands corresponding to C=O and C-N because of the weak chemical bonding environment and variations in dipole motions. When nanourea was compared to urea, the spectral peak shift in H-N-H (3200-3430 cm−1) and peak widening were detected, indicating an induced change in the HAP nanostructures. When compared to bare HAP nanoparticles, the phosphate sites of the nanourea were found with lower intensities. Intramolecular interactions generated by urea doped on the surface of HAP nanoparticles changed the vibrational rhythm and caused spectrum shifts in the phosphate bands as reported by Sharma et al. [23]. The phosphate sites in the nanohybrids were occupied by the chemical reaction with urea. The data suggested that hydroxyapatite and urea interacted, presumably via hydrogen bonding. Kottegoda et al. [18] reported similar findings.Fig. 3 FTIR spectra of PO43−, HPO42−, NH2, C=O, and N−C−N regions of urea, nanourea, and HAP.

Fig. 3

3.2.1 TEM

Fig. 4 shows the comparison of particle morphology of SW and SH in the TEM images. In both Fig. 4, Fig. 1 represent the rod-shaped and clustered nanourea structures. Due to the hydrophilicity of HAP, SW, and SH were in an elongated spindle-like cluster [23]. The average particle length and width of SH were 74.69 ± 25 nm and 20.44 ± 6.17 nm, respectively which was 38.92 % and 30.88 % larger than SW both in length and width. Figs. 4–1 shows the promoted growth of nanoparticles during hydrothermal synthesis because of high temperature and pressure Gyergyek et al., 2012 [24]. In addition, concentric diffraction rings in selected area (electron) diffraction (SAED) patterns of SH showed the highest crystallinity than SW (Fig. 4-b, 4-2). Synthesis of HAP in high temperature increased crystallinity which also supports our XRD results. A similar result was reported by Huang et al., 2019 [25].Fig. 4 TEM images of SW (a–h) and SH (1–8).

Fig. 4

Fig. 4 (c-h; 3–8) presents STEM-EDS elemental mapping of SW and SH where denotation of C, O, P, Ca, and N is blue, cyan, red, green, and yellow respectively. It can be concluded the presence of all molecules in the nanourea from the images. Images of a high-angle annular dark-field (HAADF) detector depict clear and wide edges of SH due to its high crystallinity.

3.3 Thermal properties of nanourea

Fig. 5 shows the DTA/TGA traces of SW and SH samples in the bare state, urea, and SW and SH. Thermogravimetric traces for urea exhibited three stages: the first 146.4–275.0 °C with a 70.2 % weight loss corresponded to the formation of biuret, eqn (2), the second 308.9–395.6 °C with a 13.3 % weight loss corresponded to the decomposition of urea to biuret and cyanic acid, and the production of a negligible amount of ammelide, eqn (4); and the third >395.6 °C with a 3.2 % weight loss represented the formation of cyanuric acid eqn (3). The reactions are given below:eqn (1) H2N–CO–NH2(m) + heat → NH+4 NCO−(m) → NH3(g) + HNCO (g) … … … … … … … …

Fig. 5 TG/DTA result showing thermal property and weight loss of nanourea and urea.

Fig. 5

Urea ammonium cyanate ammonia cyanic acideqn (2) H2N–CO–NH2(m) + HNCO (g) → H2N–CO–NH–CO–NH2(m) … … … … … … … …

Urea cyanic acid biureteqn (3) CYA(s) + NH3(g) → ammelide (s) + H2O(g) … … … … … … … …

Cyanuric acid ammoniaeqn (4) H2N–CO–NH–CO–NH2(m) + HNCO (g) → ammelide (s) + H2O(g) … … … … … … … …

Biuret cyanic acid.

