Effects of Nitrogen Source on Mineral Element, Phytochemical Content, and Antioxidant Activity of Short-Day Onion (Allium cepa) Bulbs

Abstract

Onion (Allium cepa) is one of the world’s most consumed, nutrient-dense foods, low in calories and containing a rich amount of major bioactive compounds, vitamins, and minerals. The purpose of this study was to determine the influence of different nitrogen (N) fertilizer sources on the nutritional and phytochemical qualities of short-day onions. A white-type onion (cv. Texas Grano) was subjected to different fertilizer application treatments, namely (i) pre-plant base application of 80 kg ha⁻¹ N from CaCN₂, alone or in combination with (ii) top-dressing with 50 kg ha⁻¹ N from limestone ammonium nitrate (LAN), or (iii) top-dressing with 50 kg ha⁻¹ N from urea, (iv) pre-plant base application of 80 kg ha⁻¹ N from LAN and top-dressing with 50 kg ha⁻¹ N from LAN, (v) pre-plant base application of 80 kg ha⁻¹ N from urea and top-dressing with 50 kg ha⁻¹ N from urea, and (vi) 0 kg ha⁻¹ N. Pre-plant application of CaCN₂ (80 kg ha⁻¹ N) outperformed standard onion fertilizers, urea (130 kg ha⁻¹ N) and LAN (130 kg ha⁻¹ N), significantly enhancing total phenolic content, antioxidant activity, and calcium (Ca) and potassium (K) content in onion bulbs. Applying 50 kg ha⁻¹ N from urea as top-dressing with the pre-plant application of CaCN₂ (80 kg ha⁻¹ N) elevated total phenolics (5.48 mg GAE g⁻¹) and flavonoids (0.741 mg CE g⁻¹) in the onion bulbs. The highest antioxidant activity (55.9%) and free radical scavenging activity (26.3%) were achieved with top-dressing 50 kg ha⁻¹ N from LAN following CaCN₂ pre-plant application. Application of CaCN₂ + urea also significantly increased onion bulb potassium (2335 mg kg⁻¹) and calcium (828 mg kg⁻¹) contents, while CaCN₂ combined with LAN improved magnesium (123.3 mg kg⁻¹) content. This study recommends pre-plant CaCN₂, top-dressed with either LAN or urea, for improved phytochemical components, antioxidant activities, and certain mineral content in onion bulbs. These findings present a practical approach for cultivating nutrient-rich and phytochemically abundant onion bulbs, promoting improved human health.

1. Introduction

Onion (Allium cepa) is one of the most essential vegetables grown worldwide. In 2019, onion consumption per capita reached 11.6 kg worldwide [1]. There are three types of onion based on color, namely white, red, and yellow, and all have different flavors and pungency, from mild to highly strong [2,3]. Raw onions exhibit antibiotic properties that are known to reduce bacterial contamination in salads [4]. It is an indispensable ingredient for flavoring many dishes. Moreover, it is considered a good source of nutrients and is used in traditional medicine [5].

Not only does onion have moderate levels of protein, fat, and fiber, but it also boasts significant amounts of calcium (Ca), phosphorus (P), potassium (K), iron (Fe), magnesium (Mg), sodium (Na) [6,7], and vitamins B₆ and C [6]. Every segment of an onion, from the bulb to the stalk, is consumable. Frequently ignored, the stalk, in particular, proves to be a nutritional powerhouse. Abundant in carotene and iron, it serves as a valuable reservoir of these essential nutrients [6]. Several studies have described the benefits of onions for human health, including their antioxidant [5,8], antibacterial [9], anticancer [10], anti-hypercholesterolemic [11], anti-obesity [12], blood-pressure lowering [13], heart disease prevention [14], and anti-inflammatory [15,16] properties. The active compounds of onions contribute to their various biological activities. For example, the antioxidant capacity of onions is highly correlated with their total phenolic content, whereas their antibacterial activity is attributable to allicin [9]. Flavonoids in onions, including quercetins and quercetin glycosides, have been reported to be associated with their anticancer and antioxidant properties [10,17]. The quantity and quality of bioactive compounds contained in an onion bulb can vary according to the variety and cultivation practices.

