Contributions of Nano-Nitrogen Fertilizers to Sustainable Development Goals: Comparison
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Please note this is a comparison between Version 2 by Peter Tang and Version 1 by Abhiram Gunaratnam.

Nano-nitrogen fertilizers (NNFs) have emerged as a promising technology in the field of agriculture, offering potential solutions to improve nutrient uptake efficiency, enhance crop productivity, and reduce environmental impacts. NNFs showed superior characteristics and performance on crops and, therefore, became a potential alternative to conventional nitrogen (N) fertilizers. These fertilizers enhance plant uptake while simultaneously reducing environmental losses. For example, a hydroxy appetite-based urea NNF extended the N release for 112 days, which could cover the N demand of many perennial crops, thus reducing losses. 

  • crop responses
  • environmental impacts
  • nanoparticles
  • smart fertilizers
  • synergist

1. Introduction

The ever-growing population has a substantial impact on food demand and necessitates higher agricultural production [1]. With limited arable land, increasing the agricultural input rates, including nitrogen (N) fertilizers, to obtain higher productivity is seen as a successful way to address the growing food demand [2]. Nitrogen is a major nutrient for plants that primarily influences vegetative growth and crop yield [3]. Although higher levels of N fertilizer are applied with the goal of achieving higher yields, only a small fraction is actually taken up by the plants, while a significant portion is lost through processes such as leaching, runoff, and gaseous emissions, namely N2O, NO, and NH3 [4,5][4][5]. With the main focus on increasing productivity, the negative environmental consequences of increasing the application rate or excessively applying N fertilizers have been often overlooked in the past [6]. However, after several studies showed the detrimental effects of higher N application rates on soil health, water quality, and ecosystem sustainability, a paradigm shift has taken place toward the sustainable use of N fertilizers in agricultural systems.
The current and future agriculture practices should align with the Sustainable Development Goals (SDGs) established by the United Nations to effectively tackle global challenges such as poverty, hunger, climate change, and environmental degradation. Sustainable agriculture plays a crucial role in achieving the SDGs. Achieving SDGs can be facilitated through the precise application of input resources, particularly N fertilizers. Toward this, several approaches are being used, namely split applications, integrated nutrient management, application closer to the root zone, foliar application, the application of fertilizer after substantial development of crops, and the use of smart fertilizers [6,7][6][7]. Of these, smart fertilizers are the latest technology used to minimize nitrogen losses to the environment and increase nitrogen utilization efficiency (NUE).
Smart fertilizers, also known as slow- (SRFs) or controlled-release fertilizers (CRFs), have the ability to release nutrients in a slower manner than conventional fertilizer, and their release pattern matches the crop nutrient demand [8,9][8][9]. Several materials have been used in the formulation of SRFs, and nanomaterials have emerged as a recent addition to this repertoire. These materials are within the range of 1–100 nm. Nanomaterials offer unique properties to improve nutrient releases, such as high surface area, high reactivity, and high porosity, thus advancing the field of fertilizer technology [10,11][10][11]. Owing to the high surface area to volume ratio, nanomaterials adsorb nutrients higher than bulk materials. Hence, the loading capacity of nanomaterials is very high [12]. Nanofertlizers showed higher absorbance than conventional fertilizers when they were used as foliar applications [13]. Furthermore, nanoparticles are applied as synergists along with fertilizers to increase crop performance [10,14][10][14].

