Conjoint application of nano-urea with conventional fertilizers: An energy efficient and environmentally robust approach for sustainable crop production

Abstract

One of the biggest challenges to be addressed in world agriculture is low nitrogen (N) use efficiency (<40%). To address this issue, researchers have repeatedly underlined the need for greater emphasis on the development and promotion of energy efficient, and environmentally sound novel fertilizers, in addition to improved agronomic management to augment nutrient use efficiency for restoring soil fertility and increasing farm profit. Hence, a fixed plot field experiment was conducted to assess the economic and environmental competency of conventional fertilizers with and without nano-urea (novel fertilizer) in two predominant cropping systems viz., maize-wheat and pearl millet-mustard under semi-arid regions of India. Result indicates that the supply of 75% recommended N with conventional fertilizer along with nano-urea spray (N₇₅PK+nano-urea) reduced the energy requirement by ~8–11% and increased energy use efficiency by ~6–9% over 100% nitrogen through prilled urea fertilizer (business as usual). Furthermore, the application of N₇₅PK+ nano-urea exhibited ~14% higher economic yields in all the crops compared with N₅₀PK+ nano-urea. Application of N₇₅PK+nano-urea registered comparable soil N and dehydrogenase activities (35.8 μg TPF g^-1 24 hrs^-1 across all crops) over the conventional fertilization (N₁₀₀PK). This indicates that application of foliar spray of nano-urea with 75% N is a soil supportive production approach. More interestingly, two foliar sprays of nano-urea curtailed nitrogen load by 25% without any yield penalty, besides reducing the greenhouse gases (GHG) emission from 164.2 to 416.5 kg CO₂-eq ha^-1 under different crops. Therefore, the application of nano-urea along with 75% N through prilled urea is an energy efficient, environmentally robust and economically feasible nutrient management approach for sustainable crop production.

1. Introduction

Conventional chemical fertilizers are widely used across the globe for achieving maximum yield in agricultural systems to meet the demand of food for ever-growing population. The post green revolution era witnessed an excess and unchecked use of chemical fertilizers across the globe. With an increasing world population [1] and diminishing arable land [2], it is becoming necessary to employ larger quantities of chemical fertilizers [3], especially nitrogen (N) to meet the global food demands [4]. In 2019–20, worldwide total demand of N was 107.4 million tonnes and out of it 76.5% was supplied through urea [5]. The worldwide demand of urea is increasing day by day, which has increased from 147.5 Mt in 2009 to 178.3 Mt in 2019 [5], indicating 20.9% rise in its demand in a decade. The major concern of global urea production and use is environmental pollution. GHG emissions (urea production + use) per kg use of urea is 5.15 kg CO₂-eq [6]. This means globally huge emission of about 918 Mt CO₂-eq took place from total urea production in 2019. Water and specific electricity consumption for the production of a tonne of urea is 12.8 m³ and 173.7 kWh, respectively [7]. Hence, a huge quantity of water (228×10⁷ m³) and electricity (3097×10⁷) consumption took place in 2019 due to the production of 178.3 Mt urea worldwide. Large consumption of water and electricity, and massive emission of GHGs in the urea production may have a severe harmful impact on the environment. Consequently, identifying potential points for improving efficiency of water and electricity and reducing the GHGs emission under urea production system may be one way, but reducing total urea demand over the globe by replacing urea with energy efficient novel fertilizers [8] would be a more viable approach.

Under field condition, N use efficiencies of conventional fertilizers rarely exceed 30–35% [9]. Therefore, application of these fertilizers in excessive amount for supplying nutrient demand of the crops has environmental and ecological consequences [10]. Nutrient losses from agricultural soils in the form of leaching (NO₃¯) or gaseous emissions (NH₃ and N₂O) leads to environmental pollution [11]. Scientists, policymakers, industrialists and farmers are equally concerned over these facts [12], and are actively looking for some alternate novel nutrient sources. For the last two decades, development of nano-fertilizers has been researched upon by various researchers, organizations and institutes [13]. The novel nano-fertilizers are believed to have the potential for a paradigm shift in agriculture [8]. However, so far there is hardly any systematic study that has shown the effect of nano fertilizers or integration of nano-fertilizers with conventional fertilizers on productivity and profitability of crops under field conditions [14, 15]. A very few examples are being seen where nanotechnology has moved from the laboratory to the field level, and even fewer where it has been deployed at the commercial scale [16]. Nano-fertilizers are nutrient carriers [15], developed using the substrates of nano-dimensions (1–100 nm) [17]. The nano-fertilizers possess more surface area [15] to volume size ratio and the feature of surface functionalization [18] along with slow or plant response-based release [19]. Therefore, nano-fertilizers can reduce nutrient losses through leaching and gaseous emissions, ensuring a sustainable production system [20]. The nano-clay-based fertilizer formulations [21] are capable of releasing N for a much longer period [22] than the conventional fertilizers. Coating of fertilizer molecules with nano-membranes also facilitates slow or controlled release fertilizers [23]. The introduction of nano-fertilizer in Indian or world agriculture can prove to be an important step towards sustainable agriculture, which promotes minimalistic use of agro-chemicals, curbing environmental pollution, promoting ecological preservation, and maintain and improving soil health with a subsequent increment in crop productivity and farmers’ income.