The decomposition began at the temperature of 146.4 °C, eqn (1) [26], and at about 160 °C, the reaction between intact urea and the urea decomposition product HNCO started to form biuret, eqn (2) [27]. The direct reaction of cyanuric acid that might produce cyanuric acid with accessible NH3 (ammination) was the conceptually simplest method of producing ammelide. This was a seemingly obvious method, but it could only happen in high pressure or at temperatures above 300 °C which existed during the decomposition process [28]. When NH3 was taken out of the system during the commercial preparation of cyanuric acid in a kiln, less ammelide was typically produced. Although this would appear to be compatible with eqn (3), it is more likely that the scenario reflected how cyanuric acid is prepared in the laboratory as opposed to an industrial setting, where temperature gradients in the kiln may provide circumstances that are conducive to the creation of ammelide in this way. The direct reaction of biuret with HNCO, eqn. (4), which resulted in the synthesis of ammelide in our experiment, is a more likely parallel laboratory route that results in cyanuric acid [29,30]. There was an abrupt decrease in weight loss after 395.6 °C, which is the final decomposition temperature of urea [31]. For the nanourea, weight loss decreased gradually due to the loss of associated water and there was a sudden change at the temperature of 276.2 °C where most of the urea was decomposed. This result supports the finding of [32] revealing the attachment of urea nearly 45.9 %. The doses of nanourea were calibrated from this finding (referring to the section on materials and methods).

3.4 Yields of rice grain and straw

All the treatments showed significant effects on rice yields for SW (both grain and straw) (p < 0.001). Grain yields ranged from 4.56 t ha−1 to 6.52 t ha−1 for different treatments (Table 1). Treatment T3 had a significantly higher yield (6.52 t ha−1) than the treatments T1 (6.51 t ha−1) and T2 (6.47 t ha−1) which were statistically alike. In addition, among the nanourea treatments, the highest (6.52 t ha−1) and the lowest (6.20 t ha−1) yields were generated by T3 and T4, respectively, which were statistically different. Considering all the treatments, the lowest yield was found in T0 (4.56 t ha−1). Overall, the grain yields in T3, T2, and T4 were 42.99 %, 42.06 %, and 36.01 % higher than in T0. In the case of straw, treatments also showed significant effects (p < 0.001). Straw yield varied from 5.41 t ha−1 to 7.93 t ha−1 for different treatments. The treatment T1 produced the highest yield (7.93 t ha−1) followed by the second highest T2 (7.21 t ha−1) which were statistically comparable. However, T3 (6.85 t ha−1) and T4 (6.83 t ha−1) were statistically different from T1. Among the urea-HAP treatments, the highest and the lowest straw yields were found in T2 (7.21 t ha−1) and T4 (6.83 t ha−1) which were statistically analogous. Straw yields in T2, T3, and T4 were 33.29 %, 26.73 % and 26.23 % higher than T0.Table 1 Effect of different nanourea treatments on grain and straw yield of Boro rice (Here, T0 = Control, T1 = 100 % Urea, T2 = 100 % urea-HAP, T3 = 75 % urea-HAP, and T4 = 50 % urea-HAP.).

Table 1Treatments	Chemical Co-Precipitation	Hydrothermal	
Grain yield (t ha−1)	Straw yield (t ha−1)	Grain yield (t ha−1)	Straw yield (t ha−1)	
T0	4.56 ± 0.1 c	5.41 ± 0.20 c	4.61 ± 0.14 c	5.47 ± 0.11 d	
T1	6.51 ± 0.09 a	7.93 ± 0.36 a	6.47 ± 0.04 b	8.44 ± 0.07 a	
T2	6.47 ± 0.1 a	7.21 ± 0.02 ab	6.84 ± 0.08 a	7.73 ± 0.11 b	
T3	6.52 ± 0.07 a	6.85 ± 0.18 b	6.43 ± 0.1b	6.97 ± 0.07 c	
T4	6.20 ± 0.12 b	6.83 ± 0.34 b			
Statistics (P < 0.05)					
Treatments	***	***	***	***	