2. Materials Used in Nanoparticle Preparation

2.1. Clay Minerals

Clay minerals are used in nanofertilizers due to their unique properties such as high cation exchange capacity (CEC), porous structure, high surface-to-volume ratio, colloidal property, and ease of modification. Additionally, they are readily available and cheap materials compared to their counterparts. Bentonite [15], attapulgite [16], kaoline [17], and glauconite [18] are a few of the clay minerals used for the formulation of NNFs. Bentonite is an aluminum phyllosilicate clay that is primarily composed of montmorillonite, a member of the smectite group of minerals [19]. It is derived from volcanic ash deposits that have undergone weathering and transformation over time. The lamellar structure of bentonite consists of two silica tetrahedral sheets with an alumina octahedral sheet sandwiched in between. The sheet structure of attapulgite contains interlayer spaces that are occupied by various cations. These cations have the ability to be replaced by other cations [20]. Therefore, this provides the opportunity to incorporate several macro and micronutrients with this clay. The CEC of this clay ranged between 60 and 150 meq 100 g−1 of soil [21]. Several NNFs were developed using bentonite as a raw material [11,15][11][15]. In a study, Umar et al. [11,15][11][15] developed an SRF by coating urea with Zn-fortified nano-bentonite. To enhance the presence of Zn2+ on the active sites of bentonite, nano-bentonite was treated with varying concentrations of a ZnSO4 solution. Subsequently, urea was coated using two different methods: firstly, by employing vegetable oil and nano-bentonite (referred to as ZnBenVegU), and secondly, using stearic acid, paraffin oil, Ca(OH)2, and nano-bentonite (referred to as ZnBenParU). The results of a soil incubation study indicated that the release of urea was effectively controlled for up to 10 days with ZnBenVegU and up to 15 days with ZnBenParU. The authors propose that the network structure of bentonite potentially increases the distance water must traverse, thereby reducing urea dissolution. Liu et al. [11,15][11][15] developed an SRF by reacting biochar together with bentonite and polyvinyl alcohol (PVA) and impregnated the urea into it. Through a reaction between the -OH group of bentonites and biochar along with polyvinyl alcohol (PVA), new bonds were formed. The findings of this restudyearch demonstrated that bentonite and urea were successfully incorporated within the cavities and channels of biochar and underwent polymerization with PVA. This impregnation and polymerization process effectively slowed down the dissolution of urea in water for a period of up to 42 days. In another study, urea was intercalated with quaternary ammonium lignin (QAL) modified nano-bentonite and then mixed with sodium alginate to form NNFs [22]. Attapulgite, also known as palygorskite, is a hydrated aluminum–magnesium silicate (Mg,Al)4(Si)8(O,OH,H2O)26·nH2O). This is a naturally occurring nano-clay with a nanorod structure, and the average length and width of this rod are 800–1000 and 30–40 nm, respectively [16]. Attapulgite exhibits a ribbon-layer structure with a 2:1 arrangement in which ribbons are interconnected through the inversion of SiO4 tetrahedra (Figure 1). This arrangement allows for the formation of channels and tunnels [34][23], which could accommodate foreign materials or other minerals. The cation exchange capacity of attapulgite ranged between 11 and 33 meq 100 g−1 of soil [35][24].
Figure 1. A schematic diagram of nanosheet preparation from natural attapulgite [34].
A schematic diagram of nanosheet preparation from natural attapulgite [23].
Various NNFs were created by combining modified attapulgite with urea (WNLCU) and ammonium chloride (WNLCN), along with the addition of sodium polyacrylate (P) and polyacrylamide (M) [16]. Sodium polyacrylate has high water retention ability and polyacrylamide forms a stronger bond with the nanorod structure of attapulgite. Therefore, the addition of these compounds enhances the network structure of the matrix and extends the nutrient release by 66 and 90% more than the control treatment. In a study, modified attapulgite was mixed with polyacrylamide (P) and then loaded into urea or ammonium chloride (NH4Cl) to obtain loss control urea (LCU) or loss control NH4Cl (LCN) [26][25]. This forms a 3D skeleton structure that is mainly formed by modified attapulgite, and P and urea also take part in the structure (Figure 2). The morphology of the 3D skeleton Is pH dependent, and more H-bonds form when pH increases. These fertilizers were reported to decrease N leaching losses by 50% compared to urea and NH4Cl.
Figure 2. Scanning electron microscopy (SEM) images of (a) a micro/nano network surface of LCU, and (b) a transmission electron microscope (TEM) of LCU with a nanorod structure in the skeleton [26][25].
Kaolin, which is also referred to as china clay, is a clay mineral primarily composed of kaolinite. It possesses a white color, soft consistency, and fine-grained texture with a smooth feel. Kaolin originates from the weathering and erosion processes of rocks, specifically those rich in feldspar, like granite. The chemical formula of kaolin is Al2O3·2SiO2·2H2O. The structural arrangement of kaolinite is with one tetrahedral sheet of silica (SiO4) linked through oxygen atoms to one octahedral sheet of alumina (AlO6) octahedra [36][26]. It can interact with other molecules in the inter-layer space. Kaolin and abandoned plastic (polystyrene) nanocomposites were used as coating materials for SRFs [17]. The combination of sodium polyacrylate and polyacrylamide resulted in the formation of a honeycomb structure, as confirmed by SEM images. This structural arrangement led to an increased surface area, enabling effective adsorption and absorption of nutrients, while also functioning as a slow-release fertilizer (SRF). Another NNF was developed by intercalating urea with kaolin and then granulating it with chitosan, as described by Roshanravan et al. [28][27]. Notably, the native form of kaolin was utilized instead of a modified form in this preparation. As a result, the release of urea was extended beyond 30 days, and the NNF exhibited a controlled release of urea that was 60–77% lower compared to urea. Additionally, the study demonstrated that this NNF effectively reduced NH3 release by 6% compared to urea. Similar urea intercalated NNF with kaolin was produced by co-grinding these raw materials [28][27]. This study found that increasing milling speed and milling time increased the incorporation of urea into the amorphous structure of kaolin. Further, increasing milling speed and time prolonged the urea release time from the NNF and the best NNF controlled the release up to 7 days.