Recently, the Indian Farmers Fertiliser Cooperative (IFFCO) invented and patented (Indian patent application number 201921053828, and application number 201921044499) a nano-fertilizer, viz. nano-urea for use as an alternative to commercial urea. The fertilizer nano-urea has a size of particle in nanometre (nm) in one dimension (minimum 50% of the material), physical particle size ranging between 20 and 50 nm, and hydrodynamic particle size varying from 20 to 80 nm [24]. Nano-urea contains 4% N, has a shelf-life of about 2 years, and has a zeta potential > 30 [24]. It contains functional nutrients derived primarily from urea which are treated with non-ionic surfactants and further stabilised in polymer matrices to produce nano clusters of less than 100 nm size. The developed nano-fertilizer was tested in laboratory conditions, and small-scale pot studies to check its effectiveness [19, 25]. The efficacy of IFFCO invented nano-urea was tested based on multi-location (11,000 locations) and multi-crop (94 crops) trials in different crop seasons, both by the researcher’s and progressive farmers’ in India. It was found that the application of nano-urea enhanced yields in wheat [26–28] maize [29] and Indian mustard [30–32] across the locations.

The initial studies indicated a possibility of curtailing fertilizer doses with subsequent application of nano-urea. Hence, there may be possibility about use of nano-urea can reduce other forms of N fertilization application and also it can positively influence soil health thereby it might pave the way for widespread adoption of nano-fertilizers across the globe and curtail urea requirement at the same time. Therefore, a full-fledged experimental trial was conducted at ICAR-Indian Agricultural Research Institute, New Delhi (India) to assess the effectiveness of nano-urea with the objectives: 1) to assess the effect of nano-urea on crop productivity, profitability, energetics and N uptake under maize-wheat and pearl millet-mustard systems; and 2) to study the effect of nano-urea on mineral N and enzymatic activities.

2. Materials and methods

2.1 Site description

Field experiments were conducted at the research farm of ICAR-Indian Agricultural Research Institute, New Delhi (N 28.38.0838 and E 077.09.1441). The sandy loam soil of the experimental site was mildly alkaline (pH 8.22) and non-saline (EC 0.24 dS m^-1). The plough layer (0–15 cm) soil contained 0.58% organic C, 272 kg ha^-1 available N, 22.3 kg ha^-1 available P, and 311 kg ha^-1 available K. DTPA-extractable Zn, Fe, Mn and Cu contents in the soil were 0.84 mg kg^-1, 4.72 mg kg^-1, 19.9 mg kg^-1 and 1.91 mg kg^-1, respectively.

2.2 Experimental details

The field experiments under maize-wheat and pearl millet-mustard systems were conducted with 8 treatments comprising of 4 rates of N (0, 50, 75 and 100% of recommended N) with and without nano-urea in a randomized complete block design (RCBD) with 3 replications. Fertilizer P and K were applied uniformly at recommended rates to all plots. The complete treatment details are listed in Table 1. The maize, wheat, pearl millet and mustard crops were grown in respective cropping systems for two cropping cycles. The crop management details are given in the Table 2.

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[Figure omitted. See PDF.]

2.3 Nutrient management

The recommended fertilizer doses in terms of N, P₂O₅ and K₂O were 150, 75 and 75 kg ha^-1 respectively for maize crop. In wheat, N, P₂O₅ and K₂O were applied @ 120, 60 and 60 kg ha^-1, respectively. In pearl millet, N, P₂O₅ and K₂O were applied @ 60, 60 and 30 kg ha^-1, respectively. In mustard, N, P₂O₅ and K₂O were applied @ 80, 40 and 30 kg ha^-1, respectively. The source of fertilizers were urea, single superphosphate and muriate of potash to supply N, P and K, respectively. In mustard and pearl millet, 50% of N (as per treatment) and 100% of recommended P and K were applied as basal, and the remaining 50% N was top-dressed in one split. In wheat and maize, 50% of N (as per treatments) and 100% of P and K fertilizers were applied as basal, and the remaining N was top-dressed in two equal splits. The Nano- urea was sprayed to each crop twice, first at 30 days after germination and second at one week before flowering. The nano-urea was applied @ 4 mL L^-1. Foliar sprays of nano-urea were undertaken through hand operated knapsack sprayers with flat fan nozzles for complete coverage of leaves. During kharif (rainy) season, whenever rain occurred within 12 hours of spray, the process was repeated. In winter, the spraying was done during evening hours, when there was no dew present on the leaves.

2.4 Harvesting and yield monitoring

All crops were harvested manually and aboveground biomass was removed from the plots. The grain yields of all crops were measured from 16 m² net plot area and finally the yield of net plot was converted into tonne ha^-1. The harvested produce was sun-dried, and thrashed using mechanical thrasher. The straw and grain yields were reported separately on dry-weight basis. All crop residues were removed from the field after harvest.

2.5 Collection and processing of soil samples

At flowering stage of each crop in both systems, representative soil samples were collected from each plot at the depth of 0–15 cm using a core sampler (with a core of 3.9 cm diameter and 179.2 cm³ volume). A sub-set of the fresh undisturbed moist sample was used for analysis of mineral N, microbial biomass carbon (MBC) and dehydrogenase activity (DHA). The sub-set was stored at 4°C in sealed containers. The rest of the samples were air dried, ground in a mortar and pestle, passed through a 2-mm sieve and ultimately stored for further analysis. Another set of soil samples were taken after harvesting of each crop in both systems. One sub-set of undisturbed field-moist soil samples were used for analysis of mineral N, whereas, rest of these samples were air dried and processed for further analysis.