Onions can be grown through direct seeding, seedlings/transplants, or by utilizing sets, as documented by Lazić et al. [18] and Vojnović et al. [19]. Irrespective of the method used for cultivation, onions have a significant requirement for nitrogen (N). Inefficient use of applied N fertilizers is a common concern due to the shallow root system of onions, which often leads to nitrate leaching [1]. These losses contribute significantly to the escalating problem of environmental contamination [20]. To combat this issue, environmentally conscious methods are being explored, such as integrating slow-release fertilizers into cultivation practices [20,21,22].

Nitrogen is one of the most vital nutrients essential for plant growth. Across the globe, N fertilizers significantly impact the productivity and yield of diverse crops such as beetroot [23], maize [24], and wheat [25]. The impact of different N fertilizers on secondary metabolite production in plants has not been extensively studied. However, some studies have suggested a link between N availability and the production of secondary plant metabolites, such as antioxidants, in red beet leaves [26] and amaranth [27]. Different N sources have the potential to influence the secondary metabolism of plants. For example, numerous researchers have indicated that N increased bulb yield, quality, and flavors. Jurgiel-Malecka et al. [28] reported that the applied N fertilization significantly affected microelements in all the tested onions. The content of iron and manganese in onion bulbs increased with the increasing N doses, while that of copper and zinc decreased. Barrales-Heredia et al. [29] showed that the use of N fertilizers resulted in a remarkable improvement in the antioxidant capacity of onion bulbs. Kołota et al. [30] highlighted the significant impact of nutrient supply on both the nutritional value and yield of onion bulbs. Abdissa et al. [31] reported a significant improvement in onion yield when N fertilizer was used compared to an unfertilized onion. In contrast, Mofunanya et al. [32] reported that higher antioxidant and phenolic concentrations of organically grown vegetables are mainly due to the lower N present in the substrate. In another study, reduced N supply was associated with higher phenolic levels in onion [33].

Zhao et al. [8] reported that the application of nitrogen at 260 kg/ha significantly increased the phenolic content of onion bulbs compared to the control treatment. Additionally, nitrogen had a notable impact on the flavonoid content of onion bulbs, with the 130 kg/ha treatment showing the highest content [8]. This pattern was also observed in the DPPH scavenging capacity of Welsh onions, where the application of nitrogen resulted in the highest value, while the control treatment recorded the lowest [5,8]. The findings highlight the influence of nitrogen on the biochemical composition of onion bulbs and suggest potential implications for nutrient management in agriculture.

The integration of slow-release and conventional fertilizers has been acknowledged as a method to enhance nutrient management in agriculture [20,22]. Conventional fertilizers such as urea play a crucial role in supplying the essential nutrients necessary for various plant growth stages. However, their effectiveness is often hampered by the fact that they tend to leach easily into the environment [34], while slow-release counterparts provide nutrients gradually, minimizing wastage [20,22] and reducing environmental leaching [35,36]. Despite these advancements, a notable gap exists in research concerning the impact of different N sources on phytochemical components, antioxidant activities, and the overall nutritional value of onion plants. Ahmadi et al. [37] reported an increase in plant growth and the phytochemical properties of Echinacea purpurea (L.), a medicinal plant, by the use of slow-release N as compared to urea.

Within the realm of slow-release fertilizers, Perlka^® (AlzChem, Trostberg, Germany), also recognized as calcium cyanamide, emerges as a promising contender, especially suited for crops in regions characterized by high rainfall [21,38]. This fertilizer not only stimulates beneficial soil organisms, thereby enhancing soil fertility, but also provides calcium in a readily accessible form, ensuring robust cell walls and healthier plants [21,38,39,40]. Despite its significant positive impact on various crops, there exists a dearth of research investigating its effects on the mineral composition and secondary metabolite production in onions.

Therefore, this study was undertaken to bridge this knowledge gap and assess the influence of calcium cyanamide, functioning as an alternative N source, on the phytochemical quality and mineral element composition of short-day onions. The primary objective of this research was to evaluate the effect of N source on mineral elements, polyphenols, and flavonoid contents as well as antioxidant capacity of onions, thereby contributing valuable insights to the realm of sustainable agriculture.

2. Materials and Methods

2.1. Description of the Study Area

The field experiment was conducted from March to November in 2019 and repeated in 2020 under overhead (sprinkler) irrigation at the Hygrotech Experimental Farm, Dewagensdrift (coordinates 25.4580° S, 28.6411° E, altitude 1214 m), located 60 km from Pretoria, South Africa.