Different results were obtained from SH. Grain yield responded positively to the treatments ranging from 4.61 t ha−1 to 6.84 t ha−1 and the effects were significant (p < 0.001). From Fig. 5, it is evident that T2 showed the highest grain yield (6.84 t ha−1) while T0 supplied the lowest (4.61 t ha−1). The treatments T1 and T3 provided a yield of 6.47 t ha−1 and 6.43 t ha−1 which were statistically similar but differed from T2. Nanourea-treated pots (T2 and T3) showed 48.4 % and 39.5 % higher yields than T0. In the case of straw yield, treatment effects were significant (p < 0.001) and the yield ranged from 5.47 t ha−1 to 8.44 t ha−1. The highest yield was generated by T1 followed by T2, T3, and T0 (7.73 t ha−1, 6.97 t ha−1, and 5.37 t ha−1), respectively, where the latter three were statistically different from T1. Nanourea-treated pots (T2 and T3) provided 41.3 % and 35.6 % higher yields than T0.

In this experiment, the conventional urea fertilizer was provided following the recommended dose in three splits [33] whereas the nanourea was applied in different doses in two splits. There was a large gap of 26 days between the first and the last split of the nanourea application whereas the urea was applied at a 14-day interval. The experiment revealed that a 75 % supply of SW produced a significantly higher yield where 100 % SW and 100 % synthetic urea shared similar kinds of data statistically. For SH, a 100 % supply of nanourea statistically outperforms the grain yield by 100 % synthetic urea. Therefore, the yield penalty was checked by nanourea in both cases. It happened because of the synchronization of the physiological N demand of the rice plants and N supply by the nanourea. Rice plant needs a relatively continuous N supply during their whole life cycle to sustain vegetative and reproductive growth. Only a fertilizer that has the property of a slow-release nutrient supplier can fit this criterion. Higher yield from nano fertilizer was reported by Al-Khuzai & Al-Juthery., 2020 [34] which aligns with the findings of this experiment. Yields can be variable as they depend on several factors, for instance, terrain, soil properties, variety, and climate [35,36]. In terms of straw and grain yield, the synthetic urea showed higher straw yield with lower grain yield compared to the nanourea produced in both methods. More vegetative growth is induced as synthetic urea was applied in three splits up to the 50 days of transplanting, whereas the application of nanourea in two splits created a stressed condition for rice plants to shift their growth from vegetative to reproductive. It was evident that flowering first occurred in nanourea-treated pots. The flowering of rice is delayed by one to four days due to N application and further application of increased amount of N fertilizer led to more late flowering [37]. Therefore, it is better to assess the potentiality of the nanourea from the following perspectives discussed below.

3.5 Nitrogen loss investigations

3.5.1 Total nitrogen concentration

The effects were significant (p < 0.001) for all the treatments on total N in soil (%), total N in grain (%), and total N in straw (%) (Fig. 6). For SW, TN% in soil ranged from 0.09 % to 0.11 % for different treatments. T2 generated significant TN% in soil (0.11 %) followed by T3 (0.10 %), T1 (0.10 %), and T4 (0.09 %) where the latter three were statistically analogous. TN% in soil for the nanohybrid treatments (T2, T3, and T4) were approximately 37.5 %, 25 %, and 12.5 % higher compared to T0, respectively. The residual N was 1 g per pot higher in T2 treatments, estimated to be 200 kg ha−1, than the sole application of urea. TN% in grain ranged from 1.04 % to 1.54 % for different treatments. The highest value of TN% in grain (1.54 %) was found in both T2 and T3 followed by T1 (1.22 %) and T4 (1.11 %) where the latter two were significantly different (p < 0.001). The lowest value was found in T0 (1.04 %) which was different from all other treatments statistically. T2 and T3 showed nearly one and a half times higher TN% in grain compared to T0. Turning to the TN% in straw, the treatments showed significant effects (p < 0.001). TN% in straw ranged from 0.55 % to 0.91 % for different treatments. T2 rendered significant TN% in straw (0.91 %) followed by T3 (0.88 %) and they were statistically comparable. the effect of T4 (0.70 %) was statistically analogous to T1 (0.67 %). The lowest value was found in T0 (0.55 %). TN% in straw for T2, T3, and T4 were around 1.67, 1.62, and 1.28 times higher compared to T0.Fig. 6 Total Nitrogen, Total N uptake, and % NUE obtained from SW and SH treatments (SW = nanourea synthesized by chemical co-precipitation method; SH = nanourea synthesized by hydrothermal method; T0 = Control; T1 = 100 % urea; T2 = 100 % urea-HAP; T3 = 75 % urea-HAP; T4 = 50 % urea-HAP).