2.6 Soil and plant analysis

Dehydrogenase activities (DHA) were monitored in soil at flowering stage (S1 Table in S1 File) under maize-wheat and pearl millet-mustard system. The DHA was determined in the same samples by monitoring the rate of production of tri-phenyl formazan (TPF) from tri-phenyl tetrazolium chloride (TTC) in anaerobic conditions using the method of Casida [33]. Briefly, moist soil (2 g) was treated with 2.5 ml 3% TTC–Tris buffer (pH 7.6), and then incubated in darkness for 24 h at 25°C. All results were expressed on an oven-dried (105°C for 24 hrs followed by 65°C till constant weight gained) soil weight basis.

The mineral N was extracted with 2 M KCl from undisturbed soil samples [34] collected at flowering (S1 Table in S1 File) and post-harvest stages, and estimated following steam distillation method [35]. The available Zn and Cu were extracted from processed soil samples collected at both stages using 0.005 M diethylene tri-amine penta acetic acid (DTPA), 0.1 M tri-ethanolamine (TEA) and 0.01 M calcium chloride (CaCl₂⋅2H₂O), buffered at pH 7.3 [36], and then determined by using atomic absorption spectrophotometer. Grain/seed and straw/stover samples were analysed for N content following the micro-Kjeldahl method as described by Jackson [37].

2.7 Nitrogen uptake

Nitrogen (N) uptake by grain/seed and straw/stover of different crops was calculated in kg ha^-1 in relation to dry matter production ha^-1 by using the following formula [38]:

2.8 Economic analysis

The economic analysis included studying the cost of cultivation, gross returns, net returns, and benefit: cost (B:C) with or without subsidy of urea in different treatments. The cost of cultivation was calculated for all treatments with the prevailing market prices of inputs and worked out by considering all the expenses incurred in the cultivation of each crop and summed up with the common costs of various operations and inputs. Gross profit was calculated by multiplying the grain and straw yield ha^-1 with the prevailing market prices of grain and straw, respectively. Benefit-cost ratio (B:C) was calculated by dividing the net return to the cost of cultivation of the individual treatment combination.

2.9 Energy use efficiency

All inputs (fertilizers, seeds, fuel, human, agro-chemicals, implements, machine etc.) and outputs (main and by-product) were taken for energy computations. Physical unit of inputs were translated in to energy units [39] by multiplying with energy equivalents (S2 Table in S1 File) as suggested by Devasenapathy et al. [40] for the estimation of energy inputs. Similarly, by multiplying the amount of grain/seed and straw/stover yield by its corresponding energy equivalent [40] the energy output [39] was calculated. The net energy and energy use efficiency were calculated as described below.

2.10 GHG emission

Greenhouse gas (GHG) emissions was calculated using reference values 5.15 kg CO₂-eq kg^-1 product (urea production + use) [6], and value for GHG emission was taken 0.248 kg CO₂-eq L^-1 product based on the steam and power consumption of nano-urea plant (measured).

2.11 Statistical analysis

The data were subjected to analysis of variance (ANOVA) of a RCBD and tested at 5% level of significance (Tables 3–6, 8; S3-S6 Tables in S1 File, and Figs 1 and 2) using SAS 9.3. Post-hoc mean separation was done using Duncan’s Multiple Range Test. The analysis was made following fixed effect model [41]where, y_ij is the measurement on the unit in block j that received treatment i, μ is the mean, τ_i is the i^th treatment effect, β_j is the j^th block effect, and ϵ_ij is the random error of the observation.

[Figure omitted. See PDF.]

*Values of means followed by different capital letter(s) based on duncan’s multiple range tests are significantly different at p ≤0.05.

[Figure omitted. See PDF.]

*Values of means followed by different capital letter(s) based on duncan’s multiple range tests are significantly different at p ≤0.05.

[Figure omitted. See PDF.]

[Figure omitted. See PDF.]

[Figure omitted. See PDF.]

[Figure omitted. See PDF.]

3. Results

3.1 Productivity

Highest grain yields of maize were recorded under N₁₀₀PK+ nano-urea in the year 2019–20 and 2020–21 (Fig 1). The treatments N₁₀₀PK and N₇₅PK+ nano-urea registered comparable grain yields in both cropping seasons. Treatments with applications of 50% of recommended N dose along with nano-urea sprays exhibited significantly lower yields compared with N₁₀₀PK plots. Irrespective of nano-urea applications, all treatments with application of recommended N registered similar grain yields in both cropping seasons. The plots under no fertilizer N application, irrespective of nano-fertilizer spray registered very low grain yields, which were significantly lower compared with recommended NPK application.

Similar yield trends were observed for the wheat crop across seasons. Application of 75% recommended N (90 kg N ha^-1) + PK along with two sprays of nano-urea recorded significantly higher grain yield (4.93 and 4.91 t ha^-1 in 2019–20 and 2020–21, respectively) over no nitrogen + PK and remained at par with the recommended dose of fertilizers (100% N i.e., 120 kg N + PK) (5.02 and 5.14 t ha^-1 in 2019–20 and 2020–21, respectively) (Fig 1). Application of recommended NPK along with spraying of nano-urea exhibited higher grain yield of wheat. Two sprays of nano-urea along with 50% of recommended N + PK application registered significantly lower grain yields compared with N₁₀₀PK. Whereas, application of nano-urea without N fertilizer application yielded very low grain yields.