In 2019, an approximate total of 600 mm of irrigation was administered. Initially, plants received 25 mm of irrigation in the first week, followed by a weekly application of 35 mm thereafter. In the subsequent growing season of 2020, a total of 620 mm of irrigation was provided, averaging around 35 to 40 mm per week. The irrigation ceased when approximately 50% of the plants exhibited leaf drops. Subsequently, the plants were uprooted, soil was shaken off, and they were laid out to cure by being suspended in sacks under a shelter. Throughout both growing seasons, the annual average rainfall received amounted to 700 mm (Figure 1).

2.2. Experimental Design and Treatments

In this experiment, the short-day onion variety ‘Texas Grano’ was used. The treatments in the trial comprised various N sources, namely

(i)

pre-plant base application of 80 kg ha⁻¹ N from CaCN₂,

(ii)

pre-plant base application of 80 kg ha⁻¹ N from CaCN₂ + top-dressing with 50 kg ha⁻¹ N from limestone ammonium nitrate (LAN),

(iii)

pre-plant base application of 80 kg ha⁻¹ N from CaCN₂ + top-dressing with 50 kg ha⁻¹ N from urea,

(iv)

pre-plant base application of 80 kg ha⁻¹ N from LAN and top-dressing with 50 kg ha⁻¹ N from LAN,

(v)

pre-plant base application of 80 kg ha⁻¹ N from urea and top-dressing with 50 kg ha⁻¹ N from urea, and

(vi)

0 kg ha⁻¹ N.

These treatments were administered following a randomized complete block design with four replications.

The application of fertilizer was conducted as described by Simelane et al. [21]. Pre-plant fertilization was determined by soil analysis and included 2000 kg ha⁻¹ calcitic lime (except for CaCN₂ treatments), 300 kg ha⁻¹ superphosphate (10.5% P), 150 kg ha⁻¹ potassium sulfate (22.4% K and 18.4% S), and 150 kg ha⁻¹ magnesium sulfate (10% Mg and 13% S), which were broadcasted. Additional fertilizer was applied according to the N source treatments, with LAN (28% N, 285.7 kg ha⁻¹) and urea (46% N, 173.9 kg ha⁻¹). Perlka^®, a calcium cyanamide (CaCN₂, 19.8% N), was applied at 80 kg ha⁻¹ N by broadcasting and incorporated into the soil to a depth of 10 cm (Perlka; AlzChem, Trostberg, Germany). Calcium cyanamide was watered and kept moist for 8 days before seeding. Top-dressing of LAN and urea was applied at 35.71 and 22 kg ha⁻¹ per week, respectively, for five applications from week 4 to 8 after seeding, depending on the treatments. The control treatment received no N fertilizer. Additionally, potassium sulfate was applied at 50 kg ha⁻¹ week⁻¹ from week 6 to 10 after sowing, comprising five applications for all treatments, regardless of the N source.

2.3. Determination of Onion Bulb Mineral Composition

For each treatment, six bulbs of uniform size were meticulously chosen for mineral analysis. Subsequently, freeze-dried onion samples ground into fine powder underwent wet digestion in order to determine their mineral composition, encompassing essential elements such as phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), sulfur (S), iron (Fe), zinc (Zn), and manganese (Mn).

The powdered onion sample (0.5 g) in a Teflon beaker was digested in 10 mL of aqua regia [HNO₃: HCl, (1:3 v/v)] on a hot plate at 95 °C for 2 h. Both nitric acid (65%) and hydrochloric acid (37%) were obtained from Merck (Darmstadt, Germany). Each digested sample was carefully transferred and the beaker was rinsed several times using deionized water (18 Ω, Opurite System, Lasec, Cape Town, South Africa) into a 100 mL calibrated volumetric flask. Each sample was digested in triplicate. The mineral element analysis procedure involved utilizing an aliquot of the digested sample for ICP-AES (inductively coupled plasma atomic emission spectrometry) analysis. This analysis was executed using the ICPE-9820 (Shimadzu Corporation, Kyoto, Japan) at the ARC-Vegetable, Industrial and Medicinal Plants, Pretoria. Scandium (Sc, wavelength of 361.384 nm) was included as an internal standard. Each mineral element was quantified at a specific wavelength with no or minimum interferences using a calibration curve generated from multi-element standards, with concentrations ranging from 0.0016 ppm to 1000 ppm. Each element quantified was expressed in mg per 100 g dry weight.