2.2. Minerals

Glauconite is a green-colored mineral belonging to the mica group. It is a hydrous potassium, iron, and aluminum silicate mineral ((K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2). It is commonly found in sedimentary rocks, such as sandstones and shales. The structure of glauconite consists of a 2:1 layered arrangement, with an octahedral layer sandwiched between two tetrahedral layers [37][28]. In preparing NNF, Rudmin et al. [28][27] found that glauconite was activated using chemical methods, mechanical methods, or a combination of both. Ammonium dihydrogen phosphate (ADP) was then loaded into the activated glauconite. Various analyses confirmed that ADP molecules were adsorbed by the surface and meso and macro pores, as well as intercalated between layers of glauconite. In a soil leaching column test, this formulation demonstrated an extended release of nutrients for more than 56 days. Zeolites are characterized as three-dimensional, microporous, crystalline solids that possess distinct structures composed of aluminum, silicon, and oxygen within their framework. Additionally, the pores of zeolites contain cations and water molecules [32][29]. Zeolite’s high pore density and anion exchange capacity allow for the incorporation of a significant number of anions within its structure. Two different zeolites, synthesized zeolite clinoptilonite (SZC) and synthesized zeolite montmorillonite (SZM), were prepared using silica and aluminum nitrate and loaded with ammonium nitrate (AN) [30]. The nitrogen release of this NNF was 35% lower than AN. Lateef et al. [28][27] prepared another nano-zeolite with sodium silicate and ethylene glycol, and then sodium nitrate (SN), urea, and other macro and micronutrients were doped into it. The water incubation and soil leaching study exhibited that these fertilizers could extend the N release for more than 7 and 16 days, respectively. Nano ZnO is extensively utilized as a nanomaterial in various applications, including the formulation of NNFs. Due to its commercial availability, nano-ZnO is easily accessible and can be conveniently used in various applications within the agricultural sector. Milani et al. [31] developed nao-ZnO-coated and regular-ZnO-coated urea/mono ammonium phosphate (MAP) NNFs and compared their characteristics. Results revealed that coated urea slightly dissolved and dispersed in the soil compared to the coated MAP, possibly due to the high ionic strength of the urea solution and high pH [38][32]. This study provides evidence that the solubility of nano-ZnO-coated fertilizers is influenced by the acidity generated by the main nutrient used in the fertilizer. Further, this study revealed that ZnO in coated MAP underwent different speciation like ZnSO4, Zn(NH4)PO4, (CaZn2(PO4)2·2H2O), and Zn(OH)2. Interestingly, the speciation of ZnO differs between nano- and bulk-coated ZnO, suggesting that the speciation of ZnO is dependent on particle size. Indeed, the interactions among ZnO, fertilizers, and soil have a significant impact on the formation of various soluble constituents, which, in turn, affect the controlled release ability of the coatings. These reactions play a crucial role in determining the release kinetics and availability of nutrients from the coated fertilizers in the soil environment. In a previous study by these researchers, it was found that Zn solubility was not significantly influenced by the size of the ZnO particle used for coating urea or MAP [33].