Plots treated with 75% recommended N (45 kg N ha^-1) + PK along with two sprays of nano-urea recorded significantly higher grain yields of pearl millet (3.29 and 3.25 t ha^-1 in 2019–20 and 2020–21, respectively) compared to recommended NPK application (100% N i.e., 60 kg N + PK) (Fig 2). Maximum grain yields were obtained under N₁₀₀PK+ nano-urea in both cropping seasons. The treatment N₇₅PK+ nano-urea registered ~14% higher grain yields compared with N₅₀PK+ nano-urea in both cropping seasons. All the other treatments with 75% recommended N application along with combinations of nano-urea spray also registered significantly higher grain yields compared with 50% recommended N application with nano-fertilizer sprays. Nano-fertilizer spray without recommended N application resulted in significantly lower grain yields, compared with recommended NPK application, with values ranging from 1.37 to 1.44 t ha^-1.

The mustard seed yields under N₅₀PK + nano-urea and N₇₅PK + nano-urea were similar in 2019–20 (Fig 2). On the other hand, in 2020–21, treatments with 50% recommended N application with nano-urea sprays registered significantly lower seed yields compared with recommended NPK application. The treatments with 75% recommended N application with nano-urea sprays registered similar seed yields compared with recommended NPK application. In 2020–21, treatment N₇₅PK+ nano-urea registered significantly higher seed yield compared with N₅₀PK+ nano-urea. Lowest seed yields were found under non-fertilized plots, irrespective of nano-urea applications.

3.2 Soil mineral nitrogen

Soil mineral N varied from 16.5 to 31.5 mg kg^-1 across the treatments and sampling times in the maize-wheat system. Recommended NPK application along with nano-urea spray registered highest values of soil mineral N (31.5 and 30.4 mg kg^-1 at flowering and post-harvest stages, respectively) in maize crop during 2019–20 (Table 3). During 2020–21, highest mineral N was registered under N₁₀₀PK, and N₁₀₀PK +nano-urea in flowering and post-harvest stages, respectively. In both the years, N₁₀₀PK and N₇₅PK+ nano-urea had comparable mineral N in post-harvest soils of maize crop. Overall, the recommended application of NPK had similar soil mineral N across the cropping seasons irrespective of nano-urea application. Treatment N₇₅PK+ nano-urea registered ~13% and ~17% lower mineral N compared with N₁₀₀PK in soils at flowering stage of maize crop in 2019–20 and 2020–21, respectively. The N₀ treatments irrespective of P, K and nano-fertilizer application reported similar mineral N, with values ~41 and 33% lower compared with recommended NPK application and N₇₅PK+ nano-urea respectively. In wheat, N₁₀₀PK and N₇₅PK+ nano-urea registered similar values of mineral N in flowering and post-harvest soils of 2020–21. In 2019–20, treatments with 50% recommended N doses with sprayings of nano-urea registered significantly lower mineral N compared with recommended NPK application (Table 3). However, the recommended NPK application irrespective of nano-urea application had similar soil mineral N content in wheat crop. Soil mineral N varied from 17.9 to 33.9 mg kg^-1 across the treatments and sampling times in the pearl millet-mustard system (Table 4). The N₇₅PK treatments along with two sprays of nano-urea had similar mineral N as compared with recommended NPK application, irrespective of time of sampling. Treatments without N application, irrespective of nano-urea spray registered ~36% lower mineral N compared with N₇₅PK+ nano-urea.

3.3 Dehydrogenase activities

Under maize, N₇₅PK+ nano-urea registered similar DHA activity (31.9 and 30.3 μg TPF g^-1 24 h^-1 in first and second years, respectively) compared to N₁₀₀PK+ nano-urea (35.1 and 34.6 μg TPF g^-1 24h^-1 in first and second years, respectively) and N₁₀₀PK (33.4 and 33.8 μg TPF g^-1 24h^-1 in first and second years, respectively) (Table 5). On the other hand, N₇₅PK+ nano-urea had significantly higher values of DHA, compared with N₅₀PK (24.3 and 22.9 μg TPF g^-1 24h^-1 in first and second years, respectively) and N₅₀PK+ nano-urea (26.7 and 24.5 μg TPF g^-1 24h^-1 in first and second years, respectively).

Application of recommended N doses (N₁₀₀PK) along with nano-urea spray registered significantly higher DHA (38.3 and 36.0 μg TPF g^-1 24h^-1 in first and second years, respectively) in soil at flowering stage of wheat compared with application of 50% of recommended N doses (Table 5). On the other hand, treatments with application of 75% of recommended N doses registered similar values of DHA as compared with N₁₀₀PK. Under nano-urea spraying treatments, N₁₀₀PK, N₇₅PK and N₅₀PK had similar DHA, with values significantly higher than that under N₀PK + nano-urea.

Soil DHA varied from 20.4 to 44.0 μg TPF g^-1 24h^-1 across the treatments and sampling times in the pearl millet-mustard system (Table 5). Irrespective of nano-urea spray, plots received 50% recommended N application registered significantly lower values of DHA compared with those under recommended N application along with nano-urea spray (42.2 and 42.2 μg TPF g^-1 24h^-1 in pearl millet, and 44.0 and 43.2 μg TPF g^-1 24h^-1 in mustard during first and second years, respectively). Application of N₇₅PK along with nano-urea spray noted at par DHA (38.5 and 38.5 μg TPF g^-1 24h^-1 in pearl millet, and 40.2 and 39.7 μg TPF g^-1 24h^-1 in mustard during first and second years, respectively) over N₁₀₀PK (39.5 and 39.5 μg TPF g^-1 24h^-1 in pearl millet, and 39.2 and 36.9 μg TPF g^-1 24h^-1 in mustard during first and second years, respectively).