This meticulous approach ensured an accurate and reliable data collection, contributing to a thorough understanding of the onion samples’ mineral element content.

2.4. Total Phenolic and Flavonoid Contents

Freeze-dried onion samples weighing 0.2 g were subjected to extraction with 10 mL of 50% methanol using a sonication bath for 20 min. After centrifugation, the resulting supernatant was utilized for the analysis of total phenolic and flavonoid contents. Total phenolic content was determined using the Folin Ciocalteu colorimetric method [41]. The reaction mixtures were incubated at 25 °C for 40 min, and the absorbance was measured at 725 nm against a blank using a spectrophotometer (SPECORD^® 210 PLUS, Analytik Jena, Jena, Germany). Gallic acid (0.1 mg mL⁻¹) served as the standard for creating the calibration curve, and the results were expressed in milligrams of gallic acid equivalent (GAE) per gram of dry weight. The assay was performed in triplicate.

For the quantification of flavonoid content, the aluminum chloride method was employed [42]. Absorbance was measured at 510 nm against a freshly prepared reagent blank using a spectrophotometer. Standard calibration curves were generated using suitable aliquots of catechin (0.1 mg mL⁻¹). The quantification was conducted in triplicate. The results were reported in milligrams of catechin equivalent (CE) per gram of dry weight.

2.5. Antioxidant Assays

Analyzing antioxidants in natural compounds can be challenging due to their varied chemical structures, biological functions, and diverse mechanisms of action [42]. Consequently, different antioxidant methods can yield varying results as each assay operates based on its unique thermodynamics and kinetics [43].

2.5.1. DPPH (2,2-Diphenyl-1-picrylhydrazyl) Free Radical Scavenging Activity

Samples were extracted using the method described previously by Amoo et al. [44]. The extract volume was condensed on a rotary evaporator at 40 °C before air-drying. The antioxidant activity was determined using the DPPH method [45], with modifications [46]. An aliquot (30 μL) of 50% MeOH extract was diluted with 720 μL MeOH followed by an addition of 750 μL DPPH solution. Ascorbic acid was used as a positive control. The sample final concentration in the assay was 600 µg/mL. The mixture was incubated at room temperature (25 ± 2 °C) for 40 min before recording absorbance at 517 nm. The assay was conducted in triplicate and the percentage of free radical scavenging activity (RSA) was calculated using Equation (1):

$$RSA\left(\%\right)=\left\{1-\left.\left(\frac{{Abs}_{517nm}Sample-{Abs}_{517nm}Blank}{{Abs}_{517nm}NegControl}\right)\right\}\times 100\right.$$

(1)

where ${Abs}_{517nm}Sample$ is the absorbance of the sample mixture; ${Abs}_{517nm}NegControl$ is the absorbance of the negative control (MeOH); and Abs_{517 nm} Blank is the absorbance of the blank (50%MeOH in place of DPPH).

2.5.2. Antioxidant Activity Using β-Carotene Linoleic Acid Assay

Following extraction with 50% methanol [46], the extracts were concentrated in vacuo after filtration through Whatman No. 1 filter paper. The concentrated 50% methanol extracts were then air-dried at room temperature. Dried extracts were then dissolved in methanol at a known sample concentration in glass vials [46,47,48]. A 2.4 mL aliquot of β-carotene emulsion, comprising 5 mg of β-carotene dissolved in 1 mL of chloroform, 100 µL of linoleic acid, 1 mL of Tween 20, and 248 mL of distilled water, was dispensed into reaction tubes containing 100 µL of the sample extract at a predetermined concentration. As a positive control, butylated hydroxytoluene was prepared at a concentration of 6.25 mg mL⁻¹. A negative control was prepared using aqueous methanol (50%) instead of the sample. The assay was conducted in triplicates. Absorbance readings were taken immediately at 470 nm, and a second reading at 470 nm was obtained after incubation in a water bath at 50 °C for 1 h.