2.3. Nano-Biochar

Nano-biochar has gained significant attention in the field of agriculture due to its diverse applications. It has been recognized for its potential in various areas such as wastewater treatments, soil amendment, environmental remediation, pesticide formulation, and nutrient delivery [39][34]. Khan et al. [23][35] produced biochar from wheat straw and prepared the nano-biochar by mechanical grinding. An NNF was prepared by impregnating sodium nitrate and other macro- and micronutrients into nano-biochar. The XRD and FTIR analysis confirmed that nutrients impregnated well into the nano-biochar. It controlled the nitrate release for more than 10 days. In a similar method, corn-based nano-biochar was prepared, and nutrients (N, Ca, P, K, Mg) were impregnated [25][36]. The nutrient release from this NNF extended for more than 14 days.

2.4. Other Nano-Materials

In addition to the materials discussed in this section, other substances, such as nanocellulose [24][37], and hydroxyapatite [29][38], were also used for preparing NNFs. Nanocellulose, derived from cellulose fibers, offers unique properties, including high surface area, biodegradability, and stability, which make it a promising material for NNF formulations [40][39]. Nanocellulose was prepared from eucalyptus pulp and it was mixed with sodium alginate, FeCl3.6H2O (Ferric Chloride; FC), and urea to form a hydrogel as a pH-sensitive NNF [24][37]. Under microscopic examination, the gel without FC (only nanocellulose) displayed a smooth structure, whereas the introduction of FC resulted in an increase in surface coarseness and roughness. The optimum level of FC is important for the correct level of cross-linking. Increasing the FC content above optimum level led to a weakening of the bonding within the gel matrix. This was evident in the nutrient release in water and soil as well. The NNF with 5%, 10%, and 20% FC extended the 80% of urea release by 3.5, 25, and 5 h, respectively. Therefore, this study concluded that 10% of FC and pH 11 was conducive for longer urea retention. Hydroxyapatite (HA) is a naturally occurring mineral form of calcium apatite, and its chemical formula is Ca10(PO4)6(OH)2. Hydroxyapatite finds extensive use in the medical field, specifically in diverse dental applications such as toothpaste, dental fillings, and coatings for dental implants. However, there has been a recent increase in its application within the agricultural sector, especially in developing NNFs [29][38]. The first nano-HA urea was developed with a ratio of urea:HA of 1:1 using the oven drying method [41][40].

3. Methods of Nanoparticle Formulations and Modifications for Preparing NNFs

There are several nanoparticle formulation and modification methods used in the preparation of NNFs, which are summarized in Figure 3.
Figure 3.
Nanoparticle formulation and modification methods.

3.1. Nanoparticle Formulation Methods

There are several methods available for formulating nanoparticles. They are mainly categorized into two methods: top-down and bottom-up approaches. The top-down approach involves breaking down larger particles or bulk materials physically or mechanically to obtain nanoparticles. This approach involves breaking down the starting material into smaller particles through processes such as milling, grinding, or lithography. In the bottom-up approach, nanoparticles are formulated from smaller building blocks or molecular precursors. This approach involves the controlled growth or self-assembly of these building blocks to form nanoparticles with desired properties. The sol–gel method, self-assembly, template-assisted synthesis, microfluidic synthesis, and biomimetic synthesis are a few examples of bottom-up approaches.