3.4 Grain/Seed nitrogen uptake

Grain/seed N uptake was significantly influenced by different treatments of nano-urea under maize-wheat and pearl millet-mustard system (Table 6). The highest grain/seed N uptake was noted under N₁₀₀PK + nano-urea sprays (87.2 and 84.7 kg ha^-1 for maize, 65.1 and 65.3 kg ha^-1 for wheat, 59.7 and 62.1 kg ha^-1 for pearl millet, 65.8 and 69.7 kg ha^-1 for mustard in first and second years, respectively) and it remained at par with N₇₅PK + nano-urea (76.2 and 76.7 kg ha^-1 for maize, 59.5 and 57.8 kg ha^-1 for wheat, 56.4 and 58.3 kg ha^-1 for pearl millet, 63.3 and 61.3 kg ha^-1 for mustard in first and second years, respectively) and N₁₀₀PK (80.5 and 81.0 kg ha^-1 for maize, 63.0 and 64.5 kg ha^-1 for wheat, 59.3 and 62.0 kg ha^-1 for pearl millet, 64.7 and 66.5 kg ha^-1 for mustard in first and second years, respectively). Furthermore, N₇₅PK + nano-urea was at par with N₁₀₀PK with respect to grain/seed N uptake irrespective of crops.

3.5 Profitability

The cost of cultivation with subsidized urea was comparatively lower than unsubsidized urea while net returns and B:C were high across the crops. The cost of cultivation, gross and net returns were highest under N₁₀₀PK + nano-urea in all the tested crops. Significantly (P<0.05) higher net returns (US $ 1044 with and US $ 911 without subsidised urea) and B:C (2.80 with and 2.28 without subsidised urea) were recorded under N₁₀₀PK + nano-urea over N₅₀PK (S3 Table in S1 File) while it remained at par with N₇₅PK + nano-urea (net return of US $ 911 with and US $ 811 without subsidised urea, and B:C of 2.59 with and 2.21 without subsidised urea) and N₁₀₀PK (net return of US $ 958 with and US $ 825 without subsidised urea, and B:C of 2.75 with and 2.21 without subsidised urea). N₁₀₀PK was again statistically at par with N₇₅PK + nano-urea with respect of net returns and B: C.

The N₇₅PK with spray of nano-urea recorded similar gross (US $ 1294) and net returns (US $ 787 with and US $ 707 without subsidised urea) with N₁₀₀PK (gross return of US $ 1334, and net return of US $ 852 with and US $ 745 without subsidised urea) for wheat (S4 Table in S1 File). B:C based on subsidised urea was significantly superior under N₁₀₀PK (2.77) than N₇₅PK + nano-urea (2.55) but B:C based on unsubsidised urea was statistically at par under N₇₅PK + nano-urea treatments (2.21).

The treatment N₁₀₀PK+ nano-urea registered the highest gross (US $ 1309) and net returns (US $ 929 with and US $ 876 without subsidised urea) among the treatments but the same remained at par with N₇₅PK + nano-urea (gross return of US $ 1219, and net return of US $ 842 with and US $ 802 without subsidised urea) and N₁₀₀PK (gross return of US $ 1250, and net return of US $ 901 with and US $ 848 without subsidised urea) for pearl millet (S5 Table in S1 File). However, highest B:C (with 3.58 and without 3.11 subsidised urea) was observed under recommended N₁₀₀PK and it was statistically at par with N₇₅PK + nano-urea (2.92) when unsubsidized urea was taken into account.

In mustard, highest gross return (US $ 1639) as well as net return (US $ 1240 with and US $ 1169 without subsidised urea) were also obtained under N₁₀₀PK+ nano-urea treatment (S6 Table in S1 File) however, with respect to gross and net returns this treatment was statistically at par with N₇₅PK + nano-urea (gross return of US $ 1524, and net return of US $ 1130 with and US $ 1077 without subsidised urea) and N₁₀₀PK treatments (gross return of US $ 1575, and net return of US $ 1207 with and US $ 1136 without subsidised urea). Perusal of the data revealed that by the application of N₁₀₀PK the maximum B:C (4.28 with and 3.59 without subsidised urea) was obtained but, it remained at par with N₇₅PK + nano-urea treatment (3.40) while unsubsidized urea was taken into consideration.

3.6 Nano-urea additional price/saving

An additional cost ranging from US $ 9.10 to 12.76 was incurred due to application of nano-urea under N₇₅PK + nano-urea treatments over N₁₀₀PK across the crops while subsidized urea was taken into account (Table 7). On the other hand, when unsubsidized urea was taken into consideration an additional saving ranging from US $ 7.42 to 41.99 was noted under N₇₅PK + nano-urea plot over N₁₀₀PK. Grain/seed yield of maize, wheat, pearl millet and mustard under N₇₅PK + nano-urea treatment was at par with N₁₀₀PK.

[Figure omitted. See PDF.]

3.7 Energy efficiency

Based on the energy equivalents (S2 Table in S1 File), energy input, energy output, net energy returns and energy use efficiency (Table 8) were calculated. Energy input was highest under N₁₀₀PK + nano-urea across the different crops under the investigation. Energy outputs under N₁₀₀PK + nano-urea treatments (241997, 167846 and 162131 MJ ha^-1 under maize, wheat and pearl millet, respectively) were significantly superior over rest of the treatments except in pearl millet where it remained at par with N₁₀₀PK.