The rate of β-carotene bleaching was calculated using Equation (2).

$$Bleachingrate(R)=\left\{\mathrm{l}\mathrm{n}\left.\left(\frac{A_{t=0}}{A_{t=t}}\right)\right\}\right.\times \frac{1}{t}$$

(2)

where $A_{t=0}$ is the absorbance of the emulsion at 0 min and $A_{t=t}$ is the absorbance of the emulsion at 60 min. The bleaching rate was used to calculate the percentage of antioxidant activity (ANT), expressed as a percentage inhibition of the rate of β-carotene bleaching using Equation (3):

$$\%ANT=\left.\left(\frac{R_{control}-R_{sample}}{R_{control}}\right.\right)\times 100$$

(3)

where $R_{control}$ and $R_{sample}$ are the average β-carotene bleaching rates for the control and plant extract or BHT, respectively.

2.6. Statistical Analysis

The collected data were transformed, where applicable, before they were subjected to analysis of variance (ANOVA) using GenStat^®, version 11.1. Where there were significant differences observed, mean values were separated using Fisher’s least significant difference (LSD) protected t-test at a 5% level of significance.

3. Results and Discussion

3.1. Biochemical Composition

3.1.1. Effect of N Source Fertilizer on Total Phenolic Content

This study revealed significant differences (p ≤ 0.05) in total phenolic content among various N fertilizer sources (

Table 1. Effect of different N sources on onion bulb phytochemical content and antioxidant activities.

Fertilizer Source	Total Phenolics (mg GAE g−1)	Flavonoids (mg CE g−1)	Free Radical Scavenging Activity (%)	Antioxidant Activity (%)
Control	2.530 ± 0.036 f	0.315 ± 0.002 e	17.14 ± 0.77 d	18.57 ± 0.06 f
LAN	4.414 ± 0.006 e	0.653 ±0.025 c	21.73 ± 0.74 c	37.47 ± 0.07 d
Urea	4.556 ± 0.011 d	0.695 ± 0.003 b	22.26 ± 1.60 c	23.73 ± 0.16 e
CaCN2	4.672 ± 0.069 c	0.630± 0.010 d	23.63 ± 1.53 bc	48.63 ± 0.10 c
CaCN2 + Urea	5.484 ± 0.008 a	0.741 ± 0.002 a	24.53 ± 1.06 ab	55.90 ± 0.10 b
CaCN2 + LAN	5.134 ± 0.080 b	0.701 ± 0.002 b	26.33 ± 0.32 a	58.30 ± 0.03 a
LSD 0.05	0.0885	0.02076	1.918	1.675

Values (mean ± standard deviation, n = 3) followed by the same letter, within a column, are not significantly different (p > 0.05); values followed by the same letter, within a row, are not significantly different (p > 0.05). LSD: least significant difference; LAN: limestone ammonium nitrate.

). Phenolic content is crucial for flavor, aroma, and oxidative stability in foods [49]. The control treatment (0 kg ha⁻¹ N) exhibited the lowest phenolic content (2.53 mg GAE g⁻¹), while the highest content (5.48 mg GAE g⁻¹) was found in the group treated with pre-plant 80 kg ha⁻¹ N ha⁻¹ CaCN₂ top-dressed with 50 kg ha⁻¹ N urea, followed by pre-plant application of CaCN₂ top-dressed with LAN. Notably, the pre-plant application of CaCN₂ at a rate of 80 kg ha⁻¹ N outperformed the conventional N sources, urea (130 kg ha⁻¹ N) and LAN (130 kg ha⁻¹ N), in enhancing the total phenolic content of the onion bulbs. Comparable documented findings [50] indicated a noteworthy 13.3% rise in strawberry phenolic content with the utilization of CaCN₂ as a N source. In contrast, a prior study revealed that the supply of N had minimal impact on the total phenolic content of onions [51].

Applying 50 kg ha⁻¹ N from LAN or urea as top-dressing with the pre-plant application of CaCN₂ (80 kg ha⁻¹ N) led to an increase (0.72 mg GAE g⁻¹ with LAN and 0.928 mg GAE g⁻¹ with urea) in the total phenolic content of the onion bulbs. This increase could be attributed to a combination of factors. These factors include the role of calcium from CaCN₂ in maintaining cell wall integrity [52,53], reduced leaching due to CaCN₂ [21,40], and balanced nutrient uptake [50]. This balance results from the immediate availability of N from urea and LAN, as well as the continuous supply of N from the slow-release CaCN₂.