3.2. Nanoparticle Modification Methods

The surface properties, composition, or structural changes to increase its desired characteristics are achieved through the modification of nanoparticles. This can be achieved through various methods including surface modification, chemical functionalization, and coating techniques. Several modification techniques, namely high-energy electron beam irradiation, ozone treatment, applying surfactants, and catalytic oxidation, are a few methods employed in the formulation of NNFs. Although the nanonature of attapulgite is beneficial in the formulation of NNFs, the rod structures tend to aggregate with each other due to the high surface area and nano-effect [16,26][16][25]. This necessitates modifications of attapulgite to improve the dispersion and retain its nanocarrier property. Toward this, Zhou, et al. [16] applied high-energy electron beam (HEEB) irradiation to natural attapulgite, which separated the rods from each other and increased the effective surface area (Figure 4). However, Cai, et al. [26][25] applied ozone (O3) oxidation and hydrothermal processes to increase the dispersion of nanorods and increase –OH active sites on the surface. The authors reported that increased active sites might help to form a micro-nano network with urea.
Figure 4.
A schematic diagram of aggregated attapulgite dispersed after irradiation.
Zeolites exhibit high loading ability for anions, but they typically have a low capacity for loading cations onto their pores. However, this limitation can be overcome through structural modifications, which enhance their cation loading capacity. Methods such as applying surfactants and thermal treatments are commonly employed for these purposes [32][29]. In a study conducted by Milani et al. [33], it was observed that nano-ZnO particles exhibited a similar tendency for particle aggregation in water suspensions, forming clumps at the micrometer scale. This aggregation process was found to be faster for nanoparticles compared to bulk particles, primarily due to Brownian motion and the uniform particle size of nano-ZnO [33,42][33][41]. Additionally, the high ionic strength and low surface charge of ZnO also facilitate this aggregation process.

4. Crop Responses for Nano-Nitrogen Fertilizers

Several studies have been conducted to investigate the responses of NNFs on different crops, considering different climate and soil conditions. These studies primarily examined agronomic performance, yield response, nitrogen uptake, and physiological changes.

4.1. Yield Responses

Ever growing population, limited arable land, and declining land productivity pose significant challenges to the agriculture sector. Henceforward, it is crucial to focus on increasing crop yield per unit application of nutrients [1]. Several studies showed that NNFs significantly increased crop yield compared to conventional N fertilizers. For example, Rudmin et al. [18] formulated new nano-ADP-glauconite fertilizers and tested them on oats. It was found that these fertilizers significantly (p < 0.05) increased the yield by 4.6% compared to non-fertilized treatment. The application rate of NNF influences the yield of crops. A greenhouse experiment conducted by Rop et al. [44][42] showed that a lower application rate (266 kg ha−1) of NNFs significantly (p < 0.05) decreased maize yield by 91–191% compared to urea (the application rate was 532 and 1064 kg ha−1). However, maize yield significantly (p < 0.05) increased the yield by 11% for NNFs compared to urea when a higher application rate (1064 kg ha−1) was employed. This observation was consistent with the yield of capsicum and kale in the same study [44][42]. A higher application rate of 1064 kg ha−1 NNFs increased capsicum and kale yield by 14% and 18.6%, respectively, for NNFs compared to conventional fertilizers. In a study, nano-Zno and vegetable oil-coated urea significantly (p < 0.05) increased the grain yield of wheat than non-fertilized wheat [45][43]. Karoline–urea NNFs applied to rice increased the yield by 80% compared to single urea application [46][44].

4.2. Crop Nitrogen Uptake

The uptake of nitrogen by crops is vital for promoting healthy plant growth, protein synthesis, and photosynthesis, ultimately leading to increased crop productivity. Efficient nitrogen uptake also contributes to environmental sustainability by minimizing nutrient waste and reducing the environmental impact of agricultural practices. Studies showed mixed effects of NNFs on crop nitrogen uptake. For instance, 15N labeled urea in an attapulgite sodium polyacrylate polyacrylamide complex was tested against urea alone on corn in a pot experiment [16]. This NNF showed significantly (p < 0.05) higher plant total 15N compared to urea. Rop et al. [44][42] developed a cellulose-graft-poly(acrylamide)/nanohydroxyapatite NNF and tested it on maize, kale, and capsicum. At a 1064 kg ha−1 application rate, NNF significantly (p < 0.05) lowered capsicum herbage N compared to urea. However, maize and kale herbage N were not significantly different between NNF and urea treatment.