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Significantly higher net energy returns (220922, 147489, 148846 and 156103 MJ ha^-1 under maize, wheat, pearl millet and mustard, respectively) were recorded under N₁₀₀PK + nano-urea over rest of the treatments and remained at par with N₁₀₀PK in wheat (138728 MJ ha^-1), pearl millet (142361 MJ ha^-1) and mustard (147909 MJ ha^-1). The treatment N₇₅PK + nano-urea recorded significantly higher energy use efficiency (11.8, 8.4, and 11.4 under maize, wheat and mustard, respectively) over N₁₀₀PK (Table 8). While, energy use efficiency under N₇₅PK + nano-urea treatment (12.2) was statistically at par with N₁₀₀PK (11.7) and N₁₀₀PK + nano-urea (12.2).

3.8 Greenhouse gas emission (GHG)

Greenhouse gas emission influenced by 2 treatments (N₁₀₀PK and N₇₅PK + nano-urea) was quantified and presented in Table 9. The GHG emission under N₁₀₀PK was high (1678.9, 1344.2, 669.5 and 896.1 kg CO₂-eq ha^-1 in maize, wheat, pearl millet and mustard, respectively) while 25% reduction in N doses along with nano-urea spray (N₇₅PK + nano-urea) recorded less GHG emission (1262.4, 1010.0, 505.3 and 670.1 kg CO₂-eq ha^-1 in maize, wheat, pearl millet and mustard, respectively). The reduction in GHG emission due to nano-urea application (N₇₅PK + nano-urea) was 416.5, 334.1, 164.2 and 226.0 kg CO₂-eq ha^-1 in maize, wheat, pearl millet and mustard, respectively.

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4. Discussion

4.1 Productivity and N uptake

Application of 75% recommended dose of N + PK along with two sprays of nano-urea recorded statistically at par results with N₁₀₀PK for the yields of maize, wheat, pearl millet and mustard during both the years (Figs 1 and 2). Although the yields of wheat and mustard were statistically at par during first year, yield of both crops significantly decreased under N₇₅PK than N₁₀₀PK during second study year. This might be due to decline of inherent fertility status of the soil which supplemented the N nutrition to both the crops during first year (the year of start of the experiment). Whereas, the application of nano-urea had advantage over no application of nano-urea in the same treatment (N₇₅PK). Hence, up to 25% of recommended N dose can be curtailed without any yield penalty, with nano-urea application. In the current study, nano-urea was sprayed on leaves, leading to direct penetration through stomatal pores, and transportation through plasmodesmata [25]. Diminutive surface property and size of nano-urea enable its penetration into the plants through leaves. After entry in plant systems, nano-urea releases N in a controlled manner. The uptake efficacy of nano-urea by plant is 80% higher [25] than conventional urea. However, the efficiency of nano-urea depends on the concentration, method of application and weather conditions. Babu et al. [17] opined that, efficiency of nano-urea was higher under warm weather condition due to better acquisition and translocation of nano-urea by plants.

Nano-urea boosts speedy nutrients availability to growing plant parts, ensuing increased dry matter accumulation, chlorophyll production, plant growth, development (data not reported) and yield. Nitrogen is an important structure of chlorophyll and higher N content can positively influence chlorophyll biosynthesis [42] which in turn can reflect as increased photosynthesis and subsequent increase in dry matter accumulation, this can explain the increased yield seen in the foliar applied Nano N treatment. The enhancement in the crop yield is possibly due to the synchronous release of N from the nano-urea following the demand of the crops (maize, wheat, pearl millet and mustard). Smart release of N from nano-urea improved photosynthesis by acquiring sufficient light-harvesting chlorophyll-protein complexes and also did not cause any stress in the crops resulting in the enhanced growth and ultimately yield [17]. Nano-urea discharges nutrients in 40–50 days [24], and it is applied on the leaves instead of soil; whereas conventional urea is applied in soil and discharges nutrients in 2–7 days [43]. Leaching and volatilization accounts more than 70% of applied conventional urea and leaving only <20% [44] readily available for plants growth. Nano-urea releases nitrogen 12 times slower than urea and thus is available for functional metabolic interaction for a longer time and this can be one of the reasons for increased grain yields [21].

Our results are consistent with Kumar et al. [24] who reported that foliar sprays of nano-fertilizer at critical crop growth stages either in isolation or in combination with fertilizers increases crop yields even at reduced levels of application of their conventional analogues. Al-Juthery et al. [27] and Abdel-Aziz et al. [45] indicated that foliar spray of nano-fertilizers significantly improved the plant growth characteristics and yield of wheat. Yield attributes viz., number of effective tillers per metre row length, ear length (cm), grains per ear, test weight etc. of crop were also higher in nano-fertilizer applied plots [46]. Nano-NPK applications were reported to stimulate the porphyrin molecules present in metabolic compounds, in turn, increasing plant biomass, yield and yield attributes of maize [47, 48]. Irrespective of crops, N uptake under N75PK + nano-urea treatment was at par with N100PK (Table 6). This was mainly due to the statistically similar level of yield observed in all crops under said treatments vis-à-vis statistically at par N content was recorded under both the treatments. It indicates that uptake mechanism is also triggered by the application of nano-urea as foliar spray. Similar finding was also reported by [17, 48].

Nano-urea contains nanoscale particles of 18–30 nm size range, which has ~ 10,000 times more surface area compared with conventional prilled urea [24]. Thus, the nano-urea particles have high surface area to volume ratio, unique magnetic properties, electronic states and catalytic reactions [19]. These unique properties manifest in exceptionally high reactivity of nano-fertilizers compared with conventional fertilizers [19, 48]. The nano particles have the ability of binding with carrier proteins through aquaporin and ion channels, and in turn, tend to facilitate different metabolic processes in plant systems, resulting in higher production of photosynthates [24]. Furthermore, nano-fertilizers mediate nutrient absorption in synchrony with plant demand for nutrients [49]. The yield enhancement due to Nano-fertilizers were reported in wheat [26–28] maize [29] and Indian mustard [30–32].