In a study by Zhao et al. [8] it was observed that onion extracts treated with 260 kg ha⁻¹ N exhibited the highest phenolic content (22.66 ± 0.50 mg GAE 100 g⁻¹ dry weight), while the 130 kg ha⁻¹ N treatment showed a slight increase compared to the control group. Onyango et al. [54] indicated that the extent of change in phenolic content is dependent on N source and its application rate.

3.1.2. Effect of N Source Fertilizer on Onion Bulb Flavonoids

Flavonoids hold significant dietary importance due to their involvement in various human physiological functions, including the activation of phagocytic cells and their potential as antioxidants in both animal and human systems [55].

The application of N fertilizers yielded a notable increase in the average flavonoid content within the onion bulbs (Table 1). Diverse N-based fertilizer sources exhibited higher flavonoid levels compared to the control treatment. Notably, the use of CaCN₂ + urea as an N source resulted in a significantly (p ≤ 0.05) higher flavonoid content (0.7413 mg CE g⁻¹), followed by CaCN₂ + LAN (0.7013 mg CE g⁻¹), while the lowest flavonoid content was found in the control treatment (0.3153 mg CE g⁻¹) (Table 1). It is noteworthy that no significant differences were observed between CaCN₂ + LAN and urea (Table 1). Matrella et al. [56] also observed the highest flavonoid levels when multiple N fertilizer sources were used, while the control treatments showed the lowest levels.

Contrary to the findings of the present study, Ibrahim et al. [57] found a 19.07% decrease in total phenolic content and a 21.13% decrease in total flavonoids in Labisia pumila Blume, a Malaysian medicinal plant, treated with 270 kg ha⁻¹ N compared to the control. This reduction in flavonoids might be due to high N fertilizer levels (270 kg ha⁻¹ N) disrupting the plant’s nutrient balance, favoring rapid growth over secondary metabolite production like flavonoids. Excessive N inhibits genes and enzymes related to flavonoid biosynthesis, a phenomenon observed in other studies [58,59].

Top-dressing with 50 kg ha⁻¹ N from LAN or urea, to pre-plant application of 80 kg ha⁻¹ N from CaCN₂, significantly boosted onion bulb flavonoid content. The increase was significant at 0.048 mg CE g⁻¹ with LAN and 0.046 mg CE g⁻¹ with urea. Incorporating CaCN₂ as a N source not only supplied N but also essential calcium to the plants [38]. Sufficient calcium levels enhance enzymatic pathways for flavonoid production. The slow-release N from CaCN₂ could contribute to amino acid formation, vital for flavonoid synthesis [40]. It is worth noting that the flavonoid content reported in this study was notably in the same range reported by Amal et al. [60], whose findings indicated 0.67 mg CE g⁻¹ in white onions.

3.1.3. Free Radical Scavenging Activity

Table 1 illustrates the impact of different fertilizer sources on the free radical scavenging activity of onion extracts. Significant differences were observed among the various fertilizer sources tested. The treatment that received 80 kg ha⁻¹ N from CaCN₂ and top-dressed with 50 kg ha⁻¹ N from LAN (CaCN₂ + LAN) exhibited the highest free radical scavenging activity (26.33%). This activity level was comparable to that of CaCN₂ + urea, with no significant difference noted between the two. On the contrary, the treatment receiving no N (0 kg ha⁻¹ N, control) displayed the lowest free radical scavenging activity.

Comparing these findings with prior research, a study [61] reported a range of free radical scavenging activity, from 18% to 35%, in white onions. Additionally, another study [62] reported a free radical scavenging activity of 20% in onions. It is crucial to note that the variations in reported activities might stem from differences in extract concentrations used in each study, potentially confounding the results.

Nonetheless, our current study distinctly highlights variations in free radical scavenging activity based on the choice of fertilizer source, shedding light on the intricate relationship between agricultural practices and the bioactive properties of onions.

3.1.4. Effect of N Source Fertilizer on Antioxidant Activity

The impact of N fertilizer sources on the antioxidant activity of onions was profound. Specifically, when onions were treated with a pre-plant 80 kg ha⁻¹ N from CaCN₂ and top-dressed with 50 kg ha⁻¹ N from LAN (referred to as CaCN₂ + LAN), a remarkable increase in antioxidant activity was observed. This combination elevated the overall antioxidant activity by 20.83% in comparison to LAN alone and by 34.57% in comparison to urea.