4.3. Nitrogen Utilization Efficiency (NUE)

Nitrogen utilization efficiency (NUE) refers to the ability of crops to convert the applied nitrogen to a useful component (i.e., yield). Improving NUE is essential for sustainable agriculture, as it helps minimize N losses, thus reducing environmental pollution and optimizing crop productivity. Only a limited number of studies have reported the NUE of NNFs. For example, a study reported that the NUE of winter wheat increased by 5–22% by applying nano-carbon and nano-CaCO3 as synergists [10]. An HA-urea nanohybrid applied to tea in Sri Lanka significantly (p < 0.05) improved NUE [48][45]. In a field study, an NNF was applied to lettuce as a foliar application at different levels, with ammonium nitrate (AN) as a compensator for AN [52][46]. NUE was significantly (p < 0.05) three times higher for a 100% NNF application than a 100% AN application in both years. Some other studies suggested that this could be due to the regulation of important nitrogen metabolism-related unigenes [56][47].

4.4. Germination of Seeds

Few studies have reported that NNFs contribute to an increase in germination percentage in many plant species. A study reported that a urea/carboxylated nanocellulose NNF increased the germination percentage of wheat by 20–27% compared to urea [24][37]. Nano-ADP-glauconite, an NNF, significantly (p < 0.05) increased the germination percentage of oats by 6% for the best treatment [18]. Badran et al. [51][48] conducted a comprehensive study to examine the influence of urea, ammonium sulfate, and NNFs (urea surface-modified hydroxy appetite nanoparticles) on the germination of almond seeds under varying levels of saline conditions. The study found that the germination rate was significantly (p < 0.05) higher for NNFs compared to other fertilizers at all application rates and under all levels of saline conditions (1, 3, and 5 ds m−1). However, germination percentage significantly (p < 0.05) improved only at higher application levels of NNFs under all saline conditions. Therefore, this study shows that NNFs can be used to alleviate the effect of saline water on seed germination. The higher germination exhibited by NNFs could potentially be attributed to the intake of nanomaterials by seeds, leading to an increase in the micropores for water intrusion that could increase germination [57][49].

4.5. Chlorophyll Content

Chlorophyll is a color pigment that is responsible for capturing light energy during photosynthesis, and its content directly influences energy production and essential metabolic processes. Studies have shown that the application of NNFs enhances chlorophyll content in plants compared to conventional nitrogen fertilizers. For example, a 25% AN and 50% NNF application significantly (p < 0.05) increased the chlorophyll content of lettuce by 10 SPAD compared to AN application alone in two years [52][46]. This observation is directly correlated with the leaf N content. Quaternary ammonium lignin (QAL)-modified nano-bentonite-coated urea applied to tomatoes significantly (p < 0.05) increased the leaf chlorophyll content compared to urea at higher application rates [22]. However, at lower-level applications, the chlorophyll content is only comparable to urea.

4.6. Gene Expression

Several studies reported the positive influence of NPs on gene expression [58,59,60,61][50][51][52][53]. However, only a few studies reported that NNFs induce gene expression, which is beneficial for crops. Glutamine synthase (GS) genes are responsible for N assimilation in crops, which induce nitrate transportation family genes. Yang et al. [10] found that nanocalcium carbonate (NCa) and nano-carbon (NC) synergists applied to wheat with N fertilizers significantly (p < 0.05) increased the expression of GS, such as TaGS1, TaGS2, TaNRT2.2, and TaNRT2.3, compared to N fertilizer alone. Therefore, these synergists increased the N transportation and accumulation in wheat. Chew et al. [62][54] reported that biochar-based nano-iron-fortified compound fertilizer significantly (p < 0.05) downregulated the expression of ammonium transportation genes such as OsAMT1.1, OsAMT1.2, and OsAMT1.3 in roots compared to the control, whereas this NNF significantly (p < 0.05) increased nitrate transporter genes, such as OsNAR2.1 and OsNRT2.3, in roots compared to the control. It is evident that an NNF promotes the transportation of nitrates over ammonium ions.

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