4.2 Soil mineral nitrogen

Results indicated the possibility of curtailing up to 25% of recommended N dose through nano-urea spray (Tables 3 and 4). The nano-urea contains ~ 4% N by weight [24], which by itself cannot possibly supply the remaining ~ 25% of recommended N dose to plants. As the experimental soil is deficient in N, we obtained yield responses to nano-urea as well as N fertilizer application in both cropping systems. Nano-urea application possibly enhanced root morphology and growth [50]. As a result, use efficiency of native soil N and applied N increased by virtue of greater nutrient adsorption surface of the crops [50]. Application of nano-urea tapped into the vast pool of non-mineral (apart from ammonium and nitrate) N present in soil which is ~96–98% of total soil N. Therefore, the treatments N₁₀₀PK and N₇₅PK+nano-urea registered similar values of mineral N across the seasons (Tables 3 and 4), implying non-occurrence of nutrient mining. Nonetheless, the recommended N applications along with nano-urea spray reported highest soil mineral N. Further, 50% of recommended N applications depleted soil mineral N, irrespective of nano-urea applications. On the other hand, treatments with no N applications recorded very less mineral N across the seasons even with the nano-urea spray. Therefore, at least 75% of the recommended N should be applied to avoid N mining.

4.3 Dehydrogenase activities (DHA)

The N application is directly linked with greater shoot and root biomass production [51]. Enhanced root growths under recommended N applied plots might have caused greater production of root exudates and other secretions. These carbonaceous materials often act as substrates for microbial growth and metabolism. Therefore, the recommended N applied plots registered highest values of DHA, which supports this contention (Table 5) [52]. Further, DHA is considered to be proportional to the biomass of the microorganisms present in the soil [52]. The application of nano-urea further stimulated root growth and activities, in turn favouring soil enzymatic activities. On the other hand, 50% curtailing of N fertilizer doses, irrespective of nano-spray registered significant decline in crop yield (Figs 1 and 2) vis-à-vis root growth and activities. The decline in root growth in turn registered significant decline in DHA in N₅₀ and N₀ plots. The nano-urea application did not register any negative impact on soil microbial health.

4.4 Profitability and additional price/saving

The cost of cultivation with unsubsidized urea was comparatively higher than subsidized urea while net returns and B:C were lower across the crops (S3-S6 Tables in S1 File). This is due to high cost of unsubsidized urea (US $ 28.38 per 45 bag) where cost of subsidized urea was US $ 3.36. As far as urea-price is concerned government of India (GoI) is giving huge subsidy on it. According to Fertilizer Statistics 2019–20 [53] total requirement of N source as urea was 33.9 mt (24.8 mt produced in India and 9.1 mt imported from different countries) and government provided US $ 478 billion subsidies to the Indian Farmers. That indicates whether we should enhance the efficiency of urea or should have some alternatives as novel fertilizers to cut the additional economic burden on government as well as on farmers.

Overall, the economics in terms of net returns and B:C under N₇₅PK + nano-urea treatment over N₁₀₀PK were almost similar across the different crops during both the years (S3-S6 Tables in S1 File). This indicates application of two sprays of nano-urea can curtail up to 25% of the recommended dose of N. In this way out of total requirement of urea (33.9 mt) in the country like India 8.5 mt of its use can be replaced with nano-urea which can save all the subsidised cost borne by importing urea of around US $ 384 billion. Although, the spraying cost of nano-urea incurred additional burden on farmers but it can be minimized using drone on custom hiring basis.

Application of nano-urea along with N₇₅PK (Table 7) based on subsidised urea incurred additional cost ranging from US $ 9.10 to 12.76 per ha over recommended dose of nitrogen (N₁₀₀PK) under different test crops used for the study. This is due to huge subsidy on urea fertilizer provided by the government of India. In contrast, N₇₅PK along with 2 times nano-urea sprays resulted additional saving ranges from US $ 7.42 to 41.99 based on unsubsidized urea price. The saving was more in case of maize where higher recommended dose of N (150 kg ha^-1) was applied while less saving was noted in pearl millet where recommended N dose was less i.e., 60 kg ha^-1. The maximum B:C ratio under 50% recommended NPK application along with nano-urea spray were reported by [24, 54].

4.5 Energy efficiency

Energy input was highest under N₁₀₀PK + nano-urea across the four crops under the investigation followed by N₁₀₀PK and the difference between these two treatments was only 11 MJ ha^-1 due to addition of nano-urea. It indicates energy requirement for the production of nano-urea is very less or in other words it is a relatively energy efficient technology. As energy equivalent for production of one bottle of nano-urea is 2.13 MJ ha^-1 per 500 mL. In this way total 5 bottles of nano-urea (needed for 2 sprays in 1 ha) required 10.65 MJ ha^-1 energy. One the other hand, energy equivalent for the production of 1 kg N is 60.6 MJ ha^-1. In this way energy input required for supplementing N in maize, wheat, pearl millet and mustered is 9090, 7272, 3636 and 4848 MJ ha^-1. Therefore, curtailing 25% demand of N in crops by the intervention of nano-urea would be one of the best energy saving practices. Energy use efficiency is one of the indicators which show the potential of any production system in terms of its effectiveness [17]. Based on the results of this parameter N₇₅PK + nano-urea was found to be an energy efficient option over N₁₀₀PK irrespective of crops. This is due to statistically at par yields among all crops were noticed under N₇₅PK + nano-urea over N₁₀₀PK along with reduction of 25% dose of N as observed in the former case over latter.