Notably, pre-plant CaCN₂ at 80 kg ha⁻¹ N showed superior results when compared to conventional onion fertilizers such as urea and LAN. This finding echoed previous research [63] in which heightened N concentration in the nutrient solution led to an increase in antimicrobial activity in onion bulb extracts. Furthermore, the rise in N concentration correlated with augmented antioxidant activity, total phenolic, total flavonoids, and ascorbic acid levels [63].

Similarly, Rigueira et al. [64] documented significant improvements in antioxidant compounds when 250 kg ha⁻¹ N of urea was utilized. Contrastingly, Rigueira et al. [65] observed heightened antioxidant activity and elevated levels of phenolic compounds in collard greens due to different fertilizer sources. This disparity in findings indicates the complexity of plant responses to various N fertilizers.

In contrast to the present study, El-Mergawi. [66] reported a unique observation where low N levels (control treatment) led to an increase in the antioxidant capacity and phenolic content of lettuce. However, this phenomenon did not extend to tomato fruits, where antioxidant capacities, phenolic compounds, carotenoids, and lycopene levels remained largely unaffected by varying N levels.

The intricate relationship between N fertilization and antioxidant activity in different plants underscores the importance of understanding specific plant responses to diverse N sources. These findings not only enrich our knowledge of plant physiology but also have potential implications for agricultural practices aimed at enhancing the nutritional quality of crops.

3.2. Bulb Mineral Composition

As depicted in

Table 2. Effect of different nitrogen fertilizer sources on onion bulb mineral element composition.

Nitrogen Fertilizer Source	Ca	Na	Mg	Mn	P	K	Cu	Fe	Zn
(mg 100 g−1 Dry Weight)
Control	540.67 ± 5.51 d	423.0 ± 1.2	92.9 ± 2.2 d	1.17 ± 0.02	303.33 ± 2.08	1711 ± 48 e	2.17 ± 0.01	5.98 ± 0.02	6.02 ± 0.05
Urea	717.32 ± 30.29 c	423.7 ± 0.5	119.3 ± 1.1 b	1.18 ± 0.02	303.67 ± 1.53	1869 ± 43 d	2.17 ± 0.02	6.01 ± 0.01	6.04 ± 0.01
LAN	698.00 ± 24.25 c	423.8 ± 1.1	119.1 ± 0.6 b	1.19 ± 0.02	304.67 ± 1.51	1893 ± 32 cd	2.17 ± 0.01	5.43 ± 0.01	6.05 ± 0.02
CaCN2	745.33 ± 5.03 bc	423.9 ± 0.4	110.2 ± 2.2 c	1.18 ± 0.01	303.00 ± 2.64	1935 ± 5 c	2.17 ± 0.03	6.00 ± 0.01	6.05 ± 0.01
CaCN2 + Urea	828.00 ± 17.09 a	423.7 ± 0.4	122.3 ± 1.0 a	1.19 ± 0.01	304.67 ± 1.52	2335 ± 22 a	2.18 ± 0.01	6.30 ± 0.03	6.04 ± 0.01
CaCN2 + LAN	792.00 ± 7.21 ab	424.0 ± 0.2	123.3 ± 0.5 a	1.19 ± 0.01	304.00 ± 1.00	2261 ± 54 b	2.18 ± 0.01	6.03 ± 0.01	6.05 ± 0.01
LSD 0.05	33.95	NS	2.766	NS	NS	51.80	NS	NS	NS

Mean values (±standard deviation, n = 3) in a column with the same letter are not significantly different at p < 0.05. NS: non-significant at p < 0.05; LSD: least significant difference; LAN: limestone ammonium nitrate; CaCN₂: calcium cyanamide.