Fig. 6

For SH, the effects were also significant (p < 0.001) for all the treatments on total N in soil (%), total N in grain (%), and total N in straw (%). Surprisingly, the treatment T2 had significantly higher N content in soil (0.12 %) compared to T1 (0.10 %) and T2 (0.11 %) where the latter two were statistically comparable. The treatment T0 has a lower amount of soil TN (0.08 %). Therefore, T2 and T3 increased total soil N by 400 and 200 kg ha−1. For grain TN%, the treatment effects were also significant (p < 0.001). The treatment T2 generated the highest TN (1.54 %) followed by T3 (1.47 %) which was statistically analogous. T1 produced 1.29 % TN in grain which was statistically different. T0 showed the lowest TN. In the case of straw, the trend was similar to grain for TN%. The treatment effects were significant (p < 0.001). The highest TN (0.84 %) was generated by T2 followed by T3 (0.78 %) which was statistically analogous. T1 produced 0.64 % TN in grain which was statistically different from T2 and T3. T0 showed the lowest TN in straw. As a slow-release fertilizer, SW & SH perfectly supplied the nutrients according to the plant's necessity and rendered a considerable amount of N in the soil. The treatment T2 of SW and T2 and T3 of SH provided the soil with 200, 400 kg, and 200 kg N ha−1, respectively, which is enough to sustain the next season's rice production.

3.5.2 Total N uptake

The overall uptake of N was significantly influenced by the application of urea and different rates of nanourea (p < 0.001) (Fig. 6). For SW, the uptake of N ranged from 70.26 to 151.44 kg ha−1 for different treatments. The T2 treatment (151.44 kg ha−1) generated the maximum N uptake, which was statistically analogous to the T3 treatment (146.91 kg ha−1), followed by T1 (121.75 kg ha−1), T4 (107.00 kg ha−1), and T0 (70.26 kg ha−1) where the rest three were statistically different. The T2, T3, and T4 treatments had approximately two and one and a half times higher uptake in contrast to T0, respectively. Turning to SH, the N uptake ranged from 73.85 to 155.61 kg ha−1. Here, The T2 treatment (155.61 kg ha−1) also generated the maximum N uptake, followed by T3 (136.04 kg ha−1) and T1 (126.18 kg ha−1) where the rest two were statistically analogous. The treatment T0 showed the minimum uptake (73.85 kg ha−1). The T2 and T3 treatments had approximately two times higher uptake compared to T0.

3.5.3 N use efficiency (NUE)

N use efficiency (NUE) was significantly altered by the treatment effect (p < 0.001) (Fig. 6). The SW treatments generated the maximum NUE compared to SH where T3 of SW achieved maximum use efficiency (71.0 %), which was statistically different from T2 (56.4 %) and T4 (51.1 %) treatment. Besides that, T1 treatment provided the minimum NUE (35.8 %). All the nanourea treatments of SW (T2, T3, T4) showed around 1.5 to 2 times greater NUE compared to urea (T1). In the case of SH, T3 achieved maximum use efficiency (57.6 %), which was statistically equivalent to T2 (56.8 %). However, T1 provided the minimum NUE (36.3 %). All the nanourea treatments of SH (T2 and T3) had shown around 1.5 times greater NUE compared to urea (T1).