4.6 Greenhouse gas emission (GHG)

Greenhouse gas emission due to treatments (N₁₀₀PK and N₇₅PK+ nano-urea) was calculated using reference values of urea production + use (5.15 kg CO₂-eq kg^-1 product) [6]. GHG emission reference value 0.248 kg CO₂-eq L^-1 product was taken into consideration for nano-urea. Comparatively higher GHG emission was recorded in N₁₀₀PK treatments over N₇₅PK + nano-urea due to 25% reduction in nitrogen doses because per kg production of urea emits more GHG over nano-urea.

5. Conclusion

Based on the present investigation, it can be inferred that conjoint application of nano-urea with conventional nitrogen fertilizer have potential to enhance crop yields, nutrient uptake, and lowering GHG emissions. Application of 75% recommended N through conventional N fertilizer (urea) in maize (three equal split), wheat (three equal split), pearl millet (two equal split) and mustard (two equal split) + full dose of P₂O₅ and K₂O along with 2 time sprays of nano-urea (1250 ml ha^-1 spray^-1) resulted statistically at par grain yield over recommended dose of fertilizer (100%N + full dose of P₂O₅ and K₂O). Therefore, there is a possibility of curtailing up to 25% of the recommended dose of N by application of two sprays of nano-urea in maize-wheat and pearl millet-mustard cropping systems was observed. In this way, nano-urea seemed to have the potential to bring forth paradigm shift in the fertilizer consumption scenario of India and world. However, the findings of the nano-urea need to be further validated across the crops and locations before reaching to the policy.

Supporting information

S1 File.

https://doi.org/10.1371/journal.pone.0284009.s001

(DOCX)

S1 Graphical abstract.

https://doi.org/10.1371/journal.pone.0284009.s002

(TIF)

Acknowledgments

The authors are highly thankful to the Director, IARI for providing necessary facilities for this investigation. The authors also expressed their amiable gratitude to learned scientist Late Dr. Gulab Singh Yadav, ICAR-IARI, New Delhi, India for his contribution in the project.

Citation: Upadhyay PK, Dey A, Singh VK, Dwivedi BS, Singh T, G. A. R, et al. (2023) Conjoint application of nano-urea with conventional fertilizers: An energy efficient and environmentally robust approach for sustainable crop production. PLoS ONE 18(7): e0284009. https://doi.org/10.1371/journal.pone.0284009

About the Authors:

Pravin Kumar Upadhyay

Roles: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing – original draft

Affiliation: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India

Abir Dey

Roles: Conceptualization, Data curation, Methodology, Writing – original draft

Affiliation: Division of Soil Science and Agricultural Chemistry, ICAR-Indian Agricultural Research Institute, New Delhi, India

Vinod Kumar Singh

Roles: Conceptualization, Funding acquisition, Methodology, Writing – original draft, Writing – review & editing

E-mail: [email protected]

Affiliations: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India, ICAR-Central Research Institute for Dryland Agriculture, Hyderabad, India

ORICD: https://orcid.org/0000-0003-1208-4857

Brahma Swaroop Dwivedi

Roles: Funding acquisition, Methodology, Writing – review & editing

Affiliations: Division of Soil Science and Agricultural Chemistry, ICAR-Indian Agricultural Research Institute, New Delhi, India, Agricultural Scientists Recruitment Board, New Delhi, India

Tarunendu Singh

Roles: Funding acquisition, Writing – review & editing

Affiliation: Indian Farmers Fertiliser Cooperative Limited, New Delhi, India

Rajanna G. A.

Roles: Data curation, Methodology

Affiliations: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India, ICAR- Directorate of Groundnut Research, Regional Station- Ananthapur, Ananthapur, India

ORICD: https://orcid.org/0000-0003-3786-4956

Subhash Babu

Roles: Data curation, Methodology, Writing – review & editing

Affiliation: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India

Sanjay Singh Rathore

Roles: Conceptualization, Supervision, Writing – original draft

Affiliation: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India

Rajiv Kumar Singh

Roles: Methodology, Writing – original draft, Writing – review & editing

Affiliation: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India

Kapila Shekhawat

Roles: Investigation, Writing – original draft, Writing – review & editing

Affiliation: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India

Meenakshi Rangot

Roles: Data curation, Investigation

Affiliations: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India, GD Goenka University, Sohna, Haryana, India

Pradeep Kumar

Roles: Data curation, Formal analysis

Affiliation: Nano Fertilizer Plant, Indian Farmers Fertiliser Cooperative Limited, Prayagraj, India

Dhinu Yadav

Roles: Formal analysis, Investigation

Affiliation: Division of Soil Science and Agricultural Chemistry, ICAR-Indian Agricultural Research Institute, New Delhi, India

ORICD: https://orcid.org/0000-0002-7700-9682

Devendra Pratap Singh

Roles: Visualization, Writing – review & editing

Affiliation: Chandra Shekhar Azad University of Agriculture & Technology, Kanpur, India

Debarshi Dasgupta

Roles: Formal analysis, Investigation, Resources

Affiliations: Division of Soil Science and Agricultural Chemistry, ICAR-Indian Agricultural Research Institute, New Delhi, India, North Dakota State University, Fargo, ND, United States of America

Gaurav Shukla

Roles: Data curation, Formal analysis

Affiliation: Division of Agronomy, ICAR-Indian Agricultural Research Institute, New Delhi, India

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