, the potassium (K) content of onion bulbs was notably affected by the N source. Applying either LAN or urea as top-dressing on pre-plant CaCN₂ substantially enhanced the K content in the onion bulbs. The application of CaCN₂ + urea resulted in the highest K content, followed by CaCN₂ + LAN. The lowest K content of 1711 mg 100 g^-1 was observed in the control treatment (0 kg ha⁻¹ N). The reason for this could be that CaCN₂ contains calcium, which can enhance the cation-exchange capacity (CEC) of the soil [38,39,40]. A higher CEC allows the soil to retain more cations, including potassium (K⁺) [67]. As a result, K⁺ ions are less prone to leaching and remain available in the root zone for onion plants to uptake. This combination of fertilizers helps ensure that potassium remains available to the onion plants, resulting in higher potassium concentrations in the plant tissues.

This finding aligns with the results of Edet et al. [68], who noted that N application significantly increased the potassium percentage in onion bulbs. The current results also corroborate the findings of Stagnari et al. [69], who observed that potassium content in spinach plants was influenced by the application of different N sources. The International Potash Institute (IPI) [70] reported that consuming a potassium-rich diet is crucial for regulating heartbeat and maintaining the optimal functioning of the heart muscle. This, in turn, lowers the risk of heart-related disorders like strokes and heart attacks.

The application of N fertilizer significantly increased both calcium (Ca) and magnesium (Mg) concentrations compared to the control treatment (Table 2). Top-dressing with either 50 kg ha⁻¹ N from urea or LAN or 80 kg ha⁻¹ N pre-plant CaCN₂ resulted in superior Ca and Mg concentrations in the onion bulbs (828.0 and 122.3 mg 100 g⁻¹, respectively), surpassing levels observed with LAN (698 and 119.1 mg 100 g⁻¹), urea (717.32 and 119.3 mg 100 g⁻¹), and CaCN₂ (745.33 and 110.2 mg 100 g⁻¹). The lowest Ca and Mg content (540.67 and 92.9 mg 100 g⁻¹, respectively) in the bulbs was found with the control treatment. Interestingly, pre-application of CaCN₂ at a rate of 80 kg ha⁻¹ N, followed by top-dressing with either LAN or urea at 50 kg ha⁻¹ N, led to an enhanced onion yield when compared to using urea or LAN alone as N sources [40].

Considering that the daily recommended magnesium intake for adults and children aged four and older is 400 mg [71], short-day onions emerge as a rich magnesium source. This increase in Ca and Mg concentration is attributed to CaCN₂, which contains 35.7% calcium [38]. When applied to the soil, CaCN₂ releases calcium ions, enhancing calcium availability for onion plants. CaCN₂ also reduces nutrient leaching, including calcium and magnesium [39]. Its slow-release properties, coupled with enhanced soil CEC, retain Ca and Mg ions in the root zone, minimizing the risk of nutrient loss due to rainfall or irrigation, and ensuring a plentiful supply for onion plants.

Nitrogen source treatments did not significantly influence onion bulb phosphorus (303.00 to 304.67 mg 100 g⁻¹), iron (5.43 to 6.3 mg 100 g⁻¹), zinc (6.02 to 6.05 mg 100 g⁻¹), copper (2.17 to 2.18 mg 100 g⁻¹), sodium (423.0 to 424.0 mg 100 g⁻¹), and manganese (1.17 to 1.19 mg 100 g⁻¹) content (Table 2).

The influence of the fertilizer source appears to have a positive impact on calcium, magnesium, and potassium levels. Remarkably, this effect did not lead to any noteworthy negative consequences or adverse effects on the micronutrient content. This indicates that, despite the enhancement in essential minerals, there was no detrimental impact on the presence of micronutrients, underscoring the overall beneficial nature of the fertilizer source.

4. Conclusions

This study highlights the significant potential of optimizing N fertilizer sources to enhance the nutritional quality of white short-day onions. Specifically, the combination of pre-plant CaCN₂ (80 kg ha⁻¹ N) with subsequent top-dressing with either LAN or urea emerges as a potent strategy. This approach significantly enhances vital components in onions such as phenolics, flavonoids, free radical scavenging activity, and antioxidant activity, elevating the crop’s potential health benefits. Notably, this study revealed substantial improvements in essential mineral element contents including potassium, magnesium, and calcium, reinforcing the overall nutritional value of onions. This highlights the potential of calcium cyanamide as an alternative N source, significantly improving the growth, yield, and quality of onions. These findings underscore a practical and effective method for cultivating onions that are not only nutrient-rich but also packed with health-promoting phytochemicals contributing to improved human health.