Qureshi et al., 2018 [38] reported nanofertilizers as a controlled nutrient supplier that supports the properties of SW and SH. Nano fertilizers are highly reactive due to their large surface area [39,40], helping nutrients spread out in the soil and become more available to plants. This might be the possible rationale for enhanced N uptake in T2 suggesting the treatment provided the rice plants with the necessary N, thus nutrient mining did not take place. The T3 of SW can also be taken under consideration as it showed lower TN in soil but similar TN in grain and straw suggesting possible nutrient mining. In contrast, T3 of SH showed higher TN in soil with nearly similar TN in grain and straw suggesting more consistency in sustaining soil fertility and nutritional enrichment. Amanullah et al., 2020. [41] found that nanofertilizers are available in soil for a longer period often covering the entire growing season. That is why nanourea was able to compensate for the soil TN while boosting the N content in grain and straw. Thus, it can be a sustainable option.

The TN in grain was notably higher in nanourea treatments compared to sole application of urea which certainly enhanced the nutritional quality of the grains. Increased N content in grains will ensure quality food for an increased population by enhancing gene expression and protein synthesis. Deo et al., 2022 [42] observed enhanced N content in grain for nano-DAP and Behboudi et al., 2018 [43] and Cakmak, 2000 [44] recorded similar findings for nano-zinc which may also fit for the nanourea. Additionally, the straw showed higher TN for nanourea treatments than sole urea. Therefore, the harvested straw can be incorporated into the soil as a way of conservation agriculture which may supply N to the next crop readily [45].

Rice plants in the T3 of SW and SH showed the highest NUE (71.0 % and 57.6 %, respectively). Kottegoda et al., 2017 [18] reported 48 %, Sharmila, 2010 [46] reported 58 % and Gupta & Prakash, 2020 [47] reported a three-fold increase in NUE which is aligned with our findings. Since the TN in grain and straw was higher in nanourea treatments while the yield was quite similar to sole urea, N uptake was higher for those treatments generating higher NUE. However, T3 of SH cannot meet the demand for other parameters, for instance, yield, TN in grain and straw, and N uptake compared to T3 of SW. The T3 of SW generated the highest efficiency when 25 % less N was supplied to the rice plants than the conventional practice. However, T3 of SH showed higher N in soil compared to T3 of SW. Higher N content in the soil means lower uptake and lower NUE. It can be due to the much slower N release and dissolution of the N in soil which becomes available to plants, lack of harmonization with the growth stages of rice plants, and way of reaction in the field temperature as they were also synthesized in high temperature.

For controlled-release prilled fertilizer, there are two different ways of N release, (1) fertilizer dissolution within the prilled form, and (2) N diffusion from the prilled form into the soil solution. This process is predominantly mediated by soil temperature, soil moisture, and time, with soil temperature identified as the primary factor determining the controlled release of N from fertilizers [48,49]. Additionally, T4 of SW also generated considerable NUE due to increased uptake by the plants while having 50 % less N rate, indicating maximum N mining from the soil comparing the TN with initial soil N. That means, though the uptake and NUE were higher in terms of dose in T4, N mining in soil occurred simultaneously which is not a sustainable practice.

Overall, the NUE was high for the nanourea treatments (T2, T3, T4) both in SW and SH. Higher NUE means lower doses, supporting the hypothesis of reducing the number of split applications of urea (n = 2) and decreasing the rapid solubility of conventional urea for checking N loss. Thus, T3 of SW will be environmentally feasible as it has higher NUE which prevents N loss through different pathways, for instance, volatilization, leaching, and denitrification which is in agreement with Solanki et al., 2015 [50] and Chhipa & Joshi, 2016 [51]. Also, T3 can be a sustainable solution where lower N input (25 %) will be possible with a similar output of conventional fertilizer. The total soil N content (%) in T3 is similar to T1, however, accounts for nearly similar uptake as T2. The yield of T3 was also higher among the nano urea-treated pots. The NUE of T3 was higher than any other treatment. It is evident, that plants that received T3 treatment had a positive response to the nanourea along with a necessity of N which was supplied by the soil. Therefore, T3 of SW is a sustainable option. For SH, the treatment T2 had higher yield, TN in soil, grain, and straw, uptake, and nearly similar NUE to T3. Therefore, T2 performed the best among different SH treatments. Considering both types of nanourea, T3 of SW with 25 % less N content and two split applications outperforms all other treatments suggesting environmental sustainability.

3.6 Economic feasibility of nanourea

Bangladesh is one of the largest importers of Urea in the world. The annual demand for urea is 2.9 million tons on average where 70 % of the demand is met with import. The average import cost of urea is $645 per ton in Bangladesh which is definitely mining our foreign reserve every year. Additionally, the government of Bangladesh has to subsidize $128 per ton of urea so that farmers can buy them at a lower cost to sustain crop production Uzzaman, 2023 [52]. However, it can be easily comprehended that any disruption in the supply chain of urea will negatively impact the country's overall production system. Therefore, it is necessary to seek alternatives to resolve the issue of sustainable rice production as the security of rice is the security of food for our country. Nanourea can take a leading role here. As nanourea can be synthesized in Bangladesh with the help of local experts and our recommendation is to use 25 % less than the amount farmers are currently using, our foreign reserve can be saved since less amount of urea has to be bought and transported. Farmers also have to buy less amount of fertilizer and transport less of the usual amount. Thus, it can save money for the farmers and reduce the overall cost of production. Moreover, nanourea can be produced in the country so the fertilizer cost would be lower. Therefore, the government doesn't have to subsidize more money here. All we need is to commercialize the production of nanourea. The country has several state-run urea production factories that can take the role of establishing and supplying overall infrastructure for the local experts. Last but not least, the indirect economic saving would be the lower environmental hazards due to the excessive use of urea. The indiscriminate use of urea could lead to different natural hazards that might cost a huge amount of public funds. This can be checked by this nanourea. Further investigation is necessary in this regard.

4 Conclusions

Our research on nanourea has revealed positive implications for sustainable rice production. The study has clarified the structure and properties of nanourea using a thorough set of investigations, including X-ray Diffraction, Fourier Transform Infrared Spectroscopy, and Thermogravimetric/Differential Thermal Analysis. The results have shown that nanourea has a slow-release tendency because of the urea molecules coated on nHAP. This slow-release property is crucial because it tackles the common problem of N loss in conventional urea fertilization methods. The field trial has provided more evidence of the usefulness of nanourea in rice farming. In comparison to standard urea applications, rice plants treated with nanourea have shown improved yields, greater N content in the grains and straw, and much higher NUE. The most effective treatment group, T3 of SW, has suggested that nanourea can provide a sustainable nitrogen supply with a 25 % reduced N input, opening up a promising new path for agricultural methods that are both economically and environmentally sound. Given the growing global need for N fertilizers and the resulting difficulties in preserving soil fertility, minimizing nutrient losses, and assuring sustainable food production, this research is extremely pertinent. A potential answer to these problems can be found in the creative use of nanotechnology, as shown by the effective production and use of nanourea. However, as the very first field trial of nanourea that has been synthesized and trialed in Bangladesh, we acknowledge our limitations. A larger sample size would have given us more data and a robust statistical analysis. That is why we believe that further research is necessary to fully confirm its long-term effects, scalability, and economic sustainability within the diversified ecosystem.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Md Abdullah Al Asif: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Farah Mahjabin: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Sourav Kumar Singha: Writing – review & editing, Methodology, Formal analysis, Data curation. Mohammad Mofizur Rahman Jahangir: Validation, Supervision, Funding acquisition. Sheikh Manjura Hoque: Writing – review & editing, Visualization, Validation, Supervision, Resources, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge Bangladesh Atomic Energy Commission, Dhaka, greatly